Dlr. PAPARA Yl PY Fv:P, ]Lr IBiG ALl lD~ TVl1lH Aito ETiO )UOTt AICT A'IND EIr IE BlOGCAI.PtICE-0L iSOTICt OF TP'Ei HItEF Pi OA BSOTCO b. S OP THE:NEW' tiEWi'. EDWARD L. YOU~MS, N% 12) -: ho t icA i phy;usietl acocow whnc'h o fa'c.iticti'qs te'naipermit ocl to'pes eive —the: NEsW YO KE fAPPL ETON AND:;: COXt)AIAN" -..8 &s 44&5 BROADIW'AY~ i. 4g5 Br:irxErsm, acco'rding to Act of Congress, it ti:L yea it 4 186,t -by D). A'PPLETOI'S AT'D CO):ia' 2AY, I1 the Clerfk's Offite, o tlhe Districe. ourt of the U.tnitedt States tor. t'otouherx Distr'iet of Tew ~Yortc JO.... IN t IJ DiRtLR. L o... IL t D. P]ROPFSSOR OF CtEMISTRY AND PHYSIO:LOGY liN:nH UNIRSTY OF N or R Ko DKr SIRIr seems peculiarly t)propriate that this volumte should be dedicated to you, Knowing- t.he eminent esteema int-. whic you a. re held in tihe ireles of EntroDac.n s"cienee, I can ot doubt that the distinguished au:athors of the fob lowing essays woulvd eor itdly approve tis connection of your natme twith their introduction to the American public. There is, besides a [t tlher reason for thlis hi tmt atrge coicidence of purposo which is manlifest in. their labors and your own, For while the perp vading design of the present collection is to widetn the rmalge of tIhot; b'y unfoldin-g t, broater philosophy of the energies of natrc, your onu. olnprehelnsie colrseo of research-beginning with n cxtsended series of eixperineu.tl.d invefi giations in. chemical physics and plnh sioogy, and rSholng to the consideration. of [ttd spleindid problem, trhe bearing of science upon'the historyof'the Intellectual Development of Europe-h —as, poweroifly contributed to the samne noble end; hat of elevating the nafit. a.nd enflarging the scope of scientific tnqunhi I gladly tasl m Yself of this occasion to say how greatly I am indebted to your w vritings, in which accurate eand profound instruction i.s so often and happivy blended with the chiarms of poeti.c lonuene. Ttt yqou" uay live long to enjoy yoiur well-wo hon0ors, atd'to contribute still further to tile triumtphan.t advance of scientfic trut, is the heat.ftlt Nrwish of Y'ours trdyv.'..i,. P 1R E F A C E Ix hi address before th e British Association for the Adva nc te ment of Science last year, the President tsertke tla t the elnew views of the Correlation and Conservation of FIorces constitute tthe most important discovecry of the present centur'y The rernmciak is probably- just, prolific as has been this period in graend scientific.re. dsits.l b,one c gan thknslet gh the current scientific p'rfbliae.e tions without perceiving that these vews are statti cling; e'ipro, mfond attention of t1 he most thoughtful. ni nds. -, Te, livelyy ont — troversy tbha't has l been c aried on Lfor the last latwo or tthree years res!ectring - te share tlfait difiarent.tnen of di'fferent coun tsries haver had in their establsbi.-ment, s'till further;attests the estitate pltaed, upon them in the scieentific world. But little, howevaer, has been plublished in this country apon tfhe subject; no comidplete work, I elieve, except tite admaiurtable volume of Profi TyntLdall ot on "Heat as a Mlode of M[oti on" i.n wii(fi> PCe new philosophy is adopted, and applied to -le e xpl nation of llhervia pthenotontena in a very clear andto rcible imaniner I laxe t'herefore%, tlhought ist woudd be a useful service to the public to reissue sonae of tihe ablest presenttations of these views which have a.pe.-red in Eitope, in. a compact, and convenient,:forIn~ 1 he,seleec tion of these disscussions has been determinted by a desire to coinbime clearness of expositior with authority of cstatement,. I.n tfhe fir.st of these res'pets the esstays will speak for thentse.lves;.ft regard, to li] last 4 I tay r emak that all the autthors q(uoted stand hi gh as tuxbnders of the new thleory of forces. Although I anm not awsare that Prof. Liebig has made any claims in fhis direction, yet it can scaceely be doubted that his original researches in Anhilat Chemistry tended strongly tow1ard the promotion of the science of vtal dYnamics, The work of Professor Grove, which is here reprinted in full has a high Eu ropean reputation, having passed to the fourth edition in Englanda, and boeen translated into severTi con inenttal i -a guag'esa It is hardly to the. crecdit of science in our countrya that this is thie first American edition. The oleqnent and interestnig' paper of Heinhoiltz, thonugh delivered as a popular lect'urer was translated for the Philosophical r agazine% and has'ben very higthly appreci t;ed. n scientific circles. The trec articles f M yc r which were also tiraunsated for the Ithilosophical Lagazincc will have interest not only becanse of the great abiflity witnh which th-e snbjects are treated but as cmana ting front a na n'wo ti ansxds per-.haps pre'1ise-rt- amiong the explorers in this new tract of inquiry The researches of Faraday in this field ive becan conspicuous and impor0ant, and. his argiml ent is marked by the, dcpth aind clearness which. characterize in an eminent degre the'-tefritisgus of tils i ex traordiniarv man Th.e essay of Liebig cforis: c hapt, eir s tfhe last edition of his invaluabile PFaniatir Leticers o0n Chemisutry' which has not been irepublished here; and, as it touces the relantoioln of tl-.te sub)ject t;o orga1ice proesc se, it eir38 a fit intioduction) to the final article of thie series by Dr. oIarpe iter on the Correlatioo n of the Ph fysical ai.d Vlital Forces. The eminiet pigf: h. ph ysiologis t has woirk ed out this branchl of the subject indepe'ndldintlyr, ai.td the pai per q uoted gives evidence of being prepared,with his Iusual care anld ability, Ai certtiX aimoun.t 0of rpe tip ein is of curse uni avoidable in su-tch a collection yet the readeir will fadl]. lnuch less o -this thian hle uoight be inclined ito look for, as each writ;a5 in etItboratoimg tfhee sunjeci, has stamped it iv iiL his own or(ig:altty.t. iu. tl.e introdiuetlion I have att0eipted to brin0 fora0d et. ain tfacts in thIe histor of these. discoieries in,l7which w'e.s Aniericans have a special u steirest, and also to ildicate several applicatiocuns of -(he ne principles which4 ae not treated in the vsolumie. it senited'bie t t confine t he gseneralM discussio ls to -those aspect. of the susljectl pn. wc.lh most tugt had' been expended, and which ray be1 rewarded is settled amoing advane d' scientific sien, Busit t.ohre are other applications of the doctrine, of06tiUefm higitest interest wlfieh though c inomplete are; yet irtaiT, and. these wiSl bet found j5''omji u:'c.tii brieflyt noticed in the introdct-ory observatio:.ls o h.aocj'briefxy i fun to be satisctoatOryv Those, however, who desireo to Pitrsiste still Aurther toi-s branciLh of the iu ltiry — he correlation of t.he ital, lenta.l an, ocial force s —are referrodt to the last editioln of OCla pen ter's "1 Ir'inG iples of IHuman Phys');ology;x i tlorell' "''1 at2lnles of l.ental hilsosophy; 1" tLaycock's " Colrrelations of COIscio usneTss a'nd Orgt'a tisonr;i Sir J7 K. Shitsftlewortl's address before fthli Social Scinceo Congress of 180, on tn Ul Co brltfcion of flae I oral, "ad Physical ForciTces;" t-intons Life sTr N ature," an.d V Flirst Prainciples " of iHerbesh Spceerts new systumn of PB.ilosophy, The first and lrst of thee c works are. the only ones ift is beieved, tI hat lhave appesesod in an A.ttericsll frmi ng tea e t ash t is suznh. she ablest of l; ~ t wars chiefly indebted to it in preparing' tlhe bites part of the Iftnroouction0 The biograblephi noices, b)Sief andti u perfect ais they sare it is; i hoped ismay en hane tB reader's inte e st int the volanme I havs7 been "pecsl'ly inicted to pto procure."o p:bhiitafots osf ts wxorki of b his ki ind by the "sase emotive that I).as s iplled.' e tto wr e upon lihe subject elsexwherre; a conviction of ounr etducstxso.al needs io this- direction, The trea. tr ent of a va sal subject likeT this in osrdiina school text-bhooLks is at best quite. t;oo lss initd'or the treqiiemment s of ti3.o active-minde3d ttesacheir to'S. J. a vr'11i.ne tlike tne present mayi prove i.vauiable But as more serious cliffGilty is thfat, until. compelled, tby demnatids of itBllitghcut teaehies the coimpt so sc - o' hohbool:bks xill pass new vic-ws entiarety by or give t heim smere.,hasty "sid cae, lsC sototice, whilo contstinsl.g to inculcate, t he old erroni s eous doctrin esT And sthus -i; is that frol. inv terat, i habit or'a it.UestI d t lUg:gs i.'' ness, oi a shrtwd caletulation of- he idiffer ence 0of teCehesers, outworn. trd ef"et ideas consftine to drag -throtifgh sch'ot-oo)ks fa3 half a centuarysv tex they have been aiploded o i tddUihe aworsld oif Iv.tag science' lie xwho continues to teach tlhe hypothlies-is oxf ealoric,.falsiiies the present truth'i o science as l abs o utely as iltoe odd do ian teaching the hypothesis of j s.ogFaton; in f.act- the reoson" of. fe ed for perslistig a o -he erroneou's notions of the i at;bial of eat —-conve nience of'tachin. g, unsiettledsness of the neo'w 3:oc. i. sary, &e, pire isy v those that wiore ofi red s.o cnginag c it,'.o gi"ston and rejecting'the Lavoiserl ti a I chemist. ry oxf r......'.oBoth conceptions ha've sno doubt been of' service, but boths. As,,e.i transitional and having donie tthellir wftor l tthey bec'ome hin dr c'.mtesCi :istlead of hel]ps, W'e can nlow see tat wlhe tri teh ttraue ichemistry of combustnionlt - as once reahed, tlTe notion of &phlogs t:on O V as,- of no ftxrt.he.r nise, ad if retuained eo4 ion only prloduee coialSiouIa anid prevtelit'Us reception of correct ideas So with ooi e ori nd ti se false concceptions of the materiality of forees, whiuhe it i mplies: not only are they errors, but the ideas they invol1v e' tre re fadically incomipatible wi3 t thee higher truntns to whiach sc.tien has adv anced; so that whit1e tIhe errors are ret-ained the truths cannot be receive(e. N or wrill it answer merely to maenxtxio3n the new vi ews while adopttino the od on the plea Lthat the facts are the Le same in both cas. The facts are very -far fromp beng the same ai both eases. It is precisely becanse t:ve old ideas a't:e out of halrmtonyl with thle ~itcs) u'td e),a ano longer correct.ly explain and express,thei., hi.t na w ideas'wdae soug.t'Was not phlogiston a.b,tldoned boerauise t no loanger agPreed'w: Ifhxe tfactis? o WithI the concegptio0 of ofhe raierii.tyv, of'fthe forces; it contradicts ate iacts and therefore, -for scientific pa:poses, cart no loager represent then, I-i.t the i-vorki:hop it viimY perhaps be very well to magn tafi t fcts, and depretiste their tie )oretveal expl1anat.ions but, not i uhe scdhool-roo m; hthe b-siness is here not:wA-'or:kia g, tt tfAhimxl go It is the aim of'art to a.s vaets bhtat of sceice to ~ulvcrsa'tnd them~ And it is simply because scitene,goes beyond f:he f tc to io ts.explanatibon, and ia c ver strivaing atter ive higles -truth that it is fitted to disciplt:i the Ltb dint anv.(i reasoli-ng facvlti.es" and timrerfore has imperative edueationa-Il ol lmaS I Ie therefore briging forwardt these able nd at thorit ati.v't ex peositions in a for. m xreadily accessible to teaesrs Ir - ttrust I "'i not,only d(loing them a helpmful srtoice, bnt tthat they' ill be led to rkequire of the pxreparers of selo00-fbook0 s a ft re c0OnsB'le1ouas perforlnlac oft te.ir tasks and that the ianterestso of sou ntd odued:-atiov will be tftereby proimoted ttXx Yoxra, oct. t, 151. CIN: TIN AL T T'i COiBELA.?ON F 1-YSIC.AL. FORCES,: By'W,.!L Lrov. Prei'aee,'" O.;....... g Y.-:ntuhodutomry ie -'s, oak9 woi, — ot on,........o h25 IIL-4.e%,..... 89lo v I -[ —it-anotsm) 4 9 Vi' T-heomicrdt Affi.ty, >...n t 5; A Vii':fL-OtGiser iodos of orce, 1. 69 I,~Co lding -.Coaoldm:. c. 5.. ",8 otes;and elle'tences, 00 ON THE INTEBACTIO N OF NA1TURAL F02OESICE, iB-:o:c, E'iAtRiES ON THE FORCES OF INORGANIC NAS.I.U,:By t0, J. R.. AYER...) 2 5't 22 0NO1TE1NTS1 ON CELESTIAL D1YN1AMICS, BY DtJ. BoJ' -i3. E/P.2, — i.:nt,;roleLct ionl,..Y 269 — Somuces of -lteat, 2 o61 IIL -'reasuore of the Sun's He at, 264 IV.-.0rig i of the Sans I-featS, 2nH6? GCoow.stdt cy of the Sn2:s xtSsac e a8,2 VI. —The Spots on the un s vDisc, 2 86 IL- The Tidal Wae A v - 29.1 T'VT, —1P.-e, r-he's Interior Heat,. 00 E'gitt t'RKS ON THE IECH0.A& CA IEQUIVALEN'T OF EATi, By D1) o~ RM jL1131 f I~S- i aj~ 16, S0' "E TH'1 0'U1g SN Til"fi' C3OIe~ JLrUEIC OF F4H2TE BIy D-. 4Ax,~ A...AY, 359 THE COiNN-EC~TION AND EQCUItVALENCE OF POR10C]S, B'.PnoPs~ Lxn~msn, 387 UP QTHE CO'i R'ELATtON3 O THE Pit YS'CAL L N1'AND VITA12L F0101Esi 12Y DIL C1ARPE"NeTEaiR 0161 i..1.elaiton of Light and Heat to the "Vit.al Forces of Phant,'401'IA-I-elaetion of Legh-t and Heat to t het VialI Forcs of A.nA> -m gaI G, G.......420 I N T I U C 0 H~OI ON T nr uare many' who deplore what the- y regrad,as the iangat;Gtt.1l izi. ng'tetencies ofl modern. sien:e~ They m.aintaitn iat this plro:fend a nd. ineretasing, engrossmen.i of the mLind swith ma.terial objeets is fatal to i. refining'i and spiritualizing influen.ce. The corns rectness o- this con.eounsion is open to serious u..estion: ihndeed, tlhe history of 1scientific thonght not o.n!y fails to jt.'s i.t, b'ult proves ft.e reverse toe o be tre~ I; shoevs that the tendency' of -ins kind of mgnutry is ever'ro-i the lmJateri i.l) to -r,,d tstiCe alItOtl thet idetial, the spira l.P si We ary aXppeal to ^the olasdest anld mos't develoeped oE t he sciloene as for confirmatronl of. I ais stairterelnt,. The ea r lis xt ex l. hations of the t,nlestil movaements w-ere t horouhly and grostly matterial andL as]l astronoaic pogress s1e his bown towalrd more trefined and ideal vsiwers, The hea venly bodies werd e a firs rt a thiouf;I; o-be to -s)esportd and caried ound heir courses b souid byol:v0t1rt; cryst 1alle- c spheres to wlDi]'t. tbhey were ataciched, I.n.ond trepl cacd r the y ore cm nlex aund noilue'.,..nlrsm of peyicrleas To his sCethd th, hlypotheis of',;, tk in ho retr eOd the cilumy'm uehatnlct explanattion of revol.ig,-t-., whee i.-vork, ad proposed t.e more sub.itlef conception of ethereal currena:ts, w.iitld Constiantly. dla3td aLronmd in vortices, a.ud bore Kalaong thl' e h, eave 1ly tb~odfIi(es, At'le t1. h he ors of ab lstronomerst terinaltinig wit. h INewtof, atnu ak vay thesae crude levices, and csubstiette. f i3.e actieon of a aniversal imsaterinal torce. Thl cornrse of astr"onoale Scia cer haus thus been on a vast scale to withdraw at't.entio-n fio t) tlhe i.naterial and sensible and to fix it upon the invisible and slupersensuous. It has shown thit a pure principle seruis the irnmmlteri. iundation of the niverse. Froym the baldeos materialilty w.e rise at last to a truth of the spiriu ali world, of so exalted an order tbat it i as been said'to connect t'he mind of man with ti, Sbpiit of God,' The tendency thus illustrated by astronomny sis char.neteristi;ic i a markeded degree of all. iocdrn sciene, Scientifie, inquiries are becouting less and less question s of m attter", id more'and tmore q-esticus of force; rmaitewrial ideas are gi-ving place to dynamical ideas, While tihe grea; agencies of change -wirl- whicht i i1t is t'e buisiness of science to d.ea1 lheatn, ight electric ity, maglnretism,, andl( afin;ity hav e been formerly regarded as hknds of iniatter Isanpon deri ble eloemrn ts in distinction from other matnerital elejmen.ts these notijons must now be regarded as outgrown amnd( ab:andomitetl Ia:d i. tsheir place we. have an rdeer o 0 l prenlb inm talSR:il aes. Tow:tc'd tbhe close of the last cen.ry the umata ] ti-r yl.t each i the cresat, prin.ciple of tlw indestructibhliy of matter What the in'telteinatl activi ty of a'es had f led to establish by a11 st re. - sources aof re0lsoingm Cnd plotilosophl, was accormpnlc'li lshd by he,vention of a memtchaical ilmplenit the bal coance of Ltovo. l 0sir, Win.:aature was tested itn ntc chemuist?'s secaldetin, it Was. Psi.t found t'hat niever at atom is created or dcseold; o rd s td at thougoib cmatter ct mges form'switl- protea in facililq, travers.ai'g a ttius.iaad cyclrdes of c]_ange,'vaw1sings al..d reabn d ppen uars'in i.essatdl ett i;i never wears out or lapses int o athi..... The proesent oag will'be. nme imorable iln th e b'So y of scie - fo havnig demonstia"ted tiat the samleC gIreat prinrc>ile ap] lci.ves aeso to sobrcec, and for the esablishmenta of h new ]jilosophy e3onrcerniiig their.attare asid rel]ations. lteat bib't,% electriietyi, and sas.ogu.ismsni,6 are locw n o lone e.r regarded ass substainti-ve and indeipnudcl nt eXaiab tesene... subtisle fdluids with peculis:' properties. but si.m p t ls m-odes T.SgUE NEW DOCTRUINE OF FOECOER:., I Of mo10tion. in Ordlliary m tter; fornms of energy whicr h are capable of mrauetl c00mversionon, ]Ieat is a m ode of encrgy man ifeste1d yby certati effbets.: It mayns be transformed into electricitt, which. is ano'.ther torim of fre poi oducig 2 diflerent effeects Ort the, process mvy be reversed- the 3l ctericity disappearti ng and the hea at reappeating. Againn meclhanicai motion, whicih is a motion of masess ma-y be transformed into heat or electricity, whiTh is held to be a motion of the atonis of matter, while by a reoVerse p]roces the, i3 otion of atoms, that is heat or e]ectrieity may bTe turned b ack again into:mlchanicalt niotion Th-us a portion of'tin heat generatted in a locomotive is converted into the notion of the otrn awh, ile'by the napplication of tihe brad.es the motion of the train is thanged )back again into the heat of friction, Th-ese mutations are rigidly subject to the l a; t of cPanntity A g iven a:Vonnit of one force produces a definite qcua:atity of a Coother so that power or energy, like matters can neither? be, creatoe:ocr. destroyed: thouigh ever changing form, its total quantityin a the maisverse rem.ains constant and unalterable, Every.an.c..sat.t of forice sat have come from a. pre 6xisting equivalent:fore, and nmust gi0ve rise to a snbseauent and equal Ilnomunt of solhmer oher ere, Wh en, therefore, a force or e-ct appears, W e rc Lo at h6 liberty to alggg', S,' se lf-org'0].sned, ora me from nothinhig; whenz it asume tnh' ia was eif-originated,o disappears we are forbidden to conclude lth at it is annihimlatedit we must searelh and f ind whence i.t camein and whti' itCi t - a. on e a. t2 is, wuhat ptrodueed it and wha t effect it has itself producetn These relatiionms among th:e iiodes of energy are currentX y known b'i tnf phrases (/orerccdsionu and (o.n:e2,86'atat io of Force. The pieseni condition of the pluilosopah of eforcI is i e pOr'tiectly paralleled b'iy that of the phMlosophy of matter toward the closse of hlie last cetuanry So long. as it was asdmitied that macttor i.n ixts vtsioums han ges may'be created or lesat oy ed c ahemial prog.Xsess was impossible 14 in his pIrocesses) a portion of ile mlnateriaol disapp-eared~ the cemist had a re acty exdplant i'nas... l-tio tLe matter cwas ets,:?o ed hits anealysis was therefore worthles. But whaen ahe VY Ut~INTRODUeCTION. started with tie axiom that matter is indestructible, all disappear ance of uaterial during his operations warfs chargablie'to their tin perfection, lie was therefobre comipell ed to iniprove thon —-to ac-e count in his reslit for every thousandth of a grain w w h~ich he ommtenced; ad as a consequlence of fis inex orble condition, analytical ch enmistry advan ed to a hii,,h perfettion, anrd its consequences to the world are ioea.lulabole Pre-cisely so" with" the anal ysis of forces, So long as they afre considered cap)able, of bemng created and destbroyecd, the quest -for them will hbe careless and the results valueless...But the moment they are detearm-inied to be in destructible, the investigator becomes boeund to acconut for theni; all problems of power are at once affioeted ad tela' science of dy-k arl ics enters upon a new er a, The v.iews here briefly stated will lbe found filly and. variously el'caidated in the essays of the present voluae' in these inttrodue tory remtarks I propose to ofTer som.e obsiervations, on th.eir h.itory sa:d -th.e ex tended scope of their aipplieationm I have spoken of the prinSrtples of iorr. dn a 0 it-'.m. COon'er vation'a of Forces as estalilfilied; it may be well to stats the sense in which this is to b, taken They have'ieei acc pte d by the ea-ndla-' scier, tific minds of oll nations witI: remnarkable unanimiaty inheir dlscussion sorts ].us leading d olem nent in sci ntific Il era'urn' wI'hile ict:ey oc. eiUp?5y t.bhe. houf:I, tsc aid ginde t. e c invstitaotions of the itmost iphiio. sdphiacdar 3lainquirer"s, suit while srcinc e'o.'ssoeemaly her new piossesilon as a i sun-dTa-. iedni. lplu p cip]le, it.S Xva..3ious tZ -ts N:e.y.no means completely t-worked outt, NT-t onyi hlas there ee. t1,oo i.ttloe time' br this- cvenH if the views were afatr' ls Inp(3Z'~-lt, blt 1s-'f quaIestionrais started l-i at -the foaundation of I. iebran.'htes of,scie-nc nd f_ oriby be 1&tfXy delucddated as theso advt*.e inJ Ser dive' lop. tat, Th, e new doctr,dineo of roree~ s is Io u fl. iancl m R.!t' COsin. ditotin s was wa te newa astronomy of- Coe' l" tnons' i: lis t..ith.. out its diffieulties., whinch tirnme alone i.ust i Je{ a'tusted ito remove; but it simpliNties so iiianly probileins., cd ars so man'y:. obseutlties, .'.I rISTOilY0' OF EI;anTIFI~ rDiSeOVi;,. [Kv opens SO extended a iranige of new investigationt s an'd conrltasts so strongly with the conimple1Xitis and Incongrtliit't" of the older doctrinel, as to learve.itfle le liberty of choebetvweren th opposIing rleories Not onlry doos0 th rec eption of ithes view, m nark a sigfnal n?i b e i n the progress of serene but i Ifrom their onmprehl::nsive'beareings antd -the lm.inmos ihmpses which fthey open inuto the:most oelvcated regions of speoulnati:ve inlairnr xhlter have a.. pronuind inteorest for ma3ny th inker s iwho give little attention to'the spei alties of exact Science. In tie, hi.story O lL hitan fai.rs there is t growil tc l conception of the aet-toi of georalt causess i fire production of even:ts, atBd a orrespondag conviction t the p int playedt b- individuals has been mnuch exageted d far less controlling sad penianent: t than has been hitheirto supposoed So also in Lthe hiostory of science it is now aoknorwledg'ed that the progress of discoveryv is nnrc hmore indpt:pendn t of lie labors of particulrar persons thtan has been formerly nadmittedl; Gre discovrLies be l og no t co munh to i.di v'idnus as to hu. cnitl; they are litS in"piration s of geotils l tan births of cerst Asg theri e has'been a definite rinteilectual op.ires f:lhougnfht: has neeesRsaj.iRy been lintled to the subjects scuccens-ively rachd, d ~'a.y anmnCinds have 3:bee tusi oeoupied a'~ ther' S'tire'inmo wit. "sh~ina ideas, amn.d hencm1 ) tle t1ane "sr"tmeor" di&sco'et:eCs of iradependent -.:irper of wf i-chi t;hre history of soiencae i.s so futl, TTnms at th ciose of tfhe axltennth eeintury, philosophers had entered upon the investiFgaion of the laws of' motion, nd accordintgy v'oe find Gahlteo, IBenediti amnd Piccollonilni proving; hinde8pendently fit ill bodies fail to the etarf with ei x (Ulg Velocity, hratevetr their sizn( or weight, A centrory aftfer, whene science had advntlced to tole syselcmatie application of t:he hicetr mathliehmatics to ge' enee;."al: pttys-, ica, Tiewton a ild Leibnitz disceovered dnie.n.enenf iy'.e dnitcerec]ni.:i cailuns. hiun }mnced years later questionsi of axolccnla'. phyasic's.and chemis-tty were reaehed, atd oxygxecn was dis tcov red siritirrt.. neously by'Priest;ley a/tnd itSchcro arid tthe oiomposth.` lonl of'' a'ter by CaTendish aitd aWni: ta, TtIhese discoveries wora cad e rbios'lls the periods werx ripe for them nm,ad awe eImot doubt thnt if those who made them had nevar lived, the T abo. s of others wouTli have seedc ily attainehd;, e same results. The discoverer is, therefore, -ia a great degrre% but the m outhjfeiee of is time. S.omn discern clearly w'hat is dimly shadowed forth to manyu; soame waolk out thef restults more com'pletely t.ha.n others, aad son e s eize the. co-mming thou al ght so Tang before it is developed in the ge, neral consciosneess that their atouno cements zare unappreciated and unheeded,'his'view by no mo.aen-fs robs the diseoverer of his honeors but it enables Zs to place upon them a juster estimatL, and to pass a mnore enlighotened jldgmment upon tahe rival claims w.hih(h as1re constasntivy rm'i fl in.'t'he history of scienceL?rohbai.bly it.e most important evetc in te, geOneiral progress of scincae was iahe tr'sition yfrona the speculative to sthe expierlmental perisod, The ancients were preven.ted froin creating' science by a fi 1 se intellectual proiens wfro conjec tires of f ancy to facts of science,,ore not og ty An.imratis by' birt'h and ednucation, but men eminently reprsoentlat'ie of t'he'pecJ iari.i. s of.Anerican h -aracter-t:ernja min u ar}..hio andt Pejnnni Thompson, after-wards known as Co'mt R'tumford. the latte philosopher is less known than the fornerir thengh his servicesr eo scienoo and society were probably uqaite as great, The pronTinsence Wh. r lih Iis rame -now occupies in connectio1.n wi-tW th-he new rviewns of heat', and the relatiions of forces, make it desirable t oord) -, & 1 a'd, ood -in 1ti7 marrised a weahy wiedow, by whoim ihe had one deautt ter At the outbr ea of revoluitionary hostilities he appbed for a commission in the American scrtvic( ea chtrged'tifh tojryrin IeRf t-he e counry in disgnst,s ad wenot to 4'gulnind,'is tale-nts wis-x er ib ero aspreOi t-ed tad lhe t0ook ia respo tsi ba. pos. tion utder ithe g'overmeuei t a} re.tih1 held for rsome yers, it'Rer rsceiving-tthe hons. of o hthoo d hke left tngland mnIl onuorted tho s ervice of the elector of t.v'aria. e scttiled ina au nic h in 1 0384 ant d Was appoi.tin ed aai de-de-canp ie d o:l-rntberlain to,the triu,,e The tiboers whtich he now u ndter-teook xwer:e -f, it most extuensive ad leiaboriout s e.a. orter, end iouid ne ver h tve been gic complishedl beut for the rigorous habits of kOrdfer whisch it.e arried into ftdl his pursuits, 1e reorganized the entire ir-lliry estabisha — meat of Ba-svar, intiroduced not only a sanipler code of t'vaticts, ad. a new systeam of ordeF, discipline, and. econosmy v-rmong uthe troops and industrial schools for the soldiers' ehildren be ut g-ret dT improved the construction and nmodes of rcnanuaciture of crms l ai d ordaance ieo sutppressed the system of beggary which had gro wn int;o a reoognizd profession in Bava -an cd becoei an2, emori0nmous pu..el evil 3 —- one of the mos t remarkable social reforlms on record. I lso o dcevotied himself to v-crc.t a me litr ioaB ons s' a em sn s lOrt inc tleo construct3ion saedt arrsng-ement of fhe. dwex-flit —'s of the worik ing casses proVig adafor ihlP a'better edcueaton or1gasmnizzing houses of ind-as'y,'introducing suIperor breeds of horses c nd catftle sai pro-moting landhaapegardesWing, which he did by contr ertin' a1n old. atagudoned ihunthi g-gros n near Alfunich lnto a patri, wr h-er, sc-fhis departui7re:, tihe inhabtr atts eeected a a-monment to his honor~ For tihese serViees Sir Benjamin Tho'mpson received M-rnriy di.stnc, tiolns, Ian.d among otners vras made Countt of the oly lToma-n Empisre. On receiving this dionsit y he chose a title ini rei eufbrance of ftie Co0unty of tls lattriity-y, aOid vas {thenceforth kne.u.. C ounts i of tRumfobrd Iis lialith failing from te xcesesi-ro itelo sd w hat, it considered the amafvorable clims he camt te s & to Engti nd in 1798, and had Saerious t:hougltts of returning to t'he UTrUed. dStos 11evling re.ec(ivd from the An.ieri-oan govermenta' ce cotmpa.ploenet of ta bfora.inl.at4.on to reviss-it fis nat ive ltds, he xwote to sc oLd frie.d reet':1ing hlima to look out fir a 1 RtleM qi'et retreat t":for tImself an2d doa. c;'er ini the vici.nity of Bo-ston i'-s a aiten onP howrver, ailed,as he shortly after became involvted ion the ente-rprise of cundin; the ~ oyal Institution of Englcand: There was in ru' mford%' ch accter a lappy ctmbi-ination otf phnL mlthroipio imapulses, esxeeutsive p0"or in ot>:r.ag out grtat projects, and vers"tility of. talent in pthyslctd reseaTsrch, Hi.s, Sientifie intvestigaigoans w're larg/ely gidded and detenlmilac d bDy his philanthropis SCIENZTTIdFIC LABORS OF COUNT T UIFORD. LT plans and public duties. His interest in -the iore needy classes led him to the assiduous study of the physical wants of mankind, and the, best methods of relieving them; the law s and domestic management of heat accordingly engaged a large share. of his attention. He determined the amou-nt of heat arising from the combustion of different kinds of fuel, by means of a calorireter of his own in,vention. He reconstructed the fireplace, and so improv-ed t1le met.hods of heating apartments and coolking food as to produce a saving in the precious elenment, varying from one-1-half to sevenigbhths of the fuel previously consumed. He improved the construction of stoves, cooking ranges, coal grates, and chimneys; show ed that the non-conducting power of cloth is due to the air enclosed among its fibres, and first pointed ont'that mod1e of action of heat called convectdion indeed he was: the first clearly to discriminate betweeh the three modes of propagation of heat —-radia tieon' conduction, and convection, Iie determined the anhiost perfet non-conducting properties of liquids, investigated I the produc tion.' of light, and invented a mode of measuring it. He was the first" to p-'.plyV steam genaerally to the -arming of fluids and the ctlinary ar't; he experimented upon the uise of gunpowder, the strength of materials, and the maximum den.sity of water, and made niany valuable and original observations aupon an e.ten sive range of subjects. Proi. James D.O Forbes, in his able D)issertation on the recent Progress of the r:athematical and Physical Sciences, in the last edition of the Encyclopedia Britannica, gives a ffll account of iRumford's contributions to scienee, and remarks: ".All umford's experiments were made with admirable precisionc and recorded with elaborate fidelity, and in the plainest ianguage.- Everything'with bhimn was reduced to wfei4ht and mneas ure, nad i-o pains were spared. to attain the best res-ults, "u mford's namle will be ever connected with the progress of se.idnc in Eingtand d by two circumastanes: first, by th:e foundalti0on of a perpetual medal and prize in the [gift of the council of the B Boy)l Society of _Loudon, for t he rewartd of discoveries conaec'tced with heatl aend lht; a: d secondlty oy es;Cb.ish't i J, 1800 of the, Boyxl nstittion in Loandon i, dest-ied, praria-rlv-, for t.e prPo motion of origuit discovery, dan'd, ad sreodalv., G J diffalsion of a t aste.or $. al e S:CIo n.''h$e aducsl.ted'lasses Th5e pla-.. Ws cont eelved with thne sagacity w vhi h ch. Y ar. ct odt ei V.t. mfod~ a I i ts sue-.( s-s,has bee tn, great ior thian couldi hnave beaen as:.uyi.eipated. Da'lgy wesFTa.th'e brc ought in to notice bx,. T;-.um-ford.himseI'.d. fitrrnisded vitb the ineas of prroseniting his aldcmirable experhaeiLcus. l e and drn Faraday have given to that institi'ton its just celebit r with little intermissioni o i alf a cenitury"r neing -lad,. ltciintccd trook his residence itn krnce Ind he esti'matitc. in whinch he wvas held amny be judged of by the falct th1at he was' e.tced toe oe of the eight:for.ei'n associstes 0 af Ie Acad1my of S@ences,3 Count Nionuford b.equeathed to icrvarid Uniuversitiy the3 itnds otr enitd.owfii3 its profeso'0r'in.-p of the App lieC;atl of Sctiea.e to the At4; of Linsdg. e, And istisfliuted a proize to be awa-itrded by tlce Atnoeriean Acade my of 5 Sc i-en esa or the most iipoitint. discoveries eud nmpnrovXemennts rc.ainhg to henat and l Al-. In 1804 he inmari ei d the widow ao of e celebrated ch emistc r Lao -olsieir, end w -f r reti3t e redl to itho of.Autet.hl +TX n res1 enc of hs1^r fc - nrrn l'r inesba.idx w1ere he died in 18':4.: Itav"i.g th.us gilanced briefly at. his careert ir now Ipass to tbeo dins. covery npon which. COllt mIin'ordis fame in lm flture, wiV.rll cdiefLy rest. t is described. in a paper pblishIed int. ile transaction s of t.he Royal Society for t1V98 t io was leld'to it while supi eritdi nig' ime operations of t1o nnmi. i rsne. hrby observing the largee amo nt of tieat genret'.ed i bortlin'brass can.lcno0 P Refeoctig umon this, he. p oposed to lhti-, selIm ILhe olw n qnuestiints: 1i h enco conmies the heatl produced in te m necih.tanic operations abo0ve mentioned?" ts it fa rnish by the mentalc. echipus wlmich are sipr'teo'cl fixom e t ametal? " The comraeon lhypothesit afirmTed that the hea t produced had been latent in the.metl, and. had bceen forced out; bby coad,.a.scdiona of the chlips. 3ut if l-this were thfse ease the caGpscity for hea t of thel parts of ~letal so reduced. to chips ought not only to be changced ibut the charse undergone by themn should be sufficiently gmeat -to accounit tfor aci the heat produced~ With a fine saw ita-uiroid then cut away slices of the unheated metal,. and foun.d t.h:1- -at, the ad. e:ady te saie capli-atty; foir tct as tia ir e metall clap tie eatac o in this respect lhali.d occurr'ed and it was thus co-netluasively proved that the heat generated tcould n.ot hve bee n he1d latent in tlea ehiips. Having settled this preliuninr.y piv int) Rumnford piroceeds to,his principal experiments. oith th.e intuition' of the true investigat'.or he remarks that very interesting philosophical experiments may often be made al.miost without trouble or expense, by means of iranchierS contrived for mere nmeehanical purposes of the arts and maunxufactut res Accordingly, be mounted a metaillic cynder weighing 11.3.13 pournds avoirdupotis' in ", hoorizontatl positioc;.n At one end c dthberei'was a cavity three ind a half inches in diaeter, and into this nwas -jn troduced a borer, a fiat piece of hardened steel, four inches long, 0.63 inches thilek and nearly as wide as the cavity, the arca of co-. tact of the'borei with the cylinder being t-o anld a half incles, To meoasure the heat developed, a small ro-und hole was bored i-n thie cylinder niear the botton of the cavit, for the insertion of a smail mrceurial tiuhermometer. The borer was pressed aggainst tihe base of the cavity withh a force of 10,000 poundsw, uald t(.I (cylinder nrade to revolve by Ihorse-power at the rate of tL irty-tswo times per mfute, At the beginning of the expetriment the temnperature of the air in the shade and also in the cylinder was O-600F a5t the end of thirty vminutes, an d after thed ae ind0 rittOvolut.iourns the temperature was found to be 1830~ F htaving taken a+rway the borer, he found thai; 839 grains of ime ta-iic dust had been cut awvay.' iS it possible"' he exnclains s talt the vey v coneSderable quantity of heat produced il this experiAent XX11 1 TIODUCITVO:: -ta quantity which actual1y raised the temperatnure of -pward of 113 pounds of gun metal it least 70~, could have been f:urnl.ished, by so inconsiderable a quantity of metallic dust, and this merely in consequence of a change in the capacir for heat? To measure more precisely thfie heat prodced he next surrounded his cylinder by an. oblong wooden box in such a m sannuser that it could turn water-tight in the centre of the box, whbile the borer was pressed against the bottom. he boom was flled with. Wat er until the entire cylinder was covered, and the apparatus was set in action. The tempera tusre of the wate 1r on comm:ncin'g was OP l te remarks,`" The result of this beautiful experht.ent -was very striking, and fte pleasure it afforded amtply repaid mne for all the trouble I had tak.en in contriving and atrroanging the complicated machinery used in mn aking it. The cylinder. had been in -notion but a short time when I perceived, by putting na: hand in;to the water and touching the ontside of the cylinder, trha at heti waas ge.nerated." As th.e work continued the tenperatuire gradually rose; a't- two:hours,and twenty minutes from the beginning of the openrati oni the water was'at 2O0~, t and in ten minutis mnore it actuallyt boiled! Upon this result Rumford observes, "It would be diihicult to describe tihe surprise afnd astonishment expressed ir tlhe countenances of'the bvstanders on seeingr so large a qantity of wt ter heated. and Iactally made to boil withou0 t an r fire. Though there was nothing that could be considered -ver- surprnising inh this mnatter yet I acknowledge -fairly tha, it alftLorded s1e a degree of childish pleasa-re which, were I almbitious of the r putation of a grave p!.hilosophler, I ought most certainly rathe-t to bide thlint to discver."e _Rmniford estimated the total heatt generatred as sulfiimient to raise 206.58 pounds of ice-cold water I0P, Or to its boili-ng point; and he adds, ifrom the results of these coml-putations-, it appears':-::ihat ihre quanti-ty of het' produced equt ly o-r in a continluous streans,if I may use the expression, by the frietio1n of the blunt; stel- borer against the bottom of the hollow mettllic cylinder, was,greater,? 1 IUFOND tS NFE RENCES FRO M I S EXPERIIE SJTX StXII. than t hat produced in the conmbustion of nine wa3 x casn d.le each thlreequarters of an ilch in dia netert all blrning together with cleabr bright flames."' One hornse woud have.. been quna to tibe work peirformed, though -tr were actually employed, Heat ay iv hus ble produced Imerely by the strength of a horse, tnd in a case of necessity- this mitglt be sed in cooking victuals, But no eirncumistanlcs could be iuaginred in ihisicbh this method of produchmog heIt couild be advtan-;tgeou, f Ro. mt, heat -migtt be obt-aineed by using the fodder notessary fzor thte support of the horse, as fiuel " By -ueditating on the results of all these experiments, we are en tutrally broug Iht to thfat great tq estion which. has so often'been the subject of speculaSotio ong philosophers, namely, What is heat s Is thiere au a thineg as at son o fluid? IS there any titing that with priopriety can be called cnalorice? 9We itvwe seen that1 a very considerasble qgantity o heat may be excited by the friction of two metallic surfices, aGnd. given. off in a conitant stream or flux t2, alt diryections xw ithoeu.t intierruptsion or intermltmisso ait n aid withost any signs of dir, o'tt.zni ci sa.'tioeSO in reasoning on. this srubjet we mnmst not borget stcat.0os2t r esmurt, k maibe c.i"cTUm;, s.t',cc that t/he source of the beat geentrated by' friction in these experiments appeared evidently to be:in, 2vt-stiblwTe.i (The itaics are tanmiford's.) It, is hardly necessary to add, thlat any thing which any;zin:szda.tce body or system- of'bodies ctn continue'to furnish t.it./o.-out lim'titatob, cannL.ot possibly be a acZ:!t.ig sut i.stantc; mndt it appears tO -me to be extreimsely difficuIt, if cot quito impossible, to formn ny distinet idea of tany thing capable of beiug texcited and comt7unicated in those extpe.riments excep. it be sao1 a one can read the remarkably anble 0d lucid p]aper f011 wvhich these extracts are taket, without bei-ng strulck with the perfeet disft-tnct ess with w-hiich thlee problem to be soltvred sras pre,,sented, and the systematic and conelusive method of its treatment. Imumferd kept streitly wiithin t~he limits of legitimate. iqjuiry which XXiV INTRODUCTION. no man can'define better than he did. "I am very far from pretending to know how, or by what means or mechanical contrivances, that particular kind of motion in bodies, which has been supposed to constitute heat, is exerted, continued, and propagated, and I shall not presume to trouble the Society with new conjectures. But although the mechanism of heat should in part be one one of those mysteries of nature, which are beyond the reach of human intelligence, this ought by no means to discourage us, or even lessen our ardor in our attempts to investigate the laws of its operations. How far can we advance in any of the paths which science has opened to us, before we find ourselves enveloped in those thick mists, which on every side bound the horizon of the human intellect." Rumford's experiments completely annihilated the material hypothesis of heat, while the modern doctrine was stated in explicit terms. He moreover advanced the question to its quantitative and highest stage, proposing to find the numerical relation between mechanical power and heat, and obtained a result remarkably near to that finally established. The English unit of force is the footpound, that is, one pound falling through one foot of space; the unit of heat is one pound of water heated 1~ F. Just fifty years subsequently to the experiment of Rumford, Dr. J. P. Joule,* of Manchester, England, after a most delicate and elaborate series of experiments, determined that 772 units of force produce one unit of heat; that is,. 772 pounds falling through one foot produces sufficient heat to raise one pound of water 1~ F. This law is known as the mechanical equivalent of heat. Now, when we throw Rumford's results into these terms, we find that about 940 units of force produced a unit of heat, and that, therefore, on a large scale, and at the very first trial, he came within twenty per cent. of the true * JarXES PRESCOTT JOULE, born December 24th, 1818, at Salford, near Manchester, England, where he pursued the occupation of a brewer. Long and deeply devoted to scientific investigation, he became a member of the Manchester Philosophical SBo ciety in 1842, and of the Royal Society of London in 1850. SUiMtl, 0 O lMg. 1UiFOI&S CLAim..s 7st (te&nie~.~ N't o. -1toun' t Was tt ta:,ke of t:he heat lost bhy- a.iationi Whic'ti consideri g the gIdo. temnperat,re prodncede aLd t1e deO'anCon of tihe experneint, -mrrist lhtV been c onsiderle s th t as Pumnforfd himielsf no'.eeicid dcis valae mnut be too lfil.L Tha earirst rua-erical:results in sealenee are rarely mnorero tial rou. 0 l aj - prox;imactiom., yet tley mtayr gctlieto the tabishs or;rIt of great principles ( Ciertmifly 0o one could question ta ron s con.1aim to tae discovery of the lan of defincite p'roporti 7s e.c te-C i m-wturacyo of txte:-tatir abctrs u1ponl)1 Which. bha h irst retsted it, W'e'tre fc' ied ftulter to ot to nota that -lu: ords i eas uo "Ki t geanelra sub)jet 0of: forces -wore ifea i a dv anc of is a. S, tn-a -0,f sW tihe rchlatioin o fil f-e Iion to heatcm and sueggested.ahant of ih:.itds by cIhurnig jproeesses s as a means0s of prJiodi IgC it —. rew -cisei y thes eolao a yed oil nlly e l Joule in estabishhin g1' t:io nmeC amcfieai eqjsdirlenitt of a:het:.o.:Ie furiihern oe r egarOded. anainals cdyJraaz.dio.cal>,y considering t]hef' foree a s the derivntive of their'btod, and therefore as not 0c'teecd T ha t tRumford 4hld 1h4 se views in the eo-npmpreahensive and satiured senrse in whitiic'tley ii'o no-'te',ai'tained ip, of course, not asserted Thie ajdvance tfiom htis day to ours hbas beean prodigious,'0o1l\ e scien ces ha'a'e Le created, wvhidht afford h-l-te mliost beautiful exemp; ificat iro'insi of.i. lew doetrites'. [.ose, doctrines Jhiie received their skbsehaa ellt sde-velopCment h! variouns directions hby m iany muindsa bhut we i ayi be allowerd to question if thee ontribn otions oar nty of fheir prormoters v-f]_l a nrprs% if micndeei they will ecquad, the value and importence ada'ai mtmst as sign. to hthe first great e xperiment —al step in te aew direc i The etaim e of umtbord may he summarxized as follows' 1L Ie wcas the macn w ho first took the ostiontCh of the i n-t re of lieat oat- of the domai.n of meaniphyshics it w';'rl'i hai-i beet. ulat1ed -upon s-il the tim-ie of Artis-tole, ad plac.ed it upopen the -true basis of pi1ysic1 experim untet iL He firt proved t e inscfiiioleney of h'tse 2 urr ent exl- ni:t.ii s Xxvi INT rODU t'O. of the sources of heat, anttd demonstrated the ftidsity of the prOevailng view of its materiality. ITI fe first estimated the (qusantitative relation between the heat produced by friction and that by conimbustion. IV. He first showed the quantity of heat produced by a definite amount of mechanical work, and arerived at a result remarkably near the finally established law,'V. Fie pointed out other methods to be employed in determining the anmount of heat produced by the expenditure of mehanical power, instancing partiefiarly the agitation of water, or otiher licquidS, as in churning. VI He regarded thbe power of anllimals as due to their food, therefore as havling a definite source and not created, and tlus "applied his views of force to the organic world. IL. munford was the first to demonstrate the quanti.tative conc vertibility of force in an. important case, and the first to reach, experimentally, the fundamental con elusion that heat is but a mode of motion. In lias late work upon heat, Prof. Tyndall, lafter quotin copiously from Rumford's paper, remnarks: " When the history of the dynamical thieory of heat is written, the man who in opposition to the scientific belief of his time could experiment, and reason upon experiment, a did 7Ruiford in the investigation he re rferred to, eannot be lightly passed over.) Had oher English writers bteet eqtally just the re would have been less necessity for the foregoing exposition of Rumford's labors and claim'; but there has been a manifest disposition in various quarters to obscure and. 4lepreciate them. Dr, Whewiiel in his history of the Inductive Scienees, treats the subjeott of thermotics without mentioning himn, An euinent Edinburgh professor, writing recently in the Philosophieas Magazine,:'nder the confessed influence of'patriotism,' under-, 'DAV~ S tlAtIN 0'10 T E QUESTIOIGS XNvi takes to make the dyvnamice il theory of heat an English menopol33 due to Sir Isatte Newtonl Sir H-Umphry.Davy, and Dr., P Joule; while an able wr iter in a late nlnber of the North Britis1h teview in sketchmin g the historic progress of the Tnew viewsc puts Davy for ward as their foun-der, and assignIs to Rurfor)d a. minor and subsequeunt pflace Sir Humnphry Davy it is well knoW n early rejected the cttonri hy othiesis. Xn 1799, at the age of twenrlty-one, he published a trtact at Bristol, describing some ingenious experiments upon the subject. It was the publication of this pampahlet'tlh-tfich brought him to Rumford'S notice, and resulted il his subseub uent connection with the.Rloy al nstitution. But Davy's ideas upon thle ctquesti.on. were Ir' fr'o deCar, and will bear no comparison with those of Bumiford, published the year before. Indeed his eulogist remiarks: "It is certain that even TDay himself was led astraty in Iris argueient by ausirg the hypothesis of change of capacity as the basis of his.reasoning and thatf he might have'been met successfuiy by any able atlorist, who thouglh maintaining the materiality of heat, might have been wi.lin g to'throw overboard one or two of the less essentietd tenets of his school of philosophy." It was not till 1812 that Davy wro; te in his Chemical Philosophy, "The inumediato cause of the phenomena of heat thein is mo0tion, and th-ie laws of its comnmunication are precisely the saure as those of the comntmunnication of alotion," When, theerefbore we remertber that Davy's'first publication was subsequent to that of Rmnuford's, that lhe confinetd hinmself to tfie na'rowest point of the subject, the simple question of the e xistence of calorice; and that he nowhere gives evidence of haviing the, sliglhtest notion of t;he quartli'ative relation between mechanLbical force and heat, the futility of -the ctlaim. which r would make him: the experimental founder of the dynaluical theory is aoaundantliy tappa'relt. The inquiries opened by.uiBnford. and Davy were not forma lly pursued by the succeeding genernation. Even the powerful adhesion of Dr. Thomas touungper,1iaps the greatest unid in science since Neo\wton-iiled to give currency to the noew views. Bat tfhe saiIaent and improegnable demonastration of.Rumfordt and t le ingemnious experhinrents of Davy, facts which could neihier be evaxded nor harmonized with the provailiiig errors, were not wcitiount influ nc, That there wYas a general, though unco-nscious tendene toward a new philosophy of forces, in the early inquiries of tie presentl cenc turiy is shown by the fact that various scientific:men. of differentc nations,, and with no know ledge of each ot. her's labors, g'ave expression to the same views at about the same tin3e Grote and Joule of Englciand, Mrayer of Germany, and Colding of Dennm-rk,3 announced ~the general doctrine of the xutual relations of the fotees, wit more or l]ess exphliation, about 1842, crd Segain of Francie it is clahn ed, a litt.le earlier. From this titne the stubject ias closely p ursu.edt and tie names of Helmnholtz, holtzinlt, Cmlausius* Faradayj Thoempson,.anlinemj' Tyndafll, Carpenter, anld othersare intlimately associated witl its acdvancemnent. In this cou;ntry Prt ofs-sorss Henury t and Leconte ~ have contributed to illustrate the organic phase of the doctirline, -I cannot here attempt an estimate of the r epectinve sha es which these men have had in constructring the neiV thecories; t he reader will gather various intimniations upon ithis point from the sueceeding essays. The foreign periodicals~ both scientific and. lit,eraryi shown that the question is being thorougnliy sited tand neateria iceCllls mulatiung for the fiture history of tihe sub-jeet tihe parat mournt claims care however, those of Joule, tayer1 and hGrove, t i;eXa., [:f[ID3OIPI-I oUTeIet S I.IMAUrL EL a was bora- a-t: O3lhh ]Oinntumertm, Janua-ry 9i2, 1822. I-To became.rofessor of Philosophty and Physics in tire IPolyterh6nie School at Zurich in 8l n,5 acnd then Professor of the Zurich University (1857).:e -e-was alterwards teacher of Physics and Artillery in the:'School of Berlin, and tlien presiYato teachler of t'te lUniversity of that place. it e~3W.ra3WE,'WfIiMA- JoHN'lcrcq'Oa.' was born at Ednhingbgrl Jnly, 182b0, lie is t crvil engineer in Glasgow, a memaber of the thilosophieiti Soiety a.t that place, and of th1e Royal.Society of La aondo + See tt article "Meteoroloag," in the Agriculitral Rep-ort of the Pa tent Mffie, forx 8571 3 Se the Ametican JonmaI of Sieneei for 7 r. 1853f C'LA ix OF Jr sorCE, r;u0c,_A AND MAY. ER. Xi. Aeeording to the strict rule of science, thait hinn nil those cases wr here experimstenal proof is po ssibl Ihe'who fir'st spphclies it is the Itrue discoCverCr Dr. Jeole must be, RSwied t.e oirem 0ost p pacec among,eia mod ern in'estisg, eors of th -bttject tie ide a W hi the whole quiestion tpon the basis of expirimn i I.c c ItE ]Heabor3ed with great pe.rsteveranc e andt still to det-rtmin.ie tihe Leineh:t. l e( ivilentr of bhet —' the cornner-stone of te edifice; nd in aceon-rpli].in. g this res-lt bi 1850o he may- be said to have maitured the work of Ltu ftord, and bfimaly estiubl;ched cpon l an exp. erh.neintl'bsis"i the great lawr of l-.heml. o -dyniaii lesi to rema n a ei.L.o'nstrai tion of scien ac Te -eorO Pro"fessor Grcv has l so worked outt the suljJecti izn his own I. m depetndesit way, a o- e hijg 1 original expierimiental ivestigai.ions of grea atduten.ess wRth the philosophic em ployme'ti of th~e gt eerai. results of seienee1 hbe ws the first to gi pe cocapletie.d sya Steziatie expression to tho new views. His able w-Norc which oenas the present series, is c, an iuthorilativ'e exposiieon, aicid i. S a woio - edgetd class.icT -iupoi the Subject; Atain tfhte claims of Dr Mi, ayer to ain eminmi..t" anud envhable placec amoneg the pgion.eers of this great scientific -isoverment ar 0re tinquestionable. Theres. has evidently been, on ths part0a o tf sone lgcIh wricters, acic uncwort. y inclination to ldeprei. cIc hvscmrit's whlich. has given rise t;o a shi p and sesarclii og ont3roversy, Thei intic-leetual riiter s of the Ge(rmali phcilosophier hlca-y, hioswever, been dceecisirwvely vtndichted i by the cilth'hie pen of Profi Tynd iil.d and i'tA is to the pctb- li intm erest thus exciteda that, we are indebted fc r the. translation of i's syer's. papers, wihich appr ic n athis xvol1um iIe I i1/ea y did not expnimen'c to the extent of tJoe: i cnd Gro', yet he wAel knew its inportalnce, and miLade such invetigsations as shis apparat.us iad the ducties of a aboriocsi profesion would,, i ow. Yet lisa view, were not therefore mor e irnigenius and dprobable1 conjectures,'aster ot f the results of modcrii scieni.ce and of ithc mathert. itcall methlods of deaie ng with tihe,1 possessiung1' broad piolosophic graspt l tand. an. extrocrdiiinary menti d it )cl pettinAiti Dr, _iaye e, c'ntlered ear ly11 01 the iciqcuiry ad ot d ci. nt oi bha i. c'elo3ped rctmaay cf its XXX LNTPO1DU ~O I'N. prime applications in advance of (any other th.inker, but he has done his work uander circumstances and in a imnanner which awrakens the highlest admiration for his geninus* An eamineaet authority has renmarked that tt ese discoveries open a region which promises possessions richer than any hittherto granted to the intellect of man.' Involving as they do a revolatioof fundamental ideas, their consequences must be, as comprehenSive as the range of hunan thoug'ht. A principle has been dveleoped of all-perv ading application, w5hiCh brings the diverse and distant branches of knowledge into more intimate and harmo-nious ailiatncie, and affords'a profounder i tsightinto the universal, order.'ot only is science itself deeply ffected by lthe pieentiatilo n of its question.s,- i new and suggestive lights, but its mnethod is at once made universal~ There is a crude n1tion in man y minds, tfhat it is the busines of science to occupy itself merely with tfhe study of matter. When, hitherto, it has pres its inquiriese its i i nto t- hi gher Prof. Tyndall remarks: "Mayer p)robably had not the meanls of maaking experiments himself, but he ra-nsacked the records of experimental science for his data, and thus conferred upol his avritings a strengthll which mere speulation can never possess. From the extracts which I have given, the reader nay infer his strong' desire for quantitative accuracy, the clearness of his insight, and the firmness of his grasp, Regardjbg the recoilition which will be ultimately accorded to Dr. Meayer, a shade of trouble or donbt has never crossed, my mind. Individuals may seek to pull him down, but their ef.orts ill be unavailing as long as sune evidence of his geniuts exists, and as long as the general mind of hbuanity is influenced by considerat.ions of justice and truth. "The paucity of facts in Mayer's time has been urged as if it -were a reproach to hin; bhtt it oughtfi tube reemmbered tdhat he quantmity of fact necessasr to a generalilzation is different for different ninds.'A word to the w is sufit-ient for themn,' and a single fact in some minds bearms fruit that a huni(Led cannots produce in othiers lrayer's data were comparatively scaity, but his genius went far to supply the laclk of experisment, by enabling him. to see clezly the bearing of such facts as he possessead. They enabled him to think out the law of conservation, and Isis conclusions recived the stitmnp of certainty from the subsequent experimental abhors of tir. Joule. In reference to their comparative merits, I woul: say that as Seer and Genemlizer, Mayer, in my. opinion, tands first —c8 e,perf-m.etat p.ilfI opts Ja, Jon),fe, THE TRUE SC. P0 S OF SCItENE. sXXXA region. of life, mind, society, history, and eduiation, the traditional custodians of these subjects have bidden it keep within its himts and stiel to m.gtter. But science e is not to be hampered by this narrow conception; its office is nothing less thian to investig'ate the laws and universal relations of force, and its dotmain is tihereiore coextensive wiith the display of power. Indeed, as wo winor now thing of matter, except through its mnla.ifestation of forces, it is obvious that the st ud of matter itself is at last resolved into the study of forces. The establishment of a new' philosophy of forcie,, therefore by itsvast extenson of te scope and methods of science, constitutes a momentous event of intellectual progress. The discnssioiS of the present volume will rmaike fCdly apparent the importance of the new doctrines in relation to physical science, but their higher implications are but -parftially unfolded. In the concluding article Dr. Carpenter has shown the applicability of the principle of correlatlion to v-ital phenomena. iis argumnent is of interest, noe only because of the facts and priaciptles et ablished'but as opening an ihquiry which must lead to -still larger results: for, if the principle be found operative in fndamenttal ofganie processes, it will undoubtedly be traced in those w hich are higher; if in. the lower sphere of life, ttheen throhouo t that sphere. If the forces are correlated in organic growth and nultrition they mnst be inl organic actioni; and thus humtan activity, in all its forms, is brought within the operation of the law. As a creatnire of organiC nutrition. borrowing maBtter and force trou the ouotw'ad world; as a being of feeling' and sensibility, of intellectnul power anc ultiform; actiftiaes, man must be reg'arded as amenable to the great tlaw that; forces are convertible and indestructible; and as psyIchology and sociology-the science of mind and the science of societv -ave to deal constantly with different phases and forms of huma en. ergy, the new' principle must be of the profounndest import in relation to these great subjects. The forces manaifested in the living system a re of the most va.ried and unlike character, mechanical, thermal, luriunous, electric, XXXII EN TIroMooTI'oN. chemical, nervous, seonsors lemoti0onal, and intellect'ual. lThat; these rforces are perfietly cordinated-lt at. there is some defi.miite relation as-mong thetn which explans the marvn ellous dynamic lii tiyof tho hving organiusm, does not adiit of question. Tha{t this relt'rion is of the same natlure as that which is found -bo exi-st nl-anong tie purely physieal Greesan, and which is expressed by the tserm * Corre - ation seems also abundantly evident. From sthe grest com;pIlex — ity' of'he conditions, the, s ame exactness will not, of coursle be expeel.ed here as:in the inorganic field, but this is one of thle neccssars limitations of' ll' physiotlogical and psyclhologid~ isnquldry; tthus quaidfied the proeos of the correlation of the nervous'and mental ioCee-s witu. the physical, are as cia sr and declisie s those b.s i the physiel Saores alone, if a current of electricity is passed throurih a smillt wa.e it produnces hea while if heat is applied to a certain co-mnbinationi of. metalI, it reproduces a current of electricity; thlese forces te thereforei% cotrela - ed.A cuarrent of electricitr passed fhi rou.h a Nm.all. portion of a mot1or' sensory nerve will excite'th3e nervoforce inl the rnmainderi while on the other hanad as is shown in the case of the torpedol the nerve-force may generate electricity~ Ne —rve-~force mlay, produce heat, light, electricityt and as we consta'lntly ox"-;perienci, mechcanical pow-er, and these mi thcir::turn nam y also excitse nerve-borce, This to~rm of energyv is therefore clearlys ent;itled to Ia place i theo order of correlaited argacies. Agin, t- if we take the highest fori of mentae.l aetos, viz: willpower we Mnid that while it commands lthe muovemsents of the sysi tenA, it does not act directly upoll the muscles'but vupon the cerebral hemispheres of the brain. There is a dryna-nic chains of w-hich ~-volroutas'y power is but one link. The will is a poi er which excites'nerv-iforce ih tfhe brain whichi again excit es:mechanicsil po wers in... the muscles.'Will.-power is therefore torrelat;ed with nerve-jpo wero in the snme uma ner a's the latter is with musculsar pow'eor Dr. Carpenter -well ohbserves: "It is difficult to see t, hat the dyvnameal agen.yf.c l wich e term will is snore remisoved friom iner vieforce on 001-CItRELAIVUJ(N' 0OF NIERVOU'S KND Si tES EL Tac Xi{"S', XXtS i the one hand than -nerve-force is removed fro-n mo3tor force on the other. Each, in giving' origin to 1t5he nextt is itself expended or ceases to exist ast- s..c,, and. atch bears, in its oown nltensity, a preocise relation to that of its antecedent and its counseq-llent. We have here1 only space briefl-y to trace,the principye in its application to sensations,.motion, cTad intellectnal operations, The physica. agencies actingl 10po1 ll:ninm11te objects's i.n th external. wv orld, change their forrm and state, and -we regard. these ehagnges as transforlmed manifestations of the forces in action. A body is hea-ted by hanmmer-in.g; the heat is.buat traxasmnted:meccha1Nical force; or a body is put in motion by heat, a cer.~'ztain quanatit;y being transformed into mechanical effect, or motionl of th0.e mass, And so it is held that no force cail arise exceplt- by tlte exlpendituroe of a pret'xistingu force. N! ow, the, living' system is acted. upon by the samte agencies land under the same la:tw.o Impressions mad upon the organs of sense give rise to sensations, and we have the sanme warrant; in this as in tlte former cease for regerditg. n the effieets as ratnsformtations of th.e forces in action. If the ehange of molecular state in a:melted'body represents the heat trantsforimed in fusimng i., so the sensaftion of warnith in a livitng bodvy mr:ust re'present the beeat transf'ored in prod.ciun it. The im.pression 5 on the retinma, as well as that on the photographic tablet, results f'ro.ma.: the t.ransmurted imupulses of light..And thus imupressions ttmade fromn momen11 t to mnomnent on all our orgams of settse, are dhirect].ly correhated witlt external physical forces. Tlis correlationt, furftlerm1ore, is qmnutitativeae as well as qualitative. Not only does the light-force produce its peounlia seLnsationsr, but the intensity- of tthese sensations correspontds with the intensity of'the force; not onlfy is atmospheric vibrati. on transmutned into the setsense of solundl but Ot-,c energy of the vdibration determintes its loudnsess. _And, so in all other cases; the quantity of senseation dep)ends ui onl tthe quantity of t4he frce acting to proTduce3 it. M-roreover, sensations do not tertminate in theusclves, or come to lno'th.1ing; they produce certain correlated and. equivale] nt eiteets MKI'xrxwIV ~~l~ iT'RODUn GTION. The feelings of light, heatft sound( odor, tat, t ressure, are ihmuediately follow ed by physiol ogical effects, ats secretion mniuselrmtr afti. one &c, Sensations increase the contractions o the hleta.r, and it has been lately -aintainedl that eve ry sensation contIMaets the muscular fibres throuxghout the whole vascular systemo The reso pihratory muscles also respond to sensations; thte rate of breathin, t..ing increased by oth pleasurable and painful nerve-imp' s-ioes s Tthe quat tity of sensat, iont moreover, controls the quantity of emotion, Loud sounds produce violent star-ts disagreeable tastes ca use wry faces, and sharp pains give rise to violent strutggi esn Even when groans and cries are suppressed, th clenched xhands and set teeth 1sh.ow that the m-uscular excitement is only taking t another direction. Between the emotions and bodily actions the correlation and equ.dalence are a lso e qually clear,. oderate actions, like, mnoderate sensati.on0, excite the heart; te vascular systezm, nid fthe gfland Xfar organts,.As the emotions rise in as trength however tThe various systems ot muscles- are thrown into actison; ad whlen te y reach e cortairin pitch of intensity, rViolent convulsivee unloVen' ts ensue. Anger frowns anLD stamps; grief wrings its hands; joy danlces aid leaps —the amount of sensation de.termininag the quantity of correiat tive soav0esnnt. Dr. Carptenter in his Physiologye has brouggltt fasrw i rd numerous exeumplifications of this principle of lthe conrersion of er-otion into movement, as seen in the comnon wxorkings of ]un1. lnature. Most persons have experienced the difficulty of sli;tin. still uwnder high excitenment of the felings, and also the reilief alff:rded by waiking or active exercise; wvhile, on. the other hxand repression of the movements protracts the emotional excitemoento M1.avly iras cible persons get relief from their irritated feelings by a hearty exp7losion of oath, others by a violent slammg of thie doore or a prolon-Uhed fit of grumtibling'. Demnorstrative persons habitually ceped dh tir %etlings in ction, hihle those who mxanifest them less retain thenl longer: hencec the fornet c are 1more weel;< cnd trausleut hin their COR.bIELATIOI OF PHY SiCAL AND 4ENTAL FOCES. XXXV attachoments than the latter, whose unexpended etnttions bekcome persmanfent elements of character. For the same. reason, those who a'e lond and vehenment in -their lamentations seldom die -of grief; while the deep-seated emotions of sorrow wc'hi.ch others camot work off in violent demonstrations, depress the organic f-unctions, and often wear out the life, Thle intellectual operations are, also directly correlated with phliyscal activities. As in the inorganic world we kInow nothing of forces except as exhibited by imatter, so hi the higher intcilectual realm we 11know nothing of mind-force except through its materital manife.stationsv, Mental operations are dependent upon,Lmaterial changes in the ner vous sste; aend it may now be r egarded as a fimndamental. physiological principle, that "no idea or feeling can arise, save as the result of somne physical force expended in. producing it,'" The directness of this dependence is proved by the fact that'iany disturbance of the train of carebral tr ansforsra. tions isturbs menttlity, whsile their arrest destroys it. And bher% also the correlation is quantitative, Other thilangs being equal there is a relation between the size of the nerve apparatt.8nd the ranount of:mentai action of which it is capablse. Ag'ain, it is dependent upon the vigor, of the circulation; if'this is arrested by the cessation of the heanlt's action, total unconsciousness results -if it is enfeebled, mental action is low; while if it is Cquickened, mentality rises even to delirium, whetn the cerebral. activity becomes excessiveo Again, the raft of bridn activity is dependent upon the speciVal chlemical ingredients of the'blood, oxvygen and carbonn, Tlreoase of oxygea augments cerebral action while increase of carbonic acid depresscs t The degree of metality is also dependenot upon the phosphatic constituenats of the nervous system. The prroportio- s of phosphoorut in the brain is smallest't in infaicy, idiocy, ald old ageto and'greatest during the prinme of life; while the quantity of alkaline phosphates exlcreted by the kidneys rises and falls wi th the variation-rs of m'ental activity. The equivalonea of physical agencies and, mental effects is still further seen in the action of various substtimes, as alcohol, XXVi I N'TOD'UOI O I o'pinu h.ashish, nitrous ox id, etc.e wen atbsorbed into the bloodo Within the limits of their peculiar action upon the -nervous centres, the effect of e ach is strictly proportionalto'to thle quanti't-y taken, There is a constant ratio between the anteeeedents and conseq3ituents. How.thi m iet amorphosis takes place -how iocrce exi. ~tng as motion; heat, or light, can beco-me a mode. of consciousness.28 how it is possible for airial vibrations to generatoe tht s'ensation wn o til sound11 or for the forces liberated by chemical chaenges in thel brain, to give rise to emotion, these are mysteries whlich it is ii — possibfle to fudthom But, they are not prmorolunder mysteries thrn tI-r e trans.stOrmaton of the phpysical forces nito eachl oi;her, They a're not miore cotmp]etely beyond our eo-n.p"'ehension th-p lxa thle natures of muind and matt;er They have shnuply'th Sinle isl-eolh. bilhRfy as till oth.eirultnu. te questio-ns. VWe c." learn naothing "more than, I't here" is one of the uniformnities it the order of p'Ahe Iloineia.?'s The law of correl-ation hbeing uthns appeicable to l1humanl enlergy as well as to the powers of aturt.e it. ust also apply to society; where we constuXnJtly witness the conversion of:'irees oni a ominprehensive sema Te he powers of nature are t. ransoirmedlit',o thi actWiitics of society; water-power, wlind-ipoweri stet.-upower, and oleetrical-powe'r are pressed inso the SOCid setvice reducin.n h-umanI labor, nnult;tiplying reso-rcsm and carryivng on numibeLt e- industriail procosses: indeed, thte conversion of.these forces into social', tactivities is one of the chief tri-m aphs of civilizationo The tLuniver sal tfores of heat (ald libht are tiran' norned'by the'veg't'tble ingedomon1 into tites vit~a tenergy of organ'ic eonpoulnds, and thens as food, are agn-ain converted linto humanl'beinrs and. ihumian p0ower T. e very exis:.tonce as well as tihe activity of society are obviously dependent upon th fe operations of veeptabl e growthi,. nheon thati is abcunda ant, popu lotion inmay b"icouxe dense,.d social ativities mucitihffiaouas nad tomiplim eated while a scant'y vegetrtion eutails sparse populatsion and enife ebled socia aetion. Any n l ersal di.sturbance of thoe physical forces, as excessive ramis or d1routh by reducimnl th' 1i' CORRELATION OF VITAL AND SOCIAL FORCES. XXXVii vest, is felt throughout the entire social organism. Where this effect is marked, and not counteracted by free communication with more fertile regions, the means of the community become restricted, business declines, manufactures are reduced, trade slackens, travel falls off, luxuries are diminished, education is neglected, marriages are fewer, and a thousand kindred results indicate decline of enterprise and depression of the social energies. In a dynamical point of view there is a strict analogy between the individual and the social economies-the same law of force governs the development of both. In the case of the individual, the amount of energy which he possesses at any time is limited, and when consumed for one purpose it cannot of course be had for another. An undue demand in one direction involves a corresponding deficiency elsewhere. For example, excessive action of the digestive system exhausts the muscular and cerebral systems, while excessive action of the muscular system is at the expense of the cerebral and digestive organs; and again, excessive action of the brain depresses the digestive and muscular energies. If the fund of power in the growing constitutions of children is overdrawn in any special channel, as is often the case by excessive stimulation of the brain, the undue abstraction of energy from othler portions of the system is sure to entail some form of physiological disaster. So with the social organism; its forces being limited, there is but a definite amount of power to be consumed in the various social activities; Its appropriation in one way makes impossible its employment in another, and it can only gain power to perform one function by the loss of it in other directions. This fact, that social force cannot be created by enactment, and that when dealing with the producing, distributing, and commercial activities of the community, legislation can do little more than interfere with their natural courses, deserves to be more thoroughly appreciated by the public. But the law in question has yet higher bearings. More and more we are perceiving that the condition of humanity and the 2SXXll. UN T R DUTI COUT IO N% progress of civilization. are direct resulta a;nt s of.he.hreaes by whicch men are controlled,'What we tern the mu oral order1 of society, ytinlplies a strict regularity in the action of these obroes. M:".odern statistics di1sclos e a retmarkable3 constancyr ieh n mora'1 aetvites m- anlifesited in communities of iten. Crimes, aId even the modes of cmirnm% hive been observed to occur with a uniformaity which admnits of their prediction, Each period may tire te re hr said to have its definite amiount of morality and. justice. It has been maintained, for instance, with good reason, that "the degroeo of liberty a people is capable of in any given ge, is a fixedt cli itrr asad ay thsa saty rtifiacial extension of it in one diretion brings about an cjuc-ivt lent limitation in some other direction. hFrcnoht reovoluitons show scarcely any more respect for individual rights than tihe detspotin-asm s they supplaot.; and French electors nLS their freedom -oto put themselves sagain in slavery. So hi, t.hose comml-unities where State restfriraint is feeble, we may expect to find it supplemented. by the sterner restraints of public opinion." 3 But society like tir1 individual is progressive. Vlthirough at each stage of individual growth the forces of the oronufisms physiological, intelleetual and passiontal, have etae l a ceirtauin defirite amount of strengt!h, yet these ratios are coristarntly chaing n a.nd it is in this:changei t, at developmcent essentially consis t, So with soc iety;I the rteasured action of its forces gies rnic t fi xed samonunt of m-uror>alitvy antd liberty in each,e'bt 5t thart ar9loui0t. inc.reases with. social evolution. Thie savage r.s oe in wlho.-n certarin classes of feelings and emotions predom.nsn-ate, a-nd he tecmllnes oiydl-i;ed. jsat in proportion as these feelings are slowly ric y ea'yothers of a higher character. Yet the activities which d-etr rmine human advarncement are various, Nos'i- toly ru'lst we reg'ird tihe physologicaTl forces; or throse which pertain to matin ns plsical Oruanizstion land aItpacities, and the psyeohloglrca!l, or those resuloribrti ng frnom his intellectual anid emotional conscitution)'buLt the influences of Vtahe eterntal world, tand those of tirhe oei sotase, itre iTewis rea to be considered. P ann and society, therefore, a- viteiwed bv thv e ey SPENIE1}&$ O CONTIBUTION TO Til IN-QUI iY. XX, i4t, of science present series of vast and compler dynamical problems, wihich are to be -studied in the iatuinr in ti light. of t.he great laY by -whinch we ha ve reason. to'believe, al forms and phases of force are governed. A furthier aspet of the subject rem nains still to be noticedi. Ir Herbert Spencer has the honor of crowasnin this subie-no. inquiry by showing that the laws of the conservation, or as Ie prefers to term it th.e Persistence of Force, as it is the underlying prineipwle of all being, is also the fundamental. tlruth of all philosophy,. T7tirl itmasterly analytic skill he has shown that this pr inciple of which. the hunman mind has just becomne fully conscions, is itself the profoundest law of the haumn mind: the deepest;foundation of conu{scionusness. Ie has denmonstrated that t.he laEv of the Persistence of Force, of whlich the most piercing intellects of past times had. but partial and in - satisfying glimpses, and which the iatest sccienttifc researc.h has disclosed as a great principhe of nature, has a vyt more transcendent character; is, in fact, n d piori -tr uth of the thighcst order-a truth which is lsceessarily involvedl iin our neantal orgtanizati'on; which is b'roader than any possible induction, und of hi"her v;zalidity than. any other truthf whatever, This principlos which is at cnce the highest result of scientific invest t(A4o1on and.met.haphysical analysis,.tr. Spetn.cer h1as msade the'basis of his new and comlprehensive Syrste-n of Philosophy; and. in the first work of the series, entitledb First PriTciplceS he has developed t3he doctrine tl i Rts broadest philosophic aspects. The lucid reasoning by which he reaches his conclusions cannot be presented here; a brief extract or two will, however, serve to indicate the important place assigned to Pthe law by this acute and profound i nquirer: ";: migh t, WI ded, d be certain, even. in the absence of any seh analysias as th e foregoing, that tisere must exist soile principle whiicht as being the basis of science, cannot be established by sciene. A.All'easogned out conclusions whatever must rest os stme postulate;: As befobre shown, we cnmot go 0o. mer ging dorivative truths i these swridcr and wider truahs from which they arve do 9xi IINT r D UCr TiS 40 rived, wvithout eawchingi at last c 4est t wrut-il. cll can be merged inl no otheri or derived from no other, And whioever con.-'..mptlatt.es the rel ation in which it, stands to the truths of science in general, will See thalt t'his tnrut, transconding de-monstration, -is the Persist9 Once of force,.~ * * * " Such, then, is the forundation of ainy possible system of posi. dvye I nowleGdgc Deeper than deimnsnt ration —~deeper even thza dehinit.a cognit-ion-deep as the very nature of mind, is the postulate at.-which we hbave arrived. Its autlhorit traiunseends all others whateveor; for not only is it irven in tthe cons'itution of o00 own eonsciousness, but it is iumpossible to im lalgine an eaoscioasness so lins-ent; of re lations, may be readily' clncelived to go on ewnhile yet t hesei rela;tions have not been organized into tl:e abstracts we c.all space ad and i eln so there is a concerivable kind of consciousness which does not contain the'truths conl.'monly called i, Priori, in-, voived in the orgPanization of these forms of relatiolns. But thoug-;ht ca.l.nno:t be conceived to go on iwithout somne element between which its relations may be established; and so there is no onceiRalel) kindof consciousness which does not ixmply contiaued existence as its datum. Coonsciousness wiithout this or that particular form is posss bl.bl: but connsciousness wit. hor conoca-8 is impossible, "The s-le t.ru th which i; ranscends experiel-nc by underlying it, is this the Perisstence of:force This boing the basis of expcn'iuecet, must be the: basins- of any scientific organizatlion of experiences. To -thi an ititlnate an2iiss brings us diown.; and on this a rationtal sym1shesis musst be built upi' To the ql csti on What tien is the value of expertimei nItal investiaatio s upon tflhe suntbjectr if the truth sought caunot'be estatbliRshed by inductLios from, filsm t iuoij Spencer replies: "They' are of'vaIne as disclosi'ng the mrany particular picnat cion s which the oneraftl truth (does not specify; thby are off Value aS teaching -us how much of one mode of force is the equivalent of so nti.ntui of tanother noide; the are of value as d et ermining under mwihat conditions each EirTTr:EINDOUSft U.OEACPiNU ON THIE LANttlI metnamlorphosis oc. a1rs; and trhey are of value as leading u.s to ilnl u iTe' i'- r h.'.al e the remaumt of:force ]hast escaped', withert t1h apl)arent results'are ntot ecirivalent to the cause. An it may e added tint it is to these investeigations that we are lnu. o'bted bor t'.the clear and comiprsehensive establishm-ent of the prtinciple as a li: of physficJ natiure; psychological ainalysis hau"ring o "nl.y Slhoiont that it, extettds m. ucli furlther than it is the btiSness of expesri-ental scia.ence to go. Thus the law e artacter1ized by t riaday as tnshe hi-hest in physlea. sience whict e our i.culties pcrm it us to perceive; hias 1a a mo:re. extended s ay; it might well have been p)roclaimned lthe highest law of gl sience-tbhe nosi ar-tc react'ing, priciple that adventurinsg reason has discovered in the u-nisvers'. Its saupendons reach spans al orderst of existence. N=ot only does, it govuer the.rmoiyeneints of the hesiaenly bodies but it presides over the genesis of the conou2Istellair.tons; not only Tdoes it control t hose i.radit't.ifoods of poe r wh-iich fil the eternal space hln i b atin na ill iumuinting aiid viv-fn. ing u ouran put, but it rules the actions and roela'tionus of men, and regulatSe the iarLch of terrestriaS.l Sairltl..or is its do. omi-.11n. limited to physical phenomena; it prevails equallyc itf the worlId of vind controll-itg all the faculties and p rocesses of thlought and ifeeluso The star-ntals of toe remoter ga'-.laxicse dart theirf radstiahs a'eross the universe; and although the disti aces are so profoxund that hunih(eds of ccnturies nay h*ave been reTquired to tiaverse them, 3he iip U1 ses( of force enter the eys, atnd inipressing ailn atomic change upon the nerve give origin to the sense of sight. Star and nerve-tisse are parts of the s ame systo e-stellar an(d.- nertonus forces are corelaSd, T NTy Amore; sentsatioln avwakens tihonught and ki'ndles emotioni so that this i osndrous dyna-amic chait n bihds into ivin.hg ranlity' the raslit of matter and m-ind thlroughh measureless amplitudes of space and time, And if tlhese high rea.tiues aire but faint and,fitiful glimpses which science has obht ained in the dim dawn of discovervy, w.hat must lie the,thalrirs of 0he comiing day? If it;d-ed thley are t. it xZHi NTRODlCTION.'pbblesi gathered from the shores of the great ocean of trunh, what ar te th lsteries still hidden in the boson-f of the mnighty uinexplored? An.d how far transcending all stretch of thougcit tha-t Unknown and Infinite Cause of all to winch thp humana spirit turns evermoreL in solemn and mysterious worship! It retmains only to observe, that so ilunense a stp in the priograss of our knowledge of natural agetice a's the follow-ing paaes disclose, cannot be without effect upon thne ittellectual culture of the aSget To the adherents of that scholist ic and verbal education which prefers words to things; and aucient to aodtern thouught; which ignores the study of nature, and regards the progross of science wilth indifference or hostility, i matters little what viewts of the wvorld are entertained, or w-hat changes tlhese views many nndergo. But there is another and happily an in c-reasing class, who hold that it is the true destiny of mind to conprehind the ast ordeor of existence in the midst of which it is aptlted and th ta the faculties of man are divinely adapted to this suhblme -ta;k who see that- the laws of nature mnust be understood before they can be obeyed, and that only through this understanding ca. man rise to the mastery of its powers, and bring himself into final hus mony with his conditions. These will recognize tha the. discovery of new principles dwhich exptand, and elevtate, and harmonize our vi ews of the truiverse — hich involveo the, workings of the mind itseli; open a new chapter in philosophy, and touch the very foundations of knawledge, cannot be without a determining influence upon. the futatrc course a.d development of t.hought, and tlhe spirit,and methods of its acquisition. CORRELATION 01F PAJIIYSI AL 0r GO RE. 3x R GROVE, QAC, M JtS.R$ F RSTM.AA XT.ErcAN7 BROi T.E IURTHX BESGLISH BEsXiTdOe WtILLIAA ROBt Go0v, an English lawyer and physicist, was born at Swvansea, July 1.4, 1811. He graduawted at Oxford in 183-4, at during the next five years was Professor of Natural Philosophy at the Londonl Insti tution. lprofessor Grove is a rare example of the abilit, y which hbas achieved distinguished eminence in different fields of effort. hile pnrsuing wi1th marked success the profession of an advocate, he has devoted his leisure to original seientific researches, and obtained a high. distinction both as a. dis coverer and a philosophic writer upon scientific subjects, In 1.85 hle was mnade Queen's Counsel, and afterwards'iee-]President of the Royal Society. He is the inventor of the powerful galvanic battery known. by his name, and his chief researchles have been in the field of electricity. fanry of his e;xperimental reslts are referred to in the following pages8, wh.ich will also attest his high position among the founders of the new hilosophy of bforces P REFACE. IHE Phrase I Correlation of Physical Forces' in the sense in which I have used it, having become recognized by a large number of scientific writer, it would produce confusion were I now to adop; nother title. It wvould, perhaps, have been better if I had in the first instance used the Iterm Co-relation, as the words' correlate,' correlative had acquired a pccriliar metaphysical sense somewhat differing from that which I attached to the substantive correlation~, The passage in the text (p. 183) exp(latis the meaning I have given'to the term. Twenty years ha inRg elapsed since I promulgated the views contained in this Essay, which were -first advanced in a lecture at; the London mInstitution in January 1842, and subsequently more fully developed in a course of lectures in 1843, think it advisable to add a lit.le to 3h.e Preface with reference to other labouiers in rue same field. It has happened w4 ti this suibject as with many others, that s.xilar ideas have independently presented'themselves t;o diff,2rent minds about the same period. In.MIay 1842 a paper was published by M. i Mayer which I had not read when my last edition was published, and indeed only now know imperfectly by the zvivd-voce translation of a friend. It deduces very' much the same conclusions to swhich I had been led, the author starting partly from a priori reasoning and prtly f'om an experiment by w'tich water was heated by apggtation, and from another w-hich had, however, previously been made by Davy, viz. that ice can be melted by friction thoughl kept in a medium which is below the fr eezin point of waterD In 1843 a paper by Mt. Joule on the meehanical equivalent of heat appeared, which, twheough not in terms touchuig' on ithe mutual and neessary depeand nce of all.'the Phyrsical Forces, yet bears most importantly upon the doctrineo Whuile m-y third editon was going through the press I hlad the good forttuane to makle he acquaintmnce of [, Se-guin-, d who informned ne that his unle, the eminent iAfeontgolfier~ had long entertained the idea that force was indestretibtle, honugh, with the exception of one sentence, in his paper on the hydraulic ram, and'where he is apparently Speaking of nechaneieal.force, he has 1eft nothing in print on the subject. Not so, howe g ver, I, i eguin hniself; who in I183, in a work on the I Infuiencee of Railroads, has distinctly expressed his uncle's and his own views on the identity of heat and mechanical force, and has given a calculation of therl quivalent relatio n, hich -is not far from the more retent numerical results of ay er, Joule, and others, Several of t{he great mathematicians of a much earlier period advoecaed the idea of what they ternmed the Conservation of Fore, but althouogh they considered that a body in miotion would so ontinua,or ever, unless arres'ted by the impact of aother body, and, indeed in the latter case, would, if elastic, still continne to iaove (t.houg'h deflected from it' s course) with a force proportion-.ate to ivts elaSticity, yet Wth inelastic bodies the g'eneral, and, as far as I am 1 T8rel the universal belief was. that the mo'ion. was arrested and the fore: an.nifhila'ed, Mgonigtofer went a step farvther a.nd his hydraulic ram was to im a proof of the t.ruth of his preconceived idea, tha't the shock or inact; of bodies left the mechanical orce'cundestroyed. Preiously however,t to t1le disoeveries of the vOltaic lt ttery, ciect.To-imagnetism, thermo-electricity, a nd photography, it weas impossible for any -mind to perceive what, in the greater Znumber of cases, became of the force w nhich as a j arently lost.. The phenomena of heat, know from t he earliest. timnes, would have been a mode of accounting for -the resultin; force in. many cases where motion was arrested, and we and Bacon announcing a'theory that mBfion was the form, a he quaintly termed it, of heat. Rumnford and Dary adopted this view the former with a fair approximate attempt. at numerical calculationo but no one of these philosophers seems to have conmected it with the indestructibii.ty of fo-rce A pas-sag', Li il he writing s of r. Rogegt, combating the theory that mere contact of dissi.rilar bodies was the SOruceo of voltic electricity, pihilosophically supports his'argiment by the idea of non-creation of borce. As I have introduced into. the lat edit ions of ny Esst1y abo strac'ts of the different discoveries which I have found) since my first lectures, to bear upon the subject, I have been reg'arded by many rathler as the hist orian of the progrTess made in this branch of thoughbc than as one who has had andything to do with is initiation. Everyone is but a poor judge xhere he is himself ilterested, and I therefore write with diffidence, but it would be afiectming an indi'ference which I do not feel'if I dicd not stlate thait I beleve m-yself to- have been thbe r'st who introduced this subject as a generalised system of philosophy, and contiunued to enfrcee it si my lectures and wistings for many years, duringg wvhich i rnet with the opiosito-n ursu-al and proper to novel ideas. Avocations necessary to the ell-being of ohers have prevented my f.ollowing it up experlmentally, to the extent that I once hoped; but I trust and believe that this Essay, imperfect though it be, has helped materialMy to imnpress on that portion of the public which. devotes tsI attention rather to the philosophy of science than too what is now term-ed seence, the -truth of the thesi advocated. To show that the work of to-day is not substantially different from the ti.0ughts I first published on the sublject, a period when I kne w 1ttle or nothing of what had been b thonaugh before, I venture to,give a few extracts fro3m the prhited copy of my lecture of 1842;Physical Science treats of Matter, and what I shall to-night term its Affections; namely, Attractido, Ml otion, Heat, Light, Electricity, ag net isum, Ohemnical-Affnity. S hen these re-act upon matter, they constitute Forces. The presenlt tendency of theory seems to leaid to the opinion that all t.hese AT eti'ctions ar e resoilvabl into one n'mel.otiont; however should the tiheories on these subjects be ultimately so ctetuatlly gen-eralised as to become larws, they canlot avoid the necessity fobr retainimg different inames bfor these dciffrent Affections; or, as theyf would then be called, different modes of Mfotion.... d(Elrsted proved that Electriicity and Magnetism are'teo- forces which act upon eachl olther; not in straight lines, as all other k nlown fbrces do but is a rectangular direct ion: that is that bodies invested with lectricity, or dte conduits of a.n electriecurrent, tend to place magmeets at, right angles to et i and> converseyf, that magiets tend to lace bodies condiot ing e-le ristcy at right anrglles to then.. P-EF tACE. The discovery of Otrsted, by Nwhich electricity was made, sourc-e of Maagnetism, soon led philosophers to seek the converse effect; that is to educe Electricity from a permanent magnet: —h ad these experimentalists succeeded in their expectations of making a stationary inagnet a source of electric-currents, they would have realised the ancient dreams of perlpetual motion, they would have converted statics into dynamies, they waould have produced power without; expenditure; in other wordsi t hey would bave become creators. They failed, and Faraday saw thi-ur error; lie proved that to obtain Electricity from Iagnetism it was necessary to superadd to this lattter, motion; that magnets while in motion inducd elect icity in conlltiguous conductors; and that the direction of such electeic-currents was tangential to the polar direction of the magnet; that as Dynamic-clectricity may be made the source of Magnetism and Mnotion, so Magnetisi conjoined with Modion nmay be made the source of Electricity iHere originates the Science of t~ag'neto-electricRy, the true converse of Eleetro-magnetism; ancd thus between Electricity and Magnetism is shown to exist a reciprocity of force such thatS considering Either ta the primar'y avgent, the othler becomes the re-agent; viewing one in the relation of cautse, the other is dthe effect. The Science of Thermo-Electricity coimected heat with electrieity, and proved these, like all other natural forces, to be capable of mutual reaction..... Voltaic action is Chemical action taking place at a distance or transfirred through a chain of media; and the Daltonian equivalent numbers uare the exponents of the amount of voltaic action for corresponding chemical substances. By recardinog the quantity of electrical, as directly proportional to the efficient chiemical action, atnd by experimentally tracing this principle, I havre been fortunate enough to increase the power of the Voltaic-pile more thin s idcen times, as compared witth any combhAation previously knuown. I ain strongly disposed to consider that the -facts of Ca talysis depe-nd upon Voltaic action to generate which three heterogeneous substances are always necessarv Induced bv this belief I umade some experiments on the subject, and suceeded in forming a oltaic combination by ssesoius-oxygen gaseous-hydroge1, and platinum; by which a galvanometer was deflected anid water decomposed It appen s to mne that heat and 1iShht ma-y be considered s aiffections; or, according to the Undulatory-theory, vibrations of omatelrt itself and not of a distinct etlherial fluid perm.1eating it: these vibrations would be prop. agated, just as sound is propargated by tibrationis of wood or as waves by water.'To my mind, all the conseque nces of the tndulatory-theory flow as easily from thuis, as from the hypothesis of a specific other; to suppose wshieci namely, to suppose a fluid sifi oeneris, and of extreme tenuity peneltrating solid bodies, we imust assume, first, the existence of the fluid itself secondly, that bodies are without exception porous; thirdly> t/hat these pores commun.icate; efourthsly, that matter is limited in. exp sibility. None of these difficulties apply to the modification of this'theory which I venture to propose; and no other difficulty apphlies to it which does not equally apply to the received hypothesis. With regard to the planetary spaees, the diminishing periods of comets is a stronrg argunment for the exa0 istenee of an universally-diffused matter: this has the function of resist. anme, and there appears to be no reason to. divest it of'lthe fhictions comRon to all matter, or superftcidlly to appropriate it to certain affections. Again, the pheenomena of transparency and opacity, nare, to my mind,, more easily explicable by tlhe former than by the latter tneory; as resulting firom a difference in the molecsulca arrlangement of the mxtter affected. In reo gard to Sth effects of double-refraction. and'polarisetlon, the inolecular gives at once a reason for the effects upon the one theory, while upon'the other we must, in. addition to previous assumptions further assume ia dif ferent elasticity of the ether in. diferent directions within the doublyrefiaccting mediu-m. The same theory is applicable to Electricity and Magnetis; my own experiments on the itluanee of the elastic intermedi-tm on the voltAile-are, and those of Faraday on electrical induction, farnnish strong arguments in support of it., My inclination would lead me to de tain you ol this subject much longer than my judgment deems advis able I therefore content myself with offerring it to your consideration, an d, should mn -avcations permit, I mtay at a future period more fiftly develope Libhti Hott, Electricity, agnloetisn, MAotion, ad Chemical-affinity, sie all convertible mSt.erial affections; assuming either as the cause, one of the others will be the effect: thus heat may be said to prodrue electricits, electricity to produce heat; magnetism to produce electricity~ electricity magnetism;,and so of the rest. Cause and effect, therefore, in thleir abstract relation to these forces, are words solely of convenience: we are;totally unacquainted with the ultimate generiating power of eachd and all of them, and probably shall ever remain so; we can only ascertain the normne of -their action: we must hum-nbly refr their ecausation to one onmipresent influence, and content ourselves with studying their effccts atnd developing by experiment their mutual relations. I have tralnsposed the passages relating to vottaic action and, catalysis, but I have not added a word to the above quotation and, fas tr as I am now aware the t heory that the so-called imponderabltes are affections of ordinary matter, tlat they are resolvable into motion, that they are to be regarded in their action on matter as forces, and not as specific entities, and that they are capable of imutsul. t react-ion, thence alternately actig tls cause and effeet, had so' t t tatt time been publicly advanced. kffy original Essays beoig a record of lectures and being published by the mtanagers of the Instituion, I necessarily achered to the fbrm and matter which I had orally communicated. In preparing subsequent editiolns I found that, without destroyingg tLe ideat ty of the workC I could not altoer the stylet; atthou i t would htave been less difficult and more satisfactory to me to have don.e so, the work would not have been a republicateion; and I was fLr obvious reasons a xious to preserve as far as I could the orighnal text, wshich, though added to, is but little altered. The overm of lectures has necessarily continued the use of the first perSon, and.I wou ld beg my readers not to attribute to me% friom tihe modes of expression used, a dognmatfism which is fa-r fronm my tlought. If my opinions are expressed'brhoadlyx it'is t ha;, if opinions are always hedged in by qualificatOi.ons tbhe style becomes embarrassed and the meaning frequently unintelligible. As a course of.lectures can uonly be useful by inducing t;he auditor to consult works on toChe subject he hears treat ed, o the object of this Essary is more to induce a paaticularx t mrain of thougiht on'the known. f~cis of physical. science than to en.ter with minute eriticisms: Ilto each separate branch. ~i one or two of the reviews of previous editions the general idea i: iof the Nwork was obljected to. I believe, homweover, that will not now be the case; the mathematical labours of Mr. Thompson, Clauas.uu, and others, tthough not suRitale for insertion in atr Essay such as this, have awankened an iterest for ma ny portions of the SubjEt we:lh rom raises much. %r itf s future progress. The shor't and Irregular intervals which my profession permnits me to devote to science so prevent the continuity of attlention necessary for the proper evolution of a train of tlhough t, thati I certainly should not now have courage to publish for. he first ftime such alm Essay; and it is only the faveou it has received from those whose opinions I highly value, anid the, I trust pardon able, wsh not to let some favourite thoughts of my youth. lose all connection witth my name that haye induced me'to reprinu it. I ay scientiCfie readers wilI, I hope, excuse the very short notices of certain branches of science which are in-troduced., as wlthout them the wokrk would be unintelliogible to many or whomm it it intended. haive eondeavoured so'to.arange my matter' that each division shotfuld form an introduction'to those whic:h follow, a d to assuume no more preliminary knowledge to be possessed by ny readers than would be expected from persons a equainted with the elements of physical ascience. The notes contain references to the original menmoirs in rwhich the branlches of sciencc allided to are to'be tfund, as well as to those which bear on -the main argumenits;'wh.ere t'hese mnemoirs are unmorlous, or not easy of access I have refeorred to treatises in Vhich tIhey are collated. To prevent the readetrs attention being bmterrupt-ed, I have in the notes referred to the pages of the text, mastead of'to itterpolated letters. CORR0 ELATIOIYN PH SICAL F ROC ES I.-INTRPODUCTORY REMMAtNKS IJEN Aurar I henomena are fox thee orst ti-n & -'V ser'ved, a tencendeny mmediately dceivelopes itself to refer them to somnehing previouslny knowne-to t bringn tihen withfin the range of acknowledged s equences, The mnode of regarding new'facts, -rhich is most fyvomrably received by the ptublic is thatt which refers them to recogonilsed viewsstaps them into the rmonid in which the mind has beef3. aela ready shaped. The nevw fact maybe far rzemoved ifrori those to vwrhlh. it is referred7 and may belong to a dt-erent order of analogies but this amnnot then be known, as its co-ordinates are 1anting. It may be puestiontable.hiether'the nmind is 4 not so moulded by past events that it is impossible to advance an entirely new view, bnt acn itting sucn possibility, the new view, necessarily founded on insufficient data, s ikelky to be more incorrect and prejudicial thal even a stained'atcrptl to reconcile the nev discovery with known fiets. The theory consequent upon new facts, whether it be a co-ordinatinon of thexn wth knoN i1 ones, or the more difflult )10 (ORELATIO LA OF P1HYSICOAL FO(CE1, and dangerous attemlpt at remodelling the public ideas, Is generally elUunciated by the discoverers themselves of the fictds or by those to rwhose authority tle world at the per iod of the discovery defers; others are not bold enough, or if they be so, are unheded. Trle ea&rliest th eores thus enunciazted ob'ain the firnest hold::upon thle public, mind,'for at such time theore is no po wer of testing, by a suffuciernt range of experience, the truth of the theory; it is accepted solely or manly upon au thority: there being no ineans of cantradiction its reception is, in the first instance, attended with so-me degrec of doubt, but as the time in which it can fiairly be investigated far exceeds that of any lives then in being, and as neither the inividual nor the public i.ind will long tolerate a stat e of beyance, a theory shortly becomes, -for want of a better, admitted as an esteabished truth: -it is handed from father to son, and g'adually takes its place in education. Succeeding generations, whose minds are thlus formed to an establisned view are m tch less likely to abandon it. They have adopted it in the first instanoe, tupon au thority, to them unquestionable, and sibsequently to yield up their faitLh wotid involve a laborious re-modeling of ideas, a task hich the public as a body will and can ra-rely undertake, the frequent occurrence of wrtih ics indeed inconsistent witht the very:~existence of man in a social state, as it would induce an anarchy of thought-a perpet ity of lmental. revo T.his necessity has its good; b-ut the prejudicial efect -pon the advance of science is, that by this means, fleoreies tlhe most inmmature frecuently become the most pernanent; for no theory can be niore immiatur, none is likely to b1e so incorrect, as that which is formed at the first flush of a ner discovervy; and tholugh time exalts the atu thorhy of those rom vwhon it emanated, tine can never give to the illustrious dead the means of analysing and correctiing erroneous'views which subsequent di coveries conufr, 'r TRODrUCTORY REOmNL s It Take for instance the Ptolemaic. System, which we may almost litertJly explain by the expression of Shtakspeare l Ie that is gidd thinkss the world turns roiund/' WVe now see the error of this system, because we have all a an immediate opp~oiunlty of refing it; but this identical eror was received as a truth for centuries, beeanse, when Ifrnst promult g-ted, the eanus of refuting it were not at hand, and when thie means of its refutation became atta inabtle mankind had been so educated to the supposed tr.uth, that thley -rejeted the proof of its fgla;cy I have premised thie Jobove for two reasons: first to obtain a ir heariing, by requesting as far as possible a dismissal B'om the miind of my readers of preconceived views by and i Pavour of which all are iable to be prejudiced; and secondly, to defend myself from the eharage of undervaluing tauthority, or treating lightly the opinions of those to whoi and to whose memory aankind looks with. reverence. 1roperly to value anithority we should estiimate it together with its means of ifonirmation: if e a dwarf on th1e shoulders of a giant can see furtlhe t'han'thi giant, h is no less a dwiarf in contpaSri son with the giantt. The subject on which I am about to treat.-viz., the relation of the affections of matter to each olier and to matter — peculiarly demands an unprejudiced regard, The di-ffierent aspects under which tnese agencies have been contemplated; the diffi'rent views which have been taken of matter itself; the n Ctaphysieal subtleties to which these vieas r unavoidably lead, if pursued beyond fair inuctions from existing expe. rience, present difficulties al most insurmountable. ihe extet of claim which my views on thMs subject may have to originality has been stated in the irefaee; thiey be-'nae strongly impressed upo n mny mind at a X pferiod w-hen I was mac i3 engaged in experinental research, 2and we re, a's then believed, and still believe, regarding then as a sys-t'emi new: ex pressions in the works of different autthorsa, baripng 2. COROL:ATION (O'F PiYSIOAL FORtsOfJSE more or less on the subject, have subseqaently been ponmted out to mne soome of whcid go back to a distant, period,.An attempt to analyse the e in det ail and to trace how far I havte been aticipated by others, would probably but little interest fth'e reader, and in the course Of it I should constantly have to make distinctions slowing wherein I differed, and wherein I agreed with others. I fmight cite auth.or'ties rhich appear to m:n. to oppose, and others which appear to coincide with1 certain of the vie.ws I have put forthi; but this would interrupt t1he consecutive developement of nxy own ideas, nd nfiuht render ne iaole to the charge of miscon sttrulag those of ohers; I therefore think it better to avoid such discussion in the text; and in addition to the sketch g.iveu in. the refcace, to frmish in the notes at th.e conclusion snuch references to diffrent authors as bear upon the subjects treated of, w1hich I have discoveredior whfich. have been pointed out to me slilce the delivery of the lectures of wvhich -this essay is a record. The more extended our research becomes, thle nmore we find that knowledge is a thzin of slow prongession that the very notions which appear to ourselves new, have arisen, though perhapsi in a very indirect manner, from successive modilications of traditional opinions, Each. word Awe - tter, ea ch thoug);ht we. think, has in it the vestiges, is in itself the impress, of antecedent wvords and thoufghts. As each material fo:rm, could. we rightly read it, is book, containing in itself: the past history of the world.; so, different though our piilosophy mtay nowX appear to be from that of our progeuni tors, it is but theirs added to or subtracted froxm ttran.smitted drop by drop thfrough the filter of antecedent, as ours will be throxug that of sutbsequent, ages.-The relic is to the past as is the germ to the ft1ture Thoagh mnany valuable facts, and correct edudctions from them, are to be found scattered amongst the volumiaous Works of tie'anien4 t philosophers; yeet, giving them the IfNT2ODUCTOIY REM~AinKS. Jcredit whirh they prei-cminenty deserve for h aing r denvoted heir lives to purely intellectual pursuits and for haiving thought, seldomi frivolously, oftcn profoundly, nothing can be tore difficult than to seize and apprehend'the ideas of those wh.o reasoned from abstraction to abstractiont-who, a1though, as we now' believe, t:hey nmst h;gave depended upon observation for their first inductions, after'w ards raised. upon thenmn such a complex sTperstrauctre of syllogistic dcducetions, that, vithout folilowing the sanme paths, and tracin g the same sin-HU osities';whici led them to their conclusions, such concltusions are to U s Lunintell'ible, To tihink as another thoug ht, we mu'st be plaed in tihe same stuationa as he was placod: the errors of commentators generally arise os m their rth aonic' upon the airgumnents of their text, either iin blind obedience to its dicta, with out considering the ciremnst:anees under which tb.ey woerfe tutcred, or in viewing the imanes presented to.he origin1al n Ti'iter froim a different point to thl.at fromn which he viewed tlhem, Ex'periheinte philosophy keeps in check the errors both of ta p.rior-i reasoning and of coummentators, and at, all events, prevnts their becoming cumtultiv,; thou1gh tle theories ora explanations of a fact'be diffierent, the fact remains the same,'It is, moreover- itself the exponent of its dt eoverer's thoutgh4it' g hie observation of known phenomena has led.him to eliit from. nature the -new phenomelaa: and, though lhe may be wrong ina his deductions from this after its discovery, tihe reasonings -which condueted him nto it are themin seltves valnatfble andIi havinag led from lknotwn to iutkno wn truths, coan seldom be inminstiructive. ery diiffeiret views existed amongrst the ancients as to the aims to be pmursued by pysical investigation, nLd tas o the objects likiel to be attained by it. I do not here mean thie moiral ob — jec ts, such as tthe, atntainment of the suienmt, beonuti, &c. -but i'; e acauisitions in knowlcedoge which siuch investiga.tolns were likely to confer, Utility was one object in vieow and t:i w'was to som3e extent attMhned by the progress i eade in 1 CORi-RELATIO' OF PIIYSiCAL F'OBRES astronomy and mechanics; Archimedes, for instance, seems to have eonstantly had this end in view; but, wi-hile pursuing natural knowletdge for the sake of knowledge and ihe powrer which it brings with it, the greater number seened to entertamin an expectation of arriving at some ultil ate goal so me point of knowledge, which would give them ma stor overy the.mysterles of nature, and would enab tle the to asc ertaint rat vwas the most intimate structure of matter, and the causes of the changes it exhibits~ Wthese they could not discover, they speulated.Leucippu7s Democritiusi and others, l ave given us their notions of the ultimate atoms of which matter was formed, and of the mnoduts agen.tdi of nature in the various ttransfor mations whTich matter under.aoes. The expectation of arrivin at uitimate caus.8es or essences continued long after the speculations of the ancioents had beern abandoned, a~nd continues even to the present day to be a very general notion of the objects to be nttimately attained by physical ie rancir s Bacon, the great remodeller of seience ente-rtained this notion, and thoupght that, by experimentally testing natural phenomen,1 we should be enabled to trace them to certain primary essences or causes -whence the various phenomena flow. These he speaks of under the scholastic name of I forms' — term denrred'om nlhe ancient philosophy, but differently applied. I-e appears Ito have urn derstood by I form' the essence of quoalit y-I-itt in LironhtL ab — streacting everything extraneous, a given qtualit consists, or thnM t'w1hich, saoperinduced on any bocdy wotu-ld give it s le cullar quaihty: thus the form of transp ttreny is fiTat which constitutes transparencyr or tha t by whichb when discovered, transparc ency could be produced or superinducedo To take a specific example of wbhat ] may term the r5nyn thetie application of his philosophly: — n g'old,- thLere meiet. tog0ether ypello-wness, gfravity, nallea-bilityR fixedness in the fire, a determinate w ay of solution, which are the simple na tures i.n g'old; for e who ndersntnds form, and the nianner of 1~ otwi~ot~or~ Pe.]MARIsf - Supcrinducing this yellowness, gravity, ducti ity, fixednesss facult5y of fmsion, solution, &c., wi th their particia8 degrees and proportions, will consider how to join thmen together in so3me body, so that a transmutation into gold shM1l Ifollow On tihe other ha-nd, the analytic mnethod, ori, the enquiry from what origin gold or any other ietal orl stone is generated fro its fir st fluid matter or rudi-ments, ip to a perfect miimeral,' is to be perceived by what ]acon calls t1he latent pro,cess, or a search for I;what in every generation or transirr-'tartion of bodies, flies of, whvat remains behind, wnat is added, whcat separated, &ic,; also, in other alterations and mo-l tions, what gives motion, what, governs it,. and the ike.' Bacon appears to have thought that. qualities Separate fromli the substances themselves were a ttainable, and if not capable of physical isolation, were at all events capable of physicial transference aud superinduction. Su'bsecluqtntfy to Bacon a belief has generally existedl and now o to g eat extent exists, in what are called secondary causes, or consequential steps, wvherein oneT phenomenon is supposed necessarily to hancg on anotherO until at last we ar.rive at an essential cause, subject immediately to the First iause, This notion. is generally prevalent botbh on -the Contincut and ian thiFs country: nt otbing is m ore f{amiliar than the expression I study the effects in order to arrive at the causes. Iunstecad of reg arding the proper oobject of physical science as a search ater essenti al cases.T believe it ought to be, and must be, a searce after facts and relations-that although the'word Caase may be used in a secondary and concrete sense, as mitaninmg anteedent forces, yet in an abstract sense it is totally inapplicable; we cannot predicate of any physical agency thait lt'is abstraetedlt the cause of another; and If. for the sake of conveniencnc the languageo of secondary cautsation be permissibl, t should be only witlh reference to the special phenomena referred to, as it can never be generalised, rhe misuse, or rather vcatied use, of the term Cause o has 1B COR. RELATIO' OF PHtYSBOAL FO'OKS been a csource Iof ge t confusioL gn in physical theor4ies and philosophers are eTven now by no means agreed as to their conception of causation. The most generally received Yview of causation that of iHume, refers it to invariable antecedence — ei. e, we call that a cause f;hich invariably precedes, that an effeot which invariablyg succeeTds,'any ins'an'es of ina variable sequence mfil'glht however be selected, which do not present the relation of cause and effect: thus as R eed" observes, and Brown does not satisfactor'iy answer, day invariably procedes night an yet dayis notthe cause i Of night, The seed, ag'ain, precedes the plant, but is not the cause of it; so lint whae. nWit we study physical phenonmena it becomes ditficult to separate 1 te idea of causation from uthiato o fo eandt hse have been reSarded as identical by sorne pilosophers To take an era I ape 7hicl will cout ras these two views: if a floodgate e raised, the water flows out; in ordinary parlance the water is said to flow ecc'ztse tshe foodgate is raised: the sequence is invariabl e; no floodgate, properly so called, can be raised wvithout the w'te fowing out, and yet in tanotler, and perbaps more strict, sense, it is the grcavitation of the water -whicl. ca.ses it to flow. But though we, may truly s5ay that, in'this instance, gravitation causes the ater to flowe we cannot in truth abstract the proposition, and say, generally that gravitation is the cause of wavter (owntg, as water may flow from other causes, gttseous oeahsticity, for instanct, hs c illh s w cats e w toat to flow.fromn a receiver i-.ll of air into one that's exhtusted graviitation nay also, undoer certain eircumstances, arrest instead of cause the flow of water. Upon neither vierv, however, can we get at anyf)inrn like ohlstract causation. If we regard Icausaction as ninatriable se quence, we can find no case in which" a given anteeedent is the only antecedet to a given sequent:; thus if writer could flow fror no other cause than the wcithdrawcavl of a floodgate, we might. sy abstractedly that this Wns the cau.se of waterF flowing, I:F agnii, adoptinog the view which'looks to cai sa INN1OD-UOTORY BF TAKS. 1 tion as a force, we could s0 y tint wate r ecould be caused to fow only by gravitation. wve mnight say abstractedly that gray itation was the cauttLe of water flowingl fiut this we cannot say; and if we seck and examine anr other exam?ple wer shall f.ad that camsation. is only predicable of it in the particlar cas8 anda cannot be supported as an abstract proposition; yet this is constdatly attempted, Nevertheless, in each jpam t.iculcor case where we speak of Cause, we habitually refer to sone a atecedlent power or force: we never see motion or any cSiange in mat ter take effect without re-arding it as produced by soi e prieviois change; and wher we wcannot trace i to it antecedent, we mentally refer it to one; but -wh.ether this hablh it be philosophically correct is by no means clear. I-n other words, it seets questionable, not, only wihet. er cause and effeet are convertible terms with antecedence and sequenc but ~ledther in I fact cause does. precede effict, wtether force does precede the change in matter of which it Is said to be the The actal prior ity of cause to effect has been d.ou-fbted, and itheir simnuitane Ry argued with much abiity-', A.s an insance of this argumixent it um ay be said. the attracetion which canses iron'to approatch the magnet is simulItaneonus wilth and ever accompanie s the movemient of the iron; the nlove ent is evidence of the co-existing cause or fore bu, there is no11 evidence of any interval in time between the one and tIe oth; eri On ui-M view time woulId cease to be a necessary element in: caasation; the idea of c'ause, except perh'ps as referred to a prievals crea tion would cease to exist; and the saume ar gamt-is e hinch. apply to the Isinultaneitv of c;8ause -wit'h cBec't would ap plJy to the simu; ta4neity of Force wi.th 1fe 1otion x e could not, howeverY even if we avdopted this view, dispense with. the elemient of ti.me in tue sequence of phenoinc. na; th eact being tlims regartded as ever accomtpanied simnultanleously by its appropriate cause, we should still refe-r it to some an tecedent iffect; and o' reasoning as a'p-jied to the succesc sire production of all natural changes waould b te the same 18 CORRFELATIOI 0F PHYSICAL FORLOES. 1-kabit ad the identification of thoughtts with phenomena so compel tle use of recogised terms, that iywe cannot avoid using tahe word cautse even in the sense to which objeetion is taken; and if we st, ruck it out of our vocaibul.ary, our lan' gauage, in speaking of successive changes, would be unintetli4 gible- to the present generation. he coimion er'ror, if I am riaht in supposing it to be such, consists in the abstraction of cause, anud in supposing in each case a goeneral secondary cause — a something awlich' is not the first cause, but which, iwe examine it carefullyg, must have all the attr;ibutes of a first causne and an existence independent of and doaiinant over, Matter. The relations of electricity and magnetisin aflord -us a very instructive exaimple of' the belief in. secondary causation. Subsequent -to the discovery by Oersted of electro-maag netism, and prior to that by Faraday of magneto-electricity, electricity and magnetisin were believed by the highest autlhorLides to stand'in the relation of cause and eflcct-oa e. electriceity was regarded as the cause, and mauletisma as the effect; and vheare magnets existed. without any apparent electrical currents to cause their itmagnetism, hypothetical currents were supposed, for the purpose of carrying out the cansative view; but nagnetism n av now be said with equal truth to be th.e cause of eecitrictcy and electrical currents ray be reaferred to hypothetical magnetic lines: if hLerefo're electricat cause nmaetism., and magne-tism. cause electricityl, aibvy thhen electricity causes electricity, -Wxhich. becones, so to s@peak, a reductost ad absurd(wtan of tlo dote'trine To take another instan e, which. ma y roender thebse psos tions m0ore intelligible. By heating bars of bismnuthi and atil, *Mlony in citSact, a current of electricity is produced.; ancd i their extremities be united.by a fitne aire, thne wire is heated. aow here tfhe electricity in the metal is said to be ecaused by heatc, and thle heat in the wiire to be caused by electrici'ty and in a conrete s'ense s tis Is true; buat can we thence say I'NT ODUCTORY RA K 19 abstractefdly tlat heat is the cause of electricity, or that elee tricity is the cause of heat.? Certainly not; for if cither be true, botoh must be so, and the eaffect then becomes the cause of the ca-ase or, in other words, a tfiing causes itself. Alny other proposition on this subject will be fotund to involve siailar dffiiculties, Until, at length, the nind will becolule con vinced that abst'ract secondary ctausatidn does not e-xist, and that a searrh after essential causes is vain. ihe position which I seek to establish in this Essay is that the various affections of natter whicah constitute the ain objects of experi'mental physics, -viz., heat, hg'htt, eletricity, magnletism, chemnical affinity, and. motion, are all correlative, or have a recip0ocal dependence; tihat neither, taken abstractedly, cat.n be said to be the essential cauese of the others1,~ btt that' eith er may produce or be convertible into, any of the otihers: thus heat tmaty mediately or imiediatey produce electricity, electricity may produce heat,; and s of thei rest, each merging itself as the force it produces beco es developed: and that the sane mustt hold good of othler frces, it being an irresistible inference from observed phenomena tht a force cannot originate otherwise than by devolutieon from sonae pre-existing force or forces. The ter force, althoug h used in very different senses by Ui-Mfirent authors, iin ts limited sense mav be defined as that which produlces or res-sits motion Altho Ugh stronglyinlined to believe that the other affections of matt.er wrhich have abov e namesd, are, and. will ultima tely be resolved into, modes' of notiion, many argluments for hichk wil bN &ven in subsequent parts of this Essay, it would be going, too far, at present, to asstae their identity with it; I -therefore use the term fiorce in reference to'them, as meaning that active principle inseparable from matti:er vwili is supposed. to aindue its varit ous chainges. The word. orforce and e idea it aims at expressing might be objected to by t.he purely physical philosopher on sin ilay 9O s(OitCO'RRELATIOx ON F PHYSICAL'FOR0~ES. grounds to those whict. apply to the word cause, as it represents a subtle mental conception, an.d not a sei-nsnous percetp tion or phenomenon~ The objection would take something of this form. If the string of a bent bowa be -ut, the bow vill straighte n itself; we'thence say there is anf elastic force in the bow which straigitens it; but if we.apphel.e our expressions to this experimett alone the use of the term fobrce would be superfiuous, and. would not add: to our knowledge ont theb subject. All the informat'ion which our xinds could get wfld be as sufficiently obtained from t.lhe expression, wihen the stringn's i eut the beow becomes straight,8 as t3 f..rn. the.expression, tuhe bow becomes sf'aight by -its elastic f:orce, Do'we know more of the phenomena, ioewed wilthoaut refert eTee to ofther phenomena, by saying it is produced by force? Certainly not, tAJ- we know or see is Jti etrect; wet do not seeC forc- e, see motion or novng im etler If now we take a piece of caoutlhoec and stretch if, when teleased it returns to its or'ginal lengtAh, -]ere t1 hough'the sufbject-natter is ivery different, we see some analog)y in the effect or pihenonenon to that of the st8rung bowr' if aga.;in'iwe ruspead ant.,a, pple by a string, cut the st'ring, tuhe apple fallsa Here tlioughi i't is less striking, there is still an analogy to the strinng bow and t3he caoutchouc, o O' cwhen the word force is employed as coanprehendlng these frkree different phenomena we finl some nuse tile the ternl, not by its xplaining or rendering more intelligible e tnmods agerdi of mattter, but as onveyring to'he mind something which is ake.r in t.he three pheno ena, however distinct they uta be in other respects: the word becomes an abstract ior go-enralised etpreessi'on and regarded iin this light is of' hig t, ilit wt Aihough I h. ie r'ivn r tonly three exIM iplcs it is obvious that the termA' would equa ly apply to "00 oWr -3000 ex But it will be said, the te t erm force is aused not as expressa ing the, eSect, but as fhat w hi:h prodauces the effect: This is true and inthis its ordinary sense I shall use it in th es pa gei. 3But thouglh t h ttern has a potential meaning, to depart from which -oud render langia e untiligis.lt',e, re inust guard against supposing that -we know essentially more of the phe noMena by s~aing the i ae produced by somefthing, whilc sosmething is only a word dcirvoed from tfhe constancy yad sinilarity of the phenomena we seek to explain by t. The retlations of tle phenomena to which the t erms tforce or foir es Care asplied give: s re al knowled ge t these relations nay be called relations of forces; our knowledge of them is not fhereby lessened, and the conveoincce of exre ssion is greaftly in creasedo butn the separato phenomena are not more ntimat.tety sewn; no frather i sight'uto why th apple fais is aequi6red by saying it s forced to fail, or it faEis by the force of' gravifta tion; by the latter expression we are enabled to relate it most usefully to other phenomena but wie stil: know no moro of the stariuhar pahenom-eno o an tha t uander certain eircnnstances t.?e apple does itl. In the above ilustr4ations, force has seen treated as tIte producer of motion, in -which case the evidence of the force is the motion produte- d; thus we estimateo the force used to projeer a cannon ball in terms of the rmass of n atieter. a d teI voeocitv with whicht t it is projected. The evidence of force oen the term is applied to resistance to.motion is of a soml. e wat different character; the matter resg istin l menoleula.rly affected, and has its structure more or less changed; thus a strip of caouthoue to Ywvhich a weioght is suspended is elongsa ted, and its.molecules are displaced as compared wviith their position. when naffectedby the gravitat h g force. So a Cpice of glass bent by an appended weight, has its rheole structure changed; this internal change is made evident b tr Iansnsit tunfg throug.h it a beam of polarised igiht: a relationh ths becomes establisshed between the moljeeiar state of bodies and tnl external forces or m otion of mnsses, E-very pairtile of the caoutcheao uc or glass mu9st. be acting and contributing to 22 COEMILAIO OF PHIYSICAL FO'RCES esist or r'rest2 tihe.otion of thee inZas of matter appended It is di.nwcult, in such cfases, not to recognise a reality in force, xe need. somne word to express this state o tension; we know that it produces an effect, though the effect be -negative in character: although in. this effort of inanimunate mratter we can no more trace thac e mode of action to is ultimate el e mentes than we can follow out the connection of our o-wNn 1muscles wv ith the volition which calls them into action, -we are experimentally convinced that matter chxanges its state by the agency of other mater, and this agency we call force, In placing the weight on the glassx we have nmoved the form.er to,an extent e-uivalent to that which it woLld again describe if the resistance were removed, and this motion of the mass becomes an exponent or measure of the force exert' ed on the glass; while tins is in the state of tension, the force is ever existing, capable of reprodcing the original motion, and while in a state of abeyance as to actual motion, it is 1really acting on the glass. The motion is suspended, but the force is not annihilated. But it may be objected, if tension or static force'be thus motion in abeyance, there is at all times a large amount of dynamical action snbtracted from the nivtersoe, Beery stone pon a hill, every spjring that is bent, and has required force to npraise or bend it, has for a time, and possibly for ever, w.thdrawn this.force, and a nnihilated it, Not so; what takes place when we raise a weight and leavze it at the poirnt to which it has been elevated? we have changed the centre of gravit'i of the earthi and consecuently thie earth's position witnh refoernce to tohe sun, planets, and st-ars; tne enort w-e have made pervades and shakes the universse; -nor can we present to the mind any exercise of force,' which is tais not perm oanent in its dynamical efeets, If, hmstead of one weight being Iraised, we raise two weights, each placed at a point INaTRODTCTOR2Y RK IEARS.S 23 diarnetricall opposite the other, it wotd be, said5 here you have compensation, a balance, no chalge in t uie etnre of ggavity of t.ihe earth; butt we have increased the rean diameter of the earth, nd a perturbation of or planet, and of all other celestial bodies necessarily ensues. The force may be said to be in abeyance with ref renee to the cffect it would have produced,~ if not arrested, or placed in a state of tension; but in the act of inposing this sttar te, he relations of eqrilibriutm wilth other bodies have been changed, and these move in their turn, so that motion of the same tanount would seem to be ever afSecting matter conceived in its totality Press the hands violently together; the first notion may be that this is power locked up, and that no change ens.ues. Not so; the blood courses irore quickly, respiration is aecele& rated, changes which weve may not be able to itace, talke place in thie maccsles and nerves, transpiration is increased; we have g'iven off force in various ways, and naust, if the effort be p0oongeds replenish our soure s of power, by fresh chemical action Ai the stomach, In books Which treat of statics and dynai-ces, it is co - mnon and perhaps necessary to isolate the subjects of considration; to sppose, for instance, two bodies grrditating4, nct'd to ignoree tLhe rest of th.e iiversie n Br t -no suc h isolation e ists in realiy, nor could we predict the result if it did exist, Would two bodiesc gravitate towards each other in empty space, if space can be enty?9 the notion that they would is founded on the theory of attraction, whict Newton himself repudiated, furxther than as a convenient means of regarding i'te surbjete For purposes of instruction or arg-ument it may be, conyvenient to assumae isolnted mtatter: uany conlusions so arrived at.may be true, but many will be erroneous. If, i prodsucing effects of tension or of static force, tie effort made pervades the universe it may be said, wh ithe oi 02CORRELATION OF PIYSTICAL FrPCi ES bent sptring is freed, when tie raised wei lht ftls, a converse series of motioines must be effected, and this theory wonld lead to a m3ore, reciprocation wbich. would be equcaflyvy nrprodrc-e tive of perman ent change with ce annihilation of force. If rino tbhe %weigo.t hTas cbangied the cert, be of gravity of thtOe cart4,l and Athence of thethe weirht. it will be said, restores t.e orlaginal centre of graviy aand everything comes ba k to its original status. In this armYaient w-e aE n, I tlhouoht isolate our e x.periment; we neglect sur roundineg crean imstances. Between the tfime of the raising and iktling of thfe weig'ht, be, the interval never so sm.all, nay, more n dmi the rising and cdamng tihe:ali th.me -eart i.as been goinog on revolving round its axis ad e round the stir to say nothiig of otier changes such ait temleratitr, cosmitci.magneti-s &c., -vhieh'we may call accidental, btl whichb, if we knee all, wo uld probabdly be fbma.d to be k as necessary atd as reducible to law as ftie motion of the ear-ti. Cangie having taken place, the faI of the e weight does not bring back thee stata:s qfuo but other changes surpervene, and so o Nothing repeats tself, because notnting can be placed aga n i the same conditior t, he past is irrevocabl.t I L-M O T I 0 N.' TOTION —-w nich has been taken as the main expOnent L of ore'in the above examples-is the most obvions, the most distinctly conceived of all tile affections of matter. Visible motion, or relative change of position. in space, is a phenom.enon so obvious to simple apprehension, that to attempt to define Ri two.d be to render it more obscere; bnt with moetion as with all physical appearances, there are Cee. tain vaTnishing ggadationss or undefined limits, at:which athe obvious mode of action fades away; to detect the cont int ing existaence of te phlenomena wte are obliged to h-eave re course to otter than ordinary methods of investigation and'we freq(xe-ty apply other and died re.nt names to the effects so recogCnisedo Thus son;nd is m eon; and SIthoga in the earlier periods of philosophy the identity of sound and motion was not traced ont, and they weare considered distinct aff:ections of mnattelr —indeed, at the close of the last century a theory vwas advanced:that sound was transmitted by the vib.rations of ana, etier' — we now- so readity rresolve sound into notion, that to those who arex familiar with aco stics, the phenmc.erna of sound imnmediatlely present to the mind the iLdet of motion. i, e. motion of ordhnary matter, Agai~nA ti' o regard to light: no doubt now exists t hat liht iowes or is acconmpanied by motion. lTere the phe' 26 0 CORIRELATION OF PO.F YSICALS FOP RCE non ena of motion axre not rmad evident by the ordinary seon suLus perception, as foi s t is' e tace'e - motion of a visibly nmow ingr oprojteci leo would be, bct by an iraerse deduction ifome fmlown relations of moltion to t-li and s5pace: asall obs serva-hon teaches us tbat bodies in movrin@, frona onne pnoint in space to another occupy thne, we conclude tt.a, wherever a con-1nu nn phenno.lenon is rendered evidoent il two.di'.feirenti polats oof space t h dogreent tunes, mthet is miotion thougo -re cannot see the progression, A sinciatr deduction conv.inces us of te notion of electricity a As we in 0 common parlance speak of soutnd moving although so-1und is n.otion, it requires no g"'reat, stretch of imagrnation to conceive ight vand eectricity as. ot'ions, and not th as'ins Movirng If ne end of a longo bar of rnetitsl be st.uack: a sound is soon perceptible a 1te ocl.er end Thifs we n-ow lnow to be a v.ibration of the bar; sountrd is but a word expressive of the mnode of mnotion inpre'ssed on tbe bar so one end of a column of air or glass s 5bjecueod to alsuleinous RDnprdse gives a pcercept:ible effect of ligh t at tt-d other end -Lth.is can equall.y be conceived to be a sibrat wion or tram:smnitted motion of particles in the transparen4 ecolumn: -this question wllt, however, be furti er cLscussed hereafter o ir the present we nwil c0onfine ourselves to mo-tion withiE the limits to wehic h'ahe term is usually restr icted With the per'eptible phenonienla o of motn " Sthe imental conception has been invar.ably associated to -xvwich 1 haver before alluded~, ad to niih the t-erma frc is,o'iventla -twhrich conception, whoean we anaul yrs e it, relfers us to some antecedent motio1. If except tx e prodceoion of motion by heait, lighit, &e'x which wil be considered in!th[e sejnelt, -vithen weo see a body moving rwe took to moton h aingt been comuei~nicated to.it byr matter wMbicih has p reviously moved. Of absolute rest N'at-ure gives us no evidene: all tiatter, as far as we can ascertain) is ever n movenecnt noo-t merely ine masses, as wixth the planetary spheres, but also molecularly or thrLough'loUti its most intimate struct-ure'.hus every`4eration of temperature p-roduces a.moleecula chtage throlr hoholu the rhole substbance heated or cooled; slowr lenemical or electricalE a tions, actions of hiogt or invisib-le radiant forces, are always at play, so th1at as a fact -we cahtsf not predicate of any po tion of matteir t att is absoltut ely'at rest $Supposming, however that miotion is lnot a1n indispeusac bei function of mattter, butt ti mnatter can be at rest, matter at rest would never of itself cease to e at rest; it vwouil not mov e unless impelled to suach motion by some other i oving body, or body which has moved, This proposition,pplies not amerely to impulsive mrotion, as when a kbah at rest is struck by a moving body, or pressed by a spring wvinch has previously been moved, but to motion caused tby attnact0ions such as magnetisn or g;ravitation Suppose a piece of iron at- rest in contact with. a m'iagnet at rest i it be desired to -move tIe iron by the att racion of the m agneto- the nmatenet or'the iron must firsStbe nmoved; so before a boty italls it must first be rsvsed,. body at rest would therefore continue so for ever, and a'body once in'motion rwould coantoi-ne so 4for ever, in the. saime direction aand with'the same xvelocity, unless impeded by some other body, or affected by0 some otner force tfain that whih originally impr elled it. These propositions may se m tomewhat arbitrary, and it has.8, been dooted wvhether theey are 1necessary trilths; tLhe-y have LT for a long time been received as ttioms, and there ca at ll events be no ha.rm in accep'ting them as postulates. It-, is how'ever vetry gen rally believed that if the visibie or palpable xmotion of' one body be a rrested t by impact on another bo-dity,:.e n}otion ceases, and the fo;re elfich prarotced it a is rnultilat; d, Now th e view whihch I ventu tre to sabbmit ims that toirc canniot be anni.dlated, but it merely subdivided or,.lteredin m direction. or 6haract~er. First, as to direction Vaea;e your 28 - BXCORRELATTIO Of. PrYSICAL ~IoK hmand: the:moti on, which ats appareienty eec ased is takelL tn by the air fion e atoir by the walls of ftce rom r,:xe, and so by direct and react-ng w,av es, continually coraminnuted, but neve r destr yed t is tnue that, at a certain poit, lose a1l. means of det ectin the mo ion;n fin.'on.,t;s minute stod. d.vi sion0'whioh defies our ioost delcitte mueans of appreciation, bLt we can indefinitely extend our powter of detein it ac* corin -.ts w:confine its di'ectionr or incoeaase t4he delicaey of our examination, Thus, if the ha nd be.moved in unecoBn ned air, ththe 0motios of the ar would not be sensible to a 1er0 son ait a fw feeot distane; but if a piston of tfle sam e extentl of suracte -s the hand be noved vo'ilh the saine rapidity.ni a tube, the Iblast of air a.ay be distinctily flt at several yards edistance, There is no greater absolute amou, -1nt of motton in'the air in the second than the l firLst case, butS it irection s restran'ed, so to nake the meaens of detection more Cacile. By carryian on this restraint, as u1 the air-gun.o wer get a power of deteeoinng the motion, aind of moving other bodies at ifar gr'eter distances. The puff of cir wiieca -would in the rair'sun poject a bullet a quarter ofr a mile, if allowed to es cape witrbout. its direction breig roestrulned as by the bursting of a'bladderi would not be perceptible at a y ard Idistance, t-hc0ugh the same abusole te amot unt of motion bea m..ressed on tie surirounding air, It raay, however, be askexd, what becoses of'rcce w*hen sotion. is arrested orP impeded by th eoun-er-moti.on of anotfer tody? This is gtCeneral betleved to produce rcest or en ctare -destruction of raotion, and conusecjto.s t ai nunitilation of orftce: so indeed it a areg rds th.e motion. o f te nasses buts av n-ew fotrce, or new character of foree, now n;rsues, th'e ex'poRen1,t of which in stead of visible motionl is heat. I venture to regard. dte t heat wvhicth resrlts from friei-n o" percssioni as a continu-tion of the force wahich was previously assOcia. ted w ita. the moving bodyt andd hich -tin i this iampinges on MSOTi;lt 29 anoeffr'body, ceasilng to exist as gross, palpablo motion, con t-inunesi to exist as heat. Thus, let two bodies,: iA and B, be supposed to miove in opospioe directions (putting for t he moment out 01 qu o ston l resista8ne,u sut as f that of the air, &c?,), if they pass each ociher wi4 tout cntac-t eaosch will mnove on for ev er in its respectie deirection with the same ve locity, but if they touch each othert tthe velocity of the iovement of each is redsced, and each becomneps he0ted: if this contact be slight, or such as to ecoasi[on bu ~: a sligght dimi nution of their velocAity avs whien the surfces of te bodies are oiled, then the heat is slight; but if t1he contact be sutch as to occasion a great dininution of Motiotn" as in pereussion, or 8s when the surfaces are rouihened, then tne heat e s great, so that in all cases the rea s-ultin' heat is proportionate to the diminisbed. veocity, Where insteand of resisting and eonsequelttly impeding the rotion of tlo body A, the bod0y gives t7 o iy, 01 tselo takes up the slmotLion originualyv conmmunnicated to A, the)n we have less heat in proportion to thle motion of tfoe body B, for here the operation of the force continues in the fo rm of palpable motion: thus the heat result ting fronX friction in the axle of a wheel s i resseneds. bsy surro-undingff t'by rollers th ese take aip the pri iary motion of'tae caxle and the ].ess, by t.is mneans, the initLasl otion i ipeded, the les is t-he resultding leat, AK.gn if a'body m ove 1i a fluid, although some heat is pro0 duced, the heat is apparent y i trifling, because th'de psarticles of t. ne fiuJh tm, 4Qsel ve0v,:e an d continue Pie motion: originally tom.a'municated to t he mnving body: for every portion of modtion: c:ommu:: ni ctedo to thoemu tfhis loses ani equivalent, and where b oth loss sthen an equivalent of heat results..As the converse o-f this propostiion it should foUllotwi that ie Iore rigid ihe bodies impigngg onn each other the g~reatt.er shoidd be the ranOunt of heat developed by f1iction., and so we find it'. S linat, steel, hlard stones, glass", and umIetals, asAre those bodies wvhich give the greatest amount of hea't from 30 CORBRELATION OF PEHYSIAL'FO0GES@ frIctieon or percussion; while water, oil., &eC, give littl e or no het, and from the rea1y mobility of the.ir particles lessen ts detelopnmentt when interposed between ri oid moving bodies. r.is, if we ol0 the axles of: wheelse, sve have m or rapid mno tojln of ot te bodies tuhemselves, but. less heat; if'e inetease reserest;ane to Motionf as b-y rougheningo ihe points of con0~ L~act, so that each particle strikes against a nd imtpedes tl'e motoion of others, then we have dimi nished.t4'-otion butILt in creased heat; or if the bodies be smootsll but -instead. of slid ing past achc other.be pressed closelyv itogther antn Ii tn rubbed: we shall in many cases evolve m-omre heatt toan " by tihe nouthened b&oies, as we getu a g'eater nuber of particles in contact 4 and a greeatter resistance to uhe initial rmotion, I cantnot present to iya minld any case of lheaJt resiu iiS from frittion -which is not exTpiCale& e by this vitev: k'lifition a ccordi A n to it is simply impeded motiona The gre ater th.e -uimpedNi cutent the hmore force is required to ove-rcoma c0 it ano t-he greater is the resultfing heat; this resultingi heat being ae con tinugation of indestructible force, cnpable, as we shall pres-, ently see, of reproducing palpable motion, or motion of defit nite m mases. W —hatever be the nature of the bodies, rough or smooth00 solid or liquid, provided there be the sa. e initial force, and tihe w'wrhole motion'be ultimately tarrested, tlhere shouldi be tihe soine anlount of heat desoeloped, thoug'h. where the motion is cerried on throufgh a great nunsmber of poi'nts of eatter we do not so sensibly perecive the resnfiting heat from t I-ea', cL dissipation. The friction of fluids produces heat, oan eff. tiee first noticed I believe by Miayer The total heat, produiced by the f'iltion of fluids should therefore it iv Will be said,'be equal to tnat produced by the friction of solids; for alt ho.c-ut eaci. particle produces little heat, the mootion beminf readilj talSen up by the neigshbouring particles, yet by the timne the wholoe mass has attained a state of rest there has been ithe sar imupeding' of the initial motion as by the friction of sohn idw if produced by the same inital -force. If the.heat be viewed in the aggregate, and allowance be made for the spe cific therm.t c apacity of the substanes emnployed, it prPobably is the salne,, t ough ap parently less; the he at in the case of stolids being manifested at certain defined points, while in that of fluids it is dissip.ted. bothfa the titue and space during and through'wh.l thLe motion is propa oatted diffri il. tle two cases, so that the het. n the letter case is Jxmore readly. ca'ried o ff by surroun ding blodies. If the bocly be ohastic, and by its reaction the motion imt pressed on it by -the initial force be continaed, then the heat is proportionateSly less; and. were a substance perfectly elastic, and eo resistance opposed to it by lie air or other mat.ter theln the movement once impressed would'be perpetual, and no heat wroid result. A ball of caoutchouC'bandied about for xmany minutes betweenr a racket i.and a wal. 1 is not perceptibly heated, while a leaden bullet projected by a gun against a wari is rendered so hot as -to be intolerable to tie touch: in tZhe former case, the mot0ion of the mass is continued by tihe reaction due to its elastictyR; in thie Iatter, the m-otion of the mass is extinguished, and& heat ensues. A. pendulum started in. the exhausted receiver of an airpmnp contimues its oscillation for hours or even days; th friction sat its point of suspension and the resistante of the1 nt is 1m.inuImIised, and the lmeat is imiperceptitble, ba these trifling res istances in the end arrest the motion of the mauss, the oee giving it out as heat, the ort her conveying' th e force to the receiver, agnd thence to surrounding bodies. Simihar reasoning may be applied to the oscillation of a coiled spring a, ba lance -wheel. To wind ip a clock a certain asmount of force is expended by the arm; this force is given. back by the descent of t1.he eight, the wheels nove, thle pend'Ldum is kept oscillating, heat is generated at each point of friction, and the suarroundmig air is set in motion, a part of hich is made obvious to 32 CO.RRELATION OF' PHSICOAL FORCO'ES us by the ticking sound. But it will be, said, if instead of' allowing the weight to act uEpon the machineryv the cord by which it is suspended be cut, the weig'ht drops and the force is at an end. By -no means, for in this case the house is shake n by the concussion, and thus tae force and motion atre cont.ined, while in te former case the weight reaches -the ground quietly, and no evidence of force or motion is mant.fcsted by its impact, the whole ha iing been pre'vio-usly dissipated. If the initiat motion, instead of being arrested by ilhe impact f f other bodies, as in friction or pecnssnion, is impeded by confinement or compression, as awhere tlhe dilatation of a cas is prevented by mec hanical means, heat equally results thus if a piston is used to compress air in a closed, vessel, he compressed air and, from it tle sides of the ressel will be heatede the air beiing unable to take'up tad carry on the or igina.l motion communicates molecurar motion or expansion to all bodies in contact with it; and, conversely, if we expand air by mechanical motion, as by writdr, [ing th e piston cold is produced. So when a solid has its particles comr pressed or brought nearer toethiter, as whven a bar of iron is hnammered, heat is produced beyond tohat which is due to periussion alone, In this latter case wet cannot very easily ci e feet -tle converse result, or pro duce -cola by t hhe neGShnic aI diata0tio3n of a so i, thoufgh the pleno —mtena of olt iont where the pafrticles of a solid are detactled. from d atech other or dra.vni more widely asunder, give us an approxim-ation'to it: n tlle c;ase of solution. cold is produced. YWe are from a very extensiv rane of obsn ervantion and expe-riment entitled to conclude flat, ith somne crt iots exceptions to, be presently noticed whenever a body is cor-(m pressed or brought into smaller -dimensions it is heate d,. to it Cexpands t eigdtciohin subtauntesa enee it is di-lated ot increased in volme it sis cooled or conr otrats ineighbourin subs tances. 3{r. Joule has made a great nunmber of experiments for ihe purposet of ascertainin0 what quantity' of heat is prodnucPed by a (givenr -mechanical action. EIs mode of experiu-enitng' is followso Anu apparatus formed of floats or paddles of brass or iron is t to rotate in a bath of water o0 mercui ry The ower ii gives rise to this rotation is a weight raised lihke a clocdk-weight to a certain height; this by acting during its fall on a spindle and pulley co amnauicatees motion to the patddlewheel, the v water or merury serving as a friction.mredinam and calorimeter'; ad the heat is measu-red by a delicate mercurial ithermometer. The results of his experti ments hle considers provea that a fall of 772 lbs. througftth o 9spatce o.f one foot is a1I to rase t the temperature of one pound of waJter ttirough one degree of Fahrenheit's thermomn eter. Mir. Jot.e's erSperisments are of extrelme delicacy-he tabulateso to tle thtousa.ndth partof a: degree of Fabrein.'heit, and a lare na-be'er of his tIhermometric data are. com prehended wilthin the ITXumits of a single degree. Ofier exper i menetera havo g iven very different muanericalI resuilts, bnt the. general opinion. seemis to be that the anumnbers given by bimr. Jooule ar the neLarest approximation. to the trltihL yet ohtained. H ith erto I ha-ve taken ho distinction as to the physical character of the bodies mcpingin g'n each other; but Nature gives us a relma:TkaTble dierejnc0 in the character or mode, of the force elininated by.friction, accordingly as,th-e bodies wMich impinge are homoteneous or heteroeneouts: if the former~ heat alone is producted; if the latter electricity.'we find, indeed, inst ales given by at thors, of electricity resutitlcng, fi-ome the friction of homogeneous bodies; butf as I stated in nmy original Lectures, I have not fband stch facts confirmed bh my. own experim.ents,, and tis conclusion has been cortroborated by some experiments of oPress80or E'rman, coemunica ted to the meeting of the Britsh Association in the year 1.845, in which he found that no electricity' resulted fr~om, the f riction of perfectly homoegen useous Substances; as 2' 34 0C'r1ELATION- O PiYSIoCAL Fi'RCES. for instanee the ends of a broken'bar Sucht expe3riments as these will, indeed, be seldom free from slight electrical cur-o rents, on aceint, of the practical difficulty of ful.filling fte condition of perfect homogeneity in the substances thems elves, thcir size, their temperature &c,; but the eff cts produced arc very t, rif-ing and vary in direction, and te resultint; effect is nougsht, Indeed, it would be d-fficult to conceive the cont-rary How could we possibly ima'ge'to the i1mind or describe the direction of a currentt from the sae body to the stme boty, or give lnstruetions for a repetition of the ecper ime-nt? It, would be unintelligible to say that in rubbingf to a'nd. fo two pieces of bismuth, iron, or glass, a, cur ent of electricity circulated from bismuth -to'bismnuh, or from iron to.ron, or i:om ga ss to glass; for the qcestion humediately occ-rs-from v1hici bhislnuth to whinlh does it circulate? A4nd should this question be answered by calling one piece _A, nd the other B this would only apply to the pa rticular spehnen employed. the distinctive appellaPti.on denoting a distinction in faet, as otherw ise A coli d be substit uted for B, and the bar to whichl the positive ele tricity flowed would in turn becomse the bar to whicih thIe ntegattive electricit dflowed, re miay say tiat it circulates from roug gl ass to smooth,l froin cast iron to wrought, fbr here there is not hom.ogeneiy I; is moreover conceivable, tbGat when the motion is contfin uous in a de-finite direction, electricity may result finom the friction of homogeneous bodies. If A atnd B rub ag ainst each other, revolinug in opposite directions, concentric c>rrents'of positive and negatin e electri.city mayt be conceived circulating within thle etals, and be described by reference to the direction of their motion; this indeed wotald be a dif ferent phenomenon from. those we have b een considering; but rithout some distinction between the t.o substances in qualiRy or direction, the electriceal effcts are idescribable, if anot inconceivable'When, however, homogeneous bodies are fractured or MOTIONL even rubbed togetther, phenomena are observed to which the term electricity is applied; a flash or line of light appears at the point of friction wich by no e is catled electrical, by others phoshorescent, I have myself observed a remarkabile case of the kind in the caoutclhou fabri now cormmonly used for waterproof clothing - ift o folds of this substanc be allowed to cohere so as partly to unite sand. present a dffihculty of separa.ion, then, on stripping the one from the othe3r or tearing them. as-nder, a line of light will follow the line of setparation. If this class of phenomnena be electrical., it is electricity determined as it is genetrated; there is no dual hara cter impressed on the matter actinga the flash is electrical as a spark from the percussion of fint is eleetrical, or as the slow comt bustion of phosphorus, or an y other case of the development of heat and. light, I seems to be better to class this p henomenon tider1 the categories of heat and than nadoer that of electricity, t he latter wordd being etined tose cares [where a d al or polar ch9aracter of force is. manifested In experiments whiech have been made'by the friction of si-m iat r sbstances where the one 0 appears positivly and the other negativedy electrical there wilU be found some differ-' ence -in the mode of. rubbing by -vvhicEh the lee state o the bodies is in all probability changed, makinrg one a dissixm iar srubstaz.tce from the other; tls.it is said by Ber'gmann, that wIen tvo pieces of glass are rlbbed so tie' tal the parts of one pass over one part of the other- lhe bformer is positive and te t attier negative. I is obvious that in this case the rbbinug in one is confined to a line, and that must be more altered in x molelr ar structure at tfh line of friction than the one wre e the friction is spread over the rhole sirface so if a ribbon be drawn transve rsly over another ribbon, the sibstances are not, guat the rAbbing action, identical; so agai-n, in e rupture of crystals, we are deang with sub sauanes hav-nug a polar arrangement of partilesa —the surfaces 36 COtEsLAiON/ OF PHJYSICGkAL IORCtS2, of the firJgments cannot be assumed to be molecularly ideti The de velopement of electricty by the commonl eectreical machine arises, as far as I can understand i.t,, rom the sep4a ration or rupture of contiguity between dissimilar bodies a metallic surface, the amalg a of the ctushion, is in contact with yg ass these twvo bodies act upon eah other by the force of coh.esion; and when, by an external mee1chnical force, this i'rupturTed, a.s it is at each moment of the motion of the glass plate or cylinder, electricity is developed in ea;ch. were they similar bodies, heat only would be developed. Aeeortdinog to the expremermnts of 3 r,: Sullivan electricity may' be. produced by vibration alone if the substance vib'rating be conmposed either of dissimilar metals, as a wire partly of iron and partly of brass caused to emit -a mtesical sound or of thee same metal, if its parts be not homogeneous, as a piece of iron, one portion of -which is hard and crystallised and the other soft and fibrous — the etlrr'nt resultir g:appears to be eie to the vibration, and not to heat engenderedd as it ceases immediately with the ibration, We may-e sy thenv that in our present state oft knowledgeg, where the mutually impinping bodies are homogeneous, heat and not electricity is the result of friction and percussion where theP bOdies iampinging are hleterogL coneos we may safely state that electricity is,always produced by friction or pereus~ sion, although heat in a greater or less degree acfconp4 aDnies it; beut when we come to the question of ratio in which frictio2al electricity is produced, as determiend by the dilff'rent characters of. the substances employed; we find ve ry complex results, Bodies may difter in so many particulars wrhich in fluenlee more or' less the developement of elec —ricity, such as their chelical constitution, the state of their surfices, their stat.e: of aggregations their t2ransparency or opacity', fieir power of conducting electicity: &c., that the ntormce of thei action are very difficult of:attainment. As a gteneral rule it: may. be said that fhe developement of electricity is greater wen the substances enmployed are broadly distinct in them physical and chemical qualites, and more ptarticnllarly in their conducting powers; b ut p: to the present time the laws governing such developeient have not been even approximately determin edo I have said in reference t t he various forces or affeetions of matter, that either of them may, sn.eatesy os r y,nz mediately, produce the ot:hers; an d this is all I can venture to p.redicte f them. in the present state of scince; but after m nch consideraation I incline strongly to the opinion that sciene is rap~ idly progressing tbowards the establishmenut of iimediate or direct relations between al thes e forces, Where at present no imme diatte relation is established between any of there, electricity generily forms the" intervening idink or im iddle term. M-iotion, thena, will directly produce heat, and e~ectricity, and electricity, being, produced by it, will produce snaygnetism -a force which is always developed by electrical currents at right angles to the direction of those currents, as w vill be subsequlen.tyr more fully explained.,igytt also is readily produced by miotion, either directly, as when accompanlying the heat; of friction, or medliately, by electricity resm ting from motion; as in the electrical spark, which has most of -the atf ~tributes of solar libg ht, differing from it only in -those respects in which light differs when emanating from dif'rent sources or seen throutgh different media; for instance, in the position of the fixed lines in the spectrum or in fhe ratios of the spaces occupied by rays of different refi-angibiit.,y. In the decompositions and compositions which the terminal points proceeding fr'om the conductors of an electrical machine developse whsen immersed in different chemical nmledia, iwe get the production of chemicall cgTqfity by electricit, of wh ich mtotion is the initiIal source, Lastly, motion ay be again reproduced by the forces which have emanated from motion; thus, the 88 CORRELATION OF PHYSICAL B0 tOOE S divergence of tfi eClectrometer the revolution of the elect ri cal wh.eels the defiection of the magnetei need), are, wlhen resultinm frao frictional cletricity, palpable -movements reproduced by the intermediate modes of force, which have t, hemselves been originaited aby motioa IIio-HiEA T F we now take IrAT as our starting point, we s'haI find that t.he other modes of force may be readily prodaced by t. To'take motion:; st: this is so generally, I think I may say iyn.ariably, the immediate effect of heat, that we may' alm.ost, if not entirely, resolve heoadt into motion, and view it as 8 mechanically repnisive.force a force anta'onist to at'traction f cohesion lor aggre gation, and tendin g to mtove the particles of all bodies, or to separate them fr'om eaclh other. It may be well here to premise, that in using it[e teerms'particles' or moleette,' twhich will be frequently eimployed in this [Essay, I do not use hemn in the sense of the atomnist, or mean to assert I t mt atater consists of indivisible par'tieles or atoms,; Tle words will be used for the necessary Purpose of ~on-tradis tinagishinog th action of the indefinitely minute ph'sical elements of matters rom thllat of masses h.aving' a sensible mgng itude, mch in the same way as the terma liaes or' points' maBy be cused, and with advantaie in an'ats ract sense; thoaugh there does not exist, in fact, a thin twhich has lengti and. breadth withouat t, hickness, andthlough a thing wit]rit out parts or dimenSions is nothfing If we put aside the sensation. which heat produces in- ou own bodie(s ane d regard heat simply as to its effects rtpo ina orgaac@ aamttor,we tind thrat, a P very> r TX~ecpon31 wgh2i' /to CORi'oELATiO0 OF PBiYSt'ICAL FOoESg shal presently notice, the effects of what is caled- heat are simply an ep.ansion of the matter acted Lponr,:r.d that ft he imatter so expanded has the power by its o0lv contraction of cormmunicating expansion to all bodies i. contiog'ni ty with it, Thus, if the body be a solid, for instance, iron, a liquid, say water,) or a. gas, say atmospheric air-. eachd oxf -these, Then hleated, is expanded in every d irection; in the two formsner eases, by increasing the heat to a certain point, we change the physicalt c-.aracter of the subktance, the solid becomes a liquid, and.1te liquid becoimes a gas; these, however, are still expansions particularly the t Iter, when, at a certain period, the expansion becomes rapidly and indefinitRely greater. But what is in fact, commonly done, i order to h eat a substance or to increase the heat of a substanIe? -t is merely apprpoxunated to some other heated, th-at is, to some tother expanded substance, which Iatter is cooled or contracted as' he fomer expands, Let usnow divest the'mind of the ianpresision that heats in itself anythinr substatiavea, and sutppose that these phenomena are regarded -for -fithe first time, and Without any preconceived notions on the subject; let -a.s Ii trodtue no hypothesis, but merely express asimply as re can the facts of which we have becomee be coisant; to wnhat do they amount? to this tbtat matter has pertainaing to it a molectul rl pls ive poxe r, apower of d ilataiton, h hic.t is co.tmnunicable by contiguiy or prx oxirmityV Healt thus viewed, is mo3tlion, and this molecular. motion wf-e may re di;. change into the motion of lmasses, or otion in its most ordinary and plpab3e 0or m: for example, in the steam en0ine, the piston and all ts concomitanut masses of matter a Jc moved by th molecular dilatation of he vpou r of water. To prodIuce continuous motion there roust be n ai ternate action of heat und cold; a given portion of air for instance heated beyond the te mperature of the circu.eamamient hait is expanded, A. If now it be ade to act on a movable piston, it HEATg 41 moves this to a point at vc Wh the tension or elastic force of the confined aft eqtals that of the surrounding air. If the cmfined air'e kep't at th-is point, the piston' waoud -remin, xa stationary; but if it be cooled, the external air exercising then a greater relative dcegree of pressrea t.l.he piston returns toowtards its orig-inal position;. just as it will be seenA'when we come to the nimanetic force, that a magnet. placed in a particuiar position produces motionz in iron. ne.ar i, but to malke tics motion continuaous, or to obtain an avallablIe imechanical power, the magunet nust be demagnetisede or a stamble eqiulilt brinm is obtained, In the case of ln.e piston moved by heated air'the motion of the mass becomes tae exponent of tle amount of heat6e. of the expansion, or separation of tie molecules; nor do we, by any of our ordinary metho ds test heat in any other wacy than by itS purely dyneamical actaiont The various modit fications of the thermometer and pyromet are ll e saursn ers of heat by maotion: in tese instruments liquid or solid bodies are expanded and elongated, if e moved in a definite direction,, and, either by thei ONM vftiisite motionA or by the raotion of an attached index, comunniecate to our senses the amount. of the force by shivbic-h they mo ved, ~her.e cre indeed, some delicate experinme ts which. tend to prove Amt: at repadsiv e action b waeen separate nasses is nprodacedby hea Fresnet found that rmob;le bodies hecated in an exhausted rec iver repelled cah n otUer to sensible distances; an d Baden Powell found that th ie coloured rings tsually caclIed l'ew'toe_ s riugs hange their breadth and4 position, twhen th]e cglasses bet-een awrhieh tt.y apscpear are Ieated, in a manner ihsich lheowed tha tfie glasses repelled each otner A Ya Fv0~e's the. ory of comets is based on some such repellent Ifore. There is, bhowever somea difficulty in presenting these phenonaena to the lndl in the same aspect as the molecular reptf-sive action of heat - The phe nomena of what is termed IlIaut heatae ba B- CORRELATION OF PHYSICAGL FOtO5B. generally considered as strongly in avour of fha.t view wrhich regards heat either as actual matter, or, at all events, as a sbsatantive entit, and not a motlon or affection of ordina ry mattert The hypoothesis of latent matter is, I ven.tmre r -vith diffidence to think, a dangerous one —it is somnething like the old principle of Phlogiston, it is not tangible, visibe, audible; i is, in. fat, a mere subtle mental. conception, and ought, I snt mit, only to be received on the ground of absolutte neCcssity, thl nore so as these subtlet.ies are apt to be carried. on to other natural phenomena and so they ad to the hypotthle tical scaffolding whlich is seldom, re(quisite, and should'be sparingiy used, even in the early stages of discovery. As an instance, I think a striling one of the injuri0ous efects of this, I wvill mention the analogous doctrine of t invisible light;' and I do this, meaning no disrespect, to its distinguished aum thor any more thin in discussing the doctrine of latent heat, T can be supposed, in the slightest degTee, to aim at detracting from the merits of the illustrious investigators of the facts which that doctrine seeks to explain. Is not'invisible light,' a contradiction; in terms? has not light ever been regarded as that agent whfich affects our visual organs? Invisible light, then is darkness, and if it exist, then is darkn.ess lighti I lknow t may be said, that one eye can detect light wherec another cannot; tat a cat:may see where a man cannot; that an nsect may see where a cat cannot; MAt then it is not invisible ig. I to t iose who see it: tlie U ligc ot, r0 aer tdie object seen bv the cat, macy be invisible to'the man, bLte it is viisble to the cat., and, therefore, cannot abstrict edly be said to be.nvitsible If we goo further, and find an agent; which lffects certain substances similarly to light, but does not, as fuir as we are aware, affect the isuaal organs of any animal, then s it not an erroneous nomenclature which calls sauh an agent light? There are many cases M which. a i tde aiation from tlhe onceeaccepted meaning of words has so grad nally entere-d into commo1n usage as o a be tua:omidtable but I venture to think that additions to such cases sholed as far as possible be,avoided, as injurious to tiat precision of Ianguagae gwiohh is one of the saPst guards to aknowledg,, and iroma the aabsence of hich phvsical science ha s'nateri ally.Let us now shortly exaine the qaustiorn of loaent he,4 a,d see -whether the phenonena ca inaot be as wetTl if anot mnore satisfactorsify, eXplained wtlho1 t the hypothesis of itt tent matter, an idea presenting many simSlar difficulties to that of invisible light, ttough mne ore sanetoned by asage, Latent heat is supposed to b tie i atter of heat a;ssociated, in a Tuasked 0or dormant sutate, with ordinary m atter not ctapable of being detoteted by any test so long as the ma-tte. with which it is associated renmains in the same Physical stunatet, )1m coarnn Uicated to or atsorbed -froi other bodies, whAen the.iatter wit wminh it is associated chantges its state. To take a common exam-ple: a. pound or given'weig ohtt of water t 172an mixed -with an eqalu weight of water at 32, wIill accquire a mean temperature, or t02 ~ wthile water at 1.72~, mixed with an equal weight of ice: at 3?2~ wih be reduced to ~32 ] Bye dim ti heory of lent heat this phinenon. is thus aexplained -In the first ease, tat of thle mixdture of watt-er with vatt er, both the'bodies being in the tisme, physicalI satlte, no latent hett is rendered sensible, or sensibl e heat laItent; buat in the second d the ice changing its condition f'om the solid to the liquid state abstracts from the liquid as m uel heat; as i-t requires to maintain it in the liquid state iwhic. l it. relders laent, or roetais associated with itase, so long as it:rema in iquid, b t of which heat no evidnce can be a orded by eny t hernoscopic test. I believe this and similar phenomlena, whrer heat is connected with a c hange of state, may be explained and distinctly comprehended wvithout recourse to the conception of latent heat, though it require some effo)rt of the mi d to 4d 44 CORELLATION OF P'YSfSIO2AL FORiCES, vest itself of this idea, and to view the phenomena simtply in thenr dynam ical relations, To assist us in so viewingo tlheml, let us first parallel with purely rmeclanical act ons, ceitlan sic nple effects of heat, wmere change of state (I mtan sitch chantge as from t he solid to the iqaid. or liquid to the gases euns state) is not concerned. Ihs, 2place withixn a receiver a bxacider, and. heat the air vithin to a higlher tmnperat turei than that withrii.out, it, the bladder expands; so, force, thae air ccblhaIl mcsa y.into it by the air-pump, th.e bladder expaunds' cool the air o)n the outside, or remove its pressure mechatnicailsy by an exh-lausting puamp, the bladder lso explands; con-'vrseiy,. ncreasee the external -epellent force eiithe r by het-i or mechanical pressure, and the bladder contracts, In t. We teehani eal efieet-s, t'he force'which produeedthe distcen sion is derived Loin,'ad at the expense of, the mnechanical power employed, as &om Muscular force, from gra -itation, fIr'om the reacting - elasticity of springs, or any similar force by which the: afr-pump may be worked. th e h1eating efecLts, the force is derived from the elhemical actlon in the lamp or source of heat employed. Let'~us next consider the experiment so arrangeda. fhatt the force, wh ich produe s expansion in the one case, produces a correlative contraction in the other: thus, if two bladders, wlth a connectintg heee' betvieen then be half-illed wxith air as the one is made to contract by pressure I!)aeN otheir will die 8ate, and, vice verse; so a blader part~~y fil.ed awii t' cold a ir atnd cot;ain.ed within mr other filted with. hot air, expands, ihileh the Spae be etween the bladders cotntracts exhibiting a m:ere, transner of t he sa.mie a-mount of r epusive forcei, cthe mobilty of t1he ptart.les or their muitutal atttactirono being-~ -the sae in each body; in other words, thei replsive. foree acts in thle direetion of leaIst resistance mitil eqixlibritl1 is produced; it tlhen becomes a static or balanced, nstead of a dynami or motive foree. Let us now consider the case where a solid is to be haIlzr ed to a liquid, or a liquid to gas; here a mnuclh great er atonO t of' heat or replsiv e force s required, on account of the cohesion of the partieseo to be sep aated Ina n order to Separate the particles of tLhe solid4 precisely as mucih fore miunt ob parted Wih by the varmer li.quid body as keeps an equal ou'antit-y of it i its hq.pid state; it is, i-ndeed, o-nly withr) a Iore mflrt ing line of de0,-talion the case of the hot and cold bLadder- sa partI of te repellent power of the hot, part-. des is itramsflrsrred to the cold partielesl andd separates themi in their turn, hbut th:]e antago.nist force of cohiesion or'gregration nectessa'ry to'e overcorne, beIing fi this casCe rutca. stronger, eqiires and t.exhausts an exactly proportion~ate anottnt 0of rIerellent force mecehanically to overcote it; hence the dififer ent e:alet on a body such as the cormn i thermconater, -the expaunding liquid of which does not undergo a mTm.au change of sti'tle Thmus in the exa mp le o a e, giarve-n, of the inmixture of Cold -with hot w7ater the hot anfd cold water and the mercury of the t1her-mometer beinog a in a, liquid state before, and reaininang so afer con-tact thre sulting temperature is an exact i ean the ot water contracts to a certain extent, the cold w-vater expands to tihe 2ame1 extoenrt and thl theroM.om eter Che: sinks or rises the saune numaber o0f egrees8 accordgloy as it had been previotsly inanrsled i thLe cold n'th iht 0' uto n, eimtsercury gaining or losing an equivalent o —f repellent:force In the second instance, vizx the mix,t;ure of ice itii hot water, thet substance n use as a indircator i0. e, minercury'; does not underggo the same ph-rsical chan.ge as tlhose wdhose iela-tions of volunte we are exammnin.i The~.:iobrc"'-1e wino' heat simply as mechanical: rce tt hie l is enmloyed in loosening or tearing asndcler the particles of the soeid'ie, is abstracted rom'tIhe liquid watexr arnd t fro the quid terUrLt h 0o I. he t-lermom Leter, and il proportion as this'force mteets with a greater resistance in sepais latter ten pertno tusare t' hc mniolec3iles of m nixed gas are in a 0st1ate oelf istn We eqtulibnsiacrnm sonewbFhat rsimiar to that of the tii1nat ea s or sinrianr boodi. es in. m i a sligt derg em oent s ibverts ttlhe nicel-baaned foICesd If, for instan te'we su rppose four moeculesg AX), 9 to be in 2 batlanced state of equilibriurm between attraIting atiud repell3ng forces t2he application of a repusive force betm-ween B and J thoughi it may stll frther soeparate B ]tnd C, rill approxiwmate to oA a.nd C to D, and may bring' telaem respectively wthna the santo of attsactive torce; or) s pposing the repulsive force to be in the cterof.i indefinite sphlnere of patwicles, all these, esxcotig those eixmmediatiey acted on by the force, will be approximated a-nd unhang Iron attraction. assuied a state of st ate eqnilibrium, th tey wit rew tain this, becau-se thle repulsive force tivided by'fi. o Muass is inot cpteble of overtcumin it. B.ut if ti te reputsive force be o.reased in q tRitiby aid of suiciegnt intenusity, ten the at tractive orce of nil the molecules may be overcome, and doe coimposition ensue. Thus,'wter e or sQ'ani below a cortain teniperaPtue and mixed gas aboveo a certain. temperatlureie, may be supposed to be in a state of stable qenii.hbri i Whi"lst belob ti fs limPitig temperature the, eq f t libriu of o xyhy, dirog gas iS uns'ae'le. This, it must be confessed, is but a crude ode oif 0expla-4 ing the phenomena, and requires the assualption, tha"t the partsides of a gas exercise an attracttion for eat. other as do the piarticles of a solid, thocuwh lffre.'ent in degree: perhaps in kind. Wheif1.r 1t~hls'be o or not thtere can be no doubt thiat both gases amd sotids expand oDr contrrcet accordingo to cthe in verse contr-actio en or Cepansdion of ot-her nemgg)boutring botdies And so r''re. rnibe eoe ach oeett]mr 11hin r thiel r'tons -to o'eat and cold. The extent to which sob e4xpansion or contsaetion can be carried, e.ceels to te liatedc only by'ixe correlative state of. othe r bodies; thesoe aain. by othe3rs, a'd so o,:"as i'r as we raaLy' judge, throrghout tie niverse, Adopning the exp)anaition aioove given of 0the deconaposie tion of water bv heta, Leat would have the sae rt 6ation to chiemi cal a- ni-'f-y as it has to phyrsici at tractionr s ii iMMe d ia t tendency is antagonistic to hbotn and it is only by a seoendtrry anfcti-on Wtiat chemical. fan1ity is appar-. tl.ty promoted bv heatc, This view w iotdd e tx11 plain how e at ma y promtote changes of the eqculibrium of Oenemcal affini'ty namong mixed 0om3pound. s-ibsta-ncmes by deomnposing certain. COmpoo0uI s ad, sepatratino' eloentactry constituents who se afi.ty tis greater, when tChey are brought'withia the sphere of attoraction for t.c sunbstancee o B witt r which they are'mixed than.otr thaose Wvth wVhidb they ere or'lghinay chemnicItMly united: th'us an intense heat being' a)ipled to a m;xture of eldorine and thr.e iapour of water, occasions'the preoducti on of mtriatic acidt liberah -ito-' oxy-geno Cgerrinf' ou ti,' view, it w-otuld appear fthat a suffoioient int ensoit, of heat mi'htd yield indefinite po0wevrs of deco~n.osition; an6r1d tbereo seems soine probabii ty f bodic Os no1 supt 7posed to be etemTInt'ntry, beIingd deconrnposed or reso].ved. into fartulrher eclieent s- by the appliacation of heat of Stuf"iicint ielot i-nt sit; or, reason0g co0versely, it may fay b-. a-ntcipated tthat bodies, wahicl wivi.U not enter into comnbination at r a certain temperatare, will, enter into combination" -if thir t emperature be lowered and tllt thus new copon-nds ay$ be fornmed by tC68 COlglELATION OF pr] YlIOATL ORCES 0 a proper disposition of their constitx ents he epos to an extremely low temperature, and thie more so if compression be also employed. In considering the effect of heat as a mechanical fobce, it would be expected, d pr~iori, and independently of any theory of heat which mav be adopted, th'at a girven amount of heat acting on a given material must produ-ice a given amot of motive power; and the next question which occurs to the xnind is, whethier the same amount of heat wo-dd produce the same amount of mechanical power wrhatever be the material acted on or affected by the heat. I will endeavour to reason Tifs out on the view of heat which I have advocated, Heat has been considered in this essay as its ef mrotion or mechanical pow7er, and quantity of heatt as measuretd by motion, Thls, if by a giTen contraction of a body (say mercury) air within a cylinder having a moveable piston be expanded, the piston moves, and in this case the expansion or notion of fihe material (say iron) of the cylinder itself and of the air sunr rounding it is commonly neglected. As fle air dilates it beconies colder; in other words, by undergoing expansion itselK it loses its power of making neighbouring bodies expand; but if the piston be forcibly kept down, the expansive power due to the mercury continues to communicate itself to the iron and to the surrounding air, Which beco me hotter than they would if the piston had given way. iTow i'n the above case, if the air be confined and. its volaume rnchanged, will the expansion of the iront assuming fthat it can be utilised, produce an exaefly equivarlent mechanical eff3ec to that which the expansion of the air would produce if the heat be enutiely con-fined'to it? Assumimng that (with the exception of bodies wrhich ex pand in freezing, where through a limited range of tempera ture, the converse effects obtain) whenever a body is comipressed it is heated, e. it e xpands neighbonring substances; gwh enev er t is dilated or increased in volume it is cooled, L e. it EATe sio t contracts neigibourig substances-the CO.clcio ap. pears to me inevitable that the mechanical power produced by heat will be definite, or the same for a given amount and intensity of heat, whatever be the substance acted on. Thus, let A be a definite source of heat, say a pound of mercury at the temperatu.re of 400~; let B be another equal and similar source of heat: suppose A be employed to raise a pistor by the diltation of air, and B'to raise aother piston by the dilatation of the vapour of water. Imagine the pistons attached to a beam, so that they oppose each other's action, and thus represent a sort of calorific balance, If A being applied to air could conquer B, which is applied to water, it would depress or throw back the piston of the latter, and, by compressing the vapour, occasion. an incleaser of temperature; this, in its turn, wotdd raise the temnperature of the source of heat, so th1a we should have the anomaly t-hat a pound of.ercury at 400~ could heat another pound of mercury at 4000 to 4019, or to some point higher than its original temperature, and this without any adentitious aid.: it will be obvious that this is impossible, stances by athe eiect of induction. Indeed, chemical aotion or eleetrolysis mt5 as I have shote7, be transmitted by induction across a dielectric sBub, tnecci such as glass, but apparently only while the gasas is beinrg Charoed with electr'icity. A wi re passing through and hermetically sealed into a gass tube, a hort portion only pro jeetingg is itade to dip into wa ter contained in a Florence flask; the flask is immersed. in water to an equal depthll with; that within it; te wireo and another shmilar wire dipping into the outer water are made to communioc;ate medtalically with the powerful electrical machine known as Rhu mkorf's 0cui; blbbles of gas instantly asced fron the exposed pofr tions of -the wieas, but ceass after a certain time, and Mae renewed when, after an. interval of separatio, the coil is again connected witf the wires. The following interesting experin ent by ir. I arsten es a step farther in corroboration of the moleculit changest SO 6 ~CORRE.LATION OF PI-HYSICAL FORCES consequent upon electrisation: A coin is placed on a pack of thin plates of glass and then electrified. On removing the coin and breathiung on the glass plate, an imp ression of the coin is perceptible; this showvs a certain nolectdar cha nge on the surface of the glass opposed to the plate, or of the vapours condensed on such surface. This effect inigolt, and has been interpreted as arising from a film of'easy deposit, supposed to exist. on the plate; the impressions, ho-wever* have been proved to penetrate to certan deipths below the smuface, and not to be removed by polishing'. The following, exper;iment, howeveor, goos itrthor: On separatting carefully the aglss plates, iages of the coin can be developed on each of e surfaces, showin ti hat the raole cular change has been transmtted through -he substance of th glas s; and Awe may thence reasonably suppose' that a piece of glass, or other dielectric body, if it could be spl up whileaunder the lfl uenoce of electric induction, woiuld exhibit some molecular change at each side of each lamina however minutely subdli~,ded, I have succeeded in farther extending his epezrimen ad in permanently fixing the images thus produced by electriciy] Between two careftly-cleaned - glass pates is placed a word or device cut out of paper or tinfoil; sheets of tinfoil a little smaller thwan'he glass plates aie placed on the outside of each plate, and these coatings are brought into contact with the terminals of blkunklorf's oil After electrisation for a few seconds, the glasses are separated, and their interior st'faces e xposed to the vapou0r ofhydrofluoric acid, which acts hemically on glass; the por tions of the glass not protected byr the p pper device are co roded, whie those so protected are untouched or less affected by the acid, so that a permanent etching is thus preduced, which nothing but disintegration of the glass will eface, Some fP'ther experiments of mine on this subject bring out in a still more striking manner these curious nolecular ELECTSICIThY 87 vha ges.-~ One of t p s of glass havi.g been Iectrified in the manner just mentioned, is coated, on the side, impressed with the iv sible electrical image, with a film of:.dised collodion in the ianner usuatv adopted for photographic purposes; i is i, en in a dark room immersed.n a solution of nitrate of siver; ten: n xposed to diffise light foa r a few seconds, On poreingo over the collolion fi-e usual soluti3 on of pyrog'allie acid, the invisible electrical image is broug'lst out as a device o a light ground, and can b pe rmanently fixed by hyposulphite of sodat The point worthy of obsert vation in this experimett is, that this permanent image ex'[sts in the collodion film, which can be stripped off the glass, dried, and placed on any other surface, so that the molecular change consequent on electrisation has communicated, by contact or close proxi mity, i change to the KIm of collodion correspondng in >ron -with that oin the lass, but being undoubtedly of a chemical nature. Electricity haso, moreover, i1n this experiment so modified the s'urfaee of glass, tha-t it can, i its trnn, modify the structure of another sufbstanee so as -to aiter the relation of the laItter to light,' It would require a nuirJous complication of bypoietfie fluids to explain this; but if electricity.ndli.t be supposed to'be afibcfions of ordinary' ponderable matter the diftfic lty is only one of detail. If, agail we examine the electricity of tmi atmosphetre whei, as is usualwly the case, it is positive with. respect, to tha of the e arth, we find that Iea successive stratum is positive to those below it and negative to those above -4; anid the converse is thie case when the eletricity of te atmosphere is neogative with respect to that of the earth., If another electrical phenomenon be selected, anoth.er sort of cnangle will be found to have taken place, The electric sprik, the brush, and similar phenonmena, the old theories reg arded as actual emnmations of the matter or flCid, Electricity; I ventre to regard them as produced by an emission of tte material itself from whence they issue, and a molecular 88:- C~OgELATION OF PHYSICAL FBORCE& action of the gas, or intermecium, throug or across -which they are transmitted L The colour of the electric spark, or of te vltaic are (i. e. the flame which plays between the terminal points of a powerful voltaic battery), is dependent upon the substance of the mnetal, subject to certain modifications of the nter zed iumn thus, the electric spark or are from zinc is blue; from silver, greeen; fro iron, red and scintillating; precisely the colos-a af:orded bV these metals in their ordinary co rbustion. A portion of tiLe me-tal is also found to be actually trans i'tted with every electnrc or voltaic discharge: in the latter case, indeed, where the quantity of matter acted upon is gr-eater than in the former, the metallic particles emitted by the elecc trodes or terminals can be readily collected, tested, or yeen weigh d. It would thus ppr that the electrical dischare aeries,: t least in part, rom an actuXI repulsion and sever-, ance of the electrified matter itselfg which flies off at the points of least resistance. A. carefi examiniation of the phenonmena attending the el ectric spark or the oltaic aic, -:hich latter i. the electric disruptive discharge acting on greater portions of latt:er,7 tends to modifyr considerably ourP previous idea of the nature of t:he eectric force as a producer of ignition and conbustiotn The voltaic are is perhaps, strtitly spe aking neither ignition nor corbustion. It is not simply ignition; because -toe mat-. ter of the terminalts is not merely brought to a'state of- ineamdescence, but is physically separated and partially transferred from one electrode -to anotherr, -much of it being dissipated in a vaporous state. it is nolt combustion; for the phenomena will take place independently of atmospheric air, o-xygen gase, or any of the bodies usually called supporters of combustion, combustion being in fact chemical union attended twih heat and light. Li the voltaic are we may have no chemical union; for if the experiment be performed in an exhausted receiver, -or in itrogen, the substance forming' the elect'odes is cond0ensed ELECT RIcrY 89 atnd prec ipiatetd upon the inte'ior of the ve8sel int ehemcally speaking, an nnaletred state, Thus, to t he a very strilng example if the voltaic disehairge be taken between zinc teor minmas in an exthatsted rece"iver a fine black powder of zKne is deposited on the sides of the receiver; t'his can'be collected, and. takes fire readily in the air by being'tonched w-ith a smatch or ignited wire, instantly burning into'whe'e oiide of zinc. To an ordinary observer, the zinc would appear'to beo burned t5ice-fi rst in the receiver, where the phelnomtenon presen'ts.all the appearance of combustion, and seeondly in the real combstion n air. With iron the experiment is equally instructive, ron is volatilised by the voltaic art in nitrogen or in an exhanusted receiver; and when a scarcely perceptible film has lined the recei-er, tlis is washed with an acid, which then gives, w'it ferrocyanide of potassiumn the prmassian-blue precipitate. In this case we,r'eadily disstil ireon a metal by ordinary me ansusible only at a very high toeperatlure, Another strong evidence that the voltaic oishc terge consists of the material itself of which the terminalIs re composed, is tile pecuiar rotation w1hich is observed in, the light vWhen iron is employed, tle nmagnetic character of this metal eausin6g ts molencules to rotate by the influence of the voltaic c-rrent. I' we increase the number of redupiecations in a voltaic series, weincrease the hlenalgt of the arc and also increase its intensity or S power of overcoming resistancea Wth. batt1ery on0sisting of a lieited number, say 100 reduplicatieons the discharge will not pass$ fBom one terminal to'the other wi'thoutfirst bringinl them into contact-, bat ftf we increase the number of cells to 400 or 500, the dis'charge will pass femt one terminal to tohe other before they are brougght into contaet,. The ditfflrence betwoen ivwhat is called Franklinic elect:icity, or thatproduce dby an ordinary ele ctrical mah'in, and oltai electricity, or that produed by the ordinary voltaic battery, is that C0.R.'ELATION O PITe SIeCAL FORC1ELS the former is of much greater intensity than the latt c, or has a greater, power of overcon ing rehsistane', buit acts uspon a much smaller qauntity of maftter, If, then, a volltaic battery be formed with a view to increase the'utensity and lessen the quuantity,'the character of the electrical thenonmena approximatte those of the electrical machine, Iin order to efifect this the sizes of tLhe plotes of the battery ad thence the, quantity of matter acted on in each cell, must be reduced, but the numter of reduplicat'ons increased. Thus if -n a battery of 100 pairs of plates each plate be divided, andthe l batter be arrang ed so as -to firmt 200 pair s earc, beinc hatIf the orifinal sizee 1te quantitative beects'are diminished, and the effectt of intensity inreased ~ Bny carryinga on thyisn s-tfab-divisif on, dininmi cishn the sizes and ncreascing the nunb3er, as is the case in the vot.0 taie piles of Delue and Zambon~l, efftects are ultimately produced similar, to those of Franklhioc electricity, and we thus gradually pass from the voltaic re to the spartk or electric discharge, This discharge as I have airea(dy stat ed, has a coa tr dependingo in part upon the nature of the terminals employed. If thesce terminals be highly polished,'a sspt will be observed, even in the case of a small electric spark, at the points fouot whiIch the discharge emanates. The matter of the terrainals is itself affieted and a transmission of thifs matter across the interveniug space is detected by the deposition of minute quantities of the metal or substance composing the one, upon f oe other terminal If the gas or edastie meedirn betw een the ter.mninas be changed, a change takes place in the length or colour of the diselharge showing a an aection of the intervening malter. ~f tthe gas be rarefied, the t ischarge gradually changes with the degree of rarefaction, from t' spaurkt to a lmninotus g4low or difuse. ligh differing in colotr in different gases, and capable of extendingto a much greater distance than rhen it talkes pace in air of the ordinary density. Thaus in highly attenm ELECTC CITY. ated air s. discharge 2may be made to pass acros six or seven feet of space, whie in air of the ordincary density it wonuld not pacss arosa isnch. n observer regarding the bettifil phenome i exhibited by thifs lectric discharge iatten t a ted gas,7 hict, friom some degree of sinilaritt in appearance to the iAurora Borealis, has been called theleelectric - rora, wold have some ditfiCtty -in believing suchti..eict;s could be dee -to an action of ordinary m tater. The arount of gas, present is extrenmely small; and the terminals, t'o ta cursory examuination, show no change ~after long expmmeni 1n,? It is therefore not to be bwondered at that the first ohb servers of this and smuiar phnonm en c regrded electreicty as in itself somneiethir —as a specifi existence or fluid. Even:n, this externene case however, upon a more careful examnuation we shaix find'fat a change does take place, both as regards the gas and as regards the t errainal s. Let one of these consist of a highly-polished metal —a silver plateisone o o o th best ateri. as o"br the purpose —and Iet the discharges in atten-uated antmosph ceric air tal en place from a point, sray commi-ion sewinganeedl, to the surface of the polished silver plate; it will be found that this is graduar cly httnged in appearance opposite the point- is io= idated, and gradually more and more corroded as the dischaige is ontinned. if Bnow the gas be changed, and highls-rarefied hydrogen be substltuted for the rarefied air, all other things remnaining thie samae upon passing the discharges as bebre tihe oxide will be cleared off the plate, ald the polish to a great extent restored-not entirely, because the silver has been disintegrated by the oxidatioa-and the portion whil. has been a9c fetted by the discharge will present a somewhat different appearance from the rem"aainder of the plate. A qu stion will probably here occur to the reader:'ht will be the effect if there'be not an oxidasting amediutmn prese,.t and the experiment be first performed in a rarefied gas, which possesses no power of chexmically acting on the plate? 92 0 >M-ELATION OF PHYSICAL FORCES, In this case there will still be a molecular chanlge or H disfite g'ation of the plato; the portion of it acted on by the dis eharge. will present a cdiffrent appeariance froum that which is beyond Rts reaca, and 8a,rhitish fisr, someowhat snmilar to t, hat seen on the mercurialised portions of a daguerreotype, will gradulily appear on: ie portion of the plate aifected by the discharge. If the gas be a compound, as carbonic oxide, or a inixstaro asoxygen and hycog en and consequently contain elemen-lts capable of producing oxidation and redlction, t-lhen the eiffect upohe e to'will depend upon whset;her it be positive or negative; in the ftrmer case it will be oxidated, ia the lctter the oxide, if existing, will be reduced, This efct will also take place in atmtospheric air, Pi t be highly rarefied, and caa hardly be explained otaherwise tian by a molae cular potarisation of the compound gas, If, again, the nietal be reduced to a small point, and be of such matelial that the gas cannot act chemically upon it, it can yetbe shown to be disinteograted by Pthe eletric ispark Thus, let a fine platil num wire be hermetically sealed in a glass tube, and the ex0 tremity of the tube and the wire'ound to a -fat surface, so as to expose ea section only O of the wire,; atRer toakiRng trhe dischargt irom thifs forx some ime, it will be found thati the platinam wire 0s worn1z away, and that its teramination is sensibly belowthe level of the.glass If the discharges from such a phfat num wire be taken in gas contained in a anarrow tuobe, a cloud or 1ffim conilsfing of a deposit of platinum will be seen on the part of the tulbe surrounding the point. Another curious effect which, in addition to the'tbove, I h ave detected in the electrical discharge n vttenuated media, is that when passing between terminals of a certain form, as from a wire placed at right angles to a polished plate, the disL charge possesses certain phases or fits of an. alternate character, so that, instead of impressing a11 uniform rmark on a polished plate, a series of concentric rings is -formed, [Priestley observed that, after the diseharge of a Leyden R LExTR.ITy. 9 battery, ringT consistifg of fused globules of metal were formed on the termi al1plates; in. iny experiments made in attenuated mcdia, alternate rings of oxidation and deoxidation eare formed. Thus, it the plate be polished, coloured rngs'qof oxide will alternate with rings of polished or unoxi~ dated slur~eet; and if t e plrae be previously coated with an mniforn film. of oxide, the oxide will be removed in concentrie spaces, and inereased in the lternate ones, showiing a ater. I alternation of positive and negative electricity, or electriity of opposite character in the same discharge It wordd be hast to assert that in no case cnz tGe electtr cal disruptive discharge take place without t1lhe terminals being aected. I ave, however, seen no instance of such a result rher trhe discharge has been sueciently prolonged, and the termimls in such a state as could b e. pected to render mnanifest st gt. hanges. The next t luestion fhiech onuld occur in follow ing out the enquiry -which has been inaiecated, would probably be, What is the action upon t.i gas itself? is this changed in any manc ner? In 2nsweri to this, it mutcst be admitted that, in the present state of experEimental knowledge on this sub ect, rtain gases only appear to leave permalnent tmraces of 0their having been changed by the discharge, while others, iC affected by it, which., as will be presently seen, there are reasons to believe th.ey are, return to their noermal state immediately after the disch t iarge. In the formner class we may place8 mlany compound gaases cas 2 armoniia, ote-aft gas, protoxide of nitrogen, deualtoxd of uitrogen, an~dl others, fihichn are decomposed by the action of the discharge~ ]ixaed gases are 2also0 chemnically combined: for instance, ox ygcn a'nd I hydirogen unit d and frm water; comm.0on ar gvyes nitric acid; chlorine and aqueouns vapout give oxygen, the chlorine uriting with the, 1nymo gen of the wa tenr. :94'ca~~JOR ELATIg OF PTYSIOAL FOQ:0OE But, fa:rter thaE. tfis,7 in the Case of certain elementary gases a permanent change iS effected by the electrical. CdiSe charge. Thus, oxygen subni;tted to the discharge is partialy changed into the sibstance noor consideredto be an al, lotropic conditdeon of oxygen; and there is:reason to believe tlhat -when tLe changxe takes place, there is a definite polar condition of the gas and that definite por'tos of it are afected — that.h. a certain sense one port on of;the oxygen bears tetmpoorariy to the other the relation 4h.ich hydrogen ordinarily does to oxrygen if the d.scharge be passed through the vapou.r of phos phorus in the vacu um of a good ai r-t)ump a dgeposit of allotropie phoslphorus soon coats the itterior i of t.e..reeeiver, showngo an a.togos chan ge to that proa ced in o xyge n; and in thifs case a series of transvelrse bands or stratifica tios appe. ars in the discharge, showdig a most striking. alteratio in its phrsical c haracter, dependent on the inedium ac eross which.it is transmaitted. Tlese efteets were first observed by ml in the year 1852, They have since been munch examined by continentaI philosophers, and much extended by Ar {rC Gassiot; but -no satisfaetory r a.tiottnale of thein. has yet been g'ivenr There are naaDy gases which e SWD fd~z69generise-.ta lec,eggdi s fle inotio- of an ethfer eq-al difenflties are eneountexred A.ssming i etser to perC.ae cm,je pores of all bodies, is -ithe ethler a c onductor or non-conduto r I if Ixthe latter —t-g hat is, if the ethe r be incatbhi of transitnting the elet rical waxve-t ethlerea e hajpothle sis of electricity' necessarily falls; buti if t ol maot.on of the ether constitute what; e call condauc.tion of.teC, lrt5i hen to nt.ore porous bodies, or hosOe mnost pe17eabke by'tihe etlh er, should-. be the best conductors..But this is no-t the case. I" again, t mt l Aand the fair surroundinu:t are both. per vaded by ef ter, wIhy should th elec trical. wave a ffiet tleo ether in tihe metals, and not stir"that in Ithe gas? To s pport an ethe real hypothes s of olet-ir'city, nany'na dd''itonootr U hardly reconcilaoe hypotheses must be ie por ed. The fracture and comminution co: a u n uos1iondrctrmg bod.y, lthie fusion or dispersion of a8 metnallie i ire by tb e eceet'x.ieal discharge, are eflects eqnmhly dineuit to conceive upon the hypotlesis of'an ethereal vibration, as upon ehat of a fluid, ELECTRI'IT0. 103 but are necessy reislats of the su dden subversion of moletclar polari-sation, or of a sudden or irregula. vibratory to nove ment of the matter itsetIf, We see similar eifets produced byi' s1ono rou s vibrations: which mi ght be cated conol ction and nor1aconauetion of sountd, One body tra asnfmit-s suin31ld tea.si another- sps or deaenxc s its, at is i t;ietr.m(e& -- e i ds perses the lovibrations, iastead of ~ontinus. o the ern in tae stamie direction as the prinary impulso; and solid btodies o.t ay> as has beeon above observed, be Shivered by sudden iOpLitses of' so-und in those cases ewhere tix. tih parts of the bodCy cannot unif-orn.ly carry oi. t he uadullatory miotion T1hei.s prog'ressive staeies in thme iistory of IthyisiC8 a hilosog pl-y will account ai a grea't easure.for tae adoption by the early electficians of the theoories of fuids. The ancients, ten they w ritnessed a nate'tal phenomnenon, removed 0 fro ordinary anasloglee0, an-d itnexnplained by any mechanical action kno atn to t~lhet, rde ecd it te o a soul a1 spirituas or preternaturt. powver t -tThus amber ai tn magnet were suppo st d by Thales to have 2, soq';:3 fuit iont of digesstiont assimailation &c.c, wrere suppnosed. bv Ptaracelsu to be e-ected by aspirirt (thIe rch'ous). Aiir and gase"s w 17ere saiso at firust deened". spiritual, bat s sbUsequentdy became6 invest ed with a more aterial character; and.he word gas, froml yeos:6 a ghost or sps:iri a'fibrds s is an instance of the gradual tracnsmission of a spritual into a physical conceptiono The establittiment by Torricelli of the pondeblneroie e haroe ter 0f ai and gais, showed that salbstances hiceh had been eetmed spsis-tual and esseatially dftibrent rorta ponders0Ile ~matter yowere possessed of its att ributtes A less sUpersdtitous0;odel of reasoning r ensured, alnd now. a{Vriorm fiu.nds we-re shown. to be analooous in. mlny of their actions to liquids or known flu ids bie in te es. tence of other fiuids, dhife ing ofrom ais tis diff ered oraom water, grew Up, vband whbn a new pheno0enon presented itself-, reco.urse was had to a hypot0heti fl.uid for ex plaini3 the phenomenon and connectl .~04~ CORREE'LATION OF PITYSICOAL tORCES g it with othelers; the mind once possessed of the idea of a fnuid, soon invested it wth the necessary powers and pD'opmrties$, nd gTafted upon.t a Ihuxurious Y-eetatiom of nmaginarty offifhoots. In what, I an here thlrowingvd outL f wish to guard myseif fr-om being supposed to state that the theory, hlistorically viewed, followed exactly the dates of -the discoveries i- hich were effectual in chainging its character; sometimes a dis~ covery precede8, at other times it succeed to a Yc.,age,I the, general course of Ithou1nlght; sometinMes, and pme hps most fiequently, ift does both-,, e the discovery is the resu of au tenderncy of the age,and of th.e continually.-i proved nmethled of )bserwationu a;nd wae u Liade, it, st~eong8tl1en s a.nd extends ttL','views whiielh hlave led to it It thiik Ithe pthases of tholught'whic. physaeal philosophers have gone througohx wIi be found generally s uch as I have indicat ed, and that -the gradual acc mnuaation of discoveries -whilch has taken place during the more recent periods, by showing what effects can be produced by dynami cal cses- alone, is riapidly tendin to at general dynaemical t.heory into which that of the impondertable fluids promises ultimately to raerge Commencing With electricitvy as.al initiating fore we, geot.motona dir cetly produced by it in va rious forms; for in stance, in the attraction and repasieon of bodies, evidenced by mobile electrometers, s uch as that of Cuthboertson, where larBe mnasses are acted on; t:he rotation of the fiTiwheetl a.noher'fokrra of el ctrical repulsion, and the deflection of the galv:anometer needlf are also modes of palpable, visible motioen, It would follow,? from the reasoeing in this essay, that when electricity performs anly mechaan ical work whfich does not. return to th e machine, eleetrical po-wer -s lost. It, w-od be unsuitable to the scope of this ork tiork to g ive the mathemafti cal labours of L. Clausius and others here but the folloNr ing experhnent, w'ich:I devised for makin the Tesuit ev ELECTRICTY. 1.05 dent to an adience e a te oyal Institution, will, form a useful llustration -A Leyden ijar of one square foot coated srface has its interior conncted with a Cuthbertson's electanometer, betwveen which atd tilh outer coating of thle jai are a pair of discharging balls fixed at a certailn distance (abo-ut half an inch apart). Between the Leyden jca and the prime conduictor is inserted a small unit jar of nine inches surface t'he, knobs of rwhich are 0'2 inch apart, The balance of the electrometer is now fixed by a stiff wire inserted betvween the attracting knobs, and the Le]yden jar cIharged by discharges from the unit jar. After a certin number of t ohese say twenty, the discharge of the large jar takes place ~across the half-inch intervalt This may be viewed a s the expression of electrical power receii ed from the iunit jar. The experiment is now repeated, the wire bctween the balls having been reioved, and therefore the ftip, ot teo ra eis..n of the weight.L is pert Brme th.e el.ectri. cal. repu7sion aind'attiaction of the two paCrs of balls. t twenty discharges of th}e unit jar the balance is subverted, and one attrating knob drops upon the other; buta no disch/;cge go t akes ptce, showing -3that soml e electricity has been lost or converted into tthe nmehanical power hi6ch raised the hal'iance. By ianother mode of expression, the 2t tricit,: my be supposed. t'o be masked or analogons to latent heat, iand it'wo ld be restored if the ball ete7 bro-ught back without &cs charge'by extraneous force. If the discharge or other etecc trical oeffets weTre the sCone in both cases9 t,hen, since -the raisimgn of the ball or weight is an extra mnechani cal efflort, and since.the weight is capable by its fall of producing lec.triciit heat, or other force, it would see- tha t force could be,'t o'ut of noting,or perp etunalotion o btained, The a Oe exper-ment is suggestive of oI oter of a similar chara otr, which may be indefinitely varied, Thlns I have found tt:at twO balls made to diverge by eelectricity do not 106 CO~cE I'rlow oF PTysicAO FOrB.CESL give to an electrometer the same amlount of electricity as they do if,,x-hilst simiarly electrified, they are kept fobrciby together. This::expeiment is the converse of ie fornmer one. There is an advanta e en l.etrical exprinments of this class as compn-ared with those on heat viz. that though dthere is no perfect insulaion for electricity, yet our r eans of in suia tion are i measuriably superior to any attainable for heat. Electricity diectly produes heat, as shhow.A in the ignited'wire, the electric spark, and the voltaie are: in the latter the most- intense heat. wi wThich we are a-cqupain ted-so intense, ideed, that it Cannot be Measured as every sort of matter is dissipated by it. In the phenomenon of elect.rical ignition, a shown. by a'he~ated conjnuetive wre, the relation of force a'nd resistcanc and the correlati-ve character of ftle two forCes, electricitsy and heat, are strildnoE:gy emionstrated. Let a thin -wire of plathi nun jo3i the terminal s of a voltaic battery of suitable power, the wi'e will be iognited and a certain amount of chemlical action will take place in the cells of the battery- -at defixnite qujantity of zinc bein dislsolved and of hych-ogen eimitnated inga given timne If now vthe platinum wire be m-ncmersed in water, the heat cwfili from nhe circulating currBent of the liquid, be nore rapidly dissipated, and we hselul insta ntly find mihat thie chemical action in the battery w il be inreased~ core.e zinc wSil'be d.issolved, and more hydrogen elimin ated icr'the saen time; e th e heat being conveyed avray, by the xv ater, more cheicant action is required to genetate it, just as:more xue is requ.red in proportioxn as evaporation is more rapid. Peverse tihe experiment, and ins'ead of placingi the':Vre in alter, place it in'the fl5ame of a spirit lamp so thati the force of hea meets with greater resistance to its dissipation, wSe now find that the chemical action is less t'han in, the au*t or normal experiment. If the wire be placed in other differt ent gaseous ~or liquid metdia? we saht find that the cheraical ~LSc'r1mr~Y, 107 action of the.battery will be proportioned to the facility with whenh tfhe heat is circulated or radiated by these media, and we thus establish an alternating reciprocity Of action betweem these two orces- a srnil lar reciprocity may be established between electricity and motion, magnetism and motion, and so of other forces If it cannot be realised widti all, it is probably beeause we have not yet Telhinated iuterferting acsions -[If we artrfully think over the matter, we shall-, unless I am much mistaken~, rrive at the concls.tion tha t it::cannot be eotherwise, unless it be supposed that a brce can arise:romr nothing-cas n exist aithout antecedent fo'ree In the phenomenon of the voltaic are, the electric spark, &tc, to which I have already adveyrted, eetmi/city directly prodnuces lzg/itzof the greatest knowni' intensity Itt directly produtces,rtUgze.s.Sn as shown by Oersted, -'who first distinctly proved the connection between electricity anad Magnetism. hea e twe o forces act.upon each otherk not in straight tiles, tas ai ether uknowr forces do, but in a rectangular directio n; that is, bodies affected by dynamic electricity, or to.e condukits of an electric current, tend to place in agu2ets at right anglfes to them; and, conversely, mag'nets tend to place bodies conducting electricity at right angles to them, Thus an electric current appears to have a magnetic action, in a direction ctitting its oew at rig hft angles; or, supposing its section to be a cirele tant.genial to it. itf then, we reversee the position, and make tI-e electric cm-rent form a series of I tangnts to an maginary c hs clind, this cylinder should be a maget, Thi is effected in p radiice by coiling a wire as a helix or spira and tis, w when conducting an electrical caurtent is to al intents and pur-poses a gra-gnet. A soft iron core placed,Vitin snoth. a hetlx has the property of concentrating'ts power, and then we can, by cone ction or disconne tio n th'fh the setue s of elecricity, instantly make or unniale a most powerfuit TWe mayv figare to fthe mind electrifie d n magi et;ised 108 o0RRE LATION OF tPHIYSICAL FROi~B~S. matter, as lines of which the extremities repel eaeh. other in a definite direction: ths if a inue: B represent a wire affetted by lr ectricity and superposed on c n a wire affected by magnetism the extreme points A and B wi be. repelled to the farthest distances fron the points C and D, and the line A n be at right angles to the line c D; and so, if the Iines be subdivided to aly extent, each will have two extremities or poles repulsive of those of the other If t-he line of matter afected'by electricity be a liquid, and consequentlr have entire mobiity of particles, a continuous moveme nt will be produced by niagnetisn, eaoch particle successively tending, as t'wee, to fly off at a tangent firom the magnet: thus, place a ftat dish containing acidulated water on the poles of a powerfau magnet, immerse the terminals of a voltaic battery in te liquid just above the he magnetic poles, so that the lines of lecticlty atnd of magnetism coincide; the -ater ill now assaume a movement at right angles to tis lines, flowi ung con tinosly, as if blown by an equatorial wind, whicl may be made east or west with reference to the manetice ples by altering the direction of the electrical current: - a sini1har ffect may be produced with mercuery. These cases adflrd tan additional argtament to those previousliy mentioned of the particles of' natter being affected by the forces of electricity and mag.netism in wray irreconcil atle with the f uid or ethereal hypothlesis. The repr esentation of trtansv-erse direction ayr magn etism and lectricity tappears to have led Coleridge to pmzamlel it by the transverse expansion of matter, or lengthn and breadth, though he injmred the parcallel b adding galvanism as depth: whethel a third force exists which may bear this relation to Ieectricity and magegtism is a question aton which we have no evidence. The ratio whieh'ti-e attr:,active magnetic for ce produced bears to the electric etatrent producing it has been investigated by many experimentalisss and mnathemnaticians. The data ELECTRICITYo o09 are so numerous and so variable, that it is difficult to arrive at definite results Thus the r te relative ize of the coil and the ion, the temper or degree of ha rdness of the latter, its shapep or ftie proportions of length to diameter, the number of coils surrounding' it, the conducting power of the metal of which the coils are fjormed, th size of the keeper or i.ron in which magnetism is induced, the degree of constancy of the bat~. tery &c., co mplicate the experiments, The most trustworthy general relation which has been ascertained is, that the magnetic attrzaction is as the square of the electric force; a result due to the researches of Lenz an d Jacobi,: and also. of Sir W SF. i-] aerris.::Lastfly, electricity produces chemical acfinity; an d by its agency we are enabled to: obtain effects of analySis or synlthem sis w ith vhich ordinary chemistry does not furnish us. Of these effects we hare texamples in the brilliant discoveries, by Davy, of the alkaline mintals and in tie peculiar crystalline compounds made Inowvn by Crosse and Bacquerel. .-LIGH1 T. a I u en gering on the subject of LiGHT, it will be well to de, L. scribe brietfiy, and in a cianner as far as may'be inade, penide int of t.eory, the effects to which the term pclarisation has'been applied. When g ght is reflected from thle surface of water, glass, or menr other msedia, it undergoes a change h hicg disrb bles it fIrom beielg again sinmilarly reflected in a direction at right angles to th at at which it has beeni originaly reflected. Light so afctted i said to be polarised; it will Mways be capapble of being reflected in planes parallel to the plane in hich it tha8 been first reflected, but incapable of being refected in planes at right angles to that plane - A-t planes havin,' a direction interinediate between the original plane of reec'ion, aed a plane at right ang-les to it, the light Hl be caotable of be-ing partially reflected, an.d nore or less so ac8Cording fas the direction of ftlee ise cod plane of ecn is more or'leSS comncLdent wil the original plan. e Light, again, wienL passed througiSn a crystel of Iceland sptar, is what is termed doubly refacted, i e. split into two dij sions or beams, eacb. havingn half the luxiniosity of ft.e origbial incidoent light; each of these beams is polarsced in planes at right ang:les to cah other;- and if they be intercepted by the imineral tourlnain e, ne of them is absorbed, so that only one polarisea bran11 emer.ges. iminila efficts m-oay be p oduced by certain LIGHT. il other reilctions or refractions. A t y of light once polanis ed in a certain plane coutinues so affecte(. th roughout its Thole suabsequent course and at any indefinite distance fron tneo point Vnoere it originaily underwent thoe change, the dih rection of the plhea will be the same, p0rovided the menait througlh which it is transmitted'be air wBter, or certain oLther transparent sufbstances wtich need not be enunerated. if't however, tne polar:ised ray, instead of passing through wa. ter, be nade to pass thrloughC oil of tunrpentine the definitte dimrction in which itis polarised will be found to be Changed; and the ch rge of direction will be greater according to the length of the columrn of interposed liquid. inust4ead of being,an uniform ptane, it Sill have a' curvilinear direction, simil ar to that wrhdich a Strip of card wotild hiave if forced along two opposite groove s of a rife-barrel. This curious efftet is produced in different degrees by fditforent media. The direction also varies; the rotation, as it is termnied, being sometiames to tce riglt hand and sonletin-es to the Iet, accorditgo' to the pecuiar mnoleeular character of the medimn ftrough Nwiich thle polarised rs;y is transmitted, Light is, perh.aps, that code of force thi ereciprocal relat;tons of awhicrt ih tlo ole r otrs ha e been the least t raced ou, UnJtil the discoveries of Niepce, Daguerre, and Taibot, very litdte co-id. be defiitely predicated of the action of oliht in- producing oother modes of f1ore. Certain cheanical compotins a iong. whl swa nd phe.emnne-nt the salts of si..ver have thte property of s ssering decomtposfit:i.o when exposed to light. It, for instance, recently formed chloride of sl4ve-r. be sbmit ted to luminous r, I a partial deconmpositon stes the Chlorinue is separated at d expelled by the action of lighat, adA the silver is precipitated. -y this deocminposi'tion the colour of fthe substance changesrs fronm white'to bIle. If nowr paper be inpregCnated wit chhor 0of dissolving chloride of silver, blut not retiallic silver, iodide of potassium will effect thlis; and. the paeir being lwashed aud dried -will then preserve a permanent i mae of the depicted dbjects. This was the first and sirple process of 1i~ Talbot;; but itt is defective as to tihe purposes aimed at, in manay points First, it is not sufficisetly sensitive, requiring a strong light and a long time to prodiuce an image; secondly,'hte lights and shadows are reversed; and thirdly, the coarse structure of the finest p per doet s not admit of tlhe delicate traces of objects being distinctly impr-essed, These defects have been to a great extent remnedied by a procIess stubequently discovered by rB"r, Tatbot, and which bears his' na'me, and which h)as led to tlhe collodion process, and others unn cc ssary to be detailed hlere. The photographs of Daft guero re' with which all are now famitliar, are produced by holding a plate of hightly-polished silver over odine. thin fim of iodide of silver is foried on the surfact of the metal; and when, ti.ese iodized plates are exposed in th ecamera, a chemical tteration atakes place The portions of the plato on whi1h the light. has im-t pinged part with some of the iodine, or aore ot'rerwise chlanged ~-for the theory is somewhat doubtful-soas to be capable of ready an.alg'mation When, taerefoire t1he plate is placd : LIGHT. 113 over tle vapour of heated aercurty the mcretury a ttactles it. self to the portions affected by ight, ansd gives tlemn a white frosted tiappearance; V intermed iate tints are less aifeeted, and those parts vrwhere no light has fallen, by retaining their original poislh, appear dark.; he iodaide of silver is then washed off by hyposulphite of soda, which has ttte property of dissolving it, and there remains a pietmre in which the lights and shadows are as in nature, and the moleular uniformity of the metallic surfaie enables the most microscopic details to be depicted with perfect accun eracy'By' using hloride of io dine or bromide of iodine, instead of iodine, the equilibrium of chemical forces is renm dered still more unstable, so hat images may be taken in an indefinitely,shof.t perioclad period practically instantaneous. It would be foreign to the object of this essay to enter upon the mavny- beta tifid details into wvhid.c the science of photography has branched out, and the many vaia ble discowv eries and practical appica ations to which it has led. The short statement I have given above is perhaps supe fluous, as, though they were new and stuprising at the period wihen these Lectures were Tdelivered, photographic processes hav, e nowr be come familiar, not only to the citivator of science, but to the ar'tist and amnateur; 1t-he important point for consideration here is that light will1 chemiccally or molecularly affect matter.':~Not only'will the pex'ticular compounds above selected as instances be changed by the action of light; but a vast nuber. of substances, both elementa'ry and componmd, are notably affected by this agent even those apparently the ImPost unalterable in ~haracter, such as metals: so nimerousz indeed, are the substances alffeted, that it has been supposed, not without reason, that matter of every Ces ciptio is altere d by exp]osu.re to light. The permanent impression Ltamped on the molecules of matter by light can be -made to repeat itself by the same aency, but always with decreasing force. Thus a photo 1 A4 COR1REtLATION OF 0PHYSICAU FOORCES graph plaed oppposite a camera contaming ac sensitive plate will be retproduaced, but if the size of the nimago be equal -to th.e pictore, the second picture will be ratinter than the firSt and so on. Thus agia, a pihotognph taken on a dull day caumot, by being placed in bright satnsunf-ne be maade to uepro duce a second photograpl of the same size and more dcastinetmar kered thlan itself; I at least have unever suce eded in esch reproduetion and I am n ot aweare t'iat othlerr s heare tJle image loses in intensity as ligIhs itselfdoes by ezac t'ra, ns.mission.'he surface of thle metal or paper sni gavt ao brightier inragr.e ~ro its being{ exposed to a more intense lit, b the photogr1 phic d8tails are lirated tO't1 i nternittr tc e rst r im-roEslon or rather tosomethng s lort of this- A quest0on of theoretical interest arses fron t'he consideration of thlese reproduced photograp;s.e We 0know flhat 1^1e h 0minos08 y of the ieage at t~he focusi r of a'telescope is limited'by the area of he ooject-glass, The imageo of any given object cannot be intensified by throwing upon it extraneouts light; it is i deed diiminished in intensity, and when for certain purposes astronomers illumiinate the fields of ei1 teteseop they re obliged to be contented wrth a los of intensity in the telescopic image. NIow, let us suppose that the innu~test details in the imageo of an object seen in a given telescope, and wrh l a gihveno pwO7 er, are noted; that then a photogra hice plate is praced in the bc"us of the sa'me'telesope so as to obtain a permanent imn-ression. of tile iraage which las been viewed y the iee-gUlass' Coudt the observer, by tfhrowi-T a beam of condensed lighI tupon the photograpdh, enable himself to bring out fresh details or in othe'r words, could he use with n advantage a htgher power' applied to the illuminated photogr-aph? It is, perh.iap, s hardly safe to answer a pfrior.i this question; but thre experiment of reproducing photographs rould seem to show that miore than the initial light cannot be got, and ttuet. we clanno expect -to increase telescopic power b phpotography, LIGHT 11 thouana we may render observa tie ore convenient; aay by its means fix images seen on rare anud avOurable oc'asions0 and iay preserve pe rmluenrt ald infalible records of tfec pasft sat e of astronroumiica f obojee ts T.he cTe' oeof lihgh- on chlemical tmco'riz-ns sr-i's " straIiig instfanee of tie extent to'wichl a forZce ever activ"e,may c' hgoct ironua su cces"sve ages of philosophvy If wcet opp:' oe cis of t a arL.c roomn covered -wvif photo.'rap-bhie apparabaus, the smt a il "moulnt of lig t reflected from the face of a person sictta"ed in its cenete uwoul4 11 lunlsmta-.neonsly i'mpriut his jL pot ait on a mSitittde of recipiet sint fatesl0 Y" ere ethe casmeras HOsoen. bit the )room coated 1w-ith ot'ogralphie paper: r a oL an' e wofid equall y -taoe plhace in veye p ortpion of it, thoug'h not a reproduc-tion of form acnc ire~As other s bs t not commonlyt ced p -0to graph3ic a' re. kcno wvn to be affuected by ligti, the list of wi Lch Meiht be in -defiitely extended it-t becomes a curious object of c. tnmtlplaution ta consier ho-w far It ght is dialy operatingl eLhanoes i'n t pondern ble imatteor-4ow ir a force cor a ont; tie' recognised onTy in its 1asuna. t coi tu.sA, many be constanlnt prodecing oneOges in cth earth Land atmosphere, in a dd.tion to the cL cange it p rodces in organised structures wbndho cacm nowL begOniln -to bue extensiveW stsdied Thus, everyv portion of'i t n b sC pupposed eto write its eoiln history by La chan ge more or less permanent Min ponderable imat ter The Iate.ir, Georgeg Stephenson had a oufitu-ite idea, hich oi - rend now be recogiZsed as nmore philoSophieatl tn it ms ra n his day -viz, th a te li'gSht, wich we, nigjtly obtaintSe ofrnm coatl or or2ce fS e~Sel a a reproroduection of that w-Ich had at one- ti been aborl ed by vegetable stnlcti fi' 3SB. the sunon The convin ictio tai the. transient gleam leaves its per nmianent impress on'the wodrd's history, also leads the mind to ponde oer oveth e many possible agencies of whiell we of the present day ma7 y be as ignorant as the anientt s were of the chemical'action of tlighht 11[6 CO3ittLATION 0OF P-YSITCAL F'ORCES I havae used the term light, and affected by light, m speaking of pho]too'atph]: e-ects; but, though thle pheanomena de. rived their nume' frione light, it has been doubted bny manv competent investigators Whe-theBr the p11-enomnena of photography are not mainly dependent upon a separate agnet cccompa ying Irat, rter'an pon igaht itself. It, ist indeed, diffiTult ziot to believe that a pieture, taken in the focus of a canera-ob scura and which represents to the eye all the gTadations of ighto and shade shown by dIhe orignal lumlunorus image1 is not an efect of liaght; certain it i ho-wever, that t1e ei btih'enat coloired rays s ee rcis dsidrent acionls iup1on vanionS dihe mical compoiunds, anl t ha~ t'e eflcOs oinslm-ny, pei, — haps on ost of t hein, re not proportiona-te in in tensity to th t.eect uipon nhe, isuat] organs~ Those e fftects, howerver appear to be more of degree than of specifi differencee; ad, witf lout pronouacing m.yself pojitvely upon the qerstion, hitherto so Ettie examined, I thihnk it will be salfr to reaard the action on photographi c ormpouneds cas reserttux from.a filnetion of light. So viewina g it, we get EgIgt as an initia ting trce, c apable of produciing, mediately or i mmediatet the oth.er modes of force. Th.us, it ntim3editely prodates chemical action; and having. tis we'at one acqttire a eans u of prodiaeing the othe rs At mly Lectures in 84;3, I showed an expertiment by W-tTch the production of. hll 1e otier modes of torce by liglht is exiibited: I may here shortly describe it. A prepared daguerreotype plate is enclosed imn box fsilled with water, having a glass fr ont wita a shutter o-'er it. t etween this glass afnd the plate is a gnidi'roni of si'ver Twire t1le platei connected vith onmc extremity of0 a galva.nometetr coil, and the gridh'on of wire nith one extremnity of a Bre, guet's helxa -a legant instrimrent formned by a coil of two m etials, the u-nequal e uansion of e hich. indi Ctes slight changes in temperatuxre-the other e' xtrem.ities of the galvanometer and helix are conneted by a wie, and th3 needles bronugit to zero. A soon as a beam of either daylight or LIGHT. i17 the oxrfydrogen light is, by r aising the shutt er pernitr ci t impinge upon the platte the needles are defictead, Thus, light being e e iniating force, we get chemicad action on the plate, eectriccity circulating through the wires, miagnetism in ite coil, aeat ain e helx, and, imotion' in the needles. If two plates of platianuma be pl tlaed in acidul;ted water, and cI nnected ~wth delicate ato ga'lvaoterte'r, the ee of this s alxdways defoeeted a result due to fihus of gas or other matter on tae surnace of the platinum which1 no eleaning can r.}emove.o i', after the needle has returned to zero, w-hi-mch,vill< not bethe case for sore hours or even days, one of thIe plat- anum surf aes be ep osed to ligh;t, a fresha deflection of ttie needle taoes plat e dt as far as ]h ave been table to resolve it, to an augtentation of the che mcal acution which had ec] asioned the original deflection, for tbe deviation I in the same dairection, [, in Stead of white light, coloured light be permitted to rnpinge on. t4he plate:, itae deviation is greater iwith blue thaan wit h:::- red or yellow tight, showin0 g in addition to otlher tests tLhat the emect is not cdue to the heat; of the sun's ray's, a's tl'e catorfie elaects of lichtk are greater wiJth red than'wthl blu..e ligott, whie ithe chemicaaal e.iects are,te ainverse. There care other apparenttly more direct ageencies of light in producagt electrciy annd aagnetism, such as tthose observed by iHoricini an rd others, as wiell as its effects upon crystaliztlation but these results have hitherto been of so indefince a chracter, that thaey can only be regarded as presenting field's tbr expteriment and not as proving the relatTons of li-ghlt to tohe other t'orces Light wou.d seem directly to pod.uce heat in the phenom" eta of'"i a t ermeod a1bsorption of hio'S: in these la io e find that hea isa developetd in soe nporopot.on t0o the d msappearT, anec of Tioguk, )To tate'the old expernnent of placing a,se sries of i" aaerent colouretd pieces of clot-h upon. snow exosed to sunshine, the black cloth absorbing -the most lightt and d e veloping the maost heat, sinks xore deeply in the snou than 118 CO'RPE~LATION OF PHYSICAL FOlO'BES, any others; the other colours or shades of colour- sinkL the imore deeply in proportion as they absorb or cause to disappear the more titgh, l until we coe to ithe'-ite cloth, wiich remain s upon te su rintee The heating powers of dTIeroent colours are however, not by any imeans in exaet piroportion to the intensic- y of their, - oght as atfectrng he vis al organs, Thus red lop-ht, when produced by ree::action frim a prtisGm of gmass produces greater heating efect thtan yellow light it the I1heno menat. of absorfion, as has been observed b y Sir W?. a erschele The red rtas c trap r howpever, te o r podu e a d:i natriie elect gretater than any of 0 le others; thus they paEa; tiate wa/,ter to a greater depth titan. the otherA colours; but, according to Dr, Stebeckd, CTe get a futioher anomalny, viz, [.tba -, Wtc. i g t Sga8.tt,trd g 3Jr, ag. hf ligrbWf D&J:1rzj Y 9,O' ttat 4when t~g~ is reft-acted frm of w0 tel th y e l low reaiys pr'od-uce the greater h oeti-ngo efilct The sno ject. there3ore4 requires raune' more experLtment ber e wre can asserti1n rthe.rationale of the attion of t0re SOiC 0? f Uin7n;t ad1 hcat in tlhis class of phenomen In a Lformer ei tiou of this Es suggested tehe foli-o i.hn experime -Bn on this s ubjet:.'.Let a bieamn of liht be passed -herougr.e tr;wvo plates of tourinaline, or simi ar' sub> stcince ind4the tei mperat ure: oi te sef ond plateo 0o that on which the light 1cfcast impinges be examinerd by a cedeicate tihertosiope, nirsrt'leno it, iis in a position to trasrnit the polar, _ised be'an comini " from t te firstt Plate, and secondy wishen it hnas been turned round throu'gh En are of 90~r and thar po't.ola ie-el benam is absorhbed~. I e j r t'i.d f,[ if ti- 1 —.>~- ti as-'en were carefully perormed tahe teainperature of the second p a te w-ioulde be.moore raised mi the second case than in the c. irst, and that it niwght alifrd intmeresti result wien tried wifs igtht of Odife 1ertt colours. I met w.r" difi x ultiesi 11, in pr00Uclrin a8 able appar tnus and was a en deasrounii, g to overeo'm e her1cm. coetn I oi(und that IFno ola ch ha d, to somie extent, re iscod this resUt He fi- nds that, t when a solar beahn poltsed in a certain plan-e is transmi'. rutted. pp0oioeut. r t eo h aixis of of erystodl of brown quartz or tomyaline, the h at is tr ansmit ted in. a s8aller:proportion than when th beabm p-am so salong the direction of the axis of the crystaL. It is genert ll —as farI as I am s wat re Uniet sal y — ue lthat, whie lighat contrnues as light even thoug t.h refleted or tr.ansitted b cdifferent edia, little 6r no heat, is developed: a1-nd, as:ar as wve tcan judge, it would appear. t:ut,. a 1e4 diiu -twere poerfectty tr'ansparent, or if a surtcee perfectly refleeted ight,t no; th.e slightestt heating effet wouvld take 4plaee buth wmherevWer iglot is absorbed, t:hen heat t akes its pnle%, a: Bordn" us apparently an instance of tne conversion of tEO into heatn and of the fact that the 0trce of light is not, i faet, absorbed or 1 1mihi ated but merel cha.fmrted in harat'Oe becomning in. this instanlce converted into heat blay burno'i on solid ma tter, as in t~he instance.e-tioned in treatiag of heat, this force was shownt to be converted into olid'-:, by iim. pinginfr o0n.1 solid matters A, sho rwevera I'a-ve before observed~, this correlation of light iud heat is not so diostint, as with the other afiections of mastter One e:reriment, i~ndeed of' ielloni., already mentioned, v1onld seem to show h.at light may existl:i a condition in vhich it does not- produce heat, wthic b our instruments nare able to dtect; but some doubt has recen t y been thrown on the ac-uracy of ths perinentrt; probably lte substanceos thems eves throiugh1 which the li-ht is Ltrasmitted wotid bei found'to have been LeateId The.reeiaent body, or th1;t upon which lighLt o i-f gcm seems to exercise as important an influence on o pereeptions of nlig a-s t- eaittent body, or that from.tr..l. e l;igh first proceeds, The recenet e'x er'imnts o0, SJor John Herslerchel a- Mi Jr1 Stoles ashow thatt -u-nt rad im-rlis, ihl falin a on certain'bodies, give no eocbet of;rht, %ecome hu0 mr.onars when fia ing on other, bodies. Tlus, let ordinarty solar liglt be reftacted by- t prism (the best miaterial for which is quartz), and the spectrtam r-eeived on a sheet of paper, or Of wvhite poreela.n; o.ooin,(. -ti. 120 CORE"LATION OF i PHYSICA FR 0,ES, paper, ithe eye'detects no light beyond the extreme Violet rays. If therefore, an opaque body be interposed so as just to erut of the whole visible speetrumu the paper would be dark or invisible, with the exception of some slight ilumination from ligl'ht reflected by the air aind surrounding bodies. Substitute for that portion of the paper whiuc. was beyond t3e visible spectrum a piece of g;iass tin ced by the oxide of urainum, and the glass is perfectly visible; so witl a bottle of sliphate of quini ine or of the judee of horse-chelstilts or even paper soaked in these latter solutions. Otner sutbstances exhibit this effect hi dr:ifferent degrees; aind amlong the sub sta'nces which hav1-e'hitherto been considerea d per'fecty analogous as to their appearance whe-n illuminnated, notable diuer enees are discoveredo Thus it appears hatL emanations wuhicl give o impression of light to tie eye, whAen imp inw< ing on certain bodies, become luminons when inmpinging orn others, We mlight inlaxine a room so construitecd t-hat suchn emanations alone are permitted to enter it,, which would be d ark or light accordhig to the substane with ws hirch the waBl.s we'te coated, thougL inL fil "daylight the respective coadtings of the walls wn0rid appear eqallr vawhite; or, without altering' the coating of the walBs, the room1 exposed to one class of' raS nlgh't be rendered dark by window-. s lwhiel wroul d be tiansparent to another class, itf, instead of solar light, t'he electrical iglt, be enployed tor similair experiments, an. equally strikong el-fct cale actually be prordreuled. A design, drawn on,r4aite pCI'er witl a solution of sulphate of quinine anmd t'aoartie ac id, i i" vsible by ordinary light but appears with bea J-tift distiictness wrllen illhuminated by the electric lght Thuns, in pron touing'01 uppon a lumainous effeet, regarl m-aust be had to fte re.ipent as well as to the emittent body, That wjharich is or beoxes light when it tfalls uipon one bocdy is not light when it idths upon another. Probably the ret'ina of the eyes of differenpt per sons differ to soire, extent in a snilar'nmaner; and the same LmHT, 121 substance, luminated by the same spectraum, uay present dfierent appearances to different persons,'the spectrunma apt pear iLg nore elogated to the one than to the other, so t -hat what is light to the one is darkness to the other, A dependence on the recipient body May also, to a great extent, be predicated of heat, Let two vessetls of -water, thfe c tents of thel one cleair aund transparent, of the other tinged by some colouring: mintter; be suspended in a sunmer's sun; in a very short timne a notable differene of temperature ill be otserved, the coloured having beconme namclh hotter than the clear liquid. If the first vessel be placed at a considerable distance from the surfaec of the earth, and the second near the surfice, the difference i stfil more considerable, Carrying on tLhis experiment, and suspending the first over the top of a high mountain, and the second in a vall ey we ine y ot tain so great a difirence of temperatture, that animals w'hose organization is suited for the one temperature could not ivei in the other, iand yet both are exposed to th sae Isae lminous rays at the same ti.me, and substantially t the saine distance from the enmittent body-the substance nearer the s-n is in fact colider than the more remote. So0 with. regard to the mnediunm transmitting the t influence: a green-house zmay havec fis temperactuare considerably waried by changing the giass of w}fich. its r3ooC i made1 These effects have an important bearing on certain cosmicat qoestions whicht have scatehy been much disc ssed, and should induce the g'reactest caution in forx iang opinions on Such subjects as light and heat on the sn''s surface, the ternperatture of the planets, &Ce This imay depend as much upon their physic l constitution as nDonI their distncec from t"he sunt, Indeed, lie planet ilars gies us a a high:ty prcobal., ao-, ginent for this; for, notwvithstanding that it is ha s r ag ain fromn the snn as -the earth is, the increase of the white tracs a ets wat its pc ding its Inter, and their (dimarnution dIur ing its sumA er, show that the tenaperature of the surface of 1[t2.,Ox.EiLTn O O s PHYSICAL FO.RCEb3 * his planet osecrlates about that of the freezing point of water as do the ai;naloaous zones of our planet., [It is true, in this twe assumne tat the substance thus c'hanjg it's state is wat. er, butL,;osiring the:any close eanaogies of this plane t with the eari;th andr t- identity in appearance of these very e'ffects with ihat takes plac e o fc earlth,h ilt seems a hig' Iy probecble assump t iocn, So it by no means necessarily foltlows that because Venus is nearer to the sun than the earth, that p;lanet is hotertr f than our globe. The force emitted by the sun iay take a dixTer ent character at the surface of each. fi.ffTerent lanet- and requireq diferent organisms or senses for its appreciation. iyriads of organised beings im)ay exit t imperceptible to our vision, even. i.f we were among tuem; and we railghit be also imperceptible'to them!'}owever vaian it may be, in the present state of science to speculatoe upon such existences, it is e qually vain to assutme identity or elose approximations to our eor formns in those beings whMch. may people other worlds. From % altogical reasonintg, or from final causation: if tltat'be admit'edo'we may f cl convinced that the goreous cglobes ofs te universe are not unpeopled deserts; but whether the denizens of otlher worlds are more or less powerfil, more or ess iutelligent, rhether they have attributes of a higrher or lower classs than ourselves, is at present an rutterly h opeless gtuessing' Specilic gravity and intelligence have no neccess0ay coanexion, On our ovWm planet iBe ssenses, a iai ean density euFalt to that of wat;er are not in Cvariably associated with, iun telleet.ual or moral greatness, and tlhe many arguments -W1hih haxve been used to prove thaslt suns and planets oth-er than the earth are mlinhabi.'ted, or not iollabited by intellectual beings might, w fu.tatis mittandGisa,c e bquall y ethe deniz s of a sun or planet to prove that this world'was utri.ihabited. s.ten are too apt, because they C arre nen, bet.ca se their existenceL s is the ne tiing, of all ia sptanace t* o the.svls, to LIGHT. I 2) 3 frame schemes of the universe as though it was foried Afor man alone: painted by a- arst of the sun., a mann amight not represent so prominent' an object of creation as he does 4vlhen represented by his o wn pencil, Light was regarded, by what was ternred the corpuscci ar theoryt as being in itself matter or sa seific f luid emanating' fron lumimmous bodies, and producing the effects of sensation by imphlingi on the retina. This theory gave way to the uLn dulatory one, Wvnich is generally adopted in the present da.y anud which regards light as resulting from the undnlatioin of a specinc fluid to whieh the name of ether has been g-ven, hicen hypothetic fluid is supposed to pervade the universe, and to penetrate the pores of all bodties In a Lecture delivered in January 1842, Ten I firs t publicly ayvanced the views advocated in this Essay> I state;d thaft it appeared to me more consistent ivith known facts to regard light as resiting fiom a vibration orT motion of thie molecules of mratter itself, rather than froa n a specific ether peradling it; jnst as sound is propagated by tie vibrations of woo00d 0or as watves are by rWater. ]I am not heree se in of the caPracter of the vibrations of light, sound, or W-ater, *Ihich are doubtless very dife rent from eaci other, biut anm ory comparing' them so far as they illustrate the propagation of'rcee by meoton. in the matter itself I was not aware.,:t the time that I first adopted the above view, and brought it forward in mry Lectures, t iat the celebrated Leonard E ter head published a somewhaat s n.l av. theory; said, t'aoruP I suoao'ested it wr thocgih k wm g t,2n t it had been previou'sly advIanrced. I should have hesitated in reproducing it had I nrot fetid tnhat it was sanctioned by so emoinen mt a na. ne' atician as Euler, who cannot be.s.pe,- s to have overlooked. tany irresistible argurnent against it — tihe more so it a matter so aucIh ctntreverted and discnssed as the undulatory theory of light was in his time, Althougrh thi.s heo h een considered. deiective by a 112 C0ORELATION Ob PHIYSICOAL FO~iBES philosopher. of high rep-ute, I cannot see the:orce of tie arguments by which it has been assailed; and therefore, for the prsent, thouga with diftolen e, It stgf adherec to it. The fact itself of'the correlation of the different modes of force is to my mind a very cogent arg ment in f.ivour of their being aTections of the sam2e matter; and though; electricityn m agnc tis, antid heat might be viewed as produced. by unddiatlions of the same ether as that by means of which ]Sght is s upposed to be produced, yet this: hypothesis offers greater ct efictIties with regard to the other aiceetioans than with regard to light: many 0o dtese difficulties I have already atlud-ed to when t reating of lec tricit,; thus condection and non-conduction are no1t, exc plained by it; the transmissio n of electricity- throujgh long w'ires in preference to'-the air mwhic, surrounds them, and which must be at least equally pervaded by the ether, is irreconcilable with suach. an hypothesis. The phenoimena exhibited by these forces afford, as I tlhi k, equaltly strong ev-i dence with tlose of tighit, of ordin mater ti acting from partidle to particl, aetnd having no action at a dista ce. I have already instance3d. the experiments of Fr aday on electrical induction, shoFTling it to be an action of contiguous particles, which a 2e s trongy in favour of this iew, and manyc: eperiments' whice I h av e made on the voltaic arc, s0ome of wthl I have mientioned in this Essay, arto, to y.'nd, eonfirma.t tory of it, If it be aditted that one of the so-eaemp d onderabes is a mode of motion, then the fact of its being able to produice the others, a-dS be produced by tihemi renders it highly difficnlt to conceiWe some as molecular nol3,ions m s. others as fluids or undulations of in ethenr, To the, n.u.n obejecton oI Dr. Yorung, tlat all bodies woald have timh properties of soear phosphoorut if lighIt consisted in the and-iulat.ion.s of ordinary mattter it may be answered that so many bodies have thls property,. and with so great a ariety in its duration, tha t non constat al may not have it, he. o3 fogl r fa time so Siore LIGIHT 125 thatl the ye caunnot detect its duration., t E. Becq erel h as madeo inay experiments twch support this view the fact of' the phosphoresoace by insolation of a larg number of boedes, is in itself evidence of the Lmtter of whch itnc are omposed being thrown in to a State of undulation, or at a1l events moletcularly aicte. by the inpact of li11ght, and is tlherevare an atnmxent c in support of th.e view to which objection is take n Dr. Yotng adcaits that the phenomena nof sola r pehosplorus -ape- to resemble g eatly t he'sympatn'eti sotiul ~s of mutasicadsl n trurments whiclh a re agitated by otater sounods conveyed to them through the air, r5 d l am not a ware Ntat he gives anyB"' explianation of Uthese ei ects or the ethereal hypot esis Some curios experiments of 1. ~Niepce de. VS t seem alsso to present an anaalogy in luminous phenomenoa to synopathetic 0soands. An engravin. Which has been kept for somet day-r xs n the dk is half covered by -an opa3cle screen, and Ithen expOsed'to the sun; it is then remeoved frsom the ighlt, the screen taken tway, and lthe engTaving placed oppesite, and at a short distance from, photographfi paper: on inverted it age e of that portion of the engLraving which has been exposed to the sun is produaced on the photographic patoer, w'lile the part which had been covered by the sscreen ins no redprodu ted, If the ensraving, aft-r exposure, is.allowed to rent mai contact with wtite paper for some hours and. the white paper is then placed upon photographie paper, a ftint inmage of the exposed portion of the eTngraaing is. repiro t eed. Shmnilar results are produced by nmottled narble exposed to the s; annsibe tracing on paper by a fluores - cent body' s'Mdphate of quinine is, after isolgtionn, reproduced on the tphotographic papetr 1nsolated pper ret'aCins the, power. of producing an impression for a very long period, if it is kept m an opaque tube hermetically closed. it' is'ght to observe that these eto'ects are supposed by many to be due, to chemi'c' emaatiomns proceeding from the 126 COOiRtALTION 0 PHYSICIOAL FOCRES substanc es exposed to the suW,and -,,hieh are beliteved to have ntdergone some cnhemieal change by' xpour e t, is desirable 4t await frthler experiment beobrt formingr a decid ed opin0ion, The analogies in te progression of soun d and light are Ve-rVy Umron -ch earo eed in s traigit tinesh urnrtil intoerrtpted;t eac is refleeted in the satme man ner, thle ango.les of incides ce arnd reflexon being equal; each is aiternattely r tl1n'i fled and dobtiled in nte nsty by interference; each is capable of reftnact-in wohen passing f0omi media of Liferen densit a th:'laust ecfitct of somund long ago theoretica iy det(erm t n-ined, has baen expernmentatly proved by I Hr. Sonco['n'tss wh'o con0 surtucedI a lens of films of cdlodion' -which,'when filled vdith carbonic acidt enacbled himIe to he1ar the t icki r' of a w'atelh plaeed in one e fous of the lensI the etar of the experimenter beincig in'thve opposite focus. The ticking - was not heaard when th'e wvat.ch was moved. aside flioem th e Iboa point, thoungh;t remained at an en cal distance from the ear. An exper n-en' of -L. Dove seems, indeed to sho-sw an efl1ect o:f polari-, sa"'ion of sound~ The phenomena presented by hea.tt, v-iewed. according' to the (d y namic theoiyr, cannot be explained by jthe notion of a: imponderable ecther, but involve the molecular acieons of ordinary ponderab1e ronatter. The doctrine of protpagation by undulations of ordinaryimaitter is -very generaely admitted by fhlSse,o Sle ot1ert, tlhe dynamical th peory of heat; but the eanclogies of tole phenomena presented by hea;t'-nd ligght are so dose, that I cannot see how a, theory acppllied to the one asoct shotdd not be applicable to the otfi ter W'hen heat is transmited, reflected, refracted, or polarised,..ca-n we view that as an affection of ordlinarvy matter, intd wvhen the same effects -take place with ligiht, view the phenornena as pmro duced by an imponderable eflher, iand by that alone? An objection that immediately, occurs to the mlind in reference'to the e0hereal hypothesis of lglht is, that the n.0ost porouts boodes re opa cork, echarcoal punuice stone, dried and e>ist -wood~ &e, s-. very poroCs and -ery ]ight, are all opaquet This objection is not so superficia as it miiht seem. at iret sight 1the -theory wieAl assumwes t-hat light L; is an undulation of ia emire medcium per vating grs ds..t.... assurmnes -l the dist-anes bet ieen the molec vles or atoms of inater to be'very' g rea-at~ lat. ter hs'been lIkened by Deinocrit — us, and by i any smodertn philosoprhers t thte Starry:fisrmarent, in w hi teh ~ hough the divid monads n rde at, irtmmen se. distances fiont each other, yet they have in -the aggTegate a nehracter of unity, ad1 are irrmly held by attraction in their -nenpeetier position"s and at definhite diestanes,.Now, if imatter be buik u.p of separate miolecules, then, as:far as our knogwledpge ex.tends, the lighnest bodies would be those in which the molecus arets' e at the greatt es dist, and those im which any undouation of a pervading ediun would be the least:interfered with by tne se tparatted p.xicles-such bodies shouid consequently be the most transparent, If'gaoini the anallogy of the starry firm.tament held good in tnis case an uandulation or yave proportieoned to the indevi dual monads ewould be broken up'by the naumber of them, a-nd the very appearance of continuity whieln restirts, as in the ni'Iky ray, from each, point of vision being occupied.y one of the monad%, would show that at aso7me portion of its progressAthe wave is interrupted by one of therm so thao the'hole nay be viewed in some respect as a sheet of ordinary mantter interposed in the ethereal expanse, Even then, if it be achnitted that a highly elastic nedimin pervades the iinterspaces, the sepa rate masses as a whole must exercise an e iiporilant influence on the progress of the wrave. Sound or vibrations of air meeting with a soreen, or, as P. were, sponge of difusned particles, would be broken up and. dispersed by them but if they be sufficietly continuous to hke' tp the vibration and propagate it thiemselves the sound (ontinues compara ttely unimpiaired. 128.ORRELATION OF PHSICA$.I L i. FO.CEW. W ithl regard, however, to liquid and gaseotn bodies, there re very great difficunlties in viewing them as consisting of separate and distant molecules, Ift for instatance, we assume with Young that the particles in water are at teast a4s distant from each other. comiparatively as 100 imnn would be if dis persed t equal distances over the surface of Engtand, the dis, tance of these partieles, when the wrater is expanded into steam, would be increased more than Lorty tLimes so i.at la the 100 men would be redueed to two, and by finrtiee r increasing the tempersatsue this distance may be cm afistealy increased; adding to thef efictots of t- emperatu re raftctcion yby the asrpupTP wie?m asy agsan increase the distance, so thats if' we aWcs sulnle tay original distanc we ought J'y expt'sion'a to iun crease it to a point at which th e h istance between Iolecu1 and inolecule should beome.measurta1le. But. no:extent of rare.f etion, sl'whether by heat or the air-punsp, or both, makes the sligat est clhtange in the apparent continuity of maatter; and gases, I find, retain their penculr lharsater, as fa.r as 1a >jdgment ox it, can be frmeed from its effect on the electric spark, througlhout ay extent of rarefiaction which can e-xper imentalty be applied to them: thus the electsei spark in protoxide of nitrogen, however attenuated, presents a crimson tint, that in carbonic oxide a greenish th'it. A'Rithout, howrever, entering on the n.etaphysical enquiry as to the constitution of matter (or whether the te atonie pl.l osophers or the fol.lowTers of Bosecoic.h are right), a question whichl probably human appliances will never Ian'swer: and even admitt'ng thiat an etherea _ediun a not absolutely'mponderable a, asserted by many, but of erxtrem-e tenuty, pervades matter, still ordinary or non-ethere al matter itself must exercise a most important action. upon the transmission of light; and Dr, Toung, who opposed the theory of EBle'r, that light was transmitted by undalations of gross matter itsel~f just as sound is, was afterwards obliged'to call to his assistance the vibrations of the ponderable matter of ithe'efrae LIGUT. e 199 ing media, to explain why rays of all colours were not equabl' ly refCrated5 and other diffilcultis. One of his arguments in support of the existence of a permeating eter swas5, that a zmedium reSemblig in many properties that which has been denominated Ether does exist is undeniably proved by the phenomena of electricity," This seems to me, if I nmay venrare to say so of anythiug' proceeding Iron so eminert a man, scarcely logiea. it is supporting one.hypotihesis by another 824 vd 6uG.v rO9ed D1e 1tS X08t. SeQS and considering that to be proved which its most streanous advocates admnt to boe saroaunded by very many dia2acutties, If it be said that there is not sufficient eiasticity in orlti narv attser for the transnission of undulations, xwi it such vetocity as light is known tio travel, this.nay be so if the vibrations be Supposed exactly analogous to those of sound; but that mnolecular motion can. travel iw'ith equ-al andt even greater velocity: than ~lighl% is shownt by the rapit(ity withi whicih elee. tricifty traverse a Letalj ire where each particle of metal is undoubtedly affected. It has, moreover, been shtomw'n by the experiments of 13r flsatimer Clarke upon a ltencgi.t. of wire of 760 nmies, ftiati wvdhatever be the intensity of electrical curs retnts thuey saoe prsopagated with the samne velocity provided the effects of lateral induction be tuhe same-~a striking.t anaSl o0yT with one of the effects observed ini. the propagation of jrght and somud, The emfoects observed bv 1M3I,. Fizeau and Foucaullt1 of the slovwesr progresion of light i1 p1roportion as thoe transsaitting amediuma issm ore dense, seem to me in1 ivourt of the view here advocatetd; as a greate"r dee of heat sound be produced by tigmht in proporstion to the density of the imediuni rce tould be t'hu1s carried ot and the molecular systeNT disturbed so that thie p-ogress of thle motion shourld be maore slo-wv; but so i any considerations enter into this question, nId lthe phenomena are so extremnety complex, that it; vwo'ALd be rash to hazcard any positive opinion. Dr' Young ultidmately came to the conclusion that it was simplest to consider the ethereal nmedium, together with the 130 CoRREItLATION' OF PHtYSIC. O'R0ES material atoms of the substCane as consotitlting together a compDound nedium denser than pure ether, buht not more elas. tic. Ether Might thus be viewed as perforningn the functtions which oil does with traccing paer, giving contminuiy to the particdes of gTosS matter, and in the interpla netay spaces forming iitself hJe medium wrhich transm its th und tdations, Since the period when Hityghens, Euler awnd Young, the ftthers of the Undulatory theory, applied their great minds to this subject, a mass of experhiental data has acctlmutlated, all tending to establish the propositions, that whoenever matter trcansmlf:ting or reflecting light Undergoes a structuraal change, the light itselfis afected, land that there is a connection or paratelissm between the changeg in the matter vnd thte change i.n thbe a-fction of tight,: and conversely lthat light will modify or change th.e strueture of matter and impress its ns olecles with new cltracateristias. TranspXarency opacity, reftaetion, refiection, and colowre were phenomena known to the ancients, but scfcieont attention does not appe ar to have been pid by them to the olecular states of the bodies producing these eneots; thus the transp are-le. or opac )fyity of a body -appears to depend entlirely pon its Imolecair ra an teent. If stri occur In a ens or e lass * unough w-hich ohjects are viewed, the objects are disitorted: hi crease the ntMber of these sic>, the distortion is so in creased that the ojects beomc tinvisible and th.e gass cetases to b'e ansparenut,' though remtaining trauns;l t7centl but alter completely -the Iaolecilar st-ructure, as by slow solidifitcat'ion and it becomes opaq' e. Take, aegainl n example of a liqhaid anCd a gas: a soltion of soap is traunsparent ciir tra n.sparent, but nat tate tem t-ogether so as to fortm a 1ot0 0or 1lthiner, and tbis, though consisting of to transpgarent bodies, is opaaquet; nd the reflection of light from -the surfice of these bodies, -tihen so intermixed, is strikeigly different from its reie fiection before mixture in the one case giving o tohe eye a mere general'ehict of whiteness, in the other the images of objects in their proper shapes and cotoir's. LIGTt 131 To take a more refined instance: nitrogen is perfectly colouirXss oxygen. is perfeetly colourless, but &hemically uni ted in certain proportions they form nitrous aci, a gas which has a deep orange bromwn colour. [ I know not howr thn colour of this gas, or of such gases.s chlorine 0or apo0tU of iodine, an be accounted for by the ethereal hypothesis, withlout callng' in aid molecular affections o0f tihe qatter of these Colour in many instances depends upon Ithe thickness of thele late or film of transparent matter tupon t-which. light is incident; as in all those cases which are termed the colours of -tin. p]a tes, of l.ich the soap bubble affoirds a beautiful in stance. Vhen we anr-ve at fthe more recent discoveries of double refraotion and polarisat. on, thle effiets of ligh-t are ound to trace Out as it were the structure of the matter a ected, and the crystalline xormn of a body can be determin ed by Ajte f..et. whic h a minutoe portion of it exercises on a ray of Let a piece of good glass be placed ib what is caled a pol,riscope or insttrument in whichr light t'hat has undergone poiarisation is transaitted througlh the substance to be exnsamn mned, and the ermergent light is afterwards stubmited to another sth tance capable of polarising Iight, or, as it is termed, an anatlyserl; no chaoe in effect wile observed. Remove the g 0ass, heat it and suddenly orquickly cool it as to render it unaitaneat led,:d- in. which state its molettles are in a state of tension or st rain and the gla ss highly brittle, on replacing it i tIhe pox'aris0op0e a beautifLl1 series of colours is peBrceptible JInstead of suojecting the glass to heat and sadden cooling' let it be bent or strained by mechanical pressur, and the cotl ours wirl be eqally visible, modified, accordiug to the direction of -the flexure, and indicating by their course 4 te curves where the molectular state has been changed by ressure. So if tough glue be elongated and allowed to cool in a stretched 132 CORRELATION OF 3HtYSICAtL FOROES stae, it doubly reftracts light, and the colours are shown as iA the insctanee of glass. Submit a series of crystals to the sae examination, and diftfrent figures will be formed by different crystals, bearing a constant and definite relation to the structure of the particihlar crystal exantined, and to the direction in whuich with r ereernce to crystalline form, the ray crosses the crystal.. In the crystcMlised salts of paratartaric acid, [io Pasteur noticed two sets of crystals which were heinihedral in opepo ite dirfetions, i. e. the crystals of one set were to those of the other trical'cuets in suca neial.eiourino' bodies as are 0n&dutors Of e0leetricity, in diirections transverse to tae line of zottion; and if the direction of motion or the position of the magnetic poles be reversed, the current f eloectric.ty flows in a reverse direc tion So if thleJ ma-pet 1-44 COP3ELATION OF PrTYSICAL FORCES. be stationary, conducting bodies moved across a'ny of the lines of magnetic force, i. e. nes in the direction of which the mutual action of the poles of the magnet would place min te portions of iron, have currents of electricity developed in (them, the direction of which is dependent upon that of the motion of the substance with.reference to the nagnetic poles. Thus, as bodies affected by an electrical urret are definitely moved by a magmoet in proximity to thenm., so conversely bodies moved near a magnet have an electrical current developed in thems. /Iag netism can, -then, through the medium of electricity, produtce heat, light, and chev,,mical adi ity. ~Mrotion it can directly produce under the above condiBtions; i. e. a magnuet being itself moved will move other forreoes bodies these will acquire a static condition of equilibrium, and be aamn moved when the magnet is,ilso eyoved. By motion or arrested motion only, corlAd the phenomena of magnetism ever have become known~ to us. A magnet, however powerful, might rest for ever unnoticed and iunknown, unless it were.moved near to iron, or iron moved near to it, o as to come wvhin the sphere of its attraction. But en th other thani eth eithr magnetic o electrfied substances, all bodies wil be moved whlen placed near the poles of very powerful t.agnetssome - taing a position at tall y, or Pin t.e line from pole to pole of the magnet; others equiatorall or in at direction t ransverse to that line-the former being attracted, the latter apparently repeglled, by the poles of the magnet. Tlihese effects, accordingo to the t view f s of Faraday,} show a generic cdliference between tile -two classes of bodies, magneties and di4arsgnepties; asccordinS to ofhers, a eifaenre of. degree or a resuxltant of magnpetJie Iac tioin; the isless nmagneit'le subst1anee bei ng;forced into a t'anCverse posit.on by tihe magnetisation of thie nore nmagrletic nediim whicllh surrounds it.t Aecording to the view gi ven abovev magnetism may he produced by the otiher forces, just as the vans i the instance given are definitely deflected, but cannot prod-uce themn except.when il motion: r tion, therefore' is to be regarded in'this case as the initiati-ve foree. 3Iagbmeti sm will, however, directly affect the othr ebrrces —light, heat, antd chemical affin ity, and change their direction or mode of action, or, at all events, will so0 -ffect matter subjected to these fortes, that their direction is changed. $Since ese le*etnres were delivered', Faraday has disceovered a remarkanle effeet of the m-agnetic foxrce in occasioning the (eflection of a ray of polarised light~ if a ray of polarised light pass through water, or through any transparent liquid or solid which does not alter or turn aside the plane of polarisation, and the columna, say of water, through which it passes be subjected to the action of a poxerful magnet, the line of mag etic force, or that, wbhiel mwould nite the poles of the magnet,' being in the sau. e direction. as the r ay of polarised light, the water acquires, with reference to the li gt simiar, though, not qufite identical, properties to il of turpent ne —the plane of polarisation is rotated, and the direction of tffis rotation is cihanged by changing the dli ection of the imagneti -force: t.hus, f we sappose a pol'arised ray to pass first in its co-Lse the north pole of the imag net, tihen between that and the south pole it vwill be deflected. or c -rved, to the rigit; while if it meets the south pole first in its course, it will, in its journey between th-at and the nortlh pole, be turned to the left. If the substance throu'i'h which the ray is tr1ansm-nitted be of itself capabhle of deflecting- the plane of polaxlisation as, for inst ane, oil of turpentine, then he magnetie influene will increase or diminish -,this rotation, according to its direction. A. similar effect to tohis is obse3rved with polarised heat. w' hen the maeium tehrouxgh which it is transmitted is srbjected to ma guetic influence.'VheithLer tihis effect of magm xetisim is rightly termed an etfeet xupon fight and hbeat or is a molecti.ar 4hange of the:xmatter transmitting the light and heat, is a question thef resolu i46 CO.fRRPELTA?'01 OF' PHYSICAX FOi CtS. tion of'which mLust e left'to the future; at present, -the s:n 2-, swer to it vwotdd depend upon the theory we adopt, If the view of light a-nd heat which I have stated be adopted, i fen we nmay farly say that magnetism,'in t hese explerimeuts, et — rectly affets thfe other for es; for light and hea being, according to that view, notions of ordinary matterg 0lgnetism, in aSeeting these movemen ts, as ects the -forces rhic occasion them. If, howvever,'the other theories be adhered to, it would be more consistent wth the facts to vie-w these results as eAfiffbiting an actmion uo-n the matter itself, and t he test and light as secondarily affected W hen saib stances are al.nder'otng chemical changes,'and mg.aget bis brought near th.em, the direction or lines of action of t. helet force will be changted? There T are many old expcrim uents wateiah probably depended on this effect, but w-te.iCh wre erroneously considered to prove that permanent magnetisnm couild produ0ce or increase chemical action: these have reentnly been extended and explained by:ir.l Hunt and Mr. RxtVrtmn.ann, and are now better understood. The above cases are tapplictabl to the subject of the pres8 ent Essay, inasmuch as t.hey show a' rt1atio.n to exist be two een ma-mgnetic and the other forces, awhich.a roation.is, in all proba?bity, reciproc al;but in theso eases tJhere is not a producti on of tlight, heat. o-r chemin al affinity, by magtetism, but a han'ge in their direction or mode of action, Thetre Kis huo7ever, tht whic. r l- be viewed as a dT namie condition of magofetism; i. eC its condition at -the coin? mentcment and the termination, or duriug'theLe incre-lment or decrleltent of its developmerut 7hile. iron or steel is bei ng rendered uagoneti, and as k progresses from its non mag'ietci to its Hin mtutno. Dmagnetic sftote or recedes ruomn its maxiltIum to zero it exhibits a dynamic force; the aolecules are, it a'be inferred, in motion, Similar effects can then be pro? duced to those whic h are produced by a magpet whilst ina mo tion GMAGNETISIS' 14? Arn e rxipe: umet thich I" tublished in 1845 tenlds, I kfink, to illustrate this tand in somne degree to show the charaeter of the notion ilnpressed upon the molecules of a tmagmeIIic nml3Yetal at fi.e pe i od of ina gcnetksatioin A, tube fiuled 1w-. time qlqtid ni which maoneteic oxide of iron had bettnI preprn'red, and terminatced td eacC h ~en by plates of ogassj, is surrOuinded byha coil of coated wire. To a spectator lookincg t3ron i thhis tube a, flash oof i.t rs perceptible whenever the coil. i s electrised, ad less light is transmitted vhen thie electrica current eeases showing a symmetrical arran ement of the tinute paktiftees of ragrnetic oxi nd whl er the maectic intue-Cne, I-[n this eperhmen t t should be borne in. m hnd, -that the particles of oxide of iron a re not shaped bIy the hfand of:nan, as would be the case wth iron fiin-g,f or similar lmrus-te poartions of-r, 7on.a'g ic matter, but being cThelemca09lyr preip)itated, are of the iform given to thema by nature, Whfle magc neutfs.r is in the sttate of ch.nalne above descri'bed, i wi0l. pr0od:uce the, oter forces; but it may be said.a while nmat;net l Sm Is thus progressive, some other 0Corec is cti:i- on, and ttherefore it does not initiatoe: this is true, b-t the sa IM mIay be siddr. of all the other forces, t;he ha'ye no cm.Q -, mencelent that we can trace, We must ever refer them back to some antecedent force equalt in amnot to that produeed? and th.erefore the iword initiation can0ot in str1 imtess appy, but must only 3be taken as sgnifuying the force selected as the first: -tuis is 1anohmor leeason why t he idea of ab8stract en-zsation is inapplieable to physical produetion. To'this point I shall again rt Eleetricity may thus be produe'd directly b'y canetalinsm eith er when the ma ne% t a s a mass is ai nmotion, or wen magetism sa coinrmencing, increasing, decreacsing-, or coneasinug; tand heait:mnay syimhilarly be directly proadnced by mlagns tisri> I have S, nce the first edition of this E.ss3y -was p b]sued,,eomrnnmticed to, the iRoyal Society a pa3er by which I think 1..8 00CRRLATIN OF ruYSICAL FuaOEJ I have satis.tctorily proved, tihat whenever any iBtal susceptible of magnetism is magnetised or denagnetised, its temperature is raised, This'was shown, first, by subjecting a bar of iron, nickeld, or cobalt to the infnence of a powerful electromragnet, rhich weas rapidly magnetised ad and demagnetse in reverse directions, the electro-4nagnet itLself being kepti cool by cisterns of water, so t.hat the magnetic tmetal s-bjected to tlhe influence of magnetism was raised to a higher temperature than the electro-nagnlet itself, and could not; therefore, hTave acquired its increased temperature by conduc.tion or radiation of heat fron-i the electro-ma gnet; and secondly, by rotating a permanent steel magnet with its pole opposite to a bar of iron, a thermo-electric pile beting placed opposite the latter. mDr, iaggi covered a plate of homogeneous soft iron with a thin coating of wax mixed with oil., a tube traversed the centre tn Hiough which the vapour of boiling water was passed. The plate ivr.s made to rest on the poles of an electro-maanet, wiith card interposede hen the iron is not naa8gnetised, the mnelted. wax assumes a circular formn, the tu e accupying the centre, but when the electro-naganet, is put in action, the curve markinug the boundary of the melted substance chainges its form and becoma es elongated in a direction transverse'to the line jo;inuno the poles, showing that tle conductinfg power of tlhe iron for heat is changed by magnetisati on Th.us we get heat produced by m~agnetiism tacnd tahe coauIction. of heat altered by it in a direction tharing a, deffinite rre1tionu to'th.e direction of the m agnetismI. r it necessary to 11l in and e~er or othe substance' calorie' to explain pthese results? s is t not more rational to regaxrd the catorife e3fects as clha'uies in tbhe nolect-lar arran getenuts o tihe matter subjecterd to xmaSgnetisi? There is ever probablity thas magnetinsm, in the dyna:mic state either when the magnet is in uaotion, or wAhen the ma'etetic intensti y is vE ing, will.also directly prodnuce thesm MAGNETISM. 149 eal affnity and right, thou-gh, n.p to the present time, such has not been proved to be the case; -the reciprocal etfae t, also, of' the direct priduction of imagnetism by light and heat has not yet been ercrihnent.ty l established, I have used, in contradi stinctios ithe t.erms c'ynaiec and static to repres ent the di'fferent states of magnetismn The appicatioens I hmae nade of these terms:may be open to snrae excoepion buLt I know of no otfher words whfve will so nearly express my meing~n The stati c ondition of magnletism resembles the static conadition of other f orces sauch as the state of tensioon exist ing in te beam and a cord of a balance or0 aI ihaCrgoed iLeydet p iaio The old definition of force rwas, that which caused chnige in m.otion; and yet even this definition preo sects a difficu lyt.: i n a case of static equi. libriun -n C ii$l t br inslance? as that rwhich obtains in the two arims of a bIalance we go8t toe idea of force without 1any palpcbtle a-parent moti on whethb otere be really an abst-ence of motion may be a doubto ful qIoestion, as suc eh absence would involve in. this case per-:oet eiasticity, and, in all other cases, a stebilty -wYinch, i~n a lonyg' co-n0se of time, nature generally negative shoing, as I beli eve an insep arable connetion of eo*tion xwlh matte, and an impossibirity of a perfectly immobile or durable state. So "itnh.i mlagnetis m: I believe no nagonet can exist in an asbsikutely stle stte, tbhol h the tatthtion of its stabil ity will be proportionatte to its original resista.ace to asUsauming a polarised condition This, howeve, m ust be taken terely as a ncatter of opinion: we hive, ion support of it, the ogoenerI facts that magnets do deteriorate in the conrse of years; and we haythve t a- further getneral fact of the instability, or fiuxional state of all nature, h'len owe have an oppora tniit'v of fairly a investigatin it at different'and remote periods: in. nany cases, however, the bction is so slow that the ehanges escape humana observation, and, n til this can be broughlt to bear over a2 proportionate period of time, the proposetion cannot be - 5G CORiT LATION OF PtlHYSIC FORCESI said to'b experim'entall or inductively proved, b;t must be lajtf to ntoe ~m ent'a co lange which di turbs eq libriu i produces other chnanges w>hich go on without. eSnd. Thus, by the act 0of clhari a leyden phDia the cyinder, the -Lbber, and nde adjoinaing portions of tlhe electical Vtachine han e each and all ther stac tes ctianged and taence produce chantges in suntaoundingo bodiest$ ad.i, tum';z iv -eni the jar is disfcharoged, converse changes are again produced.. As with lhea light, and 3Itecid, rcity, the daily a1cc unmia ting observations toend to showy <-sl- each chaninge ino the phenomena to -wich these msianes are g'cer n is accor pani,-ed'by a chsarge cliler te'lmporatvy or poermnien ut in, the aitter anffecttid'by flhem; so many reactent eota1iSments on magonetiafsm leave conneected mgagneticu phenoiTena wo il a moleculatr change in the subject nmatter Ti ne Werf, e thelo has s ihown h-t I ncte clgasticty off h.on and steel i's altered by inagcnetis ation; the coet:iecient; of eas tic.ty in iron being temporarily, in steel pcer-manentlty d mi nised::: He has also examined the effeets of torsioc upon - npa cugnet ised iron, and conludes, from his experimeuts, that in a bar M'AGNETISM. 15 of iron arrived at a state of magnetic equilibrium, temporary torsion diminishes the magnetism, and that the untwisting or return to its primitive state restores the original degree of magnetisation. M. Guillemin observed that a bar slightly curved by its own weight is straightened by being magnetised. Mir. Page and MIr. farrion discovered that a sound is emitted when iron or steel is rapidly magnetised or demagnetised; and lJfr. Joule found that a bar of iron is slightly elongated by magnetisation. Again, with regard to diamagnetic bodies~, 2I[. eJatteucci found that the mechanical compression of glass altered the rotatory power upon a ray of polarised light which it transmitted. He further considered that a change took place in the temper of portions of glass which he submitted to th.e influence of powerful magnets. The same arguments which have been submitted to the reader as to the other affections of matter being modes of molecular utiJon, are therefore equally applicable to magaetism, VIL-CHEMICAL AFFINITY. H IEMICAL AFFINITY, or the force by which dissimiL lar bodies tend to unite and form compounds differing generally in character from their constituents, is that node of force of which the human mind has hitherto formed the least definite idea. The word itselif-c-in-y- is ill chosen, its meaning, in this instance, bearing no analogy to its ordinary sense; and the mode of its action is conveyed by certain conventional expressions, no dynamic theory of it worthy of attention having been adopted. Its action so modifies and alters the character of matter, that the changes it induces have acquired, not perhaps very logically, a generic contradistinction from other material changes, and we thus use, as contradistinguished, the terms physical and chemical. The main distinction between chemical affinity and physical attraction or aggregation, is the difference of character of the chemical compound from its components. This is, however, but a vague line of demarcation; in many cases, which would be classed by all as chemical actions, the change of character is but slight; in others, as in the effects of neutralil sation, the difference of character would be a result which would equally follow from physical attraction of dissimilar substances, the previous characters of the constituents depending upon this very attraction or affinity: thrus an acid corrodes CHEMICAL AFFINITY. 153 because it tends to unite with another body; when united, its corrosive power, i. e. its tendency to unite, being satiated. it cannot, so to speak, be further attracted, and it necessarily loses its corrosive power. But there are other cases where no such result could d priori be anticipated, as where the attraction or combining tendency of the compound is higher ithan that of its constituents: thus, who could, by physical reasoning, anticipate a substance like nitric acid from the combination of nitrogen and oxygen? The nearest approach, perhaps, that we can form to a comprehension of chemical action, is by regarding it (vaguely perhaps) as a molecular attraction or motion. It will directly produce mzotion of definite masses, by tile resultant of the molecular changes it induces: thus, the projectile effects of gunpowder may be cited as familiar instances of motion produced by chemical action. It may be a question whether, in this case, the force which occasions the motion of the mass is a conversion of the force of chemical affinity, or whether it is not, rather, a liberation of other forces existing in a state of static equilibrium, and having been brought into suchl state by previous chemical actions; but, at all events, through the medium of electricity chemical affinity may be directly and quantitatively converted into the other modes of force. By chemical affinity, then, wae can directly produce electricity; this latter force was, indeed, said by Davy to ]be chemical affinity acting on masses: it appears, rather, to be chemical affinity acting in a definite direction through a chain of particles; but by no definition can the exact relation of chemical affinity and electricity be expressed; for the latter, however closely related to the former, yet exists where the former does not, as in a metallic wire, which when electrified, or conducting electricity, is, nevertheless, not chemically altered, or, at least, not known to be chemically cti eled. Volta, the antitype of Prometheus, first enabled us de, 7 154 CORRELATION OF PHYSICAL FORCES.. finitely to relate the forces of chemistry and electricity. When two dissimilar metals in contact are immersed in a liquid belonging to a certain class, and capable of acting chemically on one of them, what is termed a voltaic circuit is formed, and, by the chemical action, that peculiar mode of force called an electric current is generated, which circulates from metal to metal, across the liquidf and through the points of contact. Let us take, as an instance of the conversion of chemical force into electrical, the following, which I made known some years ago. If gold be immersed in hydrochloric acid, no chemical action takes place. If gold be immersed in nitric acid, no chemical action takes place; but mix the two acids, and tlhe immersed gold is chemically attacked and dissolved: this an is ordinary chemical action, the resultt of a double chemical affinity. In hydrochloric acid, which is composed of chlorine and hydrogen, the affinity of chlorine for gold being less than its affinity for hydrogen, no change takes place; but when the nitric acid is added, this latter containing a great quantity of oxygen in a state of feeble combination, the affinity of oxygen for hydrogen opposes that of hydrogen for chlorine, and then the affinity of the latter for gold is enabled to act, the gold combines with the chlorline, and chloride of gold remains in solution in the liquid. I Tow, in order to exhibit this chemical force in the form of electrical force, instead of mixing the liquids, place them in separate vessels or compartments, but so that they may be in contact, which may be effected by having' a porous material, such as unglazed porcelain, amianthus, &c., between them. Immerse in each of these liquids a strip or wire of gold: as long as these pieces of gold remain separated, no chemical or electrical effect takes place; but the instant they are brought into metallic contact, either immediately or by connecting each with the same metallic wire, chemical action takes place — the gold in the hydrochloric acid is dissolved, electrical action CHEMICAL AFFINITY. 155 also takes place, the nitric acid is deoxidised by the transferred hydrogen, and a current of electricity may be detected in the metals or connecting metal by the application of a gal Manometer or any instrument appropriate for detecting such effect. There are few, if any, chemical actions which cannot be experimentally made to produce electricity: the oxidation of metals, the burning of combustibles, the combination of oxy gen and hydrogen, &c., may all be made sources of elec tricity. The common mode in which the electricity of the voltaic battery is generated is by the chemical action of water upon zinc; this action is increased by adding certain acids to the water, which enable it to act more powerfully upon the zinc, or in some cases act themselves upon it; and one of the most powerful chemical actions known-that of nitric acid upon oxidable metals-is that which produces the most powerful voltaic battery, a combination which I mnade known in the year 1839: indeed, we may safely say, that when the chemical force is utilised, or not wasted, but all converted into electrical force, the more powerful the chemical action, the more powerful is the electrical action which results. If, instead of employing manufactured products or educts, such as zinc and acids, we could realise as electricity the whole of the chemical force which is active in the combustion of cheap and abundant raw materials, such as coal, wood, fat, &c., with air or water, we should obtain one of the greatest practical desiderata, and have at our command a mechanical power in every respect superior in its applicability to the steam engine.' I have shown that the flame of the common blowpipe gives rise to a very. marked electrical current, capable not only of affecting the galvanometer, but of producing chemical decomposition: two plates or coils of platinum are placed, the one in the portion of the flame near the orifice of the jet, or at the points where combustion commences, the other in the full 156 CORRELATION OF PYSICOAL FOiRCES. yellow flame where combustion is at its maximum; this latter should be kept cool, to enable a thermoselectric current, which is produced by the different temperature of the platinum plates, to co-operate with the flame current; wires attached to the plates of platinum. form the terminals or poles. By a row of jets a flame battery may be formed, yielding increased effects; but in these experiments, though theoretically interesting, so small a fraction of the power, actually at Nwork in the combustion, has been thrown into an electrical form, that there is no immediate promise of a practical result. The quantity of the electrical current, as measured by the quantity of matter it acts upon in its different phenomenal effects, is proportionate to the quantity of chemical action which generated it; and its intensity, or power of overcoming resistance, is also proportionate to the intensity of chemical affinity when a single voltaic pair is employedor to the number of reduplications when the -well-known instrument called the voltaic battery is used. The mode in which the voltaic current is increased in intensity by these reduplications, is in itself a striking instance of the mutual relations and dynamic analogies of different forcees. Let a plate of zinc or other metal possessing a strong affinity for oxygen, and another of platinum or other metal possessing little or no affinity for oxygen, be partially immersed in a vessel, A, contaaining dilute nitric acid, but not in contact with each other; let platinum wires touching each of these plates have their extremities immersed in another vessel, B, containing also dilute nitric acid: as the acid in vessel A is decomposed, by the chemical affinity of the zinc for the oxygen of the acid, the acid in vessel B is also decomposed, oxygen appearing at the extremity of the wire which is connected with the platinum: the chemical power is conveyed or transferred through the wires, and, abstracting certain local effects, for every unit of oxygen which combines with the zinc in the one vessel, a unit of oxygen is evolved CIEMfICAL AFFINITY. I5 from the platinum wire in the other. The platinum wire is thus thrown into a condition analogous to zinc, or has a pow er given to it of determining the oxygen of the liquid to ita surface, though it cannot, as is the case with zinc, corn bine with it under similar circumstances. If we now substi tute for the platinum wire which was connected with the platinum plate, a zinc wire, we have in addition to the determining tendency by which the platinum was affected, the chemical affinity of the oxygen in vessel B for the zinc wire thus we have, added to the force which was originally produced bythe zinc of the combination in vessel A, a second force, produced by the zinc in vessel B, co-operating with the first; two pairs of zinc and platinum thus connected produce, therefore, a more intense effect than one pair; and if we go on adding to these alternations of zinc, platinum, and liquid, we obtain an indefinite exaltation of chemical power, just as in mechanics we obtain accelerated motion by adding fresh impulses to motion already generated. The same rule of proportion which holds good in chen:ical combinations also obtains in electrical effects, when these are produced by chemical actions. Dalton and others proved that the constituents of a vast number of compound substances always bore a definite quantitative relation to each other: thus, water, which consists of one part by weight of hydrogen united to eight parts of oxygen, cannot be formed by the same elements in any other than these proportions; you can neither add to nor subtract from the normal ratio of the elements, without entirely altering the nature of the compound. Further, if any element be selected as unity, the combining ratios of other elements will bear an invariable quantitative relation to that and to each other: thus if hydrogen be chosen as 1, oxygen will be 8, chlorine will be 36; that is, oxygen will unite with hydrogen in the proportion of B parts by weight to 1, while chlorine will unite with hydrogen in the proportion of 36 to 1, or with oxygen in the pro 158 CORRELATION OF PHYSICAL FORCES. portion of 36 to 8. Numbers expressing their combining weights, which are thus relative, not absolute, may by a conventional assent as to the point of unity, be fixed for all chemical reagents; and, when so fixed, it will be found that bodies, at-least in inorganic compounds, generally unite in those proportions, or in simple multiples of them: these proportions are termed zquivalents. Now, a voltaic battery, which consists usually of alternations of two metals, and a liquid capable of acting chemically upon one of them, has, as we have seen, the power of producing chemical action in a liquid connected with it by metals upon which this liquid is incapable of acting: in such case the constituents of the liquid will be eliminated at the surfaces of the immersed metals, and at a distance one-from the other. For example, if the two platinum terminals of a voltaic battery be immersed in water, oxygen will be evolved at one and hydrogen at the other terminal, exactly in the proportions in which they form water; while, to the most minute examination, no action is perceptible in the stratum of liquid. It was known before Faraday's time that, while this chemical action was going on in the subjected liquid,'a chemical action was going on in the cells of the voltaic battery; but it was scarcely if at all known that the amount of chemical action in the one bore a constant relation to the amount of action in tle other. Faraday proved that it bore a direct equivalent relation: that is, supposing the battery to be formed of zinc, platinum, and water, the amount of oxygen which united with the zinc in each cell of the battery was exactly equal to the amount evolved at the one platinum terminal, while the hydrogen evolved from each platinum plate of the battery was equal to the hydrogen evolved from the other platinum terminal. Supposing the battery to be charged with hydrochloric acid, instead of water, while the terminals are separated by water, then for every 36 parts by weight of chlorine which CHEMICAL AFFINITY. 159 united with each plate of zinc, eight parts of oxygen would be evolved from one of the platinum terminals: that is, the weights would be precisely in the same relation which Dalton proved to exist in their chemical combining weights. This may be extended to all liquids capable of being decomposed by the voltaic force, thence called Electrolytes: and as no voltaic effect is produced by liquids incapable of being thus decomposed, it follows that voltaic action is chemical action taking place at a distance, or transferred through a chain of media, and that the chemical equivalent numbers are the exponents oI the amount of voltaic action for corresponding chemical substances. As heat, light, magnetism, or motion, can be produced by the requisite application of the electric current, and as this is definitely produced by chemical action, we get these forces very definitely, though not immediately, produced by chemical action. Let us, however, here enquire, as we have already done with respect to the other forces, how far other forces may directly emanate from chemical affinity. Heat is an immediate product of chemical affinity. I know of no exception to the general proposition that all bodies sin chemically combining produce heat; i. e. if solution be not considered as chemical action, and even in that case, when cold results, it is from a change of consistence, as from the solid to the liquid state, and not from chemical action. We shall find that the same view of the expenditure of force which we have considered in treating of latent heat holds good as to the expenditure of'chemical force when regarded with reference to the amount of heat or repulsive force which it engenders, the chemical force being here exhausted by chemical expansion-that is, by heat. Thus, in the chemical action of the ordinary combustion of coal and oxygen, the expenditure of fuel will be in proportion to the expansibility of the substances heated; water passing fieely 160 CORnRELATION OF PHYSICAL FORCES. into the steam will consume more fuel than if it be confined and kept at a temperature above its boiling point. Why chemical action produces heat, or what is the action of the molecules of matter when chemically uniting, is a question upon which many theories have been proposed and which may possibly be never more than approximately resolved. Some authors explain it by the condensation which takes place; but this will not account for the many instances where, from the liberation of gases, a great increase of volume ensues upon chemical combustion, as in the familiar instance of the explosion of gunpowder: others explain it as resulting from the union of atmospheres of positive and negative electricity which are assumed to surround the atoms of bodies; but this involves hypothesis upon hypothesis. Dr. Wood has lately thrown out a view of the heat of chemical action which is more in accordance with a dynamic theory of heat, and as such demands some notice. Starting with his proposition, which I have previously mentioned,' that the nearer the particles of bodies are to each other the less they require to move to produce a given motion in the particles of another body,' his argument, if I rightly understand it, assumes something of this form. In the mechanical approximation of the particles of a homuogeneous body heat results; the particles ac a of the body A would, by their approximation, produce expansion in the neighbourinog body B, the more so in proportion as they them. selves were previously nearer to each other. In chemically combining, a Ca the particles of A are brought into very close proximity with5 b the particles of B; heat should therefore result, and the greater because the proximity may fairly be assumed to be greater in the case of chemical combination than in that of mechanical compression. In cases, then, where there is no absolute diminution of bulk ensuing on chemical combination, if the greater proximity of the conm CHEMICA~L AFFINITY. 161 bining particles be such that the correlative expansion ought to be greater (if there were no chemical combination) than that occulpied by the total volume of the new compound, an extra expanding power is evolved, and heat or expansion ought to be produced in surrounding bodies. In other words, if a a could be brought by physical attraction as near each other as they are by chemical attraction brought near to b b, tiley would, fromn their increased proximity, produce an expansive power ultra the volume occupied by the actual chemical compound A and B. The question, however, immediately occurs, why should the volume of the compound be limited and not occupy the full space equivalent to the expanding power induced by the contraction or approximation of the particles, As the distance of the particles is the resultant of the contending contracting and expanding powers, this result ought to express itself in terms of the actual volume produced by the combination, which it certainly does not. Though I see some difficulties in Dr. Wood's theory, and perhaps have not rightly conceived it, his views have to my mind great interest, his mode of regarding natural phenomena being analogous to that which I have in this Essay, and for many years, advocated, viz. to divest physical science as much as possible of hypothetic fluids, ethers, latent entities, occult qualities, &c. My own notion of the heat produced by chemical combination, though I scarcely dare venture an opinion upon a subject so controverted, is, that it is analogous to the heat of friction, that the particles of matter in close approximation and rapid motion inter se evolve heat as a continuation of the motion interrupted by the friction or intestinal motion of the particles: heat would thus be produced, whether the resulting compound were of greater or less bull than the sum of the components, though of course when the compound is of greater bulk less heat would be apparent in neighbouring bodies, the expansion taking place in one of the substances themselves-I say in one of them, for it is stated 162 CORRELATION OF PHYSICAL FORCES. in books of authority that there is no instance of two or more solids or liquids, or a solid and a liquid, combining and producing a compound which is entirely gaseous at orclinary temperatures and preessures. The substance gun-cotton, however, discovered by Dr. Schoenbein, very nearly realises this proposition. ]Dr. Andrews has arrived at the conclusion, after careful experiment, that in chemical combinations where acids and alkalies or analogous substances are employed, the amount of heat produced is determined by the basic ingredient, and his experiizents have received general assent; althoughl it should be stated that Mi. Hess arrived at contrary results, thle acid constituent according to his experiments furnishing the maeasure of the heat developed. Light is directly produced by chemical action, as in the flash of gunpowder, the burning of phosphorus in oxygen gas, and all rapid combustions: indeed, wherever intense heat is developed, light accompanies it. In many cases of slow combustion, such as the phenomena of phosphorescence, the light is apparently much more intense than the heat; the former being obvious, the latter so difficult of detection that for a long time it was a question whether any heat was eliminated; and I am not aware that at the present day, any thermic effects from certain modes of phosphorescence, such as those of phosphorescent wood, putrescent fish, &c., have been detected. Chemical action produces mazcgnetisz whenever it is thrown into a definite direction, as in the phenomenon of electrolysis. I may adduce the gas voltaic battery, as presenting a simple instance of the direct production of magnetism by chemical Synthesis. Oxygen and hydrogen in that combination chemically unite; but instead of combining by intimate molecular admixture, as in the ordinary cases, they act upon water, i. e. combined oxygen and hydrogen, placed between them so as to produce a line of chemical action; and a magnet adjacent to this line of action is deflected, and places itself at right CHEMICAL AFFINITY. 163 angles to it. What a chain of molecules does here, there can be no doubt all the molecules entering into combination would produce in ordinary chemical actions; but in such caCes, the direction of the lines of combination being irregular and confused, there is no general resultant by which the magnet can be affected. What tlle exact nature of the transference of chemical power across an electrolyte is, we at present know not, nor can we form any more definite idea of it than that given by the theory of Grotthus. We have no knowledge as to the ex act nature of any mode of chemical action, and, for the present must leave it as an obscure action of force, of which future researches may simplify our apprehension. ~W~e have seen that an equivalent or proportionate electrical effect is produced by a given amount of chemical action; if we, in turn, produce heat and magnetism andcl motion by the electricity resulting from chemical action, we shall be able to measure these forces far more accurately than when they are directly produced, and thus to deduce their equivalent relation to the i-nitial chemical action. Thus M. Favre, after ascertaining the oaantity of heat produced by the oxidation eo a quantity of zinc, and finding, as have others, that the heat is the samne when evolved from a voltaic battery by the same consumption of zinc forming its positive element, makes the following experiment. A voltaic battery and electro-magnet are immersed in calorimeters, and the heat produced when the connection with the mcagnet is effected is noted. The electroemagnet is then made to raise a weight, and thus perform mechanical work, and the heat produced is again noted. It is found in the latter case that less heat is evolved than in the former, a certain quantit~y of heat has therefore been replaced by the mechanical work; and by estimating the amount of heat subtracted, and the amount of work produced, he deduces the relative equivalent of work to 164 COIRRELATION OF PHYSICAL FORCES. heat. These experiments give a production of mechanical work by chemical action, not, it is true, a direct production, but, as the heat and work are in inverse ratios, and each has its source in chemical action, they prove that they are definite for a definite amount of chemical action, and as each is produced respectively by electricity and magnetism, these forces us-t also bear a definite relation to the initial chemical force. The doctrine of definite combining proportions, which so beautifully serves to relate chemistry to voltaic electricity, led to the atomic theory, which, though adopted in its universality by a large majority of chemists, presents great difficulties when extended to all chemical combinations. T'he equivalent ratios in which a great number of substances chemically combine, hold good in so many instances, that the atomic doctrine is believed by many to be universally applicable, and called a law; and yet, when followed in the combinations of substances whose natural chemical attractions are very feeble, the relation fades away, and is sought to be recovered by applying a separate and arbitrary multiplier to the different constituents. Thus, wvhen it was found that a vast number of substances combined in definite volumes and weights, and in definite volumes "and weights only, it was argued that their ultimate molecules or atoms had a definite size, as otherwise there was no apparent reason why this equivalent ratio should hold good: why, for instance, water should only be formed of two volumes or one unit by weight of hydrogen, and of one voluine or eight units by weight of oxygen; why, unless there were some ultimate limits to the divisibility; of its molecules, should not water, or a fluid substance approximating to water in character, be formed by a half, a third, or a tenth part of hydrogen, with eight parts of oxygen? It was perfectly consistent with the atomic view that a substance might be formed with one part combined with eight parts, or with sixteen, or with twenty-four, for in such a sub CHEMICAL AFFINITY. 165 stance there would be no subdivision of the (supposed indivisible) molecule; and this held good with many compounds' thus fourteen parts by weight, say grains of nitrogen, wil) combine respectively with eight, sixteen, twenty-four, thirtytwo, and forty parts by weight, or grains, of oxygen. So, again, twenty-seven grains of iron will combine with eight grains of oxygen or with twenty-four grains, i. e. three proportionals of oxygen. No compound is known in which twenty-seven grains of iron will combine with two proportiQuals or sixteen grains of oxygen; but this does not much affect the theory, as such a compound may be yet discovered, or there may be reasons at present unknown why it cannot be formed. But now comes a difficulty: twenty-seven parts by weight of iron will combine with twelve parts by weight of oxygen, and twenty-seven parts of iron will also combine with ten and two-third parts of oxygen. Thus if we retain the unit of iron we must subdivide the unit of oxygen, or if we retain the unit of oxygen we must subdivide the unit of iron, or we must subdivide both by a different divisor. What then becomes of the notion of an atom or molecule physically indivisible? If iron were tile only substance to which this difficulty applied, it might be viewed as an unexplained exception, or as a mixture of two oxides; or recourse might be had to a more minute subdivision to form the units or equivalents of other substances; but numerous other substances fall under a similar category; and in organic combinations, to preserve the atomic nomenclature we must apply a separate multiplier or divisor to far the greater number of the elementary constitnents, i. e. we must divide that which is, ecx 7hypothesi, indivisible. Thus, to take a more complex substance than any formed by the combination of iron and oxygen, let us select the substance albumen, composed of carbon, hydrogen, nitrogen, 166 CO-RRELATION OF PHYSICAL FOBCES. oxygen, phosphorus, and sulphur. In this case we must either divide the atoms of phosphorus and sulphur so as to reduce them to small fractions, or multiply the atoms of the other substances by extravagant numbers; thus to preserve the unit of one of the constituents of this substance, chemists say it is composed of 400 atoms of carbon, 310 of hydrogen, 120 of oxygen, 50 of nitrogen, 2 of sulphur, and 1 of phosphorus. This is a somewhat extreme case, but similar difficulties will be found in different degrees to prevail among organic compounds; in very many no constituent can be taken as a unit to which simple multiples of any of the others will give their relative proportions. By the mode of notation adopted, if any conceivable substance be selected, it could, whatever be the proportions of its constituents, be termed atomic. A solution of an ounce of sugar in a pound of water, in a pound and a half, in a pound and a quarter, in a pound and a tenth, raight be expressed in an atomic form, if wve select arbitrarily a multiplier or divisor. It is true that in the case of solution, different proportions can be united up to the point of saturation without any difference in the character of tJe compound, though the same may be predicated to some extent of an acid and an alkali; but even where the steps are sudden, and compounds only exist with definite proportions, they cannot, in a multitude of cases, be reconciled with the true idea of an atomic combination, i. e. one to one, one to two, &c. Although, therefore, nature presents us with facts which show that there is some restrictive law of combination which in numerous cases limits the ratios in which substances will combine, nay, further, shows many instances of a proportion between the combining weights of one compound and those. of another; although she shows also a remarkable simplicity in the comblning volumes of numerous gases, she also gives numerous cases to which the doctrine of atomic combinations cannot fairly be applied. CHEMICAL AFFINITY. 1i6 That there must be something in the constitution of matter, or in the forces which act on it, to account for the per salzonm manner in which chemical combinations take place, is inevitable; but the idea of atoms does not seem satisfactorily to account for it. By selecting a separate multiplier or divisor, chemists may denote every combination in terms derived from the atomic theory; but they have passed from the original law, which contemplated only definite multiples, and the very hypothetic expressions of atoms, which the apparently simple relations of combining weights first led them to adopt, they are obliged to vary and to contradict in terms, by dividing that which their hypothesis and the expression of it assumed to be indivisible. While, therefore, I fully recognise a great natural truth in the definite ratios presented by a vast number of chemical combinations, and in the jIer sactzin steps in which nearly all take place, I cannot accept as an argument in favour of an atomic theory, those combinations which are made to support it by thea application of an arbitrary notation. A similar straining of theory seems gradually obtaining in regard to the doctrine of compound radicals. The discovery of cyanogen by Gay-Lussac was probably the first inducement to the doctrine of compound radicals; a doctrine which is now generally, perhaps too generally, received in organic chemistry. As, in the case of cyanogen, a body obviously compound discharged in almost all its reactions the functions of an element, so in many other cases it was found that compound bodies in which a great number of elements existed, might be regarded as binary combinations, by considering certain groups of these elements as a compound ra'dical; that is, as a simple body when treated of in relation to the other complex substances of which it forms part, and only as non-elementary when referred to its internal constitution. 168 CORRELATION OF PHYSICAL FORCES. Undoubtedly, by approximating in theory the reactions of inorganic and of organic chemistry, by keeping the mind within the limits of a beaten path, instead of allowing it to wander through a maze of isolated facts, the doctrine of compound radicals has been of service; but, on the other hand, the indefinite variety of changes which may be rung upon the composition of an organic substance, by different associations of its primary elements, makes the binary constituents vary as the minds of the authors who treat of them, and makes their grouping depend entirely upon the strength of the analogies presented to each individual mind. From this cause, and from the extreme license which has been taken in theoretic groupings deduced from this doctrine, a serious question arises whether it may not ultimately, unless carefully restricted, produce confusion rather than simplicity, and be to the student an embarrassment rather than an assistance. VIII.-OTHEI MODES OF FORCE. CATALYSIS, or the chemical action induced by the mere presence of a foreign body, embraces a class of facts which must considerably modify many of our notions of chemical action: thus oxygen and hydrogen, when mixed in a gaseous state, will remain unaltered for an indefinite period; but the introduction to them of a slip of clean platinum will cause more or less rapid combination, without being in itself in any respect altered. On the other hand, oxygenated water, which is a compound of one equivalent of hydrogen plus two of oxygen, will, when under a certain temperature, remain perfectly stable; but touch it with platinum in a state of minute division, and it is instantly decomposed, one equivalent of oxygen being set free. Here, again, the platinum is unaltered, and thus we have synthesis and analysis effected apparently by the mere contact of a foreign body. It is not improbable that the increased electrolytic power of water by the addition of some acids, such as the sulphuric and phosphoric, where the acids themselves are not decomposed, depends upon a catalytic effect of these acids; but we know too little of the nature and rationale of catalysis to e>xpress any confident opinion on its modes of action, and possibly we may comprehend very different molecular actions under one and the same name. In no case does catalysis yield us new power or force: it only determines Dr facilitates 8 170 CORRELATION OF PHYSICAL FORCES. the action of chemical force, and, therefore, is no creation of force by contact. The force so developed by catalysis may be converted into a voltaic form thus: in a single pair of the gas battery above alluded to, onie portion of a strip of platinum is immersed in a tube of oxygen, the other in one of hydrogen, both the gases and the extremities of the platinum being connected by water or other electrolyte; a voltaic combination is thus formed, and electricity, heat, light, magnetism, and motion, produced at the will of the experimenter. In this combination we have a striking instance of corn relative expansions and contractions, analogous, though in a much more refined form, to the expansions and contractions by heat and cold detailed in the early part of this essay, and illustrated by the alternations of two bladders partially filled with air: thus, as by the effect of chemical combination in each pair of tubes of the gas battery the gases oxygen and hydrogen lose their gaseous character and shrink into water, so at the platinum terminals of the battery, when immersed in water, water is decomposed, and expands into oxygen and hydrogen gases. The correlate of the force which changes gas into liquid at one point of space, changes liquid into gas at another, and the exact volume which disappears in the one place reappears in the other; so that it would appear to an inexperienced eye as though the gases passed through solid wvires. Gravitation, inertia, and aggregation-, wcr: but cursorily alluded to in my original lectures; their relation to the other modes of force seemed to be less definitely traceable; but the phenomenal effects of gravitation and inertia, being motion and resistance to motion, in considering motion I have in some degree included their relations to the other forces. To ny mind gravitation would only produce other force when the motion caused by it ceases. Thus, if we suppose a mleteor to be a mass rotating in an orbit round the earth, OTHER MODES OF FORCE. 171 and with no resisting medium, then, as long as that rotation continues, the motion of the meteoric mass itself would be the exponent of the force impelling it; if there be a resisting medium, part of this motion would be arrested and taken up by the medium, either as motion, heat, electricity, or some other mode of force; if the meteor approach the earth sufficiently to fall upon it, the perceptible motion of the meteor is stopped, but is taken up by the earth which vibrates through its mass; part also reappears as heat in both earth and meteor, and part in the change in the earth's position consequent on its increase of gravity, and so on. Gravitation is but the subjective idea, and its relation to other modes of force seems to me to be identical with that of pressure or motion. Thus, when arrested notion produces heat, it nmatters not whether the motion has been produced by a falling body, i. e. by gravitation, or a body projected by an explosive compound, &c.; the heat will be the same, provided the mass and velocity at the time of arrest be the sarme. In no other sense can I conceive a relation between gravitation and the other forces, and, with all diffidence, I cannot agree with those who seek a more mysterious link. Mosotti has mathematically treated of the identity of gravitation with cohesive atterction, and Pllticker has recently succeeded in showing that crystalline bodies are definitely af. fected by magnetism, and take a position'in relation to the lines of magnetic force dependent upon their optical axis or axis of symmetry. -What is termed the optic axis is a fixed direction through crystals, in which they do not doubly refract light, and which direction, in those crystals which have one axis of figure, or a line around which the figure is symmetrical, is parallel to the axis of symmetry. 5Then submitted to magnetic influence such crystals take up a position, so that their optic axis points diamagnetically or transversely to the lines of magnetic force; and when, as is the case in some crystals, there is 1/2 CORRELATION OF PHYSICAL FORCES. more than one optic axis, the resultant of these axes points diamagnetically. The mineral cyanite is influenced by magnetism in so marked a manner that when suspended it will arrange itself definitely with reference to the direction of terrestrial magnetism, and may, according to Plhicker, be used as a compass-needlle. There is scarcely any doubt that the force which is concerned in aggregation is the same which gives to matter its crystalline form,; indeed, a vast number of inorganic bodies, if not all, which appear amorphous are, when closely examined, found to be crystalline in their structure: we thus get a reciprocity of action between the force which unites the molecules of matter and the magnetic force, and through the medium of the latter the correlation of the attraction of aggregation with the other modes of force may be established. I believe that the same principles and mode of reasoning as have been adopted in this essay might be applied to the organic as vell as the inorganic world; and that muscular force, animal and vegetable heat, &c., might, and at some time will, be shown to have similar definite correlations; but I have purposely avoided this subject, as pertaining to a department of science to which I have not devoted my attention. I ought, however, while alluding to this subject, shortly to mention some experiments of Professor lIatteucci, communicated to the Royal Society in the year 1850, by which it appears that whatever mode of force it be which is propagated along the nervous filaments, this mode of force is definitely affected by currents of electricity. His experiments show that Flwhen a current of positive electricity traverses a portion of the muscle of a living animal in the same direction as that in which the nerves ramify —i. e. a direction from the brain to the extremities-a muscular contraction is produced in the limb experimented on, showing that the nerve of motion is affected; while, if the current, as it is termed, be made to traverse the muscle in the reverse direction, or towards the OTHER MODES OF FORCE. 173 nervous centres, the animal utters cries, and exhibits all the indications of su-fering pain, scarcely any muscular movement being produced; showing that in this case the nerves of sensation are affected by the electric current, and therefore that some definite polar condition exists, or is induced, in the nerves, to which electricity is correlated, and that probably this polar condition constitutes nervous agency. There are other analogies given in the papers of Id. P]atteucci, and derived from the action of the electrical organs of fishes, which tend to corroborate and develope the same view. By an application of the doctrine of the Correlation of Forces, Dr. Carpenter has shown howi a difficulty arising from the ordinary notions of the developement of an organised being from its germ-cell may be lessened. It has been thought by many physiologists that the nisus forrnadivus, or organising force of an animal or vegetable structure, lies dormant in the primordial germ-cell.'So that the organising force required to build up an oak or a palm, an elephant or a whale, is concentrated in a minute particle only discernible by microscopic aid.' Certain other views of nearly equal difficulty have been propounded. Dr. Carpenter suggests the probability of extraneous Lorces, as heat, light, and chemical affinity, continuously operating upon the material germ; so that all that is required in this is a structure capable of receiving, directing, and converting these forces into those which tend to the assimfiation of extraneous matter and the definite developement of the particular structure. In proof of this position he shows how dependent the process of germ developement is upon the presence and agency of external forces, particularly heat and light, and how it is regulated by the measure of these forces supplied to it. It certainly is far less difficult so to conceive the supply of force yielded to organised beings in their gradual process of growth, than to suppose a store of dormant or latent force pent up in a microscopic monad. 174 CORRELATION OF PHYSICAL'ORCES. As by the artificial structure of a voltaic battery, chemin cal actions may be made to cooperate in a definite direction, so, by the organism of a vegetable or animal, the mode of motion which constitutes heat, light, &c., may, without extravagance, be conceived to be appropriated and chanced into the forces which induce the absorption, and assimilation of nutriment, and into nervous agency and muscular power. Indications of similar thoughts may be detected in the writings of Liebig. Some_ difficulty in studying the correlations of vital with inorganic physical forces arises fiom the effects of sensation and consciousness, presenting a similar confusion to that alluded to, when, in treating of heat, I ventured to suggest, that observers are too apt to confound the sensations with the phenomena. Thus, to apply some of the considerl/tions on force, given in the introductory portion of this essay, to cases where vitality or consciousness intervenes. When a weight is raised by the hand, there should, according to the doctrine of non-creation of force, have been somewhere an expenciture equivalent to the amount of gravitation overcome in raising the weight. That there is expenditure we can prove, though in the present state of science we cannot measure it. Thus, prolong the effort, raise weights for an hour or two, thae vital powers sink, food, i e. fresh chemical force, is required to supply the exhaustion. If this supply is withheld and the exertion is continued, yw~e see the consumption of force in the supervening- weakness and emaciation of the body. The consciousness of effort, which has formed a topic of argument by some writers when treating of force, and is by them believed to be that which has originated the idea of force, may by.the physical student be regarded as feeling is in the phenomena of heat and cold, viz, a sensation of the struggle of opposing molecular motions in overcoming the resistance of the masses to be moved. When we say we feel hot, we feel cold, we feel that we are exerting ourselves, our OTHER MODES OF FORCE. i75 expressions are intelligible to beings who are capable of experiencing similar sensations; but the physical changes accompanying these sensations are not thereby explained. Without pretending to know what probably we shall never know, the actual nodzus agendc of the brain, nerves, muscles, &cme we may study vital as we do inorganic phenomena, both by observation and experiment. Thus, Sir Benjamin Brodie has exalined the effect of respiration on animal heat by inducing artificial respiration after the spinal cord has been severed; in which case he finds the animal heat decclies, notvwithstanling the continuance of the chemical action of respiration, carbonic acid being formed as usual; but he also finds that under such circrumstances the struggles or muscular actions of the animal are very great, and sufficient probably to account for the force. eliminated by the chemical action in digestion and respiration; and Liebig, by measuring the amount of chemical action in digestion and respiration, and comparing it with the labour performed, has to some extent established their equivalent relations. Mr. Helmholtz has found that the chemical changes which take place in muscles are greater when these are made to undergo contractions than when they are in repose; and that, as would be expected, the consumption of the matter of the muscles, or, in other terms, the waste or excrementitious matter thrown off, is greater in the former than in the latter case. IM. ]atteucci has ascertained that the muscles of recently killed frogs absorb oxygen and exhale carbonic acid, and that when they are thrown into a state of contraction, and still more when they perform mechanical work, the absorption is increased; and he even calculates the equivalents of work so performed. M. Beclard finds that the quantity of heat produced by voluntary muscular contraction in man is greater when that contraction is what he terms static, that is, when it produces t176 CORRELATION OF PHYSICAL FORCES. no external wvork, but is effort alone, than when that e ffort and contraction are employed dynamically, so as to raise a weight or produce mechanical work. Thus, though we may see no present promise of being able'to resolve sensations into their ultimalte elements, or to trace, physically, the link which unites volition with exertion or effort, in t'erms of our own consciousness of it, wAe may hope to approximate the solution of these deeply interesting questions. In the same individual the chemical and physical state of the secretions in the warmn may be compared with those in the cold parts of the body. The changes in digesticn and respiration, when the body is in a state of rest, inmay be compared with those wahich obtain when it is in a state of activity. The relations with external matter, maintaining, by thle constant play of natural forces, the vital nucleus, or the organisation by means of which matter and force receive, for a definlite period, a definite incorporation and direction, may be ascertained, while the more minute structural changes are revealed to us by the ever-improving powers of the microscope; and thus step by step we may learn that which it is given to us to learn, boundless in its range and infinite in its progress, and therefore never giving a response to the ultimate — Why? As the first glimpse of a new star is caught by the eye of the astronomer while directing his vision to a different point of space, and disappears when steadfastly gazed at, only to have its position and figure ultimately ascertained by the employment of more penetrative powers, so the first scintillations of new na'tural phenomena frequently present themselves to the eye of the observer, dimly seen when viewed askance, and disappearing if directly looked for. When new powers of thought and experiment have developed and corrected the first notions, and given a character to the new image, probably very different from the first impression, fresh objects are OTHER MODES OF FORCE. 177 again glanced at in the margin of the new field of vision, which in their turn have to be verified, and again lead to newv extensions; thus the effort to establish one observation leads to the imperfect perception of new and wider fields of research; and, instead of approaching finality, the more we discover, the more infinite appears the range of the undiscovered! IX.-CONCLUDING RE iARKS. T HAVE now gone through the affections of matter for % which distinct names have been given in our received nomlenclature: that other forces may be detected, differing as much from therm as they differ from each other, is highly probable, and that when discovered, and their modes of action fully traced out, they will be found to be related inter se, and to these forces as these are to each other, I believe to be as far certain as certainty can be predicted of any future event. It may in many cases be a difficult question to determine what constitutes a distinct affection of matter or mode of force. It is highly probable that different lines of demlarca tion would have been drawn between the forces already known, had they been discovered in a different manner, or first observed at different points of the chain which connects them. Thus, radiant heat and light are mainly distinguished by the manner in which they affect our senses: were they viewed according to the way in which they affect inorganic matter, very different notions would possibly be entertained of their character and relation. Electricity, again, was named from the substance in which, and magnetism from the district where, it first happened to be observed, and a chain of intermediate phenomena have so connected electricity with galvanism that they are now regarded as the same force, CONCLUDING REMARKS. 179 differing only in the degree of its intensity and quantity, though for a long time they were regarded'as distinct. The phenomenon of attraction and repulsion by amber, which originated the term electricity, is as unlike that of the decomposition of water by the voltaic pile, as any two natural phenomena can well be. It is only because the historical sequence of scientific discoveries has associated them by a number of intermediate links, that they are classed under the same category. What is called voltaic electricity might equally, perhaps more appropriately, be called voltaic chemistry. I mention these facts to show that the distinction in the name may frequently be much greater than the distinction of the subject which it represents, and vice versa, not as at all objecting to the received nomenclature on these points; nor do I say it would be advisable to depart from it: were we to do so, inevitable confusion would result, and objections equally forcible might be found to apply to our new terminology. Words, when established to a certain point, become a part of the social mind; its powers and very existence depend upon the adoption of conventional symbols; and were these suddenly departed from, or varied, according to individual apprehensions, the acquisition and transmission of knowledge would cease. Undoubtedly, neology is more permissible in physical science than in any other branch of knowledge, because it is more progressive; new facts or new relations require new names, but even here it should be used with great caution. Si forte necesse est Indiciis monstrare recentibus abdita rerum, Fingere cinctutis non exaudita Cethegis, Continget; dabiturque licentia, sumpta pudenter. Even should the mind ever be led to dismiss the idea of ~arious forces, and regard them as the exertion of one force, 180 CORRELATION OF PHYSICAL FORCES. or resolve them definitely into motion; still we could never avoid the use of different conventional terms for the different modes of action of this one pervading force. Reviewing the series of relations between the various forces which we have been considering, it would appear that in many cases where one of these is excited or exists, all the others are also set in action: thus, when a substance, such as sulphuret of antimony, is electrified, at the instant of electrii sation it becomes mnagnetic in directions at right angles to the lines of electric force; at the same time it becomes' heated to an extent greater or less according to the intensity of the electric force. If this intensity be exalted to a certain point the sulphuret becomes luiminous, or light is produced: it expands, consequently motion is produced; and it is decomposed, therefore chemiccal action is produced. If we take anothet substance, say a metal, all these forces except the last are developed; and althougCh we can scarcely apply the term mechanical action to a substance hitherto undecomposed, and which, under the circumstances we' are considering, enters into no new combination, yet it undergoes that species of polarisation which, as far as we can judge, is the first step towards chemical action, and which, if the substance were decomposable, would resolve it into its elements. Perhaps, indeed, some hitherto undiscovered chemical action is produced in substances which we regard as undecomposable: there are experiments to show that metals which have been electrised are permanently chaanged in their molecular consti. tution. Oxygen, we have seen, is changed by the electric spark into ozone, and phosphorus into allotropic phosphorus, both which changes were for a long time unknown to those familiar with electrical science. Thus, with some substances, when one mode of force is produced all the others are simultaneously developed. With other substances, probably with all matter, some of the other forces are developed, whenever one is excited,-and all may be CONCLUDING nEMARKS. 181 so were the matter in a suitable condition for their developement, or our means of detecting them sufficiently delicate. This simultaneous production of several different forces.seems at first sight to be irreconcileable with their mutual and necessary dependence, and it certainly presents a formidable experimental difficulty in the way of establishing their equivalent relations; but when examinedl closely, it is not in fact inconsistent with the views we have been considering, but is indeed a strong argument in favour of the theory which regards them as modes of motion. Let us select one or two cases in -which this form of objection may be prominently put forward. A voltaic battery decomposing water in a voltameter, while the same current is employed at the same time to make an electro-magnet, gives nevertheless in the voltameter an equivalent of gas, or decomposes an equivalent of an electrolyte for each equivalent of chemical decomposition in the battery cells, and will give the same ratios if the electro-magnet be removed. Here, at first sig'ht, it would appear that the magnetism was an extra force produced, and that thus more than the equivalent power was obtained from the battery. In answer to this objection it may be'said, that in the circumstances under which this experiment is ordinarily performed, several cells of the battery are used, and so there is a far greater amount of force generated in the cells than is indicated by the effect in the voltameter. If, moreover, the magnet be not interposed, still the magnetic force is equally existend throug'hout the whole current; for instance, the wires joining the plates will attract iron filings, deflect magnetic needles, &c., and produce diamagnetic effects on surrounding matter. By the iron core a small portion of the force is, indeed, absorbed while it is being made a magnet, but this ceases to be absorbed when the magnet is made; this has been proved by the observation of Mr. Latimer Clarke, who has found that along the wires of the electric telegraph the magnetic needles 182 CORRELATION OF PHYSICAL'FORCES. placed at different stations remained fixed after the connection with the battery was made, and while the electric current acted by induction on surrounding conducting matter, separated from the wires by their gutta pereha coating, so that a sort of Leyden phial was formed; but as soon as this induction had produced its effect between each station, or, so to speak, the phial was charged, the needles successively were deflected: it is like the case of a pulley and weight, which latter exhausts force while it is being raised; but when raised, the force is free, and may be used for other purposes. If a battery of one cell, just capable of decomposing water and no more, be employed, this will cease to decompose while making a magnet. There mulst, in every case, be preponderating chemical affinity in the battery cells, either by the nature of its elemlents or by the reduplication of series, to effect decomposition in the voltameter; and if the point is just reached at which this is effected, and the power is then reduced by any resistance, decomposition ceases: were it otherwise, were the decomposition in the voltameter the exponent of the entire force of the generating cells, and these could independently produce magnetic force, this latter force would be got from nothing, and'perpetual motion be obtained. To take another and different example: A piece of zinc dissolved in dilute sulphuric acid gives somewhat less heat than when the zinc has- a wire of platinum attached to it, and is dissolved by the same quantity of acid. The argument is deduced that, as there is more electricity in the second than in the first case, there should be less heat; but as, according to our received theories, the heat is a product of the electric current, and in consequence of the impurity of zinc electricity is generated in the first case molecularly, in what is called local action, though not thrown into a general direction, there should be more of both heat and electricity in the second than in the first case, as the heat and electricity due to CONCLUDING REMARKS. 183 the voltaic combination of zinc and platinum are added to that excited on the surface of the zinc, and the zinc should be, as in fact it is, more rapidly dissolved; so that the extra heat and electricity is produced by extra chemical force. Many additional cases of a similar description might be suggested. But although it is difficult, and perhaps impossible, to restrict the action of any one force to the production of one other force, and of one only-yet if the whole of one force, say chemical actions be supposed to be employed in producing its full equivalent of another force, say heat, then as this heat is capable in its turn of reproducing chemical action, and in the limit, a quantity equal or at least only infinitely short of the initial force: if this could at the same time produce independently another force, say magnetism, we could, by adding the magnetism to the total heat, get more than the original chemical action, and thus create force or obtain perpetual motion. The term Correlation, which I selected as the title of my Lectures in 1843, strictly interpreted, means a necessary mutual or reciprocal dependence of two ideas, inseparable even in mental conception: thus, the idea of height cannot exist without involving the idea of its correlate, depth; the idea of parent cannot exist without involving the idea of offspring. It has been scarcely, if at all, used by writers on physics, but there are a vast variety of physical relations to which, if it does not in its strictest original sense apply, cannot certainly be so well expressed by any other teram. There are, for exam ple, many facts, one of which cannot take place without involving the other; one arm of a lever cannot be depressed without the other being elevated-the fuger cannot press the table without the table pressing the finger. A body cannot be heated without another being cooled, or some other force being' exhausted in an equivalent ratio to the production of heat; a body cannot be positively electrified without some other body being negatively electrifled, &c. 184 CORBRELATION OF PHYSICAL FORCES. The probability is, that, if not all, the greater number of physical phenomena are correlative, and that, without a duality of conception, the lind cannot iorm an idea of them: thus motion cannot be perceived or probably imagined without parallax or relative change of position. The world was believed fixed, until by comparison with the celestial bodies, it was found to change its place with regard to them: had there been no perceptible matter external to the world, we should never have discovered its motion. n sailing along a river, the stationary vessels and objects on the banks seem to movse past the observer: if at last he arrives at the conviction that he is moving, and not these objects, it is by correcting his senses by reflection derived fror a more extensive previous use of them: even then he can only form a notion of the motion of the vessel he is in, by its change of position with regard to the objects it passes-that is, provided his body partakes of the motion of the vessel, which it only does when its course is perfectly smooth, otherwise the relative change of position of the different parts of the body and the vessel inform him of its alternating, though not of its progressive movement. So in all physical phenomena, the effects produced by motion are all in proportion to the relative motion: thus, whether the rubber of an electrical machine be stationary, and the cylinder mobile, or the rubber mobile and the cylinder stationary, or both mobile in different directions, or in the same direction with different degrees of velocity, the electrical effects are, coeteris pcaribus, precisely the same, provided the relative motion is the same, and so, without exception, of all other phenomena. The question of whether there can be absolute motion, or, indeed, any absolute isolated force, is purely the metaphysical question of idealism or realism-a question for our purpose of little import; sufficient for the purely physical inquirer, the maxim I de non acpcar-entibus et non existentibus eadent est ratio.' The sense I have attached to the word correlation, in CONCLUDING REMARIS. 185 treating of physical phenomena, will, ] think, be evident from the previous parts of t;his essay, to be that of a necessary reciprocal production: in other words5 that any foce capable of producing another may, in its turn, be produced by itnay, rnore, can be itself resisted by the force it produces, in proportion to the energy of such production, as action is ever aceoimpanied and resisted by reaction: t hus, the action of an electro-magnetic machine is reacted upon by the mag'netoelectricity developed by its action. To many, however, of the cases we have been considering, the term correlation may be applied in a miore strict accordance with its original sense: thus, with reguard to the forces of electricity and miagnetism in a dynamic state, we cannot electrise a substance without magnetising it —we cannot raagnetise it without electrising it:-each molecule, the instant it is affected by one of these forces, is affected by the other; but, in transverse directions, the forces are inseparable and mutually dependent-correlative, but not identical. The evolution of one force or imode of force into anot;her has induced many to regard all the di-fferent natural agencies as reducible to unity, and as resulting -from one force which is the efficient cause of all the others: thus, one author writes to prove that electricity is the cause of every change in matter; another, that clhemical action is the cause of everything'; another, that heat is the universal cause, and so oiln. If, as I have stated it, the true expression of the fact is, that each mode of force is capable of producing the others, and thad none of them can be produced but by some other as an anterior force, then any view which regards either of them as abstractedly the efficient cause of all the rest, is erroneous; the view has, I believe, arisen from a confusion between the abstract or generalised meaning of the term cause, and its concrete or special sense; the word itself being indiscriminately used in both these senses. Another confusion of termns has arisen, and has, indeed, 86 CORRELATION OF PHYSICOAL FORCES. much embarrassed me in enunciating the propositions put forth in these pages, on account of the imperfection of scientific language; an imperfection in great measure unavoidable, it is true, but not the less embarrassing. Thus, the words light, heat, electricity, and magnetism, are constantly used i-n two senses-viz. that of the force producing, or the subjective idea of force or power, and of the effect produced, or the objective phenomenon. The word motion, indeed, is only applied to the effect, and not to the force, and the term chelnical affinity is generally applied to the force, and not to the effect; but the other four terms are, -for want of a distinct terminology, applied indiscriminately to both. II may have occasionally used the same word at one time in a sLbjective, at another in an objective sense; all I can say is, that this cannot be avoided without a neology, which I have not the presumlption to introduce, or the authority to enforce. Again, the use of the term forces in the plural mighlt be objected to by those wiho do not attach to the term force the notion of a specific agency, but of one universal power associated with matter, of which its various phenomena are but diversely modified effects. Whether the imponderable agents, viewed as force, and not as matter, ought to be regarded as distinct forces or as distinct modes of force, is probably not very material, for, as far as I am aTware, the same result would follow either view; I have therefore used the terms indiscriminately, as either happened to be the more expressive for the occasion. Throughout this essay I have placed motion in the same category as the other affections of matter. The course of reasoning adopted in it, however, appears to me to lead inevitably to the conclusion that these affections of matter are themselves modes of motion; that, as in the case of friction, the gross or palpable motion, which is arrested by the contact of another body, is subdivided into molecular motions or vibrations, which vibrations are heat or electricity, as the CONCLUDING REMARKS. 187 case may be; so the other affections are only matter moved or molecularly agitated in certain definite directions. We have already considered the hypothesis that the passage of electricity and magnetism causes vibrations in an ether perrmeating the bodies throngh which the current is transmitted, or the application of the same ethereal hypothesis to these imponderables which had previously been applied to light; many, in speaking of some of the effects, admit that electricity and magnetism cause or produce by their passage vibrations in the particles of matter, but regard the vibrations produced as an occasional, though not always a necessary, effect of the passage of electricity, or of the increment or decrement, of magnetism. Trhe view which I have taken is, that such vibrations, molecular polarisations, or motions of some sort onfom particle to particle, are themselves electricity or magnetism; or, to express it in the converse, that dynamic electricity and magnetism are themselves motion, and that permanent magnetism, and Franklinic electricity, are static conditions of force bearing a similar relation to motion which tension or g ravitation do. This theory might well be discussed in greater detail than has been used in this work; but to do this and to anticipate objections would lead into specialities foreign to my present object, in the course of this essay my principal aim having been rather to show the relation of forces as evinced by acknowledged facts, than to enter upon any detailed explanation of their specific modes of action. Probably man will never know the ultimate structure of matter or the minutim of molecular actions; indeed it is scarcely conceivable that the mind can ever attain to this knowledge; the monad irresolvable by a given microscope may be resolved by an increase in power~. Much harm has already been done by attempting hypothetically to dissect matter and to discuss the shapes, sizes, and numbers of atoms, and their atmospheres of heat, ether, or electricity. 188 COIRRELATION OF PHYSICAL FOROES. Whether the regarding electricity, ligalit, magnetism, &ce. as simply motions of ordinary matter, be or be not admissible, certain it is, that all past theories have resolved, and all existing theories do resolve, the actions of these forces into motion. Whether it be that, on account of our familiarity with motion1, we refer other affections to it, as to a language Tvwhich is most easily construed and most capable of explainu ing them; whether it be that it is in reality the only mode in which our minds, as contradistingnished from our senses, are able to conceive material agencies certain;it is, that since the period at which the imystic notions of spiritual or preternatural powers were applied to account for physical phenomena, all hypotheses framed to explain them have resolved them into motion. Take, f`r example, thle theoies of light to which I have before alluded: one of these supposes light to be a highly rare matter, emitted from-i. e. put in ~motionby-luminous bodies; a second supposes that the matter is not emitted from luminous bodies, but that it is put into a state of vibration or undulation, i. e. motion, by them; and thirdly, light may be regarded as an undulation or motion of ordinary matter, and propagated by undulation of air, glass, &c., as I have before stated. In all these hypotheses, matter and motion are the only conceptions. lTor, if we accept terms derived from our own sensations, the which sensations themselves may be but modes of motion in the nervous fla-. ments, can we find words to describe phenomena other than those expressive of matter and motion. We in vain struggle to escape from these ideas; if we ever do so, our mental powers must undergo a change of which at present we see no prospect. If we apply to any other force the mode of reasoning which we have applied to heat, we shall arrive at the same conclusion, and see that a given source of power can, supposing it to be fully utilised in each case, yield no more by employing it as an exciter of one force than of another. Let CONCLUDING REmTIrbS. 1589 us take electricity as an e'xample. Suppose a pound of mercury at 400~ be employed to produce a thermo-eleetric cur rent, and the latter be in its turn employed to produce me, chanical force; if this a-tter force be greater than that which the direct effect of heat would produce, then it could by coim pression raise the temperature of the mercury itself, or of a simnilar quantity equally heated, to a higher point than it5s original temperature, the 4000 to 401~, for exarmple, which is obviously impossible; nor, if we admit forde to be indestructible, can it produce less than 400~, or cool the second body except by some portion of it being converted into another form or mode of force. But as' the mechanical effect here is produced through the mediumr of electricity, and the mechanical effect is definite, so the quantity of electricity producing it must be definite also, for unequal quantities of electricity could only produce an equal mechanical effect by a loss or gain of their own force into or out of nothing. The same reasoning will apply to the other forces, and will lead, it appears to me, necessarily and inevitably to the conclusion, that each force is definitely and equivalently convertible into any. other, and that where experiment does not give the full equivalent, it is because the initial force has been dissipated, not lost, by conversion into other unrecognised forces. The equivalent is the limit never practically reached. The great problem which remains to be solved, in regard to the correlation of physical forces, is this establishment of their equivalents of power, or their measurable relation to a given standard. The progress made in some of the branches.of this inquiry has been already noticed.'Viewed in their static relations, or in the conditions requisite for producing equilibriu-mL or quantitative equality of force, a remarkable relation between chemical affinity and heat is that discovered in many simple bodies by Dulong and Petit, and extended to compounds by Neumann and Avogadro. Their researches 190 CORRELATION OF PHYSICAL FORCES. have shown that the specific heats of certain substances, when mulitiplied by their chemical equivalents, give a constant quantity as product-or, in other words, that the combining weights of such substances are those weights which require equal accessions or abstractions of heat, equally to raise or lower their temperature. To put the proposition more in accordance with the view we have taken of the nature of heat: each body has a power of communicating or receiving molecular repulsive power, exactly equal, weight for weight, to its chemical or combining power. ]For instance, the equivalent of lead is 104, of zinc 33, or, in round numbers, as 3 to 1: these numbers are therefore inversely the exponents of their chemical power, three times as much leal as zinc being required to saturate the same quantity of an acid or substance combining with it; but their power of communicating or abstracting heat or repulsive power is precisely the same, for three times as much lead as zinc is required to produce the same amount of expansion or contraction in a given quantity of a third substance, such as water. A2gain, a great number of bodies chemically combine in equal volumes, i. e. in the ratios of their specific gravities; but the specific gravities represent the attractive powers of the substance, or are the numerical exponents of the forces tending to produce motion in masses of matter towards each other; while the chemical equivalents are the exponents of the affinities or tendencies of the molecules of dissim-ilar substances to combine, and saturate each other; consequently, here we have to some extent an equivalent relation between these two modes of force-gravitation and chemical attractison. ]W/ere the above relations extended into an 1niversal law, we should have the same numerical expression for the three forces of heat, gravity, and affinity; and as electricity and magnetism are quantitatively related to them, we should have CONCLUDING REMARKS. 191 a similar expression for these forces: but at present the bodies in which this parity of force has been discovered, though in themselves nunmerous, are small compared with the excep. tions, and, therefore, this point can only be indicated as promising a generalisation, should subsequent researches alter our knowledge as to the elements and combining equivalents of mratter. With regard to what may be called dynamic equivalents, i. e. the definite relation to time of the action of these varied forces upon equivalents of matter, the difficulty of establishing them is still greater. If the proposition wsvhich ] stated at the commencement of this paper be correct, that motion may be subdivided or changed in character, so as to become heat, electricity, &c., it ought to follow that when we collect the dissipated and changed forces, and reconvert them, the initial motionl minus an infinitesimal quantity affecting the same amount of matter with the same velocity, should be reproduced, and so of the changes in matter produced by the other forces; but the difficulties of proving thLe truth of this by experiment will, in many cases, be all but insuperable; we cannot imprison motion as we can matter, thougth we may to some extent restrain its direction. The term perpetual motion, which I have not unfrequently employed in these pages, is itself equivocal. If the doctrines here advanced be founded, all motion is, in one sense, perpetuali. In masses whose motion is stopped by mutual concussion, heat or motion of the particles is generated; and thus the motion continues, so thatif we could venture to extend such thoughts to the universe, we should assume the sa me amount of motion affecting the same amount of matter for ever. Where force opposes force, as in cases of static equilibriumr the balance of pre-existing equilibrium is afcected, and fresh motion is started equivalent to that which is withdrawn into a state of abeyance. But the term perpetual motion is applied, in ordinary par 192 CORRELATION OF PHYSICAL FORCES. lance (and in such sense I have used it), to a perpetual recurrent motion, e.g. a weight which by its fall would turn a wheel, which wheel would, in its turn, raise, the ilnitial weight, and so on forever, or until the material of which the machine is made be worn out. It is strange that to cor-nmmon apprehension the impossibility of this is not self-evident: if the initial weight is to be raised by the force it has itself generated, it must necessarily generate a force greater than that of its own weight or centripetal attraction; in other words, it must be capable of raising a weight heavier than itself: so that, setting aside the resistance of friction, -c., a weig'ht, to produce perpetual recarrent motion, must be heavier than an equal weight of matter, in short, heavier than itself. Suppose two equal weights at each end of an equi-armed lever, there is no motion; cut off a fraction of one of them, and it rises while the other falls. How, now, is the lesser weight to bring back the greater without any extraneous application of force? If, as is obvious, it cannot do so in this simple form of experiment, it is c~ fortiori more impossible if machinery be added, for increased resistances have then to be overcome. Can we again mend this by employing any other force? Suppose we employ electricity, the initial weight in descending turns a cylinder against a cushion, and so generates electricity; to make this force recurrent, the electricity so generated must, in its turn, raise the initial weight, or one heavier than it, i. e. the initial weight must, tlhrough the mediun o-f elect- icity, raise a weight heavier than itself. The same problem, applied to any other forces, will involve the same absurdlty: and yet simple as the matter seems, the world is hardly yet disabused of an idea little removed from superstition. ]But the importance of the deductions to be derived from the negation of perpepetual motion seems scarcely to have impressed philosophers, and we only finld here and there a scattered hint of the consequences necessarily resulting from that CONCLUDING REMARKS. 193 which to the thinking mind is a conviction. Some of these I have ventured to put forward in the present essay, but many remain, and will crowd upon the mind of those who pursue the subject. Does not, for instance, the impossibility of perpetual motion, when thought out, involve the demonstration of the impossibility, to which I have previously alluded, of any event identically recurring? The pendulum in vacuo, at each beat leaves a portion of' the force which started it in the form of heat at its point of suspension: this force, though ever existent, can never be restored in its integrity to the ball of the pendulum, for in the process of restoration it must affect other matter, and alter the condition of the universe. To restore the initial force to its integrity, everything as it existed at the moment of the first beat of the pendulum must be restored in its integrity: but how can this be-for while the force was escaping from the pendulum by radiating heat from the point of suspension, surrounding matter has not stood still; the very attraction which caused the beat of the pendulum has changed in degree, for the pendulum is nearer to or further from the sun, or from some planet or fixed star. It might be an interesting and not profitless speculation to follow out these and other consequences; it would, I believe, lead us to the conviction that the universe is ever changing, and that notwithstanding secular recurrences which would prima facie seem to replace matter in its original position, nothing in fact ever returns or can return to a state of existence identical with a previous state. But the field is too illimitable for me to venture further. The inevitable dissipation or throwing off a portion of the initial force presents a great experimental difficulty in the way of establishing the equivalents of the various natural forces. In the steam-engine, for instance, the heat of the furnace not only expands the water and thereby produces the motion of the piston, but it also expands the iron of the boil9 194 COIREL.ATION OF PHYSICAL FORCES. er, of the cylinder and all surrounding bodies. The force ex. pended in expanding this iron to a very small extent is equal to that which expands the vapour to a very large extent: this expansion of the iron is capable, in its turn, of producing a great mechanical force, which is practically lost. Could all the force be applied to the vapour, an enormous addition of power would be gained for the same expenditure: and perhaps even with our present means more might be done in' utilising the expansion of the iron. Another great difficulty in experimentally ascertaining the dynamic equivalents of different forces arises from the effects of disruption, or the overcoming an existing force. Thus, when a part of the initial force employed is engaged in twisting or tearing asunder matter previously held together by cohesive attraction, or in overcoming gravitation or inertia, the same amount of heat or electricity would not be evolved as if such obstacle were non-existent, and the initial force were wholly employed in producing, not in opposing. There is a difficulty apparently extreme in devising experiments in which some portion of the force is not so employed. The initial force, however, that has been employed for such disruption is not lost, as at the moment of disruption the bodies producing it fly off, and carry with them their force. Thus, let two weights be attached to a cord placed across a bar; when their force is sufficient to break the cord or -the bar, the weights fall down and strike the earth, making it vibrate, and so conveying away or continuing the force expressed by the cohesion of the bar or cord. If, instead of breaking a cord, the weights be employed to bend a bar, their gravitating force, instead of making the earth vibrate, produces heat in the bar, and so with whatever other force be employed to produce effects of disruption, torsion, &c., so that, though difficult in practice, the numerical problem of the equivalent of the force is not theoretically irresolvable The voltaic battery affords us the best means of ascertain CONCLUDING REMAnKS. 195 ing the dynamic equivalents of different forces, and it is probable that by its aid the best theoretical and practical results will be ultimately attained. In investigating the relation of the different forces, I have in turn taken each one as the initial force or starting-point, and endeavoured to show how the force thus arbitrarily selected could mediately or immediately produce and be merged into the others: but it will be obvious to those who have attentively considered the subject, and brought their -minds into a general accordance with the views I have submitted to them, that no force can, strictly speaking, be initial, as there must be some anterior force which produced it: we cannot create force or motion any more than we can create matter. Thus, to take an example previously noticed, and recede backwards; the spark of light is produced by electricity, electricity by motion, and motion is produced by something else, say a steam-engine-that is, by heat. This heat is produced by chemical affinity, i.e. the affinity of the carbon of the coal for the oxygen of the air: this carbon and this oxygen have been previously eliminated by actions difficult to trace, but of the pre-existence of which we cannot doubt, and in which actions we should find the conjoint and alternating effects of heat, light, chemical affinity, &c. Thus, tracing any force backwards to its antecedents, we are merged in an infinity of changing forms of force; at some point we lose it, not because it has been in fact created at any definite point, but because it resolves itself into so many contributing forces, that the evidence of it is lost to our senses or powers of detection; just as in following it forward into the effect it produces, it becomes, as I have before stated, so subdivided and dissipated as to be equally lost to our means of detection. Can we, indeed, suggest a proposition, definitely conceivable by the mind, of force without antecedent force? I cannot, without calling for the interposition of created power, any more than I can conceive the sudden appearance of a 196 CORRELATION OF PHYSICAL FORCES. mass of matter come from nowhere, and formed from nothing. The impossibility, humanly speaking, of creating or annihilating matter, has long been admitted, though, perhaps, its distinct reception in philosophy may be set down to the overthrow of the doctrine of Phlogiston, and the reformation of chemistry at the time of Lavoisier. The reasons for the admission of a similar doctrine as to force appear to be equally strong. With regard to matter, there are many cases in which we never practically prove its cessation of existence, yet we do not the less believe in it: who, for instance, can trace, so as to re-weigh, the particles of iron worn off the tire of-a carriage wheel? who can re-combine the particles of wax dissipated and chemically changed in the burning of a candle? By placing matter undergoing physical or chemical changes under special limiting circumstances, we may, indeed, acquire evidence of its continued existence, weight for weight-and so we may in some instances of force, as in definite electrolysis: indeed the evidence we acquire of the continued existence of matter is by the continued exertion of the force it exercises, as, when we weigh it, our evidence is the force of attraction; so, again, ou r evidence of force is the matter it acts upon. Thus, matter and force are correlates, in the strictest sense of the word; the conception of the existence of the one involves the conception of the existence of the other: the quantity of matter again, and the degree of force, involve conceptions of space and time. [But to follow out these abstract relations would lead me too far into the alluring paths of metaphysical speculation. That tlie theoretical portions of this essay are open to objection I am fully conscious. I cannot, however, but think that the fair way to test a theory is to compare it with other theories, and to see whether upon the whole the balance of probability is in its favour. Were a theory open to no objection it would cease to be a theory, and bec(sme a law; and were we not to theorise, or to take generalised views of CONCLUDING REMAKRS. 197 natural phenomena until those generalizations were sure and unobjectionable-in other words, were laws science would be lost in a complex mass of unconnected observations, which would probably never disentangle themselves. Excess on either side is to be avoided; although we may often err on the side of hasty generalisation, we may equally err on the side of mere elaborate collection of observations, which, though sometimes leading to a valuable result, yet, when cumulated without a connecting link, frequently occasion a costly waste of time, and leave the subject to which they refer in greater obscurity than that in which it was involved at their commencement. Collections of facts differ in importance, as do theories: the former, in many instances, derive their value from their capability of generalisation; while, conversely, theories are valuable as methods of co-ordinating given series of facts, and more valuable in proportion as they require fewer exceptions and fewer postulates. Facts may sometimes be as well explained by one view as by another, but without a theory they are unintelligible and incommunicable. Let us use our utmost effort to communicate a fact without using the language of theory, and we fail; theory is involved in all our expressions; the knowledge of bygone times is imported into succeeding times by terms involving theoretic conceptions. As the knowledge of any particular science developes itself our views of it become more simple; hypotheses, or the introduction of supposititious views, are more and more dispensed with; words become applicable more directly to the phenomena, and, losing the hypothetic meaning which they necessarily possessed at their reception, acquire a secondary sense, which brings more immediately to our minds the facts of which they are indices. The scaffolding has served its purpose. The hypothesis fades away, and a theory, or generalised view of phenomena, more independent of supposition, but still full of gaps and difficulties, takes its place. This in 198 CORRELATION OF PHYSICAL FORCES. its turn, should the science continue to progress, either gives place to a more simple and wider generalisation, or becomes, bythe removal of objections, established as a law. Even in this more advanced stage, words importing theory must be used, but phenomena are now intelligible and connected, though expressed by varied forms of speech. To think on nature is to theorise; and difflcult it is not 01o be led on by the continuities of natural phenomena to theories which appear forced and unintelligible to those who have not pursued the same path of thought: which, moreover, if allowed to gain an undue influence, seduce us from that truth which is the sole object of our pursuit. Where to draw the line-where to say thus far we may go, and no farther, in any particular class of analogies or relations which Nature presents to us; how far to follow the progressive indications of thought, and where to resist its allurements-is a question of degree which must depend upon the judgment of each individual or of each class of thinkers; yet it is consolatory that thought is seldom expended in vain. I have throughout endeavoured to discard the hypotheses of subtle or occult entities; if in this endeavour some of my views have been adopted upon insufficient data, I still hope that this essay will not prove valueless. The conviction that the so-called imnponderables are modes of motion, will, at all events, lead the observer of natural phenomena to look for changes in these affections, wherever the intimate structure of matter is changed; and, conversely, to seek for changes in matter, either temporary or permanent, whenever it is affected by these forces. I believe he will seldom do this in vain. It was not until I had long reflected on the subject, that I ventured to publish my views: their publication may induce others to think on their subject-matter. They are not put forward with the same objects, nor do they aim at the same elaboration of detail, as memoirs on newly-discovered physical facts: they purport to be a method CONCLUDING RlEMARKS. 199 of mentally regarding known facts, some few of which I have myself made known on other occasions, but the great mass of which have been accumulated by the labours of others, and are admitted as established truths. Every one has a right to view these facts through any medium he thinks fit to employ, but some theory must exist in the minds of those who reflect upon the many new phenomena'which have recently' and more particularly during the present century, been discovered. It is by a generalised or connected view of past acquisitions in natural knowledge that deductions can best be drawn as to the probable character of the results to be anticipated. It is a great assistance in such investigations to be intimately convinced that no physical phenomena can stand alone: each is inevitably connected with anterior changes, and as inevitably productive of consequential changes, each with the other, and all with time and space; and, either in tracing back these antecedents or following up their consequents, many new phenomena will be discovered, and many existing phenomena, hitherto believed distinct, will be connected and explained: explanation is, indeed, only relation to something- more familiar, not more known-i.e. known as to causative or creative agencies. In all phenomena the more closely they are investigated the more are we convinced that, humanly speaking, neither matter nor force can be created or annihilated, and that an essential cause is unattainable. —Causation is the will, Creation the act, of God, NOTES AND REFERENCES. PAGE 13. THE reader who is curious as to the views of the ancients, regarding the objects of science, will find clues to them in the second book of ARISTOTLE'S Physics, and in the first three books of the Metaphysics. See also the Timnus of PLATO, and RITTER'S History of Ancient Philosophy, where a sketch of the Philosophy of LEUCIPPUs and DiEOCRITUS will be found. 14. BACONN's Novum Organum, book ii. aph. 5 and 6. 16 iurMaE' Enquiry concerning Human Understanding, S. 7, London, 1768. BRowN's Enquiry into the Relations of Cause and Efect, London, 1835. The illustration I have used of floodgate has been objected to, as being one to which the term cause would scarcely be applied, but after some consideration I have retained it: if cause be viewed only as sequence, it must be limited to sequence under given conditions or circumstances, and here, given the conditions, the sequence is invariable. I see no difference quoad the argument, between this illustration and that of BROWN of a lighted match and gunpowder (4th edit. p. 27), to which my reasoning would equally well apply. HEascHEL's Discourse on the Study of Natural Philosophy, pp. 88 and 149.'17. Quarterly Review, vol. lxviii. p. 212. WHEWELL, On the Question'Are Cause and Effect Successive or Simultaneous? (Cambridge Philosophical Transactions, vol. vii. p. 319.) 18. HERSCHEL's Discourse, p. 93. AMPERE, Theorie des Phenombnes Electro-dynamiques, Memoirs in NOTES AND REFERENCES. 201 PAGE the Ann. de Chimie et de Physique, and works from 1820 to 1826 Paris. 23. LAMARCK,' Sur la Matiere du Son' (Journal de Physique, vol. xlix. p. 397). 25. D'ALEMBERT, Trait' de Dynamique, pp. 3 and 4, Paris, 1796. 28. BABBAGE, On the Permanent Impression of our Words and Actions on the Globe we inhabit, 9th Bridgewater Treatise, ch. ix. 30. MAYER, Annalenl der Pharmacie Leibig und Wohler, May 1852. 33. JOULE On the Mechanical Equivalent of Heat (Phil. Trans. 1850, p. 61.) 33. ERMAN, Influence of Friction upon Thermo-electricity (Reports of the British Association, 1845.) 35. BECQUERELa IEgagement de l'Electricite par Frottement, Traite de l'Electricite, tom. ii. p. 113 et seq. 36. SULLIVAN, Currents produced by the vibration of metals (Archiv. de l'Electricite, t. 10, p. 480). LEnoux, Vibrations arrested produce heat (Cosmos, March 30, 1860). 3'7. WHEATSTONE On the Prismatic Decomposition of Electrical Light (Notices of Communications to the British Association, p. 11, 1835). 39. BACoN, De FormA Calidi, Nov. Org. book 2, aph. 20. RUMFORD, An Enquiry concerning the Source of Heat which is excited by Friction (Phil. Trans. p. 80, 1798). DAvY, On the Conversion of Ice into Water by Friction (West of England Contributions, p. 16). Of Heat or Calorific Repulsion (Elements of Chemical Philosophy, p. 69). 41. BADEN POWELL On the Repulsive Power of Heat (Phil. Trans. 1834, p. 485). FRESNEL, Annales de Chimie, tom. xxix. pp. 57 and 107. 42. MosEI on Invisible Light (Taylor's Scientific Memoirs, vol. iii. pp. 461 and 465). 43. BLACK on Latent Heat (Elements of Chemistry, p. 144 et passim, 1803). 45. The experiments of HENRY and DONNY have shown that the cohesion of liquids, as far as their antagonism to rupture goes, is much greater than has been generally believed. These experiments, however, make no difference in the view I have put forth, as, whatever be the character of the attraction, there is' a molecular attraction to be overcome in changing bodies from the solid to the liquid state, which must require and exhaust force. 202 NOTES AND REFERENCOES. PAGE DONNY, Sur la Cohesion des Liquides (MIemoires de l'Acad6mie Roy. ale de Bruxelles, 1843). HENRY, Proceedings of the American Philosophical Society, April 1844 (Silliman's Journal, vol. xlviiii. p. 215). 48. THILORIER, Solidification de l'Acide carbonique (Ann. de Ch. et de Phys. tom. Ix. p. 432). 50. I. WEDGWOOD, Thermometer for measuring the Higher Degrees of Heat (Phil. Trans. 1782, p. 305; and 1785, p. 390. TYNDALL, on the physical properties of Ice (Phil. Trans. 1858, p. 211). DESPRETZ, Recherches sur le Maximum de Densite de l'Eau pure et des Dissolutions aqueuses (Ann. de. Ch. et de Ph. tom. lxx. p. 45, and tom. lxxiii. p. 295). 51. BioT (Comptes rendus de l'Acad6mie des Sciences, Paris 1850, p. 281). The experiments on circular polarisation by water were, I believe, by Dr. Leeson. 52. I. THOMPSON, Trans. R. S. Edin. vol. xvi. p. 575. W. THOMPSON, Phil. Mag. August 1850, p. 123. BUNSEN, Pogg. Ann. vol. lxxxi. p. 562; Ann. de Ch. et de Phys. vol. xxxv. p. 383. Effects of Pressure on the Freezing Point. 53. JOULE, Phil. Trans. 1852, p. 99. Although, taking the phenomena as they are known to exist, the mechanical laws may be deduced, yet in any physical conception of the nature of heat the expansion by cold has always been a great stumbling-block to me, and I believe to many others. DULONG and PETIT, and REGNAULT. See their Memoirs abstracted and referred to in Gmelin's Handbook of Chemistry, translated by Watts for the Cavendish Society, vol. i. p. 242 et seq. 54. WooD, Phil. lag. 1851, 1852. 56. SENARMONT, Conduction of Heat by Crystals (Gmelin's Handbook vol. i. p. 222). 56. KNOBLAUCH, Ann. de Ch. et de Ph. vol. xxxvi. p. 124. TYNDALL, Transmission of Heat through Organic Structures (Phil. Trans. vol. cxliii. p. 217). 68. GROVE, Electricity produced by approximating Metals: Report of a Lecture at the London Institution (Literary Gazette, 1843, p. 39). GxssIOT, Phil. Mag. October 1844. ROGET, On the Improbability of the Contact exciting Force: Treatise on Galvanism (Library of Useful Knowledge, S. 113). FARADAY, Phil. Trans. 1840, p. 126. NOTES AND REFERENCES. 203 PAGE 60. MELLONI, Sur la Polarisation de la Chaleur: Recherches sur plusieurs Phenomenes calorifiques (Annales de Chimie et de Ph. tom. xlv. pp. 5-68; tom. xli. pp. 375-410; tom. xlviii. pp. 198, 218). FORBES, On the Refraction and Polarisation of Heat (Transactions of the Royal Society of Edinburgh, vol. xiii. pp. 131, 168). 61. KIRCHOFF Trans. Belin Acad. 1861. BALFOUR STEWART on the theory of Exchanges (Report British Association, 1861). 63. T. WEDGWOOD, On the Production of Light and Heat by different Bodies (Phil. Trans. vol. lxxxii. p. 272). 65. GROVE, On the Decomposition of Water into its Constituent Gases by Heat (Phil. Trans. 1847, p. 1). ROBINSON, On the Effect of Heat in lessening the Affinities of the Elements of Water (Transactions of the Royal Irish Academy, vol xxi. p. 2). 67. GROVE, Water decomposed by Chlorine and Heat (Phil. Trans. 1847, p. 20). 70. CARNOT, Re6fexions sur la Puissance motrice du Feu, Paris, 1824. 76. SEGUIN, Influence des Chemins de Fer, p. 378 et seq. 77. ROGERS, Consumption of Coal for Man power (Cosmos, vol. ii. p. 56). 80. Mr. WATERSTON has suggested that solar heat may arise from the mechanical action of meteoric stones falling into the sun, and Mr. THoNPsoN has written an elaborate paper on the subject (Trans. Brit. Assoc. 1853). If a number of gravitating bodies exist in the neighbourhood of the sun, and form, as is conjectured, the zodiacal light, it is difficult to conceive how comets as they approach this region steer clear of such bodies, and are not even deflected from their orbits. For Mr. THOMPSON'S various and valuable papers, see Phil. Mag. 1851 to 1854 inclusive. 81. POISSON, Comptes rendus, Paris, January 30, 1837. 83. DUFAYE, SYMMER, WATSON, and FRANKLIN, Theories of Electric Fluid and Electric Fluids (Priestley's History of Electricity, pp. 429441). 83. GROTTHUS, Sur la Decomposition de l'Eau et des Corps qu'elle tient en dissolution A aide de l'Electricit6 galvanique (Ann. de Chimie, tom. lviii. p. 54). FARADAY, On the Question whether Electrolytes conduct without Decomposition (Proceedings of the Weekly Meetings of the Royal Institution, 1855). GROVE (Comptes rendus, Paris, 1839). 204 i NOTEB AlND REFERENCESO PAGE 84. FARtADAY, On Induction as an Action of contiguous Particles (Phil. Trans. 1838, p. 30). 85. MATTEUCCI, Plates of Mica polarised by Electricity (De la Rive's Electricity, p. 140). GROVE, Electrolysis across Glass (Phil. Mag. Aug. 1860). 85. ]KARSTEN on Electrical Figures (Archiv. de l'Elec. vols. ii. iii. and iv). 87. GRovE, Etching Electrical Figures and transferring them to Collodion (Phil. Mag. January 1857). 88. FUSINIERI, Du Transport des Matibres ponderable qui s'opbre dans les D6charges 61ectriques (Archives de 1'Electricite; Suppl6ment Ah la Bibliotheque universelle de Geneve, tom. iii. p. 597). 88. GRovE, On the Voltaic Arc (Report of Lecture at the Royal Institution, Lit. Gaz. and Athenaeum, Feb. 7, 1845; Phil. Trans. 1847, p. 16). 90 to 94. GROVE, On the Electro-chemical Polarity of Gases (Phil. Trans. 1852, p. 87). 94. FnEMY and E. BECQUEREL, Oxygen changed to Ozone by the Electric Spark (Ann. de Ch. et de Phys. 1852). This subject and the nature of Ozone was first investigated by Dr. Schbnbein. See also a paper by Mr. Brodie On the Conditions of certain Elements at the Moment of Chemical Change (Phil. Trans. 1850). 95, 96. Molecular Changes in Electrised Metals (N1IRNE, Phil. Trans. 1780, p. 334, and 1793, p. 223; GROVE, Electrical M]ag. vol. i. p. 120; PELTIER, Archives de 1'Electricit6, vol. v. p. 182; FusINIERI, id. p. 516). 96. WERTHEIM, Change in Elasticity of Metals by Electrisation (Ann. de Ch. et de Phys. vol. xii. p. 623; Arch. Elec. vol. iv. p. 490). DUFOUR, Alteration in Tenacity of Metals by Electrisation (Bibl. univ. de Geneve, Fev. 1855, p. 156). 97. MATTErccI, Conduction of Electricity by Crystals (Comptes rendus de i'Acad., Paris, March, 5, 1855, p. 541). 98. E. BECQUEREL, Transmission of Electricity by heated Gases (Ann. de. Ch. et de Phys. vol. xxxix. p. 355). GROVE, Proceedings of the Royal Inst. (1854, p. 361). BECQUEREL, Divergence of Gold-Leaves in Vacuo (Traite d'Electricit6, vol. v.; part ii. p. 53). NEWTON, Thirty-first Query to the Optics. 99. GROVE, Particles of Metals and Metallic Oxids detached in Liquids by Electricity (Elec. Mag. vol. i. p. 119). 100. MATTEUCCI, Relations of Electricity and Nervous Force (Phil. Trans. NOTES AND REFERENCES. 205 PAGE 1845, p. 285, 1846, p. 497; Phenomenes physiques des Corpo vivants, p. 305; Lezioni di Fisica, p. 360). GALVANI VOLTA MARIANINI et NOBILI on Physiological Effects of Electricity (Ann. de Ch. et de Phys. vols. 23, 25, 29, 38, 40, 43, 44, 56). 102. BECQUEREL, Chemical Changes by Friction (Traite de l'Elec. vol. v. part 1, p. 16). 106. DE LA RIVE, Heat of the Voltaic Pile (Bibl. univ., vol. xiii. p. 389). DAVY, On the Properties of Electrified Bodies in their relations to Conducting Powers and Temperature (Phil. Trans. 1821, p. 428). 106. GROVE, On the Effects of surrounding Media on Voltaic Ignition (Phil. Trans. 1849, p. 49). 107. OERSTED, Exp6rience sur l'Effet du Conflict 61ectrique sur l'Aiguille aimantee (Ann. de Ch. et de Phys., tom. xiv. p. 417). 108. COLERIDGE, Table Talk, vol. i. p. 65. 109. LENZ and JACOBI, Pogg. Ann. vol. xvlii. p. 403; Bulletin de l'Acad. St. Petersburg, 1839; Harris, Magnetism, part 2, p. 63. DAvY, Decomposition of the fixed Alkalies (Phil. Trans. 1808, p. 1). BECQUEREL Des Composes electro-chimiques (Trait6 de l'Electricite, vol. iii. c. 13). CRossE, Transactions of the British Association, vol. v. p. 47; Proceedings of the Electrical Society, p. 320. 110. MALUS, Polarisation of Light by Reflection (Memloires d'Arcueil, tom. ii. p. 143). ARAGO, Circular Polarisation by Solids (Memoires de l'Institut, 1811). 111. BIOT, Circular Polarisation by Liquids (Memoires de l'Institut, 1817). 111. NIEPCE and DAGUERRE, Historique et Description des Proc6d6s du Daguerreotype, Paris, 1839. TALBOT, Photogenic Drawing and Calotype (Phil. Mag. March. 1839, and August 1841). 113. HERSCHEL, Chemical Action of the Solar Spectrum on various Substances (Phil. Trans. 1840, p. i. and 1842, p. 181). HUNT, Researches on Light, London, 1844. 116. GROVE, Other Forces produced by Light (Lit. Gaz. January 1844). 11'7. GROVE, Influence of Light on the Polarised Electrode (Phil. Mag. December 1858). SOMERVILLE (Mrs.), On the Magnetising Power of the more Refrangi. ble Solar Rays (Phil. Trans. 1862, p. 132). MORICHINI's experiments are given in Mrs. Somerville's paper. 118. HERSCHEL, On the Absorption of Light in Coloured Media viewed -206 NOTES AND REFERENCES. PAGE in connection with the Undulatory Theory (Phil. Mag. December 1863). SEEBECK, Heat of Coloured Rays (Brewster's Optics, p. 90). 118. KNOBLAUCrH (Ann. de Ch. vol. xxxvi. p. 124, and Pogg. Ann. there referred to). 119. HERSCHEL, Epipolised Light (Phil. Trans. vol. cxxxv. pp. 143, 147). STOKES, Change in Refrangibility of Light (Phil, Trans. vols. cxlii. cxliii.) 123. For the first enunciations of the Corpuscular and Undulatory Theories, see NEWTON'S Optics, HooxE's Micographia, and HUYGHENS' Tractatus de Lumine. See also BREWSTER'S Optics, p. 138. 124. YOUNG, Lectures edited by Kelland, p. 358, et seq.; Phil. Trans. 1800, p. 126; HERSCHEL, Encyc. Metro. art. Light, pp. 450 and 738; NEWTON'S Optics, p. 322; WnHEWELL'S Hist. Induc. Sc. vol. ii. p. 449; FOUCAULT, Comptes rendus, Paris, 1850, p. 65; HARRssON, Phil. fMag. November 1856; Camb. Phil. Trans. 126. SONDHAUSS, Refraction of Sound (Ann. de Ch. et de Phys. vol. xxxv. p. 505); DovrE, Polarisation of Sound (Cosmos, May 13, 1859). 132. PASTEUR, Rotation of Plane, of Polarised Light by Solutions of Hemihedral Crystals (Ann. de Ch.. et de Phys. vol. xxiv. p. 442). 134 to 135. WOLLASTON, Phil. Trans. 1822, p. 89; WHEWELL, Phil. of the Induct. Sc. vol. i. p. 419; WILSON, Trans. of the Roy. Soc. of Edin. vol. xvi. p. 79; Sir W. HERSCHEL, Phil. Trans. 1793, p. 201, and 1801, p. 300; MORGAN, Phil. Trans. vol. lxxv. p. 272; DAvw, Phil. Trans. 1822, p. 64; Elements of Chemical Philosophy, p. 97; GAssIOT, Phil. Trans. 1859, p. 157. 187. Diminishing Periods of Comets (Herschel's Outlines of Astronomy, p. 357). 140. Since writing the passage in the text, I find that STRUVE has been led, from his astronomical researches, to the conclusion that some light is lost in the interplanetary spaces. He gives as an approximation one per cent. as lost by the passage of light from a star of the first magnitude, assuming a mean or average distance (Etudes d'Astronomie Stellaire, 184"7). NEWTON, Thirtieth Query to the Optics. 142. FARADAY, Evolution of Electricity from Magnetism (Phil. Trans. 1832, p. 125). 144. FARADAY, Magnetic Condition of all Matter (Phil. Trans. 1846, p. 21; Phil. Mag. 1846, p. 249). BECqUEREL, Ann. de Ch. et de Ph. tom. xxxvi. p. 337; Comptes rendus, Paris, 1846, p. 147; and 1850, p. 201. NOTES AND REFERENCES. 20 7 PAGE 145. FARADAY, On the Magnetism of Light (Phil. Trans. 18346, p. 1). 145. WARTMANN, Rotation of the Plane of Polarisation of Heat by Magnetism (Journal de l'Institute, No. 644). PROVOSTAYE and DESSAINES, Ann. de Ch. et de Phys. October 1849. 146. HuNT, Influence of Magnetism on Molecular Arrangement (Phil. Mag 1846, vol. xxviii. p. 1; Memoirs of the Geological Society, vol. i p. 433). WARTMANN, Phil. iMag. 1847, vol. xxx. p. 263. 147. GROVE, Experiment on Molecular Motion of a Magnetic Substance (Electrical Mag. 1845, vol. i. p. 601). 14'7. On the direct Production of Heat by Magnetism (Proceedings of the Royal Society, 1849, p. 826). After this paper was communicated and ordered to be printed in the Philosophical Transactions, I found that I had been anticipated by Mr. VAN BREDA, who communicated, in 1845, a paper to the Institut on the subject: his paper appears in the Comptes rendus under an erroneous title, which accounts for its having been overlooked: he does not give thermometric measures of the heat he obtained, nor did he produce heating effects by a permanent steel magnet, or with other metals than iron. (Comptes rendus, October 27, 1845). See also an earlier experiment by Mr. JOULE (Phil. Mag. 1843), to which he called my attention after my paper was read. 148, 151. The Experiments on the effects of Magnetism on the Matter magnetised, are collected by Mr. DE LA RIVE in his recently-published Treatise on Electricity, vol. i. 153. DAVY, Electricity defined as Chemical affinity acting on Masses (Phil. Trans. 1826, p. 389). VOLTA, Electricity excited by the mere Contact of conducting Substances (Phil. Trans. 1800, p. 403). 154. GROVE, Gold-Leaf Experiment (Comptes rendus, Paris, 1839, p. 567). 155. GROVE, Voltaic Action of Sulphur, Phosphorus, and Hydrocarbons (Phil. Trans. 1845, p. 351). GROVE, New Voltaic Combination (Phil. Mag. vol. xiv. p. 388; vol. xv. p. 287). 155. GROVE, Electricity of Blowpipe Flame (Proceedings of the Royal Institution, February 1854), Phil. Mag. 157. DALTON, New System of Chemistry, London, 1810. 158. I have here and elsewhere used whole numbers, as sufficiently approxi. mate for the argument, but without intending to express any opim ion as to the law of PROUT. 158. FARADAY, Definite Electrolysis (Phil. Trans. 1834, p. 77). .208 NOTES AND REFERENCE S. PAGE 160. WVOOD, Heat disengaged in Chemical Combinations (Phil. Mag. 1862) 162. ANDREWS, Phil. Trans. 1844, p. 21. HEss, Poggendoff's Annalen, Bd. lii. p. 197. 163. FAvRE, Ann. de Ch. et de Phys. vols. 39, 40; Comptes rendus, Paris, vol. 45, p. 56, and vol. 46, p. 337. 169. CATALYSIS by Platinum (DOBEREIMER, Ann. de Ch. et de Phys. tom. xxiv. p. 93; DULONG and THENARD, Ann. de Ch. et de Phys. tom. xxiii. p. 440). 170. GROVE, Gas Voltaic Battery (Phil. Mag. February 1839, and December 1842; Phil. Trans. 1843, p. 91). 171. MOSOTTI, Forces which regulate the Internal Constitution of Bodies (Taylor's Scientific Memoirs, vol. i. p. 448). 172. PLUCKEPR, Repulsion of the Optic Axes of Crystals by the Poles of a Magnet (Taylor's Scientific Memoirs, vol. v. p. 353). Magnetic Action of Cyanite (Lit. Gaz. 1849, p. 431). 1'72. MATTEUCCI, Correlation of Electric Current and Nervous Force (Phil. Trans. 1850, p. 287). 173. CARPENTER, On the Mutual Relations of the Vital and Physical Forces (Phil. Trans. 1850, p. F751). 174. On Efort. See BROWN, Cause and Effect; HERSCHEL'S Discourse; and QUARTERLY REVIEW, June 1841. 175. HELMHOLTZ, Muller's Archives, 1845; I ATTEUCCI, Comptes rendus, Paris, 1856; BECLARD, Archives de Medicine, 1861. 189. DULONG and PETIT, Relation between Specific Heat and Chemical Equivalents (Ann. de Ch. et de Phys. tom. x. p. 395). 189. NEUMANN, Poggendorff's Annalen, Bd. xxiii. p. 1. AvOGADRO, Ann. de Ch. et de Phys. tom. Iv. p. 80. ON THE INTERACTION OF NATURAL FORCES. BY PROF. HI. L. F. HELMEOLTZ. TRANSLATED BY JOHN TYNDALL, F.R. S. HERMAN LUDWIG FERDINAND HELMHOLTZ was born at Pottsdam, August 31, 1821. He was first military physician, and afterwards assistant of the Astronomical Museum in Berlin (1848), and subsequently Professor Extraordinary of Physiology at the University of Kinigsberg (1849 to 1852). He cecame Professor of Physiology at the University of Bonn in 1855, and in 1858 accepted the physiological chair in the University of Heidelberg. The lecture wyhich follows was delivered at K6nigsberg in 1854. He is an eminent investigator, and an able promoter of the recent philosophy of forces; but of his life we have fewer particulars than of his accomplished translator. The ancestors of JOHN TYNDALL emigrated from England to the eastern or Saxon border of Ireland about the middle of the last century. He was born at the village of Leighlin Bridge in 1820, where he received his early education and acquired a taste for mathematics. In 1839 he left school and joined the Ordnance Survey as a civil assistant, where he became in turn draughtsman, computer, surveyor, and trigometrical observer. He was five years connected with the survey, and for three years occupied as railroad engineer. In 1847 he became teacher in Queenswood College in Hampshire, a school for agriculturists and engineers, where he was distinguished for his mild but efficient discipline. Professor Frankland, the chemist, was here joined with him in the work of instruction, and in 1848 the two friends left the institution and went to the University of Marburg in Hesse Cassel, to study with the eminent chemist, Bunsen. In 1851 Professor Tyndall went to Berlin and worked at the subject of diamagnetism in the laboratory of Professor Magnus. He returned to London the same year, and was elected Fellow of the Royal Society in 1852. Through the influence of Dr. Bence Jones, General Sabine, and Professor Faraday, he was appointed Professor of Natural Philosophy in the Royal Institution in 1853, an appointment which he now holds. In company with his friend, Professor Huxley, he visited the Alps in 1856; and returning each succeeding year, he accumulated the observations and adventures which are so graphically described in his "Glaciers of the Alps," published in 1860. Professor Tyndall has worked with eminent success at various scientific questions, but he is chiefly distinguished for his original and elaborate researches on the relations of radiant heat to gaseous and vaporous matter. These researches are given in his able work on " Heat as a mode of Motion," issued in 1863. As an experimenter, Professor Tyndall is marked for his caution, accuracy, and tireless perseverance under difficulties;' as a writer, for his clear, vivid, and vigorous style. INTERACTION OF NATURAL FORCES. /, NEW conquest of very general interest has been recently 17 made by natural philosophy. In the following pages, I will endeavour to give a notion of the nature of this conquest. It has reference to a new and universal natural law, which rules the action of natural forces in their mutual relations towards each other, and is as influential on our theoretic views of natural processes as it is important in their technical applications. Among the practical arts which owe their progress to the development of the natural sciences, from the conclusion of the middle ages downwards, practical mechanics, aided by the mathematical science which bears the same name, was one of the most prominent. The character of the art was, at the time referred to, naturally very different from its present one. Surprised and stimulated by its own success, it thought no problem beyond its power, and immediately attacked some of the most diffic-Ldt and complicated. Thus it was attempted to build automaton figures which should perform the functions of men and animals. The wonder of the last century was Vaucanson's duck, which fed and digested its food; the fluteplayer of the same artist, which moved all its fingers cor 212 INTERACTION OF NATURAL FOROES. rectly; the writing boy of the older, and the piano-forte player of the younger Droz: which latter, when performing, followed its hands with his eyes, and at the conclusion of the piece bowed courteously to the audience. That men like those mentioned, whose talent might bear comparison with the most inventive heads of the present age, should spend so much time in the construction of these figures, which we at present regard as the merest trifles, would be incomprehensible, if they had not hoped in solemn earnest to solve a great problem. The writing boy of the elder ]Droz was publicly exhibited in Germany some years ago. Its wheel-work is so complicated, that no ordinary head would be sufficient to decipher its manner of action. W~hen, however, we are informed that this boy and its constructor, being suspected of the black art, lay for a time in the Spanish Inquisition, and with difficulty obtained their freedom, we may infer that in those days even such a toy appeared great enough to excite doubts as to its natural origin. And though these artists may not have hoped to breathe into the creature of their ingenuity a soul gifted with moral completeness, still there were many who would be willing to dispense with the moral qualities of their servants, if, at the same time, their immoral qualities could also be got rid of; and accept, instead of the mutability of flesh and bones, services which should combine the regularity of a machine with the durability of brass and steel. The object, therefore, which the inventive genius of the past century placed before it with the fullest earnestness, and not as a piece of amusement merely, was boldly chosen, and was followed up with an expenditure of sagacity which has contributed not a little to enrich the mechanical experience which a later time knew how to take advantage of. iWe no longer seek to build machines which shall fulfil the thousand services required of one man, but desire, on the contrary, that a machine shall perform one service, but shall occupy in doing it the place of a thousand men. THE OLD MECHANICIAL PROBLEM. 213 From these efforts to imitate living creatures, another idea, also by a misunderstanding, seems to have developed itself, which, as it were, formed the new philosopher's stone of the seventeenth and eighteenth centuries. It was now the endeavour to construct a perpetual motion. Under this term was understood a machine, which, without being wound up, without consuming in the working of it, falling water, wvind, or any other natural force, should still continue in motion, the motive power being perpetually supplied by the machine itself. Beasts and human beings seemed to correspond to the idea of such an apparatus, for they moved themselves energetically and incessantly as long as they lived, were never wound up, and nobody set them in motion. A connection between the taking-in of nourishment and the development of force did not make itself apparent. The nourishment seemed only necessary to grease, as it were, the wheelwork of the animal machine, to replace what was used up, and to renew the old. The development of force out of itself seemed to be the essential peculiarity, the real quintessence of organic life. If, therefore, men were to be constructed, a perpetual motion must first be found. Another hope also seemed to take up incidentally the second place, which, in our wiser age, would certainly have claimed the first rank in the thoughts of men. The perpetual motion was to produce work inexhaustibly without corresponding consumption, that is to say, out of nothing. Work, however, is money. Here, therefore, the practical problem which the cunning heads of all centuries have followed in the most diverse ways, namely, to fabricate money out of nothing, invited solution. The similarity with the philosopher's stone sought by the ancient chemists was complete. That also was thought to contain the quintessence of organic life, and to be capable of producing gold. The spur which drove men to inquiry was sharp, and the talent of some of the seekers must not be estimated as small. The nature of the problem was quite calculated to entice por 2149 INTERACTION OF NATURAL FORCOES. ing brains, to lead them round a circle for years, deceiving ever with new expectations, which vanished upon nearer approach, and finally reducing these dupes of hope to open insanity. The phantom could not be grasped. It would be impossible to give a history of these efforts, as the clearer heads, among whom the elder Droz must be ranked, convinced themselves of the futility of their experiments, and were naturally not inclined to speak much about them. ]Bewildered intellects, however, proclaimed often enough that they had discovered the grand secret; and as the incorrectness of their proceedings was always speedily manifest, the matter fell into bad repute, and the opinion strengthened itself more and more that the problem was not capable of solution; one difficulty after another was brought under the dominion of imathematical mechanics, and finally a point was reached where it could be proved, that, at least by the use of pure mechanical forces, no perpetual motion could be generated. We have here arrived at the idea of the driving force or power of a machine, and shall have much to do with it in future. I must, therefore, give an explanation of it. The idea of work is evidently transferred to machines by comparing their arrangements with those of men and animals to replace which they were applied. We still reckon the work of steam engines according to horse-power. The value of manual labor is determined partly by the force which is expended in it (a strong laborer is valued more highly than a weak one), partly however,^ by the skill which is brought into action. A machine, on the contrary, which executes work skilfully, can always be multiplied to any extent; hence its skill has not the high value of human skill in domains where the latter cannot be supplied by machines. Thus the idea of the quantity of work in the case of machines has been limited to the consideration of the expenditure of force; this was the more important, as indeed most machines are constructed for the express purpose of exceeding, by the magnitude of their MEASUREMlENT OF MECHANICAL POWER. 215 effects, the powers of men and animals. Hence, in a mechanical sense, the idea of work is become identical with that of the expenditure of force, and in this way I will apply it. How, then, can we measure this expenditure, and compare it in the case of different machines? I must here conduct you a portion of the way-as short a portion as possible-over the uninviting field of mathematicomechanical ideas, in order to bring you to a point of view from which a more rewarding prospect will open. And though the example which I shall here choose, namely, that of a watermill with iron hammer, appears to be tolerably romantic, still, alas, I must leave tile dark forest valley, the spark-emitting anvil, and the black Cyclops wholly out of sight, and beg a moment's attention to the less poetic side of the question, namely, the machinery. This is driven by a water-wheel which in its turn is set in motion by the falling water. The axle of the water-wheel has at certain places small projections, thumbs, which, during the rotation, lift the heavy hammer and permit it to fall again. The falling hammer belabors the mass of metal, which is introduced beneath it. The work therefore done by the machine consists, in this case, in the lifting of the hammer, to do which the gravity of the latter must be overcome. The expenditure of force will, in the first place, other circumstances being equal, be proportioned to the weight of the hammer; it will, for example, be double when the weight of the hammer is doubled. But the action of the hammer depends not upon its weight alone, but also upon the height from which it falls. If it falls through two feet, it will produce a greater effect than if it falls through only one foot. It is, however, clear that if the machine, with a certain expenditure of force, lifts the hammer a foot in height, the same amount of force must be expended to raise it a second foot in height. The work is therefore not only doubled when the weight of the hammer is increased twofold, but also when the space through which it falls is doubled. From this it is easy 216 INTERACTION OFNATURAL FORCES. to see that the work must be measured by the product of the weight into the space through which it ascends. And in this way, indeed, do we measure in mechanics. The unit of work is a foot-pound, that is, a pound weight raised to the height of one foot. While the work in this case consists in the raising of the heavy hammer-head, the driving force which sets the latter in motion, is generated by falling water. It is not necessary that the water should fall vertically, it can also flow in a moderately inclined bed; but it niust always, where it has water-mills to set in motion, move from a higher to a lower position. Experiment and theory coincide in teaching, that when a hammer of a hundred weight is to be raised one foot, to accomplish this at least a hundred weight of water must fall through the space of one foot; or what is equivalent to this, two hundred weight must fall fullhalf a foot, or four hundred weight a quarter of a foot, etc. In short, if we multiply the weight of the falling water by the height through which it falls, and regard, as before, the product as the measure of the work, then the work performed by the machine in raising the hammer, can, in the most favourable case, be only equal to the number of foot-pounds of water which have fallen in the same time. In practice, indeed, this ratio is by no means attained; a great portion of the work of the falling water escapes unused, inasmuch as part of the force is willingly sacrificed for the sake of obtaining greater speed. I will further remark-, that this relation remains unchanged whether the hammer is driven immediately by the axle of the wheel, or whether-by the intervention of wheel-work, endless screws, pulleys, ropes-the motion is transferred to the hammer. We may, indeed, by such arrangements, succeed in raising a hammer of' ten hundred weight, when by the first simple arrangement, the elevation of a hammer of one hundred weight might alone be possible; but either this heavier hammer is raised to only one tenth of the height, or tenfold the TRUIE FUNCTION OF MACHINES. 21' time is required to raise it to the same height; so that, however we may alter, by the interposition of machinery, the intensity of the acting force, still in a certain time, during which the mill-stream furnishes us with a definite quantity of water, a certain definite quantity of work, and no more, can be performed. Our machinery, therefore, has, in the first place, done nothing more than make use of the gravity of the falling water in order to overpower the gravity of the hammer, and to raise the latter. When it has lifted the hammer to the necessary height, it again liberates it, and the hammer falls upon the metal mass which is pushed beneath it. But why does the falling hammer here exercise a greater force than when it is permitted simply to press with its own weight on the mass of metal? Why is its power greater as the height from which it falls is increased? We find, in fact, that the work performed by the hammer is determined by its velocity. In other cases, also, the velocity of moving masses is a means of producing great effects. I only remind you of the destructive effects of musket-bullets, which, in a state of rest, are the most harmless things in the world. I remind you of the wind-mill, which derives its force from the moving air. It may appear surprising that motion, which we are accustomed to regard as a non-essential and transitory endowment of bodies, can produce such great effects. But the fact is, that motion appears to us, under ordinary circumstances, transitory, because the movement of all terrestrial bodies is resisted perpetually by other forces, friction, resistance of the air, etc., so that motion is incessantly weakened and finally neutralized. A body, however, which is opposed by no resisting force, when once set in motion, moves onward eternally with undiminished velocity. Thus we know that the planetary bodies have moved without change, through space, for thousands of years. Only by resisting forces can motion be diminished or destroyed. A moving body, such as the hammer or the musket-ball, when 10 218 INTERACTION OF NATURAL FORCES. it strikes against another, presses the latter together, or penetrates it, until the sum of the resisting forces which the body struck presents to its pressure, or to the separation of its particles, is sufficiently great to destroy the motion of the hammer or of the bullet. The motion of a mass regarded as taking the place of working force is called the living force (vis viva) of the mass. The word " living" has of course here no reference whatever to living beings, but is intended to represent solely the force of the motion as distinguished from the state of unchanged rest-from the gravity of a motionless body, for example, which produces an incessant pressure against the surface which supports it, but does not produce any motion. In the case before us, therefore, we had first power in the form of a falling mass of water, then in the form of a lifted hammer, and, thirdly, in the form of the living force of the fallen hammer. We should transform the third form into the second, if we, for example, permitted the hammer to fall upon a highly elastic steel beam strong enough to resist the shock. The hammer would rebound, and in the most favourable case would reach a height equal to that from which it fell, but would never rise higher. In this way its mass would ascend: and at the moment when its highest point has been attained, it would represent the same number of raised foot-pounds as before it fell, never a greater number; that is to say, living force can generate the same amount of work as that expended in its production. It is therefore equivalent to this quantity of work. Our clocks are driven by means of sinking weights, and our watches by means of the tension of springs. A weight which lies on the ground, an elastic spring which is without tension, can produce no effects; to obtain such we must first raise the weight or impart tension to the spring, which is accomplished when we wind up our clocks and watches. The man who winds the clock or watch communicates to the RESERVOIR OF ACCUMIULATED POWER. 219 weight or to the spring a certain amount of power, and exactly so much as is thus communicated is gradually given out again during the following twenty-four hours, the original force being thus slowly consumed to overcome the friction of the wheels ahd the resistance which the pendulum encounters from the air. The wheel-work of the clock therefore exhibits no working force which was not previously communicated to it, but simply distributes the force given to it uniformly over a longer time. Into the chamber of an air-gun we squeeze, by means of a condensing air-pump, a great quantity of air. When we afterwards open the cock of a gun and admit the compressed air into the barrel, the ball is driven out of the latter with a force similar to that exerted by ignited powder. Now we may determine the work consumed in the pumping-in of the air, and the living force which, upon firing, is communicated to the ball, but we shall never find the latter greater than the former. The compressed air has generated no working force, but simply gives to the bullet that which has been previously communicated to it. And while we have pumped for perhaps a quarter of an hour to charge the gun, the force is expended in a few seconds when the bullet is discharged; but because the action is compressed into so short a time, a much greater velocity is imparted to the ball than would be possible to communicate to it by the unaided effort of the arm in throwing it. -From these examples you observe, and the mathematical theory has corroborated this for all purely mechanical, that is to say, for moving forces, that all our machinery and apparatus generate no force, but simply yield up the power communicated to them by natural forces, —falling water, moviig wind, or by the muscles of men and animals. After this law }had been established by the great mathematicians of the last century, a perpetual motion, which should make only use of pure mechanical forces, such as gravity, elasticity, pressure of 220 INTERACTION OF NATURAL FORCES. liquids and gases, could only be sought after by bewildered and ill-instructed people. But there are still other natural forces which are not reckoned among the purely moving forces -heat, electricity, magnetism, light, chemical forces, all of which nevertheless stand in manifold relation to mechanical processes. There is hardly a natural process to be found which is not accompanied by mechanical actions, or from which mechanical work may not be derived. Here the question of a perpetual motion remained open; the decision of this question marks the progress of modern physics. In the case of the air-gun, the work to be accomplished in the propulsion of the ball was given by the arm of the man who pumped in the air. In ordinary firearms, the condensed mass of air which propels the bullet is obtained in a totally different manner, namely, by the combustiohn of the powder. Gunpowder is transformed by combustion for the most part into gaseous products, which endeavor to occupy a much larger space than that previously taken up by the volume of the powder. Thus, you see, that, by the use of gunpowder, the work which the human arm must accomplish in the case of the air-gun is spared. In the mightiest of our machines, the steam engine, it is a strongly compressed aeriform body, water vapour, which, by its effort to expand, sets the machine in motion. Here also, we do not condense the steam by means of an external mechanical force, but by communicating heat to a mass of water in a closed -boiler, we change this water into steam., which, in consequence of the limits of the space, is developed urder strong pressure. In this case, therefore, it is the heat communicated which generates the mechanical force. The heat thus necessary for the machine we might obtain in many ways; the ordinary method is to procure it from the combustion of coal. Combustion is a chemical process. A particular constituent of our atmosphere, oxygen, possesses a strong force of IRODIUCTION OF FORCE BY COMBUSTION. 221 attraction, or, as it is named in chemistry, a strong affinity for the constituents of the combustible body, which affinity, however, in most cases, can only exert itself at high temperatures. As soon as a portion of the combustible body, for example the coal, is sufficiently heated, the carbon unites itself with great violence to the oxygen of the atmosphere and forms a peculiar gas, carbonic acid, the same which we see foaming from beer and champagne. Bythis combination, light and heat are generated; heat is generally developed by ahy combination of two bodies of strong affinity for each other; and when the heat is intense enough, light appears. Hence, in the steam engine, it is chemical processes and chemical forces which produce the astonishing work of these machines. In like manner the combustion of gunpowder is a chemical process, which, in the barrel of the gun, communicates living force to the bullet. While now the steam engine develops for us mechanical work out of heat, we can conversely generate heat by mechanical forces. A skilful blacksmith can render an iron wedge red hot by hammering. The axles of our carriages must be protected by careful greasing, from ignition through friction. Even lately this property has been applied on a large scale. In some factories, where a surplus of water power is at hand, this surplus is applied to cause a strong iron plate to rotate swiftly upon another, so that they become strongly heated by the friction. The heat so obtained warms the room, and thus a stove without fuel is provided. Now, could not the heat generated by the plates be applied to a small steam engine, which, in its turn, should be able to keep the rubbing plates in motion? The perpetual motion would thus be at length found. This question might be asked, and could not be decided by the older mnathematico-mechanical investigations. I will remark, beforehand, that the general law which I will lay before you answers the question in the negative. By a similar plan, however, a speculative American set 222 INTERACTION OF NATURAL FORCES. some time ago the industrial world of Europe in excitement. The magneto-electric machines often made use of in the case of rheumatic disorders are well known to the public. By imparting a swift rotation to the magnet of such a machine, we obtain powerful currents of electricity. If those be conducted through water, the latter will be reduced into its two components, oxygen and hydrogen. By the combustion of hydrogen, water is again generated. If this combustion takes place, not in atmospheric air, of which oxygen only constitutes a fifth part, but in pure oxygen, and if a bit of chalk be placed in the flame, the chalk will be raised to a white heat, and give us the sun-like Drummond's light. At the same time, the flame develops a considerable quantity of heat. Our American proposed to utilize in this way the gases obtained from electrolytic decomposition, and asserted that by the combustion a sufficient amount of heat was generated to keep a small steam engine in action, which again drove his magneto-electric machine, decomposed the water, and thus continually prepared its own fuel. This would certainly have been the most splendid of all discoveries; a perpetual motion which, besides the force which kept it going, generated light like the sun, and warmed all around it. The matter was by no means badly cogitated. Each practical step in tile afftir was known to be possible; but those who at that time were acquainted with the physical investigations which bear upon this subject could have affirmed, on the first hearing the report, that the matter was to be numbered among the numerous stories of the fable-rich America; and indeed, a fable it remained. It is not necessary to multiply examples further. You will infer from those given, in what immediate connection heat, electricity, magnetism, light, and chemical affinity, stand with mechanical forces. Starting from each of these different manifestations of natural forces, we can set every other in motion, for the most STATEMENT OF DYNAMIO PROBLEM. 223 part not in one way merely, but in many ways. It is here as with the weaver's web,Where a step stirs a thousand threads, The shuttles shoot from side to side, The fibres flow unseen, And one shock strikes a thousand combinations. Now it is clear that if by any means we could succeed, as the above American professed to have done, by mechanical forces, to excite chemical, electrical, or other natural processes, which, by any circuit whatever, and without altering permanently the active masses in the machine, could produce mechanical force in greater quantity than that at first applied, a portion of the work thus gained might be made use of to keep the machine in motion, while the rest of the work might be applied to any other purpose whatever. The problem was, to find in the complicated net of reciprocal actions, a track through chemical, electrical, magnetical, and thermic processes, back to mechanical actions, which might be followed with a final gain of mechanical work; thus would the perpetual motion be found. But, warned by the futility of former experiments, the public had become wiser. On the whole, people did not seek much after combinations which promised to furnish a perpetual motion, but the question was inverted. It was no more asked, How can I make use of the known and unknown relations of natural forces so as to construct a perpetual motion? but it was asked, If a perpetual motion be impossible, what are the relations which must subsist between natural forces? Everything was gained by this inversion of the question. The relations of natural forces rendered necessary by the above assumption, might be easily and completely stated. It was found that all known relations of force harmonize with the consequences of that assumption, and a series of unknown relations were discovered at the same time, the correctness of 224 INTERACTION OF NATURAL FORCES. which remained to be proved. If a single one of them could be proved false, then a perpetual motion would be possible. The first who endeavoured to travel this way was a French~ man, named Carnot, in the year 1824. In spite of a too limited conception of his subject, and an incorrect view as to the nature of heat, which led him to some erroneous conclusions, his experiment was not quite unsuccessful. He discovered a law which now bears his name, and to which I will return further on. His labors remained for a long time without notice, and it was not till' eighteen years afterwards, that is, in 1842, that different investigators in different countries, and independent of Carnot, laid hold of the same thought. The first who saw truly the general law here referred to, and expressed it correctly, was a German physician, J. R. Mayer, of Heilbronn, in the year 1842. A little later, in 1843, a Dane, named Colding, presented a memoir to the Academy of Copenhagen, in which the same law found utterance, and some experiments were described for its further corroboration. In England, Joule began about the same time to make experiments having reference to the same subject. We often find, in the case of questions to the solution of which the development of science points, that several heads, quite independent of each other, generate exactly the same series of reflections. I myself, without being acquainted with either Mlayer or Colding, and having first made the acquaintance of Joule's experiments at the end of my investigation, followed the same path. I endeavoured to ascertain all the relations between the different natural processes, which followed from our regarding them from the above point of view. MIy inquiry was made public in 1847, in a small pamphlet bearing the title, "~ On the Conservation of Force." Since that time the interest of the scientific public for this subject has gradually augmented. A great number of the PROGRESS OF THE INVESTIGATION. 225 essential consequences of the above manner of viewing the subject, the proof of which was wanting when the first. theoretic notions were published, have since been confirmed by experiment, particularly by those of Joule; and during the last year the most eminent physicist of France, Regnault, has adopted the new mode regarding the question, and by fresh investigations on the specific heat of gases has contributed much to its support. For some important consequences the experimental proof is still wanting, but the number of confirmations is so predominant, that I have not deemed it too early to bring the subject before even a non-scientific. audience. H-~ow the question has been decided you may ahlready infer from what has been stated. In the series of natural processes there is no circuit to be found, by which mechanical force can be gained without a corresponding consumption. The perpetual motion remains impossible. Our reflections, however, gain thereby a higher interest. We have thus far regarded the development of force by natural processes, only in its relation to its usefulness to man, as mechanical force. You now see that we have arrived at a general law, which holds good wholly independent of the application which man makes of natural forces; we must therefore make the expression of our new law correspond to this more general significance. It is in the first place clear, that the work which, by any natural process whatever, is performed under favourable conditions by a machine, and which may be measured in the way already indicated, may be used as a measure of force common to all. Further, the important question arises, " If the quantity of force cannot be aupgmented except by corresponding consumption, can it be diminished or lost? For the purpose of our machines it certainly can, if we neglect the opportunity to convert natural processes to use, but as investigation has proved, not for a nature as a whole." 10$ 226 INTERACTION OF NATURAL FORCES. In the collision and friction of bodies against each other, the mechanics of former years assumed simply that living force was lost. But I have already stated that each collision and each act of friction generates heat; and, moreover, Joule has established by experiment the important law, that for every foot-pound of force which is lost, a definite quantity of heat is always generated, and that when work is performed by the consumption of heat, for each foot-pound thus gained a definite quantity of heat disappears. The quantity of heat necessary to raise the temperature of a pound of water a degree of the centigrade thermometer, corresponds to a mechanical force by which a pound weight would be raised to the height of 1350 feet; we name this quantity the mechanical equivalent of heat. I may mention here that these facts conduct of necessity to the conclusion, that the heat is not, as was formerly imagined, a fine imponderable substance, but that, like light, it is a peculiar shivering motion of the ultimate particles of bodies. In collision and friction, according to this manner of viewing the subject, the motion of the mass of a body which is apparently lost is converted into a motion of the ultimate particles of the body; and conversely, when mechanical force is generated by heat, the motion of the ultimate particles is converted into a motion of the mass. Chemical combinations generate heat, and the quantity of this heat is totally independent of the time and steps through which the combination has been effected, provided that other actions are not at the same time brought into play. If, however, mechanical work is at the same time accomplished, as in the case of the steam engine, we obtain as much less heat as is equivalent to this work. The quantity of work produced by chemical force is in general very great. A pound of the purest coal gives, when burnt, sufficient heat to raise the temperature of 8086 pounds of water one degree of the centigrade thermometer; from this we can calculate that the magnitude of the chemical force of attraction between the parti AMOUNT OF FORCE IN THE UNIVERSE UNALTERABLE. 227 cles of a pound of coal and the quantity of oxygen that corresponds to it, is capable of lifting a weight of one hundred pounds to a height of twenty miles. Unfortunately, in our steam engines, we have hitherto been able to gain only the smallest portion of this work; the greater part is lost in the shape of heat. The best expansive engines give back as mechanical work only eighteen per cent. of the heat generated by the fuel. From a similar investigation of all the other known physical and chemical processes, we arrive at the conclusion that Nature as a whole possesses a store of force which cannot in any way be either increased or diminished. And that, therefore, the quantity of force in nature is just as eternal and unalterable as the quantity of matter. Expressed in this form, I have named the general law "I The Principle- of the Conservation of Force." We cannot create mechanical force, but we may help ourselves from the general store-house of Nature. The brook and the wind, which drive our mills, the forest and the coalbed, which supply our steam engines and warm our rooms, are to us the bearers of a small portion of the great natural supply which we draw upon for our purposes, and the actions of which we can apply as we think fit. The possessor of a mill claims the gravity of the descending rivulet, or the living force of the moving wind, as his possession. These portions of the store of Nature are what give Iris property its chief value. Further, from the fact that no portion of force can be absolutely lost, it does not follow that a portion may not be inapplicable to human purposes. In this respect the inferences drawn by William Thomson from the law of Carnot are of importance. This law, which was discovered by Carnot during his endeavours to ascertain the relations between heat and mechanical force, which, however, by no means belongs to the necessary consequences of the conservation of 228 INTERACTION OF NATURAL FORCES. force, and which Clausius was the first to modify in such a manner that it no longer contradicted the above general law, expresses a certain relation between the compressibility, the capacity for heat, and the expansion by heat of all bodies. It is not yet considered as actually proved, but some remarkable deductions having been.drawn from it, and afterwards proved to be facts by experiment, it has attained thereby a great degree of probability. Besides the mathematical form in which the law was first expressed by Carnot, we can give it the following more general expression:-" Only when heat passes from a warmer to a colder body, and even then only partially, can it be converted into mechanical work." The heat of a body which we cannot cool further, cannot be changed into another form of force; into the electric or chemical force, for example. Thus, in our steam engines, we convert a portion of the heat of the glowing coal into work, by permitting it to pass to the less warm water of the boiler. If, however, all the bodies in nature had the same temperature, it would be impossible to convert any portion of their heat into mechanical work. According to this, we can divide the total force store of the universe into two parts, one of which is heat, and must continue to be such; the other, to which a portion of the heat of the warmer bodies, and the total supply of chemical, mechanical, electrical, and magnetical forces belong, is capable of the most varied changes of form, and constitutes the whole wealth of change which takes place in nature. But the heat of the warmer bodies strives perpetually to pass to bodies less warm by radition and conduction, and thus to establish an equilibrium of temperature. At each motion of a terrestrial body, a portion of mechanical force passes by friction or collision into heat, of which only a part can be converted back again into mechanical force. This is also generally the case in every electrical and chemical process. From this, it follows that the first portion of the store of force, THE FORCES OF NATURE DISSIPATED IN HEAT. 229 the unchangeable heat, is augmented by every natural pros cess,- while the second portion, mechanical, electrical, and chemical force, must be diminished; so that if the universe be delivered over to the undisturbed action of its physical processes, all force will finally pass into the form of heat, and all heat come into a state of equilibrium. Then all possibility of a further change would be at an end, and the complete cessation of all natural processes must set in. The life of men, animals, and plants, cdould not of course continue if the sun had lost its high temperature, and with it his light,-if all the components of the earth's surface had closed those combinations which their affinities demand. In short, the universe from that time forward would be condemned to a state of eternal rest. These consequences of the law of Carnot are, of course, only valid, provided that the law, when sufficiently tested, proves to be universally correct. In the mean time there is little prospect of the law being proved incorrect. At all events we must admire the sagacity of Thomson, who, in the letters of a long known little mathematical formula, which only speaks of the heat, volume, and pressure of bodies, was able to discern consequences which threatened the universe, though certainly after an infinite period of time, with eternal death. I have already given you notice that our path lay through a thorny and unrefreshing field of mathematico-mechanica] developments. We have now left this portion of our road behind us. The general principle which I have sought to lay before you has conducted us to a point from which our view is a wide one, and, aided by this principle, we can now at pleasure regard this or the other side of the surrounding world, according as our interest in the matter leads us. A glance into the narrow laboratory of the physicist, with its small appliances and complicated abstractions, will not be so attractive as a glance at the wide heaven above us, the clouds, 230 INTERACTION OF NATURAL FORCES. the rivers, the woods, and the living beings around us. While regarding the laws which have been deduced from the physical processes of terrestrial bodies, as applicable also to the heavenly bodies, let me remind you that the same force which, acting at the earth's surface, we call gravity (Schwere), acts as gravitation in the celestial spaces, and also manifests its power in the motion of the immeasurably distant double stars which are governed by exactly the same laws as those subsisting between the earth and moon; that, therefore, the light and heat of terrestrial bodies do not in any way differ essentially from those of the sun, or of the most distant fixed star; that the meteoric stones which sometimes fall from external space upon the earth are composed of exactly the same simple chemical substances as those with which we are acquainted. We need, therefore, feel no scruple in granting that general laws to which all terrestrial natural processes are subject, are also valid for other bodies than the earth. We will, therefore, make use of our law to glance over the household of the universe with respect to the store of force, capable of action, which it possesses. A number of singular peculiarities in the structure of our planetary system indicate that it was once a connected mass with a uniform motion of rotation. Without such an assumption, it is impossible to explain why all the planets move in the same direction round the sun, why they all rotate in the same direction round their axes, why the planes of their orbits, and those of their satellites and rings all nearly coincide, why all their orbits differ but little fiom circles; and much besides. From these remaining indications of a former state, astronomers have shaped an hypothesis regarding the formation of our planetary system, which, although from the nature of the case it must ever remain an hypothesis, still in its special traits is so well supported by analogy, that it certainly deserves our attention. It was Kant, who, feeling great interest in the physical description of the earth and the planetary ORIGIN OF THE NEBULAR HYPOTHESIS. 231 system, undertook the labour of studying the works of Newton, and as an evidence of the depth to which he had penetrated into the fundamental ideas of Newton, seized the notion that the same attractive force of all ponderable matter which now supports the motion of the planets, must also aforetime have been able to form from matter loosely scattered in space the planetary system. Afterwards, and independent of Kant, Laplace, the great author of the fMcancique Celeste, laid hold of the same thought, and introduced it among astronomers. The commencement of our planetary system, including the sun, must, according to this, be regarded as an immense nebulous mass which filled the portion of space which is now occupied by our system, far beyond the limits of Neptune, our most distant planet. Even now we perhaps see similar masses in the distant regions of the firmament, as patches of nebuloa, and nebulous stars; within our system also, comets, the zodiacal light, the corona of the sun during a total eclipse, exhibit remnants of a nebulous substance, which is so thin that the light of the stars passes through it unenfeebled and unrefracted. If we calculate the density of the mass of our planetary system, according to the above assumption, for the time when it was a nebulous sphere, which reached to the path of the outmost planet, we should find that it would require several cubic miles of such matter to weigh a single grain. The general attractive force of all matter must, however, impel these masses to approach each other, and to condense, so that the nebulous sphere became incessantly smaller, by which, according to mechanical laws, a motion of rotation originally slow, and the existence of which must be assumed, would gradually become quicker and quicker. By the centrifugal force which must act most energetically in the neighbourhood of the equator of the nebulous sphere, masses could from time to time be torn away, which afterwards would continue their courses separate from the main mass, forming 232 INTERACTION OF NATURAL FORCES. themselves into single planets, or, similar to the great original sphere, into planets with satellites and rings, until finally the principal mass condensed itself into the sun. With regard to the origin of heat and light, this view gives us no information. When the nebulous chaos first separated itself from otler fixed star masses, it must not only have contained all kinds of matter which was to constitute the future planetary system, but also, in accordance with our new law, the whole store of force which at one time must unfold therein its wealth of actions. Indeed in this respect an immense dower was bestowed in the shape of the general attraction of all the particles for each other. This force, which on the earth exerts itself as gravity, acts in the heavenly spaces as gravitation. As terrestrial gravity when it draws a weight downwards performs work and generates vis viva, so also the heavenly bodies do the same when they draw two portions of matter from distant regions of space towards each other. The chemical forces must have been also present, ready to act; but as these forces can only come into operation by the most intimate contact of the different masses, condensation must have taken place before the play of chemical forces began. Whether a still further supply of force in the shape of heat was present at the commencement we do not know. At all events, by aid of the law of the equivalence of heat and work, we find in the mechanical forces, existing at the time to which we refer, such a rich source of heat and light, that there is no necessity whatever to take refuge in the idea of a store of these forces originally existing. When through condensation of the masses their particles came into collision, and clung to each other, the vis viva of their motion would be thereby annihilated, and must reappear as heat. Already in old theories, it has been calculated, that cosmical masses must generate heat by their collision, but it was far from any body's HEAT DEVELOPED IN THE SOLAR SYSTEM. 233 thought, to make even a guess at the amount of heat to be generated in this way. At present we can give definite numerical values with certainty. Let us make this addition to our assumption; that, at the commencement, the density of the nebulous matter was a vanishing quantity, as compared with the present density of the sun and planets; we can then calculate how much work has been performed by the condensation; we can further calculate how much of this work still exists in the form of mechanical force, as attraction of the planets towards the sun, and as vis viva of their motion, and find, by this, how much of the force has been converted into heat. The result of this calculation is, that only about the 454th part of the original mechanical force remains as such, and that the remainder, converted into heat, would be sufficient to raise a mass of water equal to the sun and planets taken together, not less than twenty-eight millions of degrees of the centigrade scale. For the sake of comparison, I will mention that the highest temperature which we can produce by the oxyhydrogen blowpipe, which is sufficient to fuse and vaporize even platina, and which but few bodies can endure, is estimated at about two thousand centigrade degrees. Of the action of a temperature of twenty-eight millions of such degrees we can form no notion. If the mass of our entire system were pure coal, by the combustion of the whole of it only the 3500th part of the above quantity would be generated. This is also clear, that such a development of heat must have presented the greatest obstacle to the speedy union of the masses, that the larger part of the heat must have been diffused by radiation into space, before the masses could form bodies possessing the present density of the sun and planets, and that these bodies must once have been in a state of fiery fluidity. This notion is corroborated by the geological phenomena of our planet; and with regard to the other planetary bodies, the flattened form of the sphere, which is the form of 234 INTERACTION OF NATURAL FORCES. equilibrium of a fluid mass, is indicative of a former state of fluidity. If I thus permit an immense quantity of heat to disappear without compensation from our system, the principle of the conservation of force is not thereby invaded. Certainly for our planet it is lost, but not for the universe. It has proceeded outwards, and daily proceeds outwards into infinite space; and we know not whether the medium which transmits the undulations of light and heat possesses an end where the rays must return, or whether they eternally pursue their way through infinitude. The store of force at present possessed by our system, is also equivalent to immense quantities of heat. If our earth were by a sudden shock brought to rest on her orbit-which is not to be feared in the existing arrangements of our system -by such a shock a quantity of heat would be generated equal to that produced by the combustion of fourteen such earths of solid coal. Making the most unfavourable assumption as to its capacity for heat, that is, placing it equal to that of water, the mass of the earth would thereby be heated 11,200 degrees; it would therefore be quite fused and for the most part reduced to vapour. If, then, the earth, after having been thus brought to rest, should fall into the sun, which of course would be the case, the quantity of heat developed by the shock would be four hundred times greater. Even now, from time to time, such a process is repeated on a small scale. There can hardly be a doubt that meteors, fre-balls, and meteoric stones, are masses which belong to the universe, and before coming into the domain of our earth, moved like the planets round the sun. Only when they enter our atmosphere do they become visible and fall sometimes to the earth. In order to explain the emission of light by these bodies, and the fact that for some time after their descent they are very hot, the friction was long ago thought of which they experience in passing through the air. We can now calculate that a velocity of 3000 feet a second, THE LIGHT AN) HEAT OF METEORS. 235 supposing the whole of the friction to be expended in heating the solid mass, would raise a piece of meteoric iron 1000~ C. in temperature, or, in other words, to a vivid red heat. Now the average velocity of the meteors seems to be thirty or forty times the above amount. To compensate this, however, the greater portion of the heat is, doubtless, carried away by the condensed mass of air which the meteor drives before it. It is known that bright meteors generally leave a luminous trail behind them, which probably consists of several portions of the red-hot surfaces. Meteoric masses which fall to the earth often burst with a violent explosion, which may be regarded as a result of the quick heating. The newly-fallen pieces have been for the most part found hot, but not red-hot, which is easily explainable by the circumstance, that during the short time occupied by the meteor in passing through the atmosphere, only a thin, superficial layer is heated to redness, while but a small quantity of heat has been able to penetrate to the interior of the mass. For this reason the red heat can speedily disappear. Thus has the falling of the meteoric stone, the minute remnant of processes which seems to have played an important part in the formation of the heavenly bodies, conducted us to the present time, where we pass from the darkness of hypothetical views to the brightness of knowledge. In what we have said, however, all that is hypothetical is the assumption of Kant and Laplace, that the masses of our system were once distributed as nebulma in space. On account of the rarity of the case, we will still further remark, in what close coincidence the results of science here stand with the earlier legends of the human family, and the forebodings of poetic fancy. The cosmogony of ancient nations generally commences with chaos and darkness. Neither is the Mosaic tradition very divergent, particularly when we remember that that which Moses names heaven is different from the blue dome above us, and is synonymous 236 INTERACTION OF ATURIAL FORCES. with space, and that the unformed earth, and the waters of the great deep, which were afterwards divided into waters above the firmament, and waters below the firmament, resembled the chaotic components of the world. Our earth bears still -the unmistakable traces of its old fiery fluid condition. The granite formations of her mountains exhibit a structure, which can only be produced by the crystallization of fused masses. Investigation still shows that the temperature in mines, and borings, increases as we descend; and if this increase is uniform, at the depth of fifty miles, a heat exists sufficient to fuse all our minerals. Even now our volcanoes project, from time to time, mighty masses of fused rocks from their interior, as a testimony of the heat which exists there. But the cooled crust of the earth has already become so thick, that, as may be shown by calculations of its conductive power, the heat coming to the surface from within, in comparison with that reaching the earth from the sun, is exceedingly small, and increases the temperature of the surface only about one thirtieth of a degree centigrade; so that the remnant of the old store of force which is enclosed as heat within the bowels of the earth, has a sensible influence upon the processes at the earth's surface, only through the instrumentality of volcanic phenomena. These processes owe their power acclmost wholly to the action of other heavenly bodies, particularly to the light and heat of the sun, and partly also, in the case of the tides, to the attraction of the sun and moon. Most varied and numerous are the changes which we owe to the light and heat of the sun. The sun heats our atmos. phere irregularly, the warm rarefied air ascends, while fresh cool air flows from-the sides to supply its place: in this way winds are generated. This. action is most powerful at the eqnator, the warm air of which incessantly flows in the upper regions of the atmosphere towards the poles: while just as persistently, at the earth's surface, the trade wind carries new and cool air to the equator. Without the heat of the sun all SOLAR FORCE PRODUCES THE WATER CIRCULATIONS. 237 winds must, of necessity, cease. Similar currents are produced by the same cause in the waters of the sea. Their power may be inferred from the influence which in some cases they exert upon climate. By them the warm water of the Antilles is carried to the British Isles, and confers upon them a mild, uniform warmth and rich moisture; while, through similar causes, the floating ice of the North Pole is carried to the coast of Newfoundland, and produces cold. Further, by the heat of the sun, a portion of the water is converted into vapour which rises in the atmosphere, is condensed to clouds, or falls in rain and snow upon the earth, collects in the form of springs, brooks, and rivers, and finally reaches the sea again, after having gnawed the rocks, carried away the light earth, and thus performed its part in the geologic changes of the earth; perhaps, besides all this it has driven our watermill upon its way. If the heat of the sun were withdrawn, there would remain only a single motion of water, namely, the tides, which are produced by the attraction of the sun and moon. How is it, now, with the motions and the work-of organic beings. To the builders of the automata of the last century, men and animals appeared as clockwork which was never wound up, and created the force which they exerted out of nothing. They did not know how to establish a connection between the nutriment consumed and the work generated. Since, however, we have learned to discern in the steam-engine this origin of mechanical force, we must inquire whether something similar does not hold good with regard to men. Indeed, the continuation of life is dependent on the consumption of nutritive materials: these are combustible substances, which, after digestion and being passed into the blood, actually undergo a slow combustion, and finally enter into almost the same combinations with the oxygen of the atmosphere that are produced in an open fire. As the quantity of heat generated by combustion is independent of the duration of the combustion and 238 ITERACTION OF NATURAL FORCES. the steps in which it occurs, we can calculate from the mass of the consumed material how much heat, or its equivalent work is thereby generated in an animal body. Unfortunately, the difficulty of the experiments is still very great; but within those limits of accuracy which have been as yet attainable, the experiments showr that the heat generated in the animal body corresponds to the amount which would be generated by the chemical processes. The animal body therefore does not differ from the steam-engine, as regards the manner in which it obtains heat and force, but does differ from it in the manner in which the force gained is to be made use of. The body is, besides, more limited than the machine in the choice of its fuel; the latter could be heated with sugar, with starchflour, and butter, just as well as with coal or wood; the animal body must dissolve its materials artificially, and distribute them through its system; it must, further, perpetually renew the used-up materials of its organs, and as it cannot itself create the matter necessary for this, the matter must come from without. Liebig was the first to point out these various uses of the consumed nutriment. As material for the perpetual renewal of the body, it seems that certain definite albumiIlous substances which appear in plants, and form the chief mass of the animal body, can alone be used. They form only a portion of the mass of nutriment taken daily; the remainder, sugar, starch, fat, are really only materials for warming, and are perhaps not to be superseded by coal, simply because the latter does not permit itself to be dissolved. If, then, the processes in the animal body are not in this respect to be distinguished from inorganic processes, the question' arises, whence comes the nutriment which constitutes the source of the body's force? The answer is, from the vegetable kingdom; for only the material of plants, or the flesh of plant-eating animals, can be made use of for food. The animals which live on plants occupy a mean position between carnivorous animals, in which we reckon man, and SOLAR ORIGIN OF ORGANIC FORCE. 239 vegetables, which the former could not make use of immedi. ately as nutriment. In hay and grass the same nutritive substances are present as in meal and flour, but in less quantity. As, however, the digestive organs of man are not in a condition to extract the small quantity of the useful from the great excess of the insoluble, we submit, in the first place, these substances to the powerful digestion of the ox, permit the nourishment to store itself in the animal's body, in order in the end to gain it for ourselves in a more agreeable and useful form. In answer to our question, therefore, we are referred to the vegetable world. Now when what plants take in and what they give out are made the subjects of investigation, we find that the principal part of the former consists in the products of combustion which are generated by the aniemal. They take the consumed carbon given off in respiration, as carbonic acid, from the air, the consumed hydrogen as water, the nitrogen in its simplest and closest combination as ammonia; and from these materials, with the assistance of small ingredients which they take from the soil, they generate anew the compound combustible substances, albumen, sugar, oil, on which the animal subsists. Here, therefore, is a circuit which appears to be a perpetual store of force. Plants prepare fuel and nutriment, animals consume these, burn them slowly in their lungs, and from the products of combustion the plants again derive their nutriment. The latter is an eternal source of chemical, the former of mechanical forces. Would not the combination of both organic kingdoms produce the perpetual motion? We must not conclude hastily: further inquiry shows, that plants are capable of producing combustible substances only when they are under the influence of the sun. A portion of the sun's rays exhibits a remarkable relation to chemical forces,-it can produce and destroy chemical combinations; and these rays, which for the most part. are blue or violet, are called therefore chemical rays. We make use of their action in the production of pho 240 INTERACTION OF NATURAL FORCES. tographs. Here compounds of silver are decomposed at the place where the sun's rays strike them. The same rays overpower in the green leaves of plants the strong chemical affinity of the carbon of the carbonic acid for oxygen, give back the latter free to the atmosphere, and accumulate the other, in combination with other bodies, as woody fibre, starch, oil, or resin. These chemically active rays of the sun disappear completely as soon as they encounter the green portions of the plants, and hence it is that in daguerrotype images the green leaves of plants appear uniformly black. Inasmuch as the light coming from them does not contain the chemical rays, it is unable to act upon the silver compounds. Hence a certain portion of force disappears from the sunlight, while combustible substances are generated and accumulated in plants; and we can assume it as very probable, that the former is the cause of the latter. I must indeed remark, that we are in possession of no experiments from which we might determine whether the vis viva of the sun's rays which have disappeared, corresponds to the chemical forces accumulated during the same time; and as long as these experiments are wanting, we cannot regard the stated relation as a certainty. If this view should prove correct, we derive from it the flattering result, that all force, by means of which our bodies live and move, finds its source in the purest sunlight; and hence we are all, in point of nobility, not behind the race of the great monarch of China, who heretofore alone called himself Son of the Sun. But it must also be conceded that our lower fellow-beings, the frog and leech, share the same ethereal origin, as also the whole vegetable world, and even the fuel which comes to us from the ages past, as well as the youngest offspring of the forest with which we heat our stoves and set our machines in motion. You see, then, that the immense wealth of ever-changing meteorological, climatic, geological, and organic processes of our earth are almost wholly preserved in action by the light DYNAMICS OF SUNLIGHT. 241 and heat-giving rays of the sun; and you see in this a remarkable example, how Proteus-like the effects of a single cause, under altered external conditions, may exhibit itself in nature. Besides these, the earth experiences an action of another kind from its central luminary, as well as from its satellite the moon, which exhibits itself in the remarkable phenomenon of the ebb and flow of the tide. Each of these bodies excites, by its attraction upon the waters of the sea, two gigantic waves, which flow in the same direction round the world, as the attracting bodies themselves apparently do. The two waves of the moon, on account of her greater nearness, are about three and a half times as large as those excited by the sun. One of these waves has its crest on the quarter of the earth's surface which is turned towards the moon, the other is at the opposite side. Both these quarters possess the flow of the tide, while the regions which lie between have the ebb. Although in the open sea the height of the tide amounts to only about three feet, and only in certain narrow channels, where the moving water is squeezed together, rises to thirty feet, the might of the phenomena is nevertheless manifest from the calculation of Bessel, according to which a quarter of the earth covered by the sea possesses, during the flow of the tide, about 25,000 cubic miles of water more than during the ebb, and that therefore such a mass of water must, in six and a quarter hours, flow from one quarter of the earth to the other. The phenomena of the ebb and flow, as already recognized by Mayer, combined with the law of the conservation of force, stand in remarkable connection with the question of the stability of our planetary system. The mechanical theory of the planetary motions discovered by Newton teaches, that if a solid body in absolute vacuo, attracted by the sun, move around him in the same manner as the planets, this motion will endure unchanged through all eternity. Now we have actually not only one, but several such 11 242 INTERACTION OF NATURAL FORCES. planets, which move around the sun, and by their mutual attraction create little changes and disturbances in each other's paths. Nevertheless Laplace, in his great work, the ]ican2tue Celeste, has proved that in our planetary system all these disturbances increase and diminish periodically, and can never exceed certain limits, so that by this cause the eternal existence of the planetary system is unendangered. But I have already named two assumptions which must be made: first that the celestial spaces must be absolutely empty; and secondly, that the sun and planets must be solid bodies. The first is at least the case as far as astronomical observations reach, for they have never been able to detect any retardation of the planets, such as would occur if they moved in a resisting medium. But on a body of less mass, the comet of Encke, changes are observed of such a nature: this comet describes ellipses round the sun which are becoming gradually smaller. If this kind of motion, which certainly corresponds to that through a resisting medium, be actually due to the existence of such a medium, a time will come when the comet will strike the sun; and a similar end threatens all the planets, although after a time, the length of which baffles our imagination to conceive of it. But even should the existence of a resisting medium appear doubtful to us, there is no doubt that the planets are not wholly composed of solid materials which are inseparably bound together. Signs of the existence of an atmosphere are observed on the Sun, on Venus, Mars, Jupiter, and Saturn. Signs of water and ice upon Mars; and our earth has undoubtedly a fluid portion on its surface, and perhaps a still greater portion of fluid within it. The motions of the tides, however, produce friction, all friction destroys vis viva, and the loss in this case can only affect the vis viva of the planetary system. VWe come thereby to the unavoidable conclusion, that every tide, although with infinite slowness, still with certainty, diminishes the store of mechanical force of the system; and as a consequence of this, the ro INFLJUENCE OF TIDES UPON THE EARTH'S ROTATION. 243 tation of the planets in question round their axes must become more slow; they must therefore approach the sun, or their satellites must approach them. What length of time must pass before the length of our day is diminished one second by the action of the tide cannot be calculated, until the height and time of the tide in all portions of the ocean are known. This alteration, however, takes place with extreme slowness, as is known by tile consequences which Laplace has deduced from the observations of Hipparchus, according to which, during a period of 2000 years, the duration of the day has not been shortened by the one three hundredth part of a second. The final consequence would be, but after millions of years, if in the mean time the ocean did not become frozen, that one side of the earth would be constantly turned towards the sun, and enjoy a perpetual day, whereas the opposite side would be involved in eternal night. Such a position we observe in our moon with regard to the earth, and also in tile case of the satellites as regards their planets; it is, perhaps, due to the action of the mighty ebb and flow to which these bodies, in the time of their fiery fluid condition, were subjected. I would not have brought forward these conclusions, which again plunge us in the most distant future, if they were not unavoidable. Physico-mechanical laws are, as it were, the telescopes of our spiritual eye, which can penetrate into the deepest night of time, past and to come. Another essential question as regards the future of our planetary system has reference to its future temperature and illumination. As the internal heat of the earth has but little influence on the temperature of the surface, the heat of the sun is the only thing which essentially affects the question. The quantity of heat falling from the sun during a given time upon a given portion of the earth's surface may be measured, and fiom this it can be calculated how much heat in a given time is sent out from the entire sun. Such measurements 244 INTERACTION OF NATURAL FORCES. have been made by the French physicist Pouillet, and it has been found that the sun gives out a quantity of heat per hour equal to that which a layer of the densest coal ten feet thick would give out by its combustion; and hence in a year a quantity equal to the combustion of a layer of seventeen miles. If this heat were drawn uniformly from the entire mass of the sun, its temperature would only be diminished thereby one and one third of a degree centigrade per year, assuming its dapacity for heat to be equal to that of water. These results can give us an idea of the magnitude of the emission, in relation to the surface and mass of the sun; but they cannot inform us whether the sun radiates heat as a glowing body, which since its formation has its heat accumulated within it, or whether a new generation of heat by chemical processes takes place at the sun's surface. At all events the law of the conservation of force teaches us that no process analogous to those known at the surface of the earth, can supply for eternity an inexhaustible amount of light and heat to the sun. But the same law also teaches that the store of force at present existing, as heat, or as what may become heat, is sufficient for an immeasurable time. With regard to the store of chemical force in the sun, we can form no conjecture, and the store of heat there existing can only be determined by very uncertain estimations. If, however, we adopt the very probable view, that the remarkably small density of so large a body is caused by its high temperature, and may become greater in time, it may be calculated that if the diameter of the sun were diminished only the ten-thousandth part of its present length, by this act a sufficient quantity of heat would be generated to cover the total emission for 2100 years. Such a small change besides it would be difficult to detect even by the finest astronomical observations. Indeed, from the commencement of the period during which we possess historic accounts, that is, for a period of about 4000 years, the temperature of the earth has not sensi CONSTANCY OF THE EARtTH'S TEMPERATURE. 245 bly diminished. From these old ages we have certainly no thermometric observations, but we have information regarding the distribution of certain cultivated plants, the vine, the olive tree, which are very sensitive to changes of the mean annual temperature, and we find that these plants at the present moment have the same limits of distribution that they had in the times of Abraham and Homer; from which we may infer backwards the constancy of the climate. In opposition to this it has been urged, that here in Prussia the German knights in former times cultivated the vine, cellared their own wine and drank it, which is no longer possible. From this the conclusion has been drawn, that the heat of our climate has diminished since the time referred to. Against this, however, Dove has cited the reports of ancient chroniclers, according to which, in some peculiarly hot years, the Prussian grape possessed somewhat less than its usual quantity of acid. The fact also speaks not so much for the climate of the country as for the throats of the German drinkers. But even though the force store of our planetary system is so immensely great, that by the incessant emission which has occurred during the period of human history it has not been sensibly diminished, even though the length of the time which must flow by, before a sensible change in the state of our planetary system occurs, is totally incapable of measurement, still the inexorable laws of mechanics indicate that this store of force, which can only suffer loss and not gain, must be finally exhausted. Shall we terrify ourselves by this thought? Men are in the habit of measuring the greatness and the wisdom of the universe by the duration and the profit which it promises to their own race; but the past history of the earth already shows what an insignificant moment the duration of the existence of our race upon it constitutes. A Nineveh vessel, a Roman sword awakes in us the conception of grey antiquity. What the museums of Europe show us o 246 INTERACTION OF NATURAL FORCES. the remains of Egypt and Assyria we gaze upon with silent astonishment, and despair of being able to carry our thoughts back to a period so remote. Still must the human race have existed for ages, and multiplied itself before the pyramids of Nineveh could have been erected. We estimate the duration of human history at 6000 years; but immeasurable as this time may appear to us, what is it in comparison with the time during which the earth carried successive series of rank plants and mighty animals, and no men; during which in our neighbourhood the amber-tree bloomed, and dropped its costly gum on the earth and in the sea; when in Siberia, Europe and iNTorth America groves of tropical palms flourished; where gigantic lizards, and after them elephants, whose mighty remains we still find buried in the earth, found a home? Different geologists, proceeding from different premises, have -sought to estimate the duration of the above creative period, and vary from a million to nine million years. And the time during which the earth generated organic beings is again small when we compare it with the ages during which the world was a ball of fused rocks. For the duration of its cooling from 2000~ to 200~ centigrade, the experiments of Bishop upon basalt show that about 350 millions of years would be necessary. And with regard to the time during which the first nebulous mass condensed into our planetary system, our most daring conjectures must cease. The history of man, therc fore, is but a short ripple in the ocean of time. For a much longer series of years than that during which man has already occupied this world, the existence of the present state of inorganic nature favourable to the duration of man seems to be secured, so that for ourselves and for long generations after us, we have nothing to fear. But the same forces of air and water, and of the volcanic interior, which produced former geological revolutions, and buried one series of living forms after another, act still upon the earth's crust. They more probably will bring about the last day of the human race than CULMINATION OF THE ARGUMENT. 241 those distant cosmical alterations of which we have spoken, and perhaps force us to make way for new and more complete living forms, as the lizards and the mammoth have given place to us and our fellow-creatures which now exist. Thus the thread which was spun in darkness by those who sought a perpetual motion has conducted us to a universal law of nature, which radiates light into the distant nights of the beginning and of the end of the history of the universe. To our own race it permits a long but not an endless existence; it threatens it with a day of judgment, the dawn of which is still happily obscured. As each of us singly must endure the thought of his death, the race must endure the same. But above the forms of life gone by, the human race has higher moral problems before it, the bearer of which it is, and in the completion of which it fulfils its destiny. I. RE1MA RKS ON THE FORC(ES OF INORGANIC NATURE. BY DR. J. R. MAYER. TRABSLATED BY J. C. FOSTER, B.A. ON CELESTIAL DYNAMICS. BY DR. J. R. MAYER. TRANSLATED BY DR. H. DEBUS, F.R.S. HTI. REMARKS ON THE MECHANICAL EQUIVALENT OF HEAT, BY DR. J. R. MAYER. TRANSLATED BY J. C. FOSTER, B.A. JULIUS ROBERT M]AYER was born at Heilbronn, November 25, 1814. He received a medical education, and became first, county wound-physician and afterwards city physician of Heilbronn. But few particulars of his life have been obtained. In 1840 he made a voyage on a Dutch freighter to Java, and it was the accident of bleeding a feverish patient in this country, and observing that the venous blood in the tropics was of a much brighter red than in colder latitudes, that led him to those investigations of natural forces, the chief results of which are given in the following essays. Two years after his attention was drawn to the subject-in 1842, he published his first paper on the "' Forces of Inorganic Nature." It was put together briefly, and published in Liebig's journal to secure the public recognition of his claims. His second publication, "On Organic Motion and Nutrition" (1845), an able essay of one hundred and twelve pages, is not yet translated. His third paper, on " Celestial Dynamics," was published in 1848; and his fourth, on the " Mechanical Equivalent of Heat," appeared in 1851. These vast and rapid labors were too much for his strength. His overtasked mind gave way, and he was taken to an insane asylum. He, however, fortunately recovered, and is now reported as occupied with the cultivation of the vine in Heilbronn. THE FORCES OF INORGANIC NATURE. IHE following pages are designed as an attempt to an swer the questions, What are we to understand by " Forces"? and how are different forces related to each other? Whereas the term matter implies the possession, by the object to which it is applied, of very definite properties, such as weight and extension; the term force conveys for the most part the idea of something unknown, unsearchable, and hypothetical. An attempt to render the notion of force equally exact with that of matter, and so to denote by it only objects of actual investigation, is one which, with the consequences that flow from it, ought not to be unwelcome to those who desire that their views of nature may be clear and unencumbered by hypotheses. Forces are causes: accordingly, we may in relation to them make full application of the principle-causa seguat effectum. If the cause c has the effect e, then cue; if, in its turn, e is the cause of a second effectf, we have e=f, and so on: c e=f... -c. In a chain of causes and effects, a term or a part of a term can never, as plainly appears from the nature of an equation, become equal to nothing. This first property of all causes we call their indestructibility. If the given cause c has produced an effect e equal to itself, it has in that very act ceased to be: c has become e; if, after the production of e, c still remained in whole or in part, 25 2 TH[E FORCES OF INORGANIC NATUE. there must be still further effects corresponding to this re maining cause: the total effect of c would thus be > e, which would be contrary to the supposition c-e. Accordingly, since c becomes e, and e becomesf, &c., we must regard these various magnitudes as different forms under which one and the same object makes its appearance. This capability of assuming various forms is the second essential property of all causes. Taking both properties together, we may say, causes are (quantitatively) indestructible and (qualitatively) converttible objects. Two classes of causes occur in nature, which, so far as experience goes, never pass one into another. The first class consists of such causes as possess the properties of weight and impenetrability; these are kinds of Matter: the other class is made up of causes which are wanting in the properties just mentioned, namely Forces, called also Imnponderables, from the negative property that has been indicated. Forces are therefore indestructible, convertible, imponderable objects. We will in the first instance take matter, to afford us an example of causes and effects. Explosive gas, H+O, and water, 11O, are related to each other as cause and effect, therefore H-+0=HO. But if H+O becomes HO, heat, cal., makes its appearance as well as water; this heat must likewise have a cause, x, and we have therefore H+0+x=HO + cal. It might, however, be asked whether H1+0 is really =HO, and x=-cal., and not perhaps H +O=cal., and x= HO, whence the above equation could equally be deduced; and so in many other cases. The phlogistic chemists recognized the equation between cal. and x, or Phlogiston as they called it, and in so doing made a great step in advance; but they involved themselves again in a system of mistakes by putting-x in place of O; thus, for instance, they obtained H=H0H +x. Chemistry, whose problem it is to set forth in equations the causal connection existing between the different kinds of MATTER AND FORCE AS CAUSES. 25 matter, teaches us that matter, as a cause, has matter for its effect; but we are equally justified in saying that to force as cause, corresponds force as effect. Since c-e, and e —c, it is unnatural to call one term of an equation a force, and the other an effect of force or phenomenon, and to attach different notions to the expressions Force and Phenomenon. In brief, then, if the cause is matter, the effect is matter; if the cause is a force, the effect is also a force. A cause which brings about the raising of a weight is a force; its effect (the raised weight) is, accordingly, equally a force; or, expressing this relation in a more general form, separation in space of ponderable objects is a force; since this force causes the fall of bodies, we call it falling force. Falling force and fall, or, more generally still, falling force and motion, are forces which are related to each other as cause and effect-forces which are convertible one into the othertwo different forms of one and the same object. For example, a weight resting on the ground is not a force: it is neither the cause of motion, nor of the lifting of another weight; it becomes so, however, in proportion as it is raised above the ground: the cause-the distance between a weight and the earth-and the effect-the quantity of motion produced-beai to each other, as we learn from mechanics, a constant relation. Gravity being regarded as the cause of the falling of bodies, a gravitating force is spoken of, and so the notions of property and of force are confounded with each other: precisely that which is the essential attribute of every forcethe union of indestructibility with convertibility-is wanting in every property: between a property and a force, between gravity and motion, it is therefore impossible to establish the equation required for a rightly-conceived causal relation. If gravity be called a force, a cause is supposed which produces effects without itself diminishing, and incorrect conceptions of the causal connections of things are thereby fostered. In 254 THE FORCES OF INORGANIC NATURE. order that a body may fall, it is no less necessary that it should be lifted up, than that it should be heavy or possess gravity; the fall of bodies ought not therefore to be ascribed to their gravity alone. It is the problem of Mechanics to develop the equations which subsist between falling force and motion, motion and falling force, and between different motions: here we will call to mind only one point. The magnitude of the falling force v is directly proportional (the earth's radius being assumed= co ) to the magnitude of the mass m, and the height d to which it is raised; that is, v —md. If the height d-1, to which the mass m is raised, is transformed into the final velocity c-=1 of this mass, we have also v —mnc; but from the known relations existing between d and c, it results that, for other values of d or of c, the measure of the force v is mce; accordingly v-=nd=mnc2: the law of the conservation of vis viva is thus found to be based on the general law of the inde structibility of causes. In numberless cases we see motion cease without having caused another motion or the lifting of a weight; but a force once in existence cannot be annihilated, it can only change its form; and the question therefore arises, What other forms is force, which we have become acquainted with as falling force and motion, capable of assuming? Experience alone can lead us to a conclusion on this point. In order to experiment with advantage, we must select implements which, besides causing a real cessation of motion, are as little as possible altered by the objects to be examined. If, for example, we rub together two metal plates, we see motion disappear, and heat, on the other hand, make its appearance, and we have now only to ask whether motion is the cause of heat. In order to come to a decision on this point, we must discuss the question whether, in the numberless cases in which the expenditure of motion is accompanied by the appearance of heat, the motion has not some other effect than the pro EFFECTS OF DESTROYED MOTION. 255 duction of heat, and the heat some other cause than the motion. An attempt to ascertain the effects of ceasing motion has never yet been seriously made; without, therefore, wishing to exclude c priori the hypothesis which it may be possible to set up, we observe only that, as a rule, this effect cannot be supposed to be an alteration in the state of aggregation of the moved (that is, rubbing, &c.) bodies. If we assume that a certain quantity of motion v is expended in the conversion of a rubbing substance qn into n, we must then have mnu+v=n, and n —m+v; and when n is reconverted into in, v must appear again in some form or other. By the friction of two metallic plates continued for a very long time, we can gradually cause the cessation of an immense quantity of movement; but would it ever occur to us to look for even the smallest trace of the force which has disappeared in the metallic dust that we could collect, and to try to regain it thence? We repeat, the motion cannot have been annihilated; and contrary, or positive and negative, motions cannot be regarded as =-O, any more than contrary motions can come out of nothing, or a weight can raise itself. Without the recognition of a causal connection between motion and heat, it is just as difficult to explain the production of heat as it is to give any account of the motion that disappears. The heat cannot be derived from the diminution of the volume of the rubbing substances. It is well known that two pieces of ice may be melted by rubbing them together in vacuo; but let any one try to convert ice into water by pressure,* however enormous. Water undergoes, as was X Since the original publication of this paper, Prof. W. Thomson has shown that pressure has a sensible effect in liquefying ice (Conf. Phil. Mag. S. 3, vol. xxxvii. p. 123); but the experiments of Bunsen and of Hopkins have shown that the melting-points of bodies which expand on becoming liquid are raised by pressure, which is all that Mayer's argument requires.-. G. C. F. 256 THiE FORCES OF INORGAIiO NATURE. found by the author, a rise of temperature when violently shaken. The water so heated (from 12~ to 13' C.) has a greater bulk after being shaken than it had before; whence now comes this quantity of heat, which by repeated shaking may be called into existence in the same apparatus as often as we please? The vibratory hypothesis of heat is an ap-proach toward the doctrine of heat being the effect of motion, but it does not favour the admission of this causal relation in its full generality; it rather lays the chief stress on uneasy oscillations (xunbehagliche Schwingungen). If it be now considered as established that in many cases (exceptio confirmat regulam) no other effect of motion can be traced except heat, and that no other cause than motion can be found for the heat that is produced, we prefer the assumption that heat proceeds from motion, to the assumption of a cause without effect and of an effect without a cause-just as the chemist, instead of allowing oxygen and hydrogen to disappear without farther investigation, and water to be produced in some inexplicable manner, establishes a connection between oxygen and hydrogen on the one hand and water on the other. The natural connection existing between falling force, motion, and heat may be conceived of as follows: We know that heat makes its appearance when the separate particles of a body approach nearer to each other; condensation produces heat. And what applies to the smallest particles of matter, and the smallest intervals between them, must also apply to large masses and to measurable distances. The falling of a weight is a diminution of the bulk of the earth, and must therefore without doubt be related to the quantity of heat thereby developed; this quantity of heat must be proportional to the greatness of the weight and its distance from the ground. From this point of view we are very easily led to the equations between falling force, motion, and heat, that have already been discussed. EQUIVALENCE OF HEAT AND MOTION. 251 But just as little as the connection between falling force and motion authorizes the conclusion that the essence of falling force is motion, can such a conclusion be. adopted in the case of heat. We are, on the contrary, rather inclined to infer that, before it can become heat, motion-whether simple, or vibratory as in the case of light and radiant heat, &c. -must cease to exist as motion. If falling force and motion are equivalent to heat, heat must also naturally be equivalent to motion and falling force. Just as heat appears as an effect of the diminution of bulk and of the cessation of motion, so also does heat disappear as a cause when its effects are produced in the shape of motion, expansion, or raising of weight. In water-mills, the continual diminution in bulk which the earth undergoes, owing to the fall of the water, gives rise to motion, which afterwards disappears again, calling forth unceasingly a great quantity of heat; and inversely, the steamengine serves to decompose heat again into motion or the raising of weights. A locomotive engine with its train may be compared to a distilling apparatus; the heat applied under the boiler passes off as motion, and this is deposited again as heat at the axles of the wheels. We will close our disquisition, the propositions of which have resulted as necessary consequences from the principle; causa sequat effectum," and which are in accordance with all the phenomena of -Nature, with a practical deduction. The solution of the equations subsisting between falling force and motion requires that the space fallen through in a given time, e. g. the first second, should be experimentally determined; in like manner, the solution of the equations subsist- ing between falling force and motion on the one hand and heat on the other, requires an answer to the question, How great is the quantity of heat which corresponds to a given quantity of motion or falling force? For instance, we must ascertain how high a given weight requires to be raised above 258 THE FORCES OF INORGANIC NATURE. the ground in order that its falling force may be equivalent to the raising of the temperature of an equal weight of water from 0~ to 1~ C. The attempt to show that such an equation is the expression of a physical truth may be regarded as the substance of the foregoing remarks. By applying the principles that have been set forth to the relations subsisting between the temperature and the volume of gases, we find that the sinking of a mercury column by which a gas is compressed is equivalent to the quantity of heat set free by the compression; and hence it follows, the ratio between the capacity for heat of air under constant pressure and its capacity under constant volume being taken as = 1-421, that the warming of a given weight of water from 0~ to 1~ C. corresponds to the fall of an equal weight from the height of about 365 metres.: If we compare with this result the working of our best steam-engines, we see how small a part only of the heat applied under the boiler is really transformed into motion or the raising of weights; and this may serve as justification for the attempts at the profitable production of motion by some other method than the expenditure of the chemical difference between carbon and oxygenmore particularly by the transformation into motion of electricity obtained by chemical means. * When the corrected specific heat of air is introduced into the calculation this number is increased, and agrees then with the experimental determinations of Mr. Joule. CELESTIAL DYNAMICS..-I N T R OD U CT IO N. VERY incandescent and luminous body diminishes in temperature and luminosity in the same degree as it radiates light and heat, and at last, provided its loss be not repaired from some other source of these agencies, becomes cold and non-luminous. For light, like sound, consists of vibrations which are communicated by the luminous or sounding body to a surrounding medium. It is perfectly clear that a body can only excite such vibrations in another substance when its own particles undergo a similar movement; for there is no cause for undulatory motion when a body is in a state of rest, or in a state of equilibrium with the medium by which it is surrounded. If a bell or a string is to be sounded, an external force must be applied; and this is the cause of the sound. If the vibratory motion of a string could take place without any resistance, it would vibrate for all time; but in this case no sound could be produced, because sound is essentially the propagation of motion; and in the same degree as the 260 CELESTIAL DYNAMICS. string communicates its vibrations to the surrounding and re, sisting medium its own motion becomes weaker and weaker, until at last it sinks into a state of rest. The sun has often and appropriately.been compared to an incessantly sounding bell. But by what means is the power of this body kept up in undiminished force so as to enable him to send forth his rays into the universe in such a grand and magnificent manner? What are the causes which counteract or prevent his exhaustion, and thus save the planetary system from darkness and deadly cold? Some endeavoured to approach'I the grand secret," as Sir Wm. Herschel calls this question, by the assumption that the rays of the sun, being themselves perfectly cold, merely cause the " substance" of heat, supposed to be contained in bodies, to pass from a state of rest into a state of motion, and that in order to send forth such cold rays the sun need not be a hot body, so that, in spite of the infinite development of light, the cooling of the sun was a matter not to be thought of. It is plain that nothing is gained by such an explanation; for, not to speak of the hypothetical 1" substance" of heat, assumed to be at one time at rest and at another time in motion, now cold and then hot, it is a well-founded fact that the sun does not radiate a cold phosphorescent light, but a light capable of warming bodies intensely; and to ascribe such rays to a cold body is at once at variance with reason and experience. Of course such and similar hypotheses could not satisfy the demands of exact science, and I will therefore try to explain in a more satisfactory manner than has been done up to this time the connexion between the sun's radiation and its effects. In doing so, I have to claim the indulgence of scientific men, who are acquainted with the difficulties of my task. SOURCES OF HEAT. 261 II.-SOURCES OF HEAT. BEFORE we turn our attention to the special subject of this paper, it will be necessary to consider the means by which light and heat are produced. Heat may be obtained from very different sources. Combustion, fermentation, putrefaction, slaking of lime, the decomposition of chloride of nitrogen and of gun-cotton, &c. &c., are all of them sources of heat. The electric spark, the voltaic current, friction, percussion, and the vital processes are also accompanied by the evolution of this agent. A general law of nature, which knows of no exception, is the following: —In order to obtain heat, something must be expended; this something, however different it may be in other respects, can always be referred to one of two categories: either it consists of some material expended in a chemical process, or of some sort of mechanical work. When substances endowed with considerable chemical affinity for each other combine chemically, much heat is developed during the process. We shall estimate the quantity of heat thus set free by the number of kilogrammes of water which it would heat 1~ C. The quantity of heat necessary to raise one kilogramme of water one degree is called a unit of heat. It has been established by numerous experiments that the combustion of one kilogramme of dry charcoal in oxygen, so as to form carbonic acid, yields 7200 units of heat, which fact may be briefly expressed by saying that charcoal furnishes 7200~ degrees of heat. Superior coal yields 6000~, perfectly dry wood from 3300~ to 39000, sulphur 2700, and hydrogen 34,600~ of heat. According to experience, the number of units of heat only depends on the quantity of matter which is consumed, and 262 CELESTIAL DYNAMICS. not on the conditions under which the burning takes place. The same amount of heat is given out whether the combustion proceeds slowly or quickly, in atmospheric air or in pure oxygen gas. If in one case a metal be burnt in air and the amount of heat directly measured, and in another instance the same quantity of metal be oxidized in a galvanic battery, the heat being developed in some other place —say, the wire which conducts the current,-in both of these experiments the. same quantity of heat will be observed. The same law also holds good for the production of heat by mechanical means. The amount of heat obtained is only dependent on the quantity of power consumed, and is quite independent of the manner in which this power has been expended. If, therefore, the amount of heat which is produced by certain mechanical work is known, the quantity which will be obtained by any other amount of mechanical work can easily be found by calculation. It is of no consequence whether this work consists in the compression, percussion, or friction of bodies. The amount of mechanical work done by a force may be expressed by a weight, and the height to which this weight would be raised by the same force. The mathematical expression for 1" work done," that is to say, a measure for this work, is obtained by multiplying the height expressed in feet or other units by the number of pounds or kilogrammes lifted to this height. We shall take one kilogramme as the unit of weight, and one metre as the unit of height, and we thus obtain the weight of one kilogramme raised to the height of one metre as a unit measure of mechanical work performed. This measure we shall call a kilogrammetre, and adopt for it the symbol Kmn. Mechanical work may likewise be measured by the velo city obtained by a given weight in passing from a state of rest into that of motion. The work done is then expressed by SOURCES OF HEAT. 263 the product obtained by the multiplication of the weight by the square of its velocity. The first method, however, because it is the more convenient, is the one usually adopted; and the numbers obtained therefrom may easily be expressed in other units. The product resulting from the multiplication of the number of units of weight and measures of height, or, as it is called, the product of mass and height, as well as the product of the mass and the square of its velocity, are called "6 vis viva of motion," " mechanical effect," dynamical effect," 6" work done," " qcuantite de travail," &c. &c. The amount of mechanical work necessary for the heating of 1 kilogramme of water 1~ C has been determined by experiment to be - 367 Km; therefore Km = 0'00273 units of heat.* A mass which has fallen through a height of 367 metres possesses a velocity of 84'8 metres in one second; a mass, therefore, moving with this velocity originates 10 C. of heat when its motion is lost by percussion, friction, &c. If the velocity be two or three times as great, 4~ or 9~ of heat will be developed. Generally speaking, when the velocity is c metres, the corresponding development of heat will be expressed by the formula 0~0001390 X c0. * This essay was published in 1845. At that time de la Roche and Berard's determination of the specific heat of air was generally accepted. If the physical constants used by Mayer be corrected according to the results of more recent investigation, the mechanical equivalent of heat is found to be 771'4 foot-pounds. Mr. Joule finds it = 772 foot-pounds. — TR. 264 CELESTIAL DYNAMIOC$ III. —MEASURE OF THE SUN'S HEAT. THE actinometer is an instrument invented by Sir John Herschel for the purpose of measuring the heating efffect produced by the sun's rays. It is essentially a thermometer with a large cylindrical bulb filled with a blue liquid, which is acted upon by the sun's rays, and the expansion of which is measured by a graduated scale. From observations made with this instrument, Sir John Herschel calculates the amount of heat received from the sun to be sufficient to melt annually at the surface of the globe a crust of ice 29'2 metres in thickness. Pouillet has recently shown by some careful experiments with the lens pyrheliometer, an instrument invented by himself, that every square centimetre of the surface of our globe receives, on an average, in one minute an amount of solar heat which would raise the temperature of one gramme of water 0'4408~. Not much more than one-half of this quantity of heat, however, reaches the solid surface of our globe, since a considerable portion of it is absorbed by our atmosphere. The layer of ice which, according to Pouillet, could be melted by the solar heat which yearly reaches our globe would have a thickness of 30'89 metres. A square metre of our earth's surface receives, therefore, according to Pouillet's results, which we shall adopt in the following pages, on an average in one minute 4'408 units of heat. The whole surface of the earth is - 9,260,500 geographical square miles*"; consequently the earth receives in one minute 2247 billions of units of heat from the sun. In order to obtain smaller numbers, we shall call the quantity of heat necessary to raise a cubic mile of water 1~ * The geographical mile = 7420 metres, and one English mile = 1608 metres. MIEASURE OF THE SUN S HEAT. 265 C. in temperature, a cubic mile of heat. Since one cubic mile of water weighs 408'54 billions of kilogrammnes, a cubic mile of heat contains 408~54 billions of units of heat. The effect produced by the rays of the sun on the surface of the earth in one minute is therefore 5'5 cubic miles of heat. Let us imagine the sun to be surrounded by a hollow sphere whose radius is equal to the mean distance of the earth from the sun, or 20,589,000 geographical miles; the surface of this sphere would be equal to 5326 billions of square miles. The surface obtained by the intersection of this hollow sphere and our globe, or the base of the cone of solar light which reaches our earth, stands to the whole surface of this hollow sphere as 4: 5326 billions, or as 1 to 2300 millions. This is the ratio of the heat received by our globe to the whole amount of heat sent forth from the sun, which latter in one minute amounts to 12,650 millions of cubic miles of heat. This amazing radiation ought, unless the loss is by some means made good, to cool considerably even a body of the magnitude of the sun. If we assume the sun to be endowed with the same capacity for heat as a mass of water of the same volume, and its loss of heat by radiation to affect uniformily, its whole mass, the temperature of the sun ought to decrease 1~08 C. yearly, and for the historic time of 5000 years this loss would consequently amount to 90000 C. A uniform cooling of -the whole of the sun's huge mass cannot, however, take place; on the contrary, if the radiation were to occur at the expense of a given store of heat or radiant power, the sun would become covered in a short space of time with a cold crust, whereby radiation would be brought to an end. Considering the continued activity of the sun through countless centuries, we may assime with mathematical certainty the existence of some compensating influence to make good its enormous loss. 12 266 CELESTIAL DYNAMICS. Is this restoring agency a chemical process? If such were the case, the most favourable assumption would be to suppose the whole mass of the sun to be one lump of coal, the combustion of every kilogramme of which produces 6000 units of heat. Then the sun would only be able to sustain for forty-six centuries its present expenditure of light and heat, not to mention the oxygen necessary to keep up such an immense combustion, and other unfavourable circumstances. The revolution of the sun on his axis has been suggested as the cause of his radiating energy. A closer examination proves this hypothesis also to be untenable. Rapid rotation, without friction or resistance, cannot in itself alone be regarded as a cause of light and heat, especially as the sun is in no way to be distinguished fromn the other bodies of our system by velocity of axial rotation. The sun turns on his axis in about twenty-five days, and his diameter is nearly 112 times as great as that of the earth, from which it follows that a point on the solar equator travels but a little more than four times as quickly as a point on the earth's equator. The largest planet of the solar system, whose diameter is about'0th that of the sun, turns on its axis in less than ten hours; a point on its equator revolves about six times quicker than one on the solar equator. The outer ring of Saturn exceeds the sun's equator more than ten times in velocity of rotation. Nevertheless no generation of light or heat is observed on our globe, on Jupiter, or on the ring of Saturn. It might be thought that friction, though undeveloped in the case of the other celestial bodies, might be engendered by the sun's rotation, and that such friction might generate enormnous quantities of heat. But for the production of friction two bodies, at least, are always necessary which are in immediate contact with one another, and which move -with different velocities or in different directions. Friction, moreover, MEASURS E OF THE SUN' S HEAT. 267 has a tendency to produce equal motion of the two rubbing bodies; and when this is attained, the generation of heat ceases. If now the sun be the one moving body, where is the other? and if the second body exist, what power prevents it from assuming the same rotary motion as the sun? But could even these difficulties be disregarded, a weightier and more formidable obstacle opposes this hypothesis. The known volume and mass of the sun allow us to calculate the vis vivca which he possesses in consequence of his rotation. Assuming his density to be uniform throughout his mass, and his period of rotation twenty-five days, it is equal to 182,300 quintillions of kilogrammetres (Km). But for one unit of heat generated, 367 ]Km are consumed; consequently the whole rotation-effect of the sun could only cover the expenditure of heat for the space of 183 years. The space of our solar' system is filled with a great number of ponderable objects, which have a tendency to move towards the centre of gravity of the sun; and in so doing, their rate of motion is more and more accelerated. A mass, without motion, placed within the sphere of the sun's attraction, will obey this attraction, and, if there be no disturbing infiuences, will fall in a straight line into the sun. In reality, however, such a rectilinear path can scarcely occur, as may be shown by experiment. Let a weight be suspended by a string so that it can only touch the floor in one point. Lift the weight up to a certain height, and at the same time stretch the string out to its full length; if the weight be now allowed to fall, it will be observed, almost in every case, not to reach at once the point on the floor towards which it tends to move, but to move round this point for some time in a curved line. The reason of this phenomenon is that the slightest deviation of the weight from its shortest route towards the point on the floor, caused by some disturbing influence such as the resistance of the air against a not perfectly uniform surface, 268 CELESTIAL DYNAMICS. will maintain itself as long as motion lasts. It is neverthe. less possible for the weight to move at once to the point; the probability of its doing so, however, becomes the less as the height from which it is allowed to drop increases, or the string, by means of which it is suspended, is lengthened. Similar laws influence the movements of bodies in the space of the solar system. The height of the fall is here represented by the original distance froml the sun at which the body begins to move; the length of the string by the sun's attraction, which increases when the distance decreases; and the small surface of contact on the floor by the area of the section of the sun's sphere. If now a cosmical mass within the physical limits of the sun's sphere of attraction begins its fall towards that heavenly body, it will be disturbed in its long path for many centuries, at first by the nearest fixed stars, and afterwards by the bodies of the solar system. Motion of such a mass in a straight line, or its perpendicular fall into the sun, would, therefore, under such conditions, be impossible. The observed movement of all planetary bodies in closed curves agrees with this. We shall now return to the example of the weight suspended by a string and oscillating round a point towards which it is attracted. The diameters of the orbits described by this weight are observed to be nearly equal; continued observation, however, shows that these diameters gradually diminish in length, so that the weight will by degrees approach the point in which it can touch the floor. The weight, however, touches the floor not in a mathematical point, but in a small surface; as soon, therefore, as the diameter of the curve in which the weight moves is equal to the diameter of this surface, the weight will touch the floor. This final contact is no accidental or improbable event, but a necessary phenomenon caused by the resistance which the oscillating mass constantly suffers from the air and friction. If all resistance could be annihilated, the motion of the weight would of course continue in equal oscillations. MEASUHE OF THE SUN'S HEAT. 269 The same law holds good for celestial bodies. The movements of celestial bodies in an absolute vacuum would be as uniform as those of a mathematical pendulum, whereas a resisting medium pervading all space would cause the planets to move in shorter and shorter orbits, and at last to fall into the sun. Assuming such a resisting medium, these wandering celestial bodies must have on the periphery of the solar system their cradle, and in its centre their grave; and however long the duration, and however great the number of their revolutions may be, as many masses will on the average in a certain time arrive at the sun as formerly in a like period of time came within his sphere of attraction. All these bodies plunge with a violent impetus into their common grave. Since no cause exists without an effect, each of these cosmical masses will, like a weight falling to the earth, produce by its percussion an amount of heat proportional to its vis viva. From the idea of a sun whose attraction acts throughout space, of ponderable bodies scattered throughout the universe, and of a resisting' a3ther, another idea necessarily followsthat, namely, of a continual and inexhaustible generation of heat on the central body of this cosmical system. Whether such a conception be realized in our solar system -whether, in other words, the wonderful and permanent evolution of light and heat be caused by the uninterrupted fall of cosmical matter into the sun-twill now be more closely examined. The existence of matter in a primordial condition ( Uimaterie), moving about in the universe, and assumed to follow the attraction of the nearest stellar system, will scarcely be denied by astronomers and physicists; for the lichness of surrounding nature, as well as the aspect of the starry heavens, prevents the belief that the wide space which separates our solar system from the regions governed by the other fixed 270 CELESTIAL DYNAMICS. stars is a vacant solitude destitute of matter. We shall leave, however, all suppositions concerning subjects so distant from us both in time and space, and confine our attention exclusively to what may be learnt from the observation of the existing state of things. Besides the fourteen known planets with their eighteen satellites, a great many other cosmical masses move within the space of the planetary system, of which the comets deserve to be mentioned first. Kepler's celebrated statement that "' there are more comets in the heavens than fish in the ocean," is founded on the fact that, of all the comets belonging to our solar system, comparatively few can be seen by the inhabitants of the earth, and therefore the not inconsiderable number of actually observed comets obliges us, according to tile rules of the calculus of probabilities, to assume the existence of a great many more beyond the sphere of our vision. Besides planets, satellites, and comets, another class of celestial bodies exists within our solar system. These are masses which, on account of their smallness, may be considered as cosmical atoms, and which Arago has appropriately called asteroids. They, like the planets and the comets, are governed by gravity, and move in elliptical orbits round the sun. When accident brings them into the immediate neighbourhood of the earth, they produce the phenomena of shooting-stars and fireballs. It has been shown by repeated observation, that on a bright night twenty minutes seldom elapse without a shootingstar being visible to an observer in any situation. At certain times these meteors are observed in astonishingly great numbers; during the meteoric shower at Boston~, which lasted nine hours, when they were said to fall "6 crowded together like snow-flakes," they were estimated as at least 240,000. On the whole, the number of asteroids which come near the earth in the space of a year must be computed to be many MEASURcE OF ATHE SUN9S HEAT. 271 thousands of millions. This, without doubt, is only a small fraction of the number of asteroids that move round the sun, which number, according to the rules of the calculus of probabilities, approaches the infinite. As has been already stated, on the existence of a resisting sether it depends whether the celestial bodies, the planets, the comets, and the asteroids move at constant mean distances round the sun, or whether they are constantly approaching that central body. Scientific men do not doubt the existence of such an rather. Littrow, amongst others, expresses himself on this point as follows:-" The assumption that the planets and the comets move in an absolute vacuum can in no way be admitted. Even if the space between celestial bodies contained no other matter than that necessary for the existence of light (whether light be considered as emission of matter or the undulations of a universal sether), this alone is sufficient to alter the motion of the planets in the course of time and the arrangement of the whole system itself; the fall of all the planets and the comets into the sun and the destruction of the present state of the solar system must be the final results of this action." A direct proof of the existence of such a resisting medium has been furnished by tile academician Encke. iIe found that the comet named after him, which revolves round the sun in the short space of 1207 days, shows a regular acceleration of its motion, in consequence of which the time of each revolution is shortened by about six hours. From tile great density and magnitude of the planets, the shortening of the diameters of their orbits proceeds, as night be expected, very slowly, and is up to the present time inappreciable. The smaller the cosmical masses are, on the contrary, other circumstances remaining the same, the faster they move towards the sun; it may therefore happen that in a space of time wherein the mean distance of the earth from the sun would diminish one metre, a small asteroid would 272 CELESTIAL DYNAMiICS. travel more than one thousand miles towards the central body. As cosmical masses stream from all sides in immense numbers towards the sun, it follows that they must become more and more crowded together as they approach thereto. The conjecture at once suggests itself that the zodiacal light, the nebulous light of vast dimensions which surrounds the sun, owes its origin to such closely-packed asteroids, However it may be, this much is certain, that this phenomenon is caused by matter which moves according to the same laws as the planets round the sun, and it consequently follows that the whole mass which originates the zodiacal light is continually approaching the sun and falling into it. This light does not surround the sun uniformly on all sides; that is to say, it has not the form of a sphere, but that of a thin convex lens, the greater diameter of which is in the plane of the solar equator, and accordingly it has to an observer on our globe a pyramidal form. Such lenticular distribution of the masses in the universe is repeated in a remarkable manner in the disposition of the planets and the fixed stars. From the great number of cometary masses and asteroids and the zodiacal light on the one hand, and the existence of a resisting sether on the other, it necessarily follows that ponderable matter must continually be arriving on the solar surface. The effect produced by these masses evidently depends on their final velocity; and, in order to determine the latter, we shall discuss some of the elements of the theory of gravitation. The final velocity of a weight attracted by and moving towards a celestial body will become greater as the height through which the weight falls increases. This velocity, however, if it be only produced by the fall, cannot exceed a certain magnitude; it has a maximum, the value of which depends on the volume and mass of the attracting celestial body. MEASURE OF THE SUN'S HE A T. 273 Let r be the radius of a spherical and solid celestial body; and g the velocity at the end of the first second of a weight falling on the surface of this body; then the greatest velocity which this weight can obtain by its fall towards the celestial body, or the velocity with which it will arrive at its surface after a fall from an infinite height, is 4/2gr in one second. This number, wherein g and r are expressed in metres, we shall call G. For our globe the value of g is 9'8164.. and that of r 6,369,800; and consequently on our earth G = i/(2X 9.8164 X 6,369,800) _ 11,183. Tilhe solar radius is 112'05 times that of the earth, and the velocity produced by gravity on the sun's surface is 28'36 times greater than the same velocity on the surface of our globe; the greatest velocity therefore which a body could obtain in consequence of the solar attraction, or G = V(28.36 x 112.05) X 11,183 = 630,400; that is, this maximum velocity is equal to 630,400 metres, or 85 geographical miles in one second. By the help of this constant number, which may be called the characteristic of the solar System, the velocity of a body in central motion may easily be determined at any point of its orbit. Let a be the mean distance of the planetary body from the centre of gravity of the sun, or the greater semidiameter of its orbit (the radius of the sun being taken as unity); and let h be the distance of the same body at any point of its orbit from the centre of gravity of the sun; then the velocity, expressed in metres, of the planet at the distance h is 2a-]h GX4/ 2aXh' At the moment the planet comes in contact with the solar sur. face, h is equal to 1, and its velocity is therefore 12* Cx9/ 2a 274 CELESTIAL DYNAMICSo It follows from this formula that the smaller 2a (or the major axis of the orbit of a planetary body) becomes, the less will be its velocity when it reaches the sun. This velocity, like the major axis, has a minimum; for so long as the planet moves outside the sun, its major axis cannot be shorter than the diameter of the sun, or, taking the solar radius as a unit, the quantity 2a can never be less than 2. The smallest velocity with which we can imagine a cosmical body to arrive on the surface of the sun is consequently.-X / -=445,750, or a velocity of 60 geographical miles in one second. For this smallest value the orbit of the asteroid is circular; for a larger value it becomes elliptical, until finally, with increasing excentricity, when the value of 2a approaches infinity, the orbit becomes a parabola. In the last case the velocity is GX /o G-1 or, 85 geographical miles in one second. If the value of the major axis become negative, or the orbit assume the form of a hyperbola, the velocity may increase without end. But this could only happen when cosmical masses enter the space of the solar system with a projected velocity, or when masses, having missed the sun's surface, move into the universe and never return; hence a velocity greater than G can only be regarded as a rare exception, and we shall therefore only consider velocities comprised within the limits of 60 and 80 miles.`The final velocity with which a weight moving in a - The relative velocity also with which an asteroid reaches the solar surface depends in some degree on the velocity of the sun's rotation. This, however, as well as the rotatory effect of the asteroid, is without moment, and may be neglected. MEASURE OF THE SUNSS nH EAT. 275 straight line towards the centre of the sun arrives at the solar surface is expressed by the formula wherein c expresses the final velocity in metres, and h the original distance from the centre of the sun in terms of solar radius. If this formula be compared with the foregoing, it will be seen that a mass which, after moving in central motion, arrives at the sun's surface has the same velocity as it would possess had it fallen perpendicularly into the sun from a distance* equal to the major axis of its orbit; whence it is apparent that a planet, on arriving at the sun, moves at least as quickly as a weight which falls freely towards the sun from a distance as great as the solar radius, or 96,000 geographical miles. What thermal effect corresponds to such velocities? Is the effect sufficiently great to play an important part in the immense development of heat on the sun? This crucial question may be easily answered by help of the preceding considerations. According to the formula given at the end of Chapter II., the degree of heat generated by percussion is - 00001390 X c2 where c denotes the velocity of the striking body expressed in metres. The velocity of an asteroid when it strikes the sun measures from 445,750 to 630,400 metres; the caloric effect of the percussion is consequently equal to from 27~ to 55 millions of degrees of heatt. An asteroid, therefore, by its fall into the sun developes *- This distance is to be counted from the centre of the sun. t Throughout this memoir the degrees of heat are expressed in the Centigrade scale. Unless stated to the contrary, the measures of length are given in geographical miles. A geographical mile = 7420 metres, and an English mile = 1608 metres.-Ta. 276 CELESTIAL DYNAMICS. from 4600 to 9200 times as much heat as would be generated by the combustion of an equal mass of coal. IV.-ORIGIN OF THE SUN'S HEAT. THE question why the planets move in curved orbits, one of the grandest of problems, was solved by Newton in consequence, it is believed, of his reflecting on the fall of an ap~ ple. This story is not improbable, for we are on the right track for the discovery of truth when once we clearly recognize that between great and small no qualitative but only a quantitative difference exists-when we resist the suggestions of an ever active imagination, and look for the same laws in the greatest as well as in the smallest processes of nature. This universal range is the essence of a law of nature, and the touchstone of tile correctness of human theories. We observe the fall of an apple, and investigate the law which governs this phenomenon; for the earth we substitute the sun, and for the apple a planet, and thus possess ourselves of the key to the mechanics of the heavens. As the same laws prevail in the greater as well as in the smaller processes of nature, Newton's method may he used in solving the problem of the origin of the sun's heat. We know the connexion between the space through which a body falls, the velocity, the vis viva, and the generation of heat on the surface of this globe; if we again substitute for the earth the sun, with a mass 350,000 greater, and for a height of a few metres celestial distances, we obtain a generation of heat exceecling all terrestrial measures. And since we have sufficient reason to assume the actual existence of such mechanical processes in the heavens, we find therein the only tenable explanation of the origin of the heat of the sun. ORIGIN OF TNE SUN'IS HEAT. 277 The fact that the development of heat by mechanical means on the surface of our globe is, as a rule, not so great, and cannot be so great as the generation of the same agent by chemical means, as by combustion, follows fron the laws already discussed; and this fact cannot be used as an argumeut against the assumption of a greater development of heat by a greater expenditure of mechanical work. It has been shown that the heat generated by a weight falling from a height of 367 metres is only 0-0'th part of the heat produced by the combustion of the same weight of coal; just as small is the amount of heat developed by a weight moving with the not inconsiderable velocity of 85 metres in one second. But, according to the laws of mechanics, the effect is proportional to the square of the velocity; if therefore the weight move 100 times faster, or with a velocity of 8500 metres in one second, it will produce a greater effect than the combustion of an equal quantity of coal. It is true that so great a velocity cannot be obtained by human means; everyday experience, however, shows the development of high degrees of temperature by mechanical processes. In the common flint and steel, the particles of steel which are struck off are sufficiently heated to burn in air. A few blows directed by a skilful blacksmith with a sledge-hammer against a piece of cold metal may raise the temperature of the metal at the points of collision to redness. The new crank of a steamer, whilst being polished by friction, becomes red-hot, several buckets of water being required to cool it down to its ordinary temperature. When a railway train passes with even less than its ordinary velocity along a very sharp curve of the line, sparks are observed in consequence of the friction against the rails. One of the grandest constructions for the production of motion by human art is the channel in which the wood was allowed to glide down from the steep and lofty sides of Mount 2T8 CELESTIAL DYNAMICS. Pilatus into the plain below. This wooden channel which was built about thirty years ago by the engineer IRupp, was 9 English miles in length; the largest trees were shot down it fromnthe top to the bottom of the mountain in about two minutes and a half. The momentum possessed by the trees on their escaping at their journey's end from the channel was sufficiently great to bury their thicker ends in the ground to the depth of from 6 to 8 metres. To prevent the wood getting too hot and taking fire, water was conducted in many places into the channel. This stupendous mechanical process, when compared with cosmical processes on the sun, appears infinitely small. In the latter case it is the mass of the sun which attracts, and in lieu of the height of Mount IPilatus we have distances of a hundred thousand and more miles; the amount of heat generated by cosmical falls is therefore at least 9 million times greater than in our terrestrial example. Rays of heat on passing through glass and other transparent bodies undergo partial absorption, which differs in degree, however, according to the temperature of the source from which the heat is derived. Heat radiated from sources less warm than boiling water is almost completely stopped by thin plates of glass. As the temperature of a source of heat increases, its rays pass more copiously through diathermic bodies. A plate of glass, for example, weakens the rays of a red-hot substance, even when the latter is placed very close to it, much more than it does those emanating at a much greater distance from a white-hot body. If the quality of the sun's rays be examined in this respect, their diathermic energy is found to be far superior to that of all artificial sources of heat. The temperature of the focus of a concave metallic reflector in which the sun's light has been collected is only diminished from one-seventh to one-eighth by the interposition of a screen of glass. If the same experiment be ORIGIN OF THE SUNS9 HEAT. 7 made with an artificial and luminous source of heat, it is found that, though the focus be very hot when the screen is away, the interposition of the latter cuts off nearly all the heat; moreover, the focus will not recover its former temperature when reflector and screen are placed sufficiently near to the source of heat to make the focus appear brighter than it did in the former position without the glass screen. The empirical law, that the diathermic energy of heat increases with the temperature of the source from which the heat is radiated, teaches us that the sun's surface must be much hotter than the most powerful process of combustion could render it. Other methods furnish the same conclusion. If we imagine the sun to be surrounded by a hollow sphere, it is clear that the inner surface of this sphere must receive all the heat radiated from the sun. At the distance of our globe from the sun, such a sphere would have a radius 215 times as great, and an area 46,000 times as large as the sun himself; those luminous and calorifie rays, therefore, which meet this spherical surface at right angles retain only 000th part of their original intensity. If it be further considered that our atmosphere absorbs a part of the solar rays, it is clear that the rays which reach the tropics of our earth at noonday can only possess from,0,-0th to 6090th of the power with which they started. These rays, when gathered from a surface of from 5 to 6 square metres, and concentrated in an area of one square centimetre, would produce about the temperature which exists on the sun, a temperature more than sufficient to vaporize platinum, rhodium, and similar metals. The radiation calculated in Chapter III. likewise proves the enormous temperature of the solar surface. From the determination mentioned therein, it follows that each square centimetre of the sun's surface loses by radiation about 80 units of heat per minute —an immense quantity in comparison with terrestrial radiations. 280 CELESTIAL DYNAMICS. A correct theory of the origin of the sun's heat must explain the cause of such enormous temperatures. This explanation can be deduced from the foregoing statements. According to Pouillet, the temperature at which bodies appear intensely white-hot is about 1500~ C. The heat generated by the combustion of one kilogramme of hydrogen is, as determined by Dulong, 34,500, and according to the more recent experiments of Grassi, 34,666 units of heat. One part of hydrogen combines with eight parts of oxygen to form water; hence one kilogramme of these two gases mixed in this ratio would produce 3850~. Let us now compare this heat with the amount of the same agent generated by the fall of an asteroid into the sun. Without taking into account the low specific heat of such masses when compared with that of water, we find the heat developed by the asteroid to be from 7000 to 15,000 times greater than that of the oxyhyclrogen mixture. From data like these, the extraordinary diathermic energy of the sun's rays, the immense radiation from his surface, and the high temperature in the focus of the reflector are easily accounted for. The facts above mentioned show that, unless we assume on the sun the existence of matter with unheard of chemical properties as a deus ex machind, no chemical process could maintain the present high radiation of the sun; it also follows from the above results, that the chemical nature of bodies which fall into the sun does not in the least affect our conclusions; the effedt produced by the most inflammable substance would not differ by one-thousandth part from that resulting from the fall of matter possessing but feeble chemical affinities. As the brightest artificial light appears dark in comparison with the sun's light, so the mechanical processes of the heavens throw into the shade the most powerful chenmical actions. The quality of the sun's rays, as dependent on his temper ORIGIN OF THE SUN S HEAT. 281 ature, is of the greatest importance to mankind. If the solar heat were originated by a chemical process, and amounted near its source to a temperature of a few thousand degrees: it would be possible for the light to reach us, whilst the greater part of the more important calorific rays would be absorbed by the higher strata of our atmosphere and then returned to the universe. In consequence of the high temperature of the sun, however, our atmosphere is highly diathermic to his rays, so that the latter reach the surface of our earth and warm it. The comparatively low temperature of the terrestrial surface is the cause why the heat cannot easily radiate back through the atmosphere into the universe. The atmosphere acts, therefore, like an envelope, which is easily pierced by the solar rays, but which offers considerable resistance to the radiant heat escaping from our earth; its action resembles that of a valve which allows liquid to pass freely in one, but stops the flow in the opposite direction. The action of the atmosphere is of the greatest importance as regards climate and meteorological processes. It must raise the mean temperature of the earth's surface. After the setting of the sun-in fact, in all places where his rays do not reach the surface, the temperature of the earth would soon be as low as that of the universe, if the atmosphere were removed, or if it did not exist. Even the powerful solar rays in the tropics would be unable to preserve water in its liquid state. Between the great cold which would reign at all times and in all places, and the moderate warmth which in reality exists on our globe, intermediate temperatures may be imagined; and it is easily seen that the mean temperature would decrease if the atmosphere were to become more and more rare. Such a rarefaction of a valve-like acting atmosphere actually takes place as we ascend higher and higher above 282 CELESTIAL DYNAMICS. the level of the sea, and it is accordingly and necessarily accompanied by a corresponding diminution of temperature. This well-known fact of the lower mean temperature of places of greater altitude has led to the strangest hypotheses. The sun's rays were not supposed to contain all the conditions for warming a body, but to set in motion the " substance" of heat contained in the earth. This " substance" of heat, cold when at rest, was attracted by the earth, and was therefore found in greater abundance near the centre of the globe. This view, it was thought, explained why the warming power of the sun was so much weaker at the top of a mountain than at the bottom, and why, in spite of his immense radiation, he retained his full powers. This belief, which especially prevails amongst imperfectly informed people, and which will scarcely succumb to correct views, is directly contradicted by tile excellent experiments made by Pouillet at different altitudes with the pyrheliometer. These experiments show that, everything else being equal, the generation of heat by the solar rays is more powerful in higher altitudes than near the surface of our globe, and that consequently a portion of these rays is absorbed on their passage through the atmosphere. Why, in spite of this partial absorption, the mean temperature of low altitudes is nevertheless higher than it is in more elevated positions, is explained by the fact that the atmosphere stops to a far greater degree the calorific rays emanating from the earth than it does those from the sun. V.-CONSTANCY OF THE SUN'S MfASS. NEWTON, as is well known, considered light to be the emission of luminous particles from the sun. In the continued emission of light this great philosopher saw a cause tend CONSTANCY OF THE SUN'S MASS. 283 ing to diminish the solar mass; and he assumed, in order to make good this loss, comets and other cosmical masses to be continually falling into the central body. If we express this view of Newton's in the language of the undulatory theory, which is now universally accepted, we obtain the results developed in the preceding pages. It is true that our theory does not accept a peculiar 6' substance of light or of heat; nevertheless, according to it, the radiation of light and heat consists also in purely material processes, in a sort of motion, in the vibrations of ponderable resisting substances. Quiescence is darkness and death; motion is light and life. An undulating motion proceeding from a point or a plane and excited in an unlimited medium, cannot be imagined apart from another simultaneous motion, a translation of the particles themselves;: it therefore follows, not only from the emission, but also from the undulatory theory, that radiation continually diminishes the mass of the sun. Why, nevertheless, the mass of the sun does not really diminish has already been stated. The radiation of the sun is a centrifugal action equivalent to a centripetal motion. The caloric effect of the centrifugal action of the sun can be found by direct observation; it amounts, according to Chapter III., in one minute to 12,650 millions of cubic miles of heat, or 5'17 quadrillions of units of heat. In Chapter IV. it has been shown that one kilogramme of the mass of an asteroid originates from 27'5 to 55 millions of units of heat; the quantity of cosmical masses, therefore, which falls every minute into the sun amounts to from 94,000 to 188,000 billions of kilogrammes. To obtain this remarkable result, we made use of a method e This centrifugal motion is perhaps the cause of the repulsion of the tails on comets when in the neighbourhood of the sun, as observed by Bessel. 284 CELESTIAL DYNAMICS. which is common in physical inquiries. Observation of the moon's motion reveals to us the external form of the earth. The physicist determines with the torsion-balance -the weight of a planet, just as the merchant finds the weight of a parcel of goods, whilst the pendulum has become a magic power in the hands of the geologist, enabling him to discover cavities in the bowels of the earth. Our case is similar to these. By observation and calculation of the velocity of sound in our atmosphere, we obtain the ratio of the specific heat of air under constant pressure and vunder constant volume, and by the help of this number we determine the quantity of heat generated by mechanical work. The heat which arrives from the sun in a given time on a small surface of our globe serves as a basis for the calculation of the whole radiating effect of the sun; and the result of a series of observations and well-:founded conclusions is the quantitative determination of those cosmical masses which the sun receives from the space through which he sends forth his rays. Measured by terrestrial standards, the ascertained number of so many billions of kilogrammes per minute appears in. credible. This quantity, however, may be brought nearer to our comprehension by comparison with other cosmical magnitudes. The nearest celestial body to us (the moon) has a mass of about 90,000 trillions of kilogrammes, and it would therefore cover the expenditure of the sun for from one to two years. The mass of the earth would afford nourishment to the sun for a period of from 60 to 120 years.. To facilitate the appreciation of the masses and the distances occurring in the planetary system, Herschel draws the following picture. Let the sun be represented by a globe 1 metre in diameter. The nearest planet (Mercury) will be about as large as a pepper-corn, 3- millimetres in thickness, at a distance of 40 metres. 78 and 107 metres distant from the sun will move Venus and the Earth, each 9 millimetres in diameter, or a little larger than a pea. Not much more than CONSTANCY OF THE SUN'S MASS. 285 a quarter of a metre from the Earth will be the Moon, the size of a mustard seed, 2l millimetres in diameter. MIars, at a distance of 160 metres, will have about half the diamneter of the Earth; and the smaller planets (Vesta, Elebe, Astrea, Juno, Pallas, Ceres, &c.), at a distance of from 250 to 300 metres from the sun, will resemble particles of sand. Jupiter and Saturn, 560 and 1000 metres distant from the centre, will be represented by oranges, 10 and 9 centimetres in diameter. Uranus, of the size of a nut 4 centimetres across, will be 2000 metres; and Neptune, as large as an apple 6 centimetres in diameter, will be nearly twice as distant, or about half a geographical mile away from the sun. From Neptune to the nearest fixed star will be more than 2000 geographical miles. To complete this picture, it is necessary to imagine finelydivided matter grouped in a diversified manner, moving slowly and gradually towards the large central globe, and on its arrival attaching itself thereto; this matter, when favourably illuminated by the sun, represents itself to us as the zodiacal light. This nebulous substance forms also an important part of a creation in which nothing is by chance, but wherein all is arranged with Divine foresight and wisdom. The surface of the sun measures 115,000 millions of square miles, or 61 trillions of square metres; the mass of matter which in the shape of asteroids falls into the sun every minute is from 94,000 to 188,000 billions of kilogrammes; one square metre of solar surface, therefore, receives on an average from 15 to 30 grammes of matter per minute. To compare this process with a terrestrial' phenomenon, a gentle rain mlay be considered which sends down in one hour a layer of water 1 millimetre in thickness (during a thunlderstorm the rainfall is often from ten to fifteen times this quantity); this amounts on a square metre to 17 grammes per minute. 286 CELESTIAL DYNAMICS. The continual bombardment of the sun by these cesmical masses ought to increase its volume as well as its mass, if centripetal action only existed. The increase of volume, could scarcely be appreciated by man; for if the specific gravity of these cosmical masses be assumed to be the same as that of the sun, the enlargement of his apparent diameter to tile extent of one second, the smallest appreciable magnitude, would require from 33,000 to 669000 years. Not quite so inappreciable would be the increase of the mass of the sun. If this mass, or the weight of the sun, were augmented, an acceleration of the motion of the planets in their orbits would be the consequence, whereby their times of revolution round the central body would be shortened, The mass of the sun is 2o1 quintillions of kilogrammes; and the mass of the cosmical matter annually arriving at the sun stands to the above as 1 to from 21 -42 millions. Such an augmentation of the weight of the sun ought to shorten the sidereal year from 42,0000th to 85sooooooth of its length, or from,-ths to "ths of a second. The observations of astronomers do not agree with this conclusion; we must therefore fall back on the theory mentioned at the beginning of this chapter, which assumes that the sun, like the ocean, is constantly losing and receiving equal quantities of matter. This harmonizes with the supposition that the vis viva of the universe is a constant quantity. VI.-TiHE SPOTS ON THE SUN'S DISC. THE solar disc presents, according to Sir John Herschel, the following appearance. "When the sun is observed through a powerful telescope provided with coloured glasses in order to lessen the heat and brightness which would be THE SPOTS ON THE SUN S DISC. 287 hurtful to the eyes, large dark spots are often seen surrounded by edges which are not quite so dark as the spots themselves, and which are called penumbrae. These spots, however, are neither permanent nor unchangeable. When observed from day to day, or even from hour to hour, their form is seen to change; they expand or contract, and finally disappear; on other parts of the solar surface new spots spring into existence where none could be discovered before, When they disappear, the darker part in the middle of the spot contracts to a point and vanishes sooner than the edge. Sometimes they break up into two or more parts that show all the signs of mobility characteristic of a liquid, and the extraordinary commotion which it seeihs only possible for gaseous matter to possess. The magnitude. of their motion is very great. An are of 1 second, as seen from our globe, corresponds to 465 English miles on the sun's disc; a circle of this diameter, which measures nearly 220,000 English square miles, is the smallest area that can be seen on the solar surface. Spots, however, more than 45,000 English miles in diameter, and, if we may trust some statements, of even greater dimensions, have been observed. For such a spot to disappear in the course of six weeks (and they rarely last longer), the edges, whilst approaching each other, must move through a space of more than 1000 miles per diem. "1 That portion of the solar disc which is free from spots is by no means uniformly bright. Over it are scattered small dark spots or pores, which are found by careful. observation to be in a state of continual change. The slow sinking of some chemical precipitates in a transparent liquid, when viewed from the upper surface and in a direction perpendicular thereto, resembles more accurately than any other phenomenon the changes which the pores undergo. The Sinmilarity is so striking, in fact, that one can scarcely resist the idea that the appearances above described are owing to a luminous medium moving about in a non-luminous atmosphere, either 288 CELESTIAL DYNAMICS. like the clouds in our air, or in wide-spread planes and flamelike columns, or in rays like the aurora borealis. "6Near large spots, or extensive groups of them, large spaces are observed to be covered with peculiarly marked lines much brighter than the other parts of the surface; these lines are curved, or deviate in branches, and are called facula, Spots are often seen between these lines, or to originate there. These are in all probability the crests of immense waves in the luminous regions of the solar atmosphere, and bear witness to violent action in their immediate neighbourhood." The changes on the solar surface evidently point to the action of some external disturbing force; for every moving power resident in the sun itself ought to exhaust itself by its own action. These changes, therefore, are no unimportant confirmation of the theory explained in these pages. At the same time it must be observed that our knowledge of physical heliography is, from the nature of the subject, very limited; even the meteorological processes and other phenomena of our own planet are still in many respects enigmatical. For this reason no special information could be given about the manner in which the solar surface is affected by cosmical masses. However, I may be allowed to mention some probable conjectures which offer themselves. The extraordinarily high temperature which exists on the sun almost precludes the possibility of its surface being solid; it doubtless consists of an uninterrupted ocean of fiery fluid matter. This gaseous envelope becomes more rarefied in those parts most distant from the sun's centre. As most substances are able to assume the gaseous state of aggregation at high temperatures, the height of the sun's atmosphere cannot be inconsiderable. There are, however, sound reasons for believing that the relative height of the solar atmosphere is not very great. Since the gravity is 28 times greater on the sun's surface than it is on our earth, a column of air on the former must THE SPOTS ON THE SUN S DISC. 289 cause a pressure 28 times greater than it would on our globe. This great pressure compresses air as much as a temperature of 8000~ would expand it. In a still greater degree than this increased gravity do the qualities peculiar to gases affect the height of the solar atmosphere. In consequence of these properties, the density of our atmosphere rapidly diminishes as we ascend, and increases as we descend. Generally speaking, rarefaction increases in a geometrical progression when the heights are in an arithmetical progression. If we ascend or descend 21, 5, or 30 miles, we find our atmosphere 10, 100, or a billion times more rarefied or more dense. This law, although modified by the unequal temperatures of the different layers of the photosphere, and the unknown chemical nature of the substances of which it is composed, must also hold good in some measure for the sun. As, however, the mean temperature of the solar atmosphere must considerably exceed that of our atmosphere, the density of the former will not vary so rapidly with the height as the latter does. If we assume this increase and decrease on the sun to be ten times slower than it is on our earth, it follows that at the heights of 25, 50, and 300 miles, a rarefaction of 10, 100, and a billion times respectively would be observed. The solar atmosphere, therefore, does not attain a height of 400 geographical miles, or it cannot be as much as 2I-th of the sun's radius. For if we take the density of the lowest strata ofthe sun's atmosphere to be 1000 times greater that that of our own near the level of the sea, a density greater than that of water, and necessarily too high, then at a height of 400 miles this atmosphere would be 10 billion times less dense than the earth's atmosphere; that is to say, to human comprehension it has ceased to exist. This discussion shows that the solar atmosphere, in comparison with the body of the sun, has only an insignificant height; at the same time it may be remarked that on the 13 290 CELESTIAL DYNAMICS. sun's surface, in spite of the great heat, such substances as water may possibly exist in the liquid state under a pressure thousands of times greater than that of our atmosphere. Since gases, when free from any solid particles, emit, even at very high temperatures, a pale transparent light-the socalled lumen philosophicun —it is probable that the intense' white light of the sun has its origin in the denser parts of his surface. If such be assumed to be the case, the sun's spots and faculoe seem to be the disturbances of the fiery liquid ocean, caused by most powerful meteoric processes, for which all necessary materials are present, and partly to be caused by the direct influence of streams of asteroids. The deeper and less heated parts of this fiery ocean become thus exposed, and perhaps appear to us as spots, whereas the elevations form the so-called faculhe. According to the experiments made by Henry, an American physicist, the rays sent forth from the spots do not produce the same heating effect as those emitted by the brighter parts. We have to mention one more remarkable circumstanlce. The spots appear to be confined to a zone which extends 30~ on each side of the sun's equator. The thought naturally suggests itself that some connexion exists between those solar processes which produce the spots and facula3, the velocity of rotation of the sun, and the swarms of asteroids, and to deduce therefrom the limitation of the spots to the zone mentioned. It still remains enigmatical by what means nature contrives to bring about the uniform radiation which pertains alike to the polar and equatorial regions of the sun. THE TIDAL WAVE. 291 VII-THE TIDAL WAVE. IN almost every case the forces and motions on the surface of the earth may be traced back to the rays of the sun. Some processes, however, form a remarkable exception. One of these is the tides. B3eautiful, and in some respects exhaustive researches on this phenomenon have been made by Newton, Laplace, and others. The tides are caused by the attraction exercised by the sun and the moon on the moveable parts of the earth's surface, and by the axial rotation of our globe. The alternate rising and falling of the level of the sea may be compared to the ascent and descent of a pendulum oscillating under the influence of the earth's attraction. The continual resistance, however weak it may be, which an instrument of this nature (a physical pendulum) suffers, constantly shortens the amplitude of the oscillations which it performs; and if the pendulum be required to continue in uniform motion, it must receive a constant supply of vis viva corresponding to the resistance it has to overcome. Clocks regulated by a pendulum obtain such a supply, either from a raised weight or a bent spring. The power consumed in raising the weight or in bending the spring, which power is represented by the raised weight or the bent spring, overcomes for a time the resistance, and thus secures the uniform motion of the pendulum and clock. In doing so, the weight sinks down or the spring uncoils, and therefore force must be expended in winding the clock up again, or it would stop moving. Essentially the same holds good for the tidal wave. The moving waters rub against each other, against the shore, and against the atmosphere, and thus, meeting constantly with re. sistance, would soon come to rest if a vis viva did not exist competent to overcome these obstacles. This vis viva is the 292 CELESTIAL DYSAMICS. rotation of the earth on its axis, and the diminution and final exhaustion thereof will be a consequence of such an action. The tidal wave causes a diminution of the velocity of the rotation of the earth. This important conclusion can be proved in different ways. The attraction of the sun and the moon disturbs the equilibrium of the moveable parts of the earth's surface, so as to move the waters of the sea towards the point or meridian above and below whfch the moon culminates. If the waters could move without resistance, the elevated parts of the tidal wave would exactly coincide with the moon's meridian, and under such conditions no consumption of' vis viva could take place. In reality, however, the moving waters experience resistance, in consequence of which the flow of the tidal wave is delayed, and high water occurs in the open sea on the average about 2~ hours after the transit of the moon through the meridian of the place. The waters of the ocean move from west and east towards the meridian of the moon, and the more elevated wave is, for the reason above stated, always to the east of the moon's mericlian; hence the sea must press and flow more powerfully from east to west than from west to east. The ebb and flow of the tidal wave therefore consists not only in an alternate rising and falling of the waters, but also in a slow progressive motion from east to west. The tidal wave produces a general western current in the ocean. This current is opposite in direction to the earth's rotation, and therefore its friction against and collision with the bed and shores of the ocean must offer everywhere resistance to the axial rotation of the earth, and diminish the vis viva of its motion. The earth here plays the part of a fly-wheel. The moveable parts of its surface adhere, so to speak, to the relatively fixed moon, and are dragged in a direction opposite to that of the earth's rotation, in consequence of which, action takes place between the solid and liquid parts of this fly THE TIDAL WAVE. 293 wheel, resistance is overcome, and the given rotatory effect diminished. Water-mills have been turned by tile action of the tides; the effects produced by such an arrangement are distinguished in a remarkable manner from those of a mill turned by a mountain-stream. The one obtains the vis vivac with which it works from the earth's rotation, the other from the sun's radiation. Various causes combine to incessantly maintain, partly in an undulatory, partly in a progressive motion, the waters of the ocean. Besides the influence of the sun and the moon on the rotating earth, mention must be made of the influence of the movement of the lower strata of the atmosphere on the surface of the ocean, and of the different temperatures of the sea in various climates; the configuration of the shores and the bed of the ocean likewise exercise a manifold influence on the velocity, direction, and extent of the oceanic currents. The motions in our atmosphere, as well as those of the ocean, presuppose the existence and consumption of vis viva to overcome thle continual resistances, and to prevent a state of rest or equilibrium. Generally speaking, the power necessary for the production of aerial currents may be of threefold origin. Either the radiation of the sun, the heat derived from a store in the interior of the earth, or, lastly, the rotatory effect of the earth may be the source. As far as quantity is concerned the sun is by far the most important of the above. According to Pouillet's measurements, a square metre of the earth's surface receives on the average 4'408 units of heat from the sun per minute. Since one unit of heat is equivalent to 367 Km, it follows that one square metre of the surface of our globe receives per minute an addition of vis vivac equal to 1620 Kmn, or the whole of the earth's surface in the same time 825,000 billions of Km. A power of 75 Km per second is called a horse-power. According to this, the effect of the solar radiation in mechanical 294 CELESTIAL DYNAMICS. work on one square metre of the earth's surface would be equal to 0'36, and the total effect for the whole globe 180 billions of horse-powers. A not inconsiderable portion of this enormous quantity of vis viva is consumed in the production of atmospheric actions, in consequence of which numerous motions are set up in the earth's atmosphere. In spite of their great variety, the atmospheric currents may be reduced to a single type. In consequence of the unequal heating of the earth in different degrees of latitude, the colder and heavier air of the polar regions passes in an under current towards the equator; whereas the heated air of the tropics ascends to the higher parts of the atmosphere, and flows from thence towards the poles. In this manner the air of each hemisphere performs a circuitous motion. It is known that these currents are essentially mlodified by the motion of the earth on its axis. The polar currents, with their smaller rotatory velocity, receive a motion from east to west contrary to the earth's rotation, and the equatorial currents one from west to east in advance of the axial rotation of the earth. The former of these currents, the easterly winds, must diminish the rotatory effect of the globe, the latter, the westerly winds, must increase the same power. The final result of the action of these opposed influences is, as regards the rotation of the earth, according to well-known mechanical principles, - 0; for these currents counteract each other, and therefore cannot exert the least influence on the axial rotation of the earth. This important conclusion was proved by Laplace. The same law holds good for every imaginable action which is caused either by the radiant heat of the sun, or by the heat which reaches the surface from the earth's interior, whether the action be in the air, in the water, or on the land. The effect of every single motion produced by these means on the rotation of the globe, is exactly compensated by the effect of another motion in an opposite direction; so that the resulb THE TIDAL WAVE. 295 ant of all these motions is, as far as the axial rotation of the globe is concerned, = 0. In those actions known as the tides, such compensation, however, does not take place; for the pressure or pull by which they are produced is always stronger from east to west than from west to east. The currents caused by this pull may ebb and flow in different directions, but their motion predominates in that which is opposed to the earth's rotation. The velocity of the currents caused by the tide of the atmosphere amounts, according to Laplace's calculation, to not more than 75 millimetres in a second, or nearly a geographical mile in twenty-four hours; it is clear that much more powerful effects produced by the sun's heat would hide this action from observation. The influence of these air-currents, however, 6n the rotatory effect of the earth is, according to the laws of mechanics, exactly the same as it would be were the atmosphere undisturbed by the sun's radiant heat. The combined motions of air and water are to be regarded from the same point of view. If we imagine the influence of the sun and that of the interior of our globe not to exist, the motion of the air and ocean from east to west is still left as an obstacle to the axial rotation of the earth. The motion of the waters of the ocean from east to west was long ago verified by observation, and it is certain that the tides are the most effectual of the causes to which this great westerly current is to be referred. Besides the tidal wave, the lower air-currents moving in the same direction, the trade'winds of the tropics especially, may be assigned as causes of this general movement of the waters. The westerly direction of the latter, however, is not confined to the region of easterly winds; it is met with in the region of perpetual calms, where it possesses a velocity of several miles a day; it is observed far away from the tropics both north and south, in regions where westerly winds pre. 296 CELESTIAL DYNAMICS. vail, near the Cape of Good Hope, the Straits of Magellan, the arctic regions, &c. A third cause for the production of a general motion of translation of the waters of the ocean is the unequal heating of the sea in different zones. According to the laws of hydrostatics, the colder water of the higher degrees of latitude is compelled to flow towards the equator, and the warmer water of the-.tropics towards the poles, in consequence of which, similar movements are produced in the ocean to those in the atmosphere. This is the cause of the cold under current from the poles to the equator, and of the warm surfacecurrent from the equator to the poles. The waters of the latter, by virtue of the greater velocity of rotation at the equator, assume in their onward progress a direction from west to east. It is a striking proof of the preponderating influence of the tidal wave that, in spite of this, the motion of the ocean is on the whole in an opposite direction. Theory and experience thus agree in the result that the influence of the moon on the rotating earth causes a motion of translation from east to west in both atmosphere and ocean. This motion must continually diminish the rotatory effect of the earth, for want of an opposite and compensating influence. The continual pressure of the tidal wave against the axial rotation of the earth may also be deduced friom statical laws. The gravitation of the moon affects without exception all parts of the globe. Let the earth be divided by the plane of the meridian in which the moon happens to be into two hemispheres, one to the east, the other to the west of this meridian. It is clear that the moon, by its attraction of the eastern hemisphere, tends to retard the motion of the earth, and by its attraction of the western hemisphere, to accelerate the same rotation. Under certain conditions both these tendencies compensate each other, and then the action of the moon on the earth's THE TIDAL WAVE. 297 rotation becomes zero. This happens when both hemispheres are arranged in a certain manner symmetrically, or when no parts of the earth can change their relative position; in the latter case a sort of symmetry is produced by the rotation. The form of the earth deviates from a perfectly symmetrical sphere on account of the three following causes:-(1) the flattening of the poles, (2) the mountains on the surface, and (3) the tidal wave. The first two causes do not change the velocity of the earth's axial rotation. In order to comprehend clearly the effect of the tidal wave, we shall imagine the earth to be a perfectly symmetrical sphere uniformly surrounded by water. The attraction of the sun and the moon disturbs the equilibrium of this mass, and two fiat mountains of water are formed. The top of one of these is directed towards the moon, and the summit of the other is turned away from it. A straight line passing through the tops of these two mountains is called the major axis of this earthspheroid. In this state the earth may be imagined to be divided into three parts-a smaller sphere, and two spherical segments attached to the opposite sides of the latter, and representing the elevations of the tidal wave. The attraction of the moon on the small central sphere does not change the rotation, and we have therefore only to consider the influence of this attraction on the two tidal elevations. The upper elevation or mountain, the one nearest the moon, is attracted towards the west because its mass is principally situated to the east of the moon, and the opposite mountain, which is to the west of the moon, is attracted towards the east. The upper tidal elevation is not only more powerfully attracted because it is nearer to the moon, but also because the angle under which it is pulled aside is more favourable for lateral deflection than in the case of the opposite protuberance. The pressure from east to west of the upper elevation preponderates therefore over the pressure from west to east of the opposite 298 CELESTIAL DYNAMICS. mountain; according to calculation, these quantities stand to each other nearly as 14 to 13. From the relative position of these two tidal protuberances and the moon, or the unchangeable position of the major axis of the earth-spheroid towards the centre of gravity of the moon, a pressure results, which preponderates from east to west, and offers an obstacle to the earth's rotation. If gravitation were to be compared with magnetic attraction, the earth might be considered to be a large magnet, one pole of which, being more powerfully attracted, would represent the upper, and the other pole the lower tidal elevation. As the upper tidal wave tends to move towards the moon, the earth would act like a galvanometer, whose needle has been deflected from the magnetic meridian, and which, while tending to return thereto, exerts a constant lateral pressure. The foregoing discussion may suffice to demonstrate the influence of the moon on the earth's rotation. The retarding pressure of the tidal wave may quantitatively be determined in the same manner as that employed in computing the precession of the equinoxes and the nutation of the earth's axis. The varied distribution of land and water, the unequal and unknown depth of the ocean, and the as yet imperfectly ascertained mean difference between the time of the moon's culmination and that of high water in the open sea, enter, however, as elements into such a calculation, and render the desired result an uncertain quantity. In the mean time this retarding pressure, if imagined to act at the equator, cannot be assumed to be less than 1000 millions of kilogrammes. In order to start with a definite conception, we may be allowed to use this round number as a basis for the following calculations. The rotatory velocity of the earth at the equator is 464 metres, and the consumption of mechanical work, therefore, for the maintenance of the tides 464,000 millions of Km, or 6000 millions of horse-powers per second. The effect of the THE TIDAL WAVE. 299 tides may consequently be estimated at 5-00th of the effect received by the earth from the sun. The rotatory effect which the earth at present possesses, may be calculated from its mass, volume, and velocity of rotation. The volume of the earth is 2,650,686,000 cubic miles, and its specific gravity, according to Reich, = 5'44. If; for the sake of simplicity, we assume the density of the earth to be uniform throughout its mass, we obtain from the above premises, and the known velocity of rotation, 25,840 quad-illions of kilogrammetres as the rotatory effect of the earth.:f, during every second in 2500 years, 464,000 millions of Km of this effect were consumed by the ebb and flow of the tidal wave, it would suffer a diminution of 36,600 trillions'of K[m, or about 70-000th of its quantity. The velocities of rotation of a sphere stand to each other in the same ratio as the square roots of the rotatory effects, when the volume of the sphere remains constant. From this it follows that, in the assulmed time of 2500 years, the length of a day has increased,4-0,00oth; or if a day be taken equal to 86,400 seconds, it has lengthened',th of a second, if the volume of the earth has not changed. Whether this supposition be correct or not, depends on the temperature of our planet, and will be discussed in the next chapter. The tides also react on the motion of the moon. The stronger attraction of the elevation nearest to, and to the east of the moon, increases with the tangential velocity of our satellite; the mean distance of the earth and the moon, and the time of revolution of the latter, are consequently augmented. The effect of this action, however, is insignificant, and, according to calculation, does not amount to more than a fraction of a second in the course of centuries. 300 CELESTIAL DYNAMICS. VIII. —THE EARTH'S INTERIOR HEAT. WITHOUT doubt there was once a time when our globe had not assumed its present magnitude. According to this, by aid of this simple assumption, the origin of our planet may be reduced to the union of once separated masses. To the mechanical combinations of masses of the second order, with masses of the second and third order, &c., the same laws as those enunciated for the sun apply. The collision of such masses must always generate an amount of heat proportional to the squares of their velocities, or to their mechanical effect. Although we are not in a position to affirm anything certain respecting the primordial conditions under which the oonstituent parts of the earth existed, it is nevertheless of the greatest interest to estimate the quantities of heat generated by the collision and combination of these parts by a standard based on the simplest assumptions. Accordingly we shall consider for tlhe present the earth to have been formed by the union of two parts, which obtained their relative motions by their mutual attraction only. Let the whole mass of the present earth, expressed in kilogrammes, be T, and the masses of the two portions T- x and x. The ratio of these two quantities may be imagined to assume various values. The two extreme cases are, when x is considered infinitely small in comparison with T, and when x T- xc = I T. These form the limits of all imaginable ratios of the parts T-x and x) and will now be more closely examined. Terrestrial heights are of course excluded' from the following consideration. In the first place, let, x, in comparison with T — x, be infinitely small. The final velocity with which x arrives on the surface of the large mass, after having THE EARTH S INTERIOR HEAT. 301 passed through a great space in a straight line, or after previous central motion round it, is, according to the laws developed in relation to the sun in Chapter III., confined within the limits of 7908 and 11,183 metres. The heat generated by this process may amount to from 8685Xsx to 17,370 Xx units, according to the value of the major axis of the orbit of x. This heat, however, vanishes by its distribution through the greater mass, because x is, according to supposition, infinitely small in comparison with T. The quantity of heat generated increases with x, and amounts in the second case, when x = I T, to from 6000 X T to 8685 XT units. If we assume the earth to possess a very great capacity for heat, equal in fact to that of its volume of water, which when calculated for equal weights = 0'184, the above discussion leads to the conclusion that the difference of temperature of the constituent parts, and of the earth after their union, or, in other words, the heat generated by the collision of these parts, may range, according to their relative magnitude, from 0~ to 32,000~, or even to 47,000~! With the number of parts which thus mechanically comn bine, the quantity of heat developed increases. Far greater still would have been the generation of heat if the constituent parts had moved in separate orbits round the sun before their union, and had accidentally approached and met each other. For various reasons, however, this latter supposition is not very probable. Several facts indicate that our earth was once a fiery liquid mass, which has since cooled gradually, down to a comparatively inconsiderable depth from the surface, to its present temperature. The first proof of this is the form of the earth. " The form of the earth is its history." According to the most careful measurements, the flattening at the poles is exactly such as a liquid mass rotating on its axis with the velocity of the earth would possess; from this we may con 302 CELESTIAL DYNAMICS. elude that the earth at the time it received its rotatory motion was in a liquid state; and, after much controversy, it may be considered as settled that this liquid condition was not that of an aqueous solution, but of a mass melted by a high temperature. The temperature of the crust of the globe likewise furnishes proof of the existence of a store of heat in its interior. Many exact experiments and measurements show that the temperature of the earth increases with the depth to which we penetrate. In boring the artesian well at Grenelle, which is 546 metres deep, it was observed that the temperature augmented at the rate of 1~ for every 30 metres. The same result was obtained by observations in the artesian well at Mondorf in Luxembourg: this well is 671 metres in depth, and its water 340 warm. Thermal springs furnish a striking proof of the high temperature existing in the interior of the earth. Scientific men are agreed that the aqueous deposits from the atmosphere, rain, hail, dew, and snow, are the sole causes of the formation of springs. The water obeying the laws of gravity, percolates through the earth wherever it can, and reappears at the surface in places of a lower situation. When water sinks to considerable depths through vertical crevices in the rocks, it acquires the temperature of the surrounding strata, and returns as a thermal spring to the surface. Such waters are frequently distinguished from the water of ordinary springs merely by their possessing a higher temperature. If, however, the water in its course meets with mineral or organic substances which it can dissolve and retain, it then reappears as a mineral spring. Examples of such are met with at Aachen, Carlsbad, &c. In a far more decided manner than by the high temperature of the water of certain springs, the interior heat of our globe is made manifest by those fiery fluid masses which sometimes rise from considerable depths. The temperature THE EARTH59S INTERIOR IEArT. 303 of the earth's crust increases at the rate of 1~ for every 30 metres we descend from the surface towards the centre. A1though it is incredible that this augmentation can continue at the same rate till the centre be reached, we may nevertheless assume with certainty that it does continue to a considerable depth. Calculation based on this assumption shows that at a depth of a few miles a temperature must exist sufficiently powerful to fuse most substances. Such molten masses penetrate the cold crust of the globe in many places, and make their appearance as lava. A distinguished scientific man has lately expressed hinm self on the origin of the interior heat of the earth as follows: N Io one of course can explain the final causes of things. This much, however, is clear to every thinking man, that there is just as much reason that a body, like the earth, for example, should be warm, warmer than ice or human blood, as there is that it should be cold or colder than the latter. A particular cause for this absolute heat is as little necessary as a cause for motion or rest. Change —that is to say, transition from one state of things to another-alone requires and admits of explanation." It is evident that this reflection is not fitted to suppress the desire for an explanation of the phenomenon in question. As all matter has the tendency to assnume the same temperature as that possessed by the substances by which it happens to be surrounded, and to remain in a quiescent state as soon as equilibrium has been established, we must conclude that, whenever we meet with a body warmer than its neighbours, such body must have received at a (relatively speaking) not far distant time, a certain degree of heat, —a process which certainly allows of, and requires explanation. Newton's theory of gravitation, whilst it enables us to de-,eroine, from its present form, the earth's state of aggregation in ages past, at the same time points out to us a source of heat powerful enough to produce such a state of aggregT 304 CELESTIAL DYNAMICS. tion, powerful enough to melt worlds; it teaches us to consider the molten state of a planet as the result of the mechanical union of cosmical masses, and thus derive the radiation of the sun and the heat in the bowels of the earth from a common origin. The rotatory effect of the earth also may be readily explained by the collision of its constituent parts; and we must accordingly subtract the vis viva of the axial rotation from the whole effect of the collision and mechanical combination, in order to obtain the quantity of heat generated. The rotatory effect, however, is only a small quantity in comparison with the interior heat of the earth. It amounts to about 4400 X T kilogrammetres, T being the weight of the earth in kilogrammes, which is equivalent to 12 2X T units of heat, if we assume the density of the earth to be uniform throughout. If we imagine the moon in the course of time, either in consequence of the action of a resisting medium or from some other cause, to unite herself with our earth, two principal effects are to be discerned. A result of tile collision would be, that the whole mass of the moon and the cold crust of the earth would be raised some thousands of degrees in temperature, and consequently the surface of the earth would be converted into a fiery ocean. At the same time the velocity of the earth's axial rotation would be somewhat accelerated, and the position of' its axis with regard to the heavens, and to its own surface, slightly altered. If the earth had been a cold body without axial rotation, the process of its combining with the moon would have imparted to it both heat and rotation. It is probable that such processes of combination between diferent parts of our globe may have repeatedly happened before the earth attained its present magnitude, and that luxuriant vegetation may have at different times been buried under the fiery debris resulting from the conflict of these masses. THE EARTHIS INTERIOR HEAT. 305 As long as the surface of our globe was in an incandescent state, it must have lost heat at a very rapid rate; gradually this process became slower; and although it has not yet entirely ceased, the rate of cooling must have diminished to a conparatively small matgnitude. Two phenomena are caused by the cooling of the earth, which, on account of their common origin, are intimately related. The decrease of temperature, and consequent contraction of the earth's crust, must have caused frequent disturbances and revolutions on its surface, accompanied by the ejection of molten masses and the formation of protuberances,; on the other hand, according to the laws of mechanics, the velocity of rotation must have increased with the diminution of the volume of the sphere, or, in other words, the cooling of the earth must have shortened the length of the day. As the intensity of such disturbances and the velocity of rotation are closely connected, it is clear that the youth of our planet must have been distinguished by continual violent transformations of its crust, and a perceptible acceleration of the velocity of its axial rotation; whilst in the present time the metamorphoses of its surface are much slower, and the acceleration of its axial revolution diminished to a very small amount. If we imagine the times when the Alps, the chain of the Andes, and the Peak of Teneriffe were upheaved from the deep, and compare with such changes the earthquakes and volcanic eruptions of historic'times, we perceive in these modern transformations but weak images of the analogous processes of bygone ages. Whilst we are surrounded on every side by the monun ments of violent volcanic convulsions, we possess no record of the velocity of the axial rotation of our planet in antediluvian times. It is of the greatest importance that we should have an exact knowledge of a change in this velocity, or in the length of the day during historic times. The investiga. 306 CELESTIAL DYNAMTCS. tion of this subject by the great Laplace forms a bright mon. ument in the department of exact science. These calculations are essentially conducted in the following manner: —In the first place, the time between two eclipses of the sun, widely apart from each other, is as accurately as possible expressed in days, and from this the ratio of the time of the earth's rotation to the mean time of the moon's revolution determined. If, now, the observations of ancient astronomers be compared with those of our present time, the least alteration in the absolute length of a day may be detected by a change in this ratio, or in a disturbance in the lunar revolution. The most perfect agreement of ancient records on the movements of the moon and the planets, on the eclipses of the sun, &c., revealed to Laplace the remarkable fact that in the course of 25 centuries, the time in which our earth revolves on its axis has not altered,th part of a sexagesimal second; and the length of a day therefore may be considered to have been constant during historic times. This result, as important as it was convenient for astronomy, was nevertheless of a nature to create some difficulties for the physicist. With apparently good reason it was concluded that, if the velocity of rotation had remained constant, the volume of the earth could have undergone no change. The earth completes one revolution on its axis in 86,400 sidereal seconds; it consequently appears, if this time has not altered during 2500 years to the extent of g th of a second, or 43,000,0th part of a day, that during this long space of time the radius of the earth also cannot have altered more than this fraction of its length. The earth's radius measures 6,369,800 metres, and therefore its length ought not to have diminished more than 15 centimetres in 25 centuries. The diminution in volume, as a result of the cooling-process, is, however, closely connected with the changes on the earth's surface. When we consider that scarcely a day passes without the occurrence of an earthquake or shock in THE EARTH9S INTERIOR HEAT. 30r one place or another, and that of the 300 active volcanos some are always in action, it woucld appear that such a lively reaction of the interior of the earth against the crust is incompatible with the constancy of its volume. This apparent discrepancy between Cordier's theory of thile connexion between the cooling of the earth and the reaction of the interior on the exterior parts, and Laplace's calculation showing the constancy of the length of the day, a calculation which is undoubtedly correct, has induced most scientific men to abandon Cordier's theory, and thus to deprive themselves of any tenable explanation of volcanic activity. The continued cooling of the earth cannot be denied, for it takes place according to the laws of nature; in this respect the earth cannot comport itself diferently from any other mass, however small it may be. In spite of the heat which it receives from the sun, the earth will have a tendency to cool so long as the temperature of its interior is higher than the mean temperature of its surface. Between the tropics the mean teimperature produLced by the sun is about 280, and the sun therefore is as little able to stop the cooling-tendency of the earth as the moderate warmth of the air can prevent the coo'lg of a red-hot ball suspended in a room. Many phenomena, for instance the melting of the glaciers near the bed on which they rest, show the uninterrupted emission of heat from the interior towards the exterior of the earth; and the question is, Htlas the earth in 25 centuries actually lost no more heat than that which is requisite to shorten a radius of more than 6 millions of metres only 15 centimetres? In answering this question, three points enter into our calculation; —(1) the absolute amount of heat lost by the earth in a certain time, say one day; (2) the earth's capacity for heat; and (3) the coefficient of expansion of the mass of the earth. As none of these quantities can be determined by direct 308 CELESTIAL DYNAMICS. measurements, we are obliged to content ourselves with probable estimates; these estimates wtill carry the more weight the less they are formed in favour of some preconceived opine ion. Considering what is known about the expansion and contraction of solids and liquids by heat and cold, we arrive at the conclusion that for a diminution of 1~ in temperature, the linear contraction of the earth cannot well-be less than,00o000th part, a number which we all the more readily adopt because it has been used by Laplace, Arago, and others. If we compare the capacity for heat of all solid and liquid bodies which have been examined, we find that, both as regards volume and weight, the capacity of water is the greatest. Even the gases come under this rule; hydrogen, however, forms an cxception, it having the greatest capacity for heat of all bodies when compared with an equal weight of water. In order not to take the capacity for heat of the mass of the earth too small, we shall consider it to be equal to that of its volume of water, which, when calculated for equal weights, amounts to 0'184A.' If we accept Laplace's result, that the length of a day has remained constant during the last 2500 years, and conclude * The capacity for heat, as well as the coefficient of expansion of matter, as a rule, increases at higher temperatures. As, however, these two quantities act in opposite ways in our calculations, we may be allowed to dispense with the influence which the high temperature of the interior of the earth must exercise on these numbers. Even if, in consequence of the high temperature of the interior, the earth's mass could have a capacity two or three times as great as that which it has from 0~ to 100~, it is to be considered, on the other hand, that the coefficient of expansion, 1TT,,r, only holds good for solids, and is even small-for them, whilst in the case of liquids we have to assume a much greater coefficient: for mercury between 0~ and 100~, it is about six times as great. Especially great is the contraction and expansion of bodies when they change their state of aggregation; and this should be taken into account when considering the formation of the earth's crust. THE EARTH' S INTERIOR HEAT. 309 that the earth's radius has not diminished II decimetre in consequence of cooling, we are obliged to assume, according to the premises stated, that the mean temperature of our planet cannot have decreased' o in the same period of time. The volume of the earth amounts to 2650 millions of cubic miles. A loss of heat sufficient to cool this mass 00 would be equal to the heat given off when the temperature of 6,150,000 cubic miles of water decreases 1~; hence the loss for one day would be equal to 6'74 cubic miles of heat. Fourier has investigated the loss of heat sustained by the earth. Taking the observation that the temperature of the earth increases at the rate of 1~ for every 30 metres as the basis of his calculations, this celebrated mathematician finds the heat which the globe loses by conduction through its crust in the space of 100 years to be capable of melting a layer of ice 3 metres in thickness and covering the whole surface of the globe; this corresponds in one day to 7'7 cubic miles of heat, and in 2500 years to a decrease of 17 centimetres in the length of the radius. According to this, the cooling of the globe would be sufficiently great to require attention when the earth's velocity of rotation is considered. At the same time it is clear that the method employed by Fourier can only bring to our knowledge one part of the heat which is annually lost by the earth; for simple conduction through terra firmac is not the only way by which heat escapes from our globe. In the first place, we may make mention of the aqueous deposits of our atmosphere, which, as far as they penetrate our earth, wash away, so to speak, a portion of the heat, and thus accelerate the cooling of the globe. The whole quantity of water which falls from the atmosphere upon the land in one day, however, cannot be assumed to be much more than half a cubic mile in volume, hence the cooling effect produced by this water may be neglected in our calculation. The heat 310 CELESTIAL DYNAMICS. carried off by all the thermal springs in the world is very small in comparison with the quantities which we have to consider here. Much more important is the effect produced by active volcanos. As the heat which accompanies the molten matter to the surface is derived from the store in the interior of the earth, their action must influence considerably the diminution of the earth's heat. And we have not only to consider here actual eruptions which take place in succession or simultaneously at different parts of the earth's surface, but also volcanes in a quiescent state, which continually radiate large quantities of heat abstracted from the interior of the globe. If we compare the earth to an animal body, we may regard each volcano as a place where the epidermis has been torn off leaving the interior exposed, and thus opening a door for the escape of heat. Of the whole of the heat which passes away through these numerous outlets, too low an estimate must not be made. To have some basis for the estimation of this loss, we have to recollect that in 1783 Skaptar-Jokul, a volcano in Iceland, emitted sufficient lava in the space of six weeks to cover 60 square miles of country to an average depth of 200 metres, or, in other words, about 1~ cubic miles of lava. The amount of heat lost by this one eruption of one volcano must, when the high temperature of the lava is considered, be estimated to be more than 1000 cubic tiles of heat; and the whole loss resulting from the' action of all the volcanos amounts, therefore, in all probability, to thousands of cubic miles of heat per annum. This latter number, when added to Fourier's result, produces a sum which evidently does not agree with the assumption that the volume of our earth has remained unchanged. In the investigation of the cooling of our globe, the influence of the water of the ocean has to be taken into account. Fourier's calculations are based on the observations of the in THE EARTH7S INTERIOR HEAT. 311 crease of the temperature of' the crust of our earth, from the surface towards the centre. But two-thirds of the surface of our globe are covered with water, and we cannot assume a priori that this large area loses heat at the same rate as the solid parts; on the contrary, various circumstances indicate that the cooling of our globe proceeds more quickly through the waters of the ocean resting on it than from the solid parts merely in contact with the atmosphere. In the first place, we have to remark that the bottom of the ocean is, generally speaking, nearer to the store of heat in the interior of the earth than the dry land is, and hence that the temperature increases most probably in a greater ratio from the bottom of the sea towards the interior of the globe, than it does in our observations on the land. Secondly, we have to consider that the whole bottom of the sea is covered by a layer of ice-cold water, which moves constantly from the poles to the equator, and which, in its passage over sand-banks, causes, as lHumboldt aptly remarks, the low temperatures which are generally observed in shallow places. That the water near the bottom of the sea, on account of its great specific heat and its low temperature, is better fitted than the atmosphere to withdraw the heat from the earth, is a point which requires no further discussion. We have plenty of observations which prove that the earth suffers a great loss of heat through the waters of the ocean. Many investigations have demonstrated the existence of a large expanse of sea, much visited by whalers, situated between Iceland, Greenland, Norway, and Spitzbergen, and extending from lat. 76~ to 80~ N., and from long. 150 E. to 15~ W. of Greenwich, where the temperature was observed to be higher in the deeper water than near the surface-an experience which neither accords with the general rule, nor agrees with the laws of hydrostatics. Franklin observed, in lat. 77~ N. and long. 120~., that the temperature of the sea near the surface was -- 0, and at a depth of 700 fathoms 312 CELESTIAL DYNAMICS. +6~. Fisher, in lat. 800 N. and long. 11~ E., noticed that the surface-water had a temperature of 0~, whilst at a depth of 140 fathoms it stood at-8~. As sea-water, unlike pure water, does not possess a point of greatest density at some distance above the freezing-point, and as the water in lat. 80~ N. is found at some depth to be warmer than water at the same depth 10~ southward, we can only explain this remarkable phenomenon of an increase of temperature with an increase of depth by the existence of a source of heat at the bottom of the sea. The heat, however, which is required to warm the water at the bottom of an expanse of ocean more than 1000 square miles in extent to a sensible degree, must amount, according to the lowest estimate, to some cubic miles of heat a day. The same phenomenon has been observed in other parts of the world, such as the west coast of Australia, the Adriatic, the Lago Maggiore, &c. Especial mention should here be made of an observation by Horner, according to whom the lead, when hauled up from a depth varying from 80 to 100 fathoms in the mighty Gulf-stream off the coast of America, used to be hotter than boiling water. The facts above mentioned, and some others which might be added, clearly show that the loss of heat suffered by our globe during the last 2500 years is far too great to have been without sensible effect on the velocity of the earth's rotation. The reason why, in spite of this accelerating cause, the length of a day has nevertheless remained constant since the most ancient times, must be attributed to an opposite retarding action. This consists in the attraction of the sun and moon on the liquid parts of the earth's surface, as explained in the last chapter. According to the calculations of the last chapter, the retarding pressure of the tides against the earth's rotation would cause, during the lapse of 2500 years, a sidereal day to be lengthened to the extent of ~6th of a second; as the THE EARTH'S INTERIOR HEAT. 313 length of a day, however, has remained constant, the cooling effect of the earth during the same period of time must have shortened thie.-"ay Ath of a second. A_ diminution of the earth's radius to the amount of 4~ metres in 2500 years, and a daily loss of 200 cubic miles of heat, correspond to this effect. H-ence, in the course of the last 25 centuries, the temperature of the whole mass of the earth must have decreased 4~. The not inconsiderable contraction of the earth resulting from such a loss of heat, agrees with the continual transfornations of the earth's surface by earthquakes and volcanic eruptions; and we agree with Cordier, the industrious observer of volcanic processes, in considering these phenomena a necessary consequence of the continual cooling of an earth which is still in a molten state in its interior. %When our earth was in its youth, its velocity of rotation must have increased to a very sensible degree, on account of the rapid cooling of its then very hot mass. This accelerating cause gradually diminished, and as the retarding pressure of the tidal wave remains nearly constant, the latter must finally preponderate, and the velocity of rotation therefore continually decrease. Between these two states we have a period of equilibrium, a period when the influence of the cooling and that of the tidal pressure counterbalance each other; the whole life of the earth therefore may be divided into three periods —youth with increasing, middle age with uniform, and old age with decreasing velocity of rotation. The time during which the two opposed influences on the rotation of the earth are in equilibrium can, strictly speaking, only be very short, inasmuch as in one moment the cooling, and in the next moment the pressure of the tides must prevail. In a physical sense, however, when measured by human standards, the influence of the cooling, and still more so that of the tidal wave, may for ages be considered constant, and there must consequently exist a period of many 14 314 CELESTIAL DYNAVICS. thousand years' duration during which these counteracting influences will appear to be equal. Within this period a sidereal day attains its shortest length, and the velocity of the earth's rotation its maximum-circumstances which, according to mathematical analysis, would tend to lengthen the duration of this period of the earth's existence. The historical times of manlind are, according to Laplace's calculation, to be placed in this period. Whether we are at the present moment still near its commencement, its middle, or are approaching its conclusion, is a question which cannot be solved by our present data, and must be left to future generations. The continual cooling of the earth cannot be without an influence on the temperature of its surface, and consequently on the climate; scientific men, led by Buffon, in fact, have advanced the supposition that the loss of heat sustained by our globe must at some time render it an unfit habitation for organic life. Such an apprehension has evidently no foundation, for the warmth of the earth's surface is even now much more deplendent on the rays of the sun than on the heat which reaches us from the interior. According to Pouillet's measurements, mentioned in Chapter III., the earth receives 8000 cubic miles of heat a day from the sun, whereas the heat which reaches the surface from the earth's interior may be estimated at 200 cubic miles per diem. The heat therefore obtained from the latter source every day is but small in comparison to the diurnal heat received from the sun. If we imagine the solar radiation to be constant, and the heat we receive from the store in the interior of the earth to be cut off, we should have as a consequence various changes in the physical constitution of the surface of our globe. The temperature of hot springs would gradually sink down to the mean temperature of the earth's crust, volcanic eruptions would cease, earthquakes would no longer be felt, and the temperature of the water of the ocean would be sensibly al THE EARTH'9S INTERIOR HEAT. 315 tered in many places-circumstances which would doubtless affect the climate in many parts of the world. Especially it may be presumed that Western Europe, with its present favourable climate, would become colder, and thus perhctps the seat of the power and culture of our race transferred to the milder parts of North America. Be this as it may, for thousands of years to come we can predict no diminution of the temperature of the surface of our globe as a consequence of the cooling of its interior mass; and, as far as historic records teach, the climates, the temperatures of thermal springs, and the intensity and firequency of volcanic eruptions are now the same as they were in the far past. It was different in prehistoric times, when for centuries the earth's surface was heated by internal fire, when mammoths lived in the now uninhabitable polar regions, and when the tree-ferns and the tropical shell-fish whose fossil remains are now especially preserved in the coal-formation were at home in all parts of the world. THE MECHANICAL EQUIVALENT OF HEAT T HE vast and magnificent structure of the experimental sciences has been erected on only a few pillars. History teaches us that the searching spirit of man required thousands of years for the discovery of the fundamental prin. ciples of the sciences, on which the superstructure wvas then raised in a comparatively short time. But these very fundamental propositions are nevertheless so clear and simple, that the discovery of them reminds us, in more than one respect, of Columbus's egg. But if, now that we are at last in possession of the truth, we speak of a method by the application of which the most essential fundamental laws might have been discovered without waste of time, it is not that we would criticize in any light spirit the efforts and achievements of our forerunners: it is merely with the object of laying before the reader in an advantageous form one of the additions to our knowledge which recent times have brought forth. The most important-not to say the only-rule for the genuine investigation of nature is, to remain firm in the conviction that the problem before us is to learn to know phenomena, before seeking for explanations or inquiring after higher TRUE NATURE OF SCIENTIFIC PROBLEMS. 317 causes. As soon as a fact is once known in all its relations, it is therein explained, and the problem of science is at an end. Notwithstanding that some may pronounce this a trite assertion, and no matter how many arguments others may bring to oppose it, it remains none the less certain that this primary rule has been too often disregarded even up to the most modern times; while all the speculative operations of even the most highly gifted minds which, instead of taking firm hold of facts as such, have striven to rise above them, have as yet borne but barren fruit. We shall not here discuss the modern naturalistic philosophy (N2atzuphllosop72ie) further than to say that its character is already sufficiently apparent from the ephemeral existence of its offspring. But even the greatest and most meritorious of the naturalists of antiquity, in order to explain, for example, the properties of the lever, took refuge in the assertion that a circle is such a marvellous thing that no wonder if motions, taking place in a circle, offer also in their turn most unusual phenomena. If Aristotle, instead of straining his extraordinary powers in meditations upon the fixed point and advancing line, as he calls the circle, had investigated the numerical relations subsisting between the length of the arm of the lever and the pressure exerted, he would have laid the foundation of an important part of human knowledge. Such mistakes, committed as they were, in accordance with the spirit of those times, even by a man -whose many positive services constitute his everlasting memorial, may serve to point us in the opposite road which leads us surely to the goal. But if, even by the most correct method of investigation, nothing can be attained without toil and industry, the cause is to be sought in that divine order of the world according to which man is made to labour. But it is certain that already immeasurably more means and more toil have been sacrificed to error than were needed for the discovery of the truth. 318 THE MECHANICAL EQUIVALENT OF HEAT. The rule which must be followed, in order to lay the foundations of a knowledge of nature in the shortest conceivable time, may be comprised in a few words. The natural phenomena with which we come into most imlmediate contact, and which are of most frequent occurrence, must be subjected to a careful examination by means of the organs of sense, and this examination must be continued until it results in quantitative determinations which admit of being expressed by numbners. These znumbers are the required foundations of an exact investigation of nature. Among all natural operations, the free fall of a weight is the most frequent, the simplest, and-witness Newton's apple -at the same time the most important. When this process is analysed in the way that has been mentioned, we immediately see that the weight strikes against the ground the harder the greater the height from which it has fallen; and the problem now consists in the determination of the quantitative relations subsisting between the height from which the weight falls, the time occupied by it in its descent, and its final velocity, and in expressing these relations by definite numbers. In carrying out this experimental investigation, various difficulties have to be contended with; but these must and can be overcome; and then the truth is arrived at, that for every body a fall of sixteen feet, or a time of descent of one second, corresponds to a final velocity of thirtytwo feet per second. A second phenomenon of daily occurrence, which is in apparent contradiction to the laws of falling bodies' is the ascent of liquids in tubes by suction. Here, again, the rule applies, not to allow the maxim, velse rerumv cognoscere causas, to lead us into error through useless and therefore harmful speculations concerning the qualities of the vacuum, and the like; on the contrary, we must again examine the phenome THE PROBLEM OF FALLING BODIES. 319 aon with attention and awakened senses; and then we find. as soon as we put a tube to the mouth to raise a liquid, that the operation is at first quite easy, but that afterwards it requires an amount of exertion which rapidly increases as the column of liquid becomes higher. Is there, perchance, an ascertainable'limit to the action of suction? As soon as we once begin to experiment in this direction, it can no longer escape us that there is a barometric height, and that it attains to about thirty inches. This number is a second chief pillar in the edifice of human knowledge. Question now follows question, and answer, answer. We have learned that the pressure exerted by a column of fluid is proportional to its height and to the specific gravity of the fluid; we have thus determined the specific gravity of the atmosphere, and by this investigation we are led to carry up our measuring-instrument, the barometer, from the plain to the mountains, and to express numerically the effect produced by elevation above the sea-level upon the height of the mercury-column. Such experiments suggest the question, Whether the laws of falling bodies, with which we have become acquainted at the surface of the earth, do not likewise undergo modification at greater distances from the ground. And if, as d priori we cannot but expect, this should be really the case, the further question arises, In what manner is the number already found modified by distance from the earth? We have thus come upon a problem the solution of which is attended with many difficulties; for what has now to be accomplished, is to make observations and carry out measurements in places where no human foot can tread. History, however, teaches that the same man who put the question was also able to furnish the answer. Truly he could do so only through a rich treasure of astronomical knowledge. But how is this knowledge to be attained by us? Astronomy is, without question, even in its first principles, the most difficult of all sciences. We have here to deal with 320 TIHE MECHANICAL EQUIVALENT OF HEAT. objects and spaces which forbid all thought of experiment, while at the same time the motions of the innumerable heavenly bodies are of so complicated a kind, that astronomical science, in its stately unfolding, is rightly considered the highest triumph whereof human intellect here below is able to boast. In accordance with the natural rule that, both in particulars and in general, man has to begin with that which is easiest and then to advance step by step to what is more difficult, it might well be supposed that astronomy must have arrived at a flourishing state of development later than any other branch of human knowledge. But it is well known that in reality the direct opposite was the case, inasmuch as it was precisely in astronomy, and in no other branch, that the earliest peoples attained to really sound knowledge, It may, indeed, be asserted that the science of the heavenly bodies had in antiquity reached as high a degree of perfection as the complete want of all the auxiliary sciences rendered possible. This early occurrence of a vigorous development of astronomy, which, indeed, was a necessary forerunner of the other sciences, since it alone furnished the necessary data for the measurement of time, is observable among the most various races of mankind: the reason of it, moreover, lies in the nature of things, and in the constitution of the human mind. It furnishes a remarkable proof that a right method is the most important condition for the successful prosecution of scientific inquiry. The explanation of -this phenomenon lies in the fact that the need which was felt at a very early period, of a common standard for the computation of time, made it necessary to institute observations such that their results required to be expressed by definite znumbers. There -was a felt necessity of determining the time in which the sun accomplishes his circuit through the heavens, as well as the time in which the moon goes through her phases, and other similar questions. FALLIiNG BODIES AT GREAT HEIGHTS. 321 In order to meet this necessity, there was no temptation to take up the Book of Nature, after the manner of expositors and critics, merely to cover it with glosses: " Mit eitler Rede wird hier nichts geschafft." It was numbers that were sought, and numbers that were found. The overpowering force of circumstances constrained the spirit of inquiry into the right path, and therein led it at once from success to success. Now that after long-continued, accurate, and fortunate observations the needful knowledge of the courses and distances of the nearest heavenly bodies, as well as of the figure and size of the earth, has been acquired, we are in a position to treat the question, What is the num6rical influence exerted by increased distance from the earth upon the known laws of falling bodies? and we thus arrive at the pregnant discovery that, at a height equal to the earth's semidiameter, the distance fallen through and the final velocity, for the first second, is four times less than on the surface of the earth. In order to pursue our inquiry, let us now return to the objects which immediately surround us. From the earliest times, the phenomena of combustion must have claimed in an especial degree the attention of mankind. In order to ex. plain them, the ancients, in accordance with the method of their naturalistic philosophy, put forward a peculiar upwardstriving element of Fire, which in conjunction with, and in opposition to, Air, Water, and Earth, constituted all that existed. The necessary consequence of this theory, which they discussed with the most acute sagacity, was, that in regard to the phenomena in question and all that related to them, they remained in complete ignorance. Here, again, it is quantitative determinations, it is numbers alone, which put the Ariadne's clue in our hand.' If wv want to know what goes on during the phenomena of combustion, we must weigh the substances before and after they 14 " 322 THE MECHANICAL EQUIVALENT OF HEAT. are burned; and here the knowledge we have already acquired of the weight of gaseous bodies comes to our aid. We then find that, in every case of combustion, substances which previously existed in a separate state enter into an intimate union with each other, and that the total weight of the substances remains the same both before and after the combination. We thus come to know the different bodies in their separate and in their combined states, and learn how to transform them from one of these states into the other; we learn, for instance, that water is composed of two kinds of air which combine with each other in the proportion of 1: 8. An entrance into chemical science is thus opened to us, and the numerical laws which regulate the combinations of matter (die Stochiornetrie) hang like ripe fruit before us. As we proceed further in our investigations, we find that in all chemical operations-combinations as well as decompositions-changes of temperature occur, which, according to the varying circumstances of different cases, are of all degrees of intensity, from the most violent heat downwards. We have measured quantitatively the heat developed, or counted the number of heat-units, and have so come into possession of the law of the evolution of heat in chemical processes. We have long known, however, that in innumerable cases heat makes its appearance where no chemical action is going on; for instance, whenever there is friction, when unelastic bodies strike one another, and when aeriform bodies are compressed. What then takes IpIace when he@t is evolved in such ways as these? We are taught by history that in this case also the most sagacious hypotheses concerning the state and nature of a peculiar " matter" of heat, concerning a " thermal s3ther," whether at rest or in a state of vibration, concerning " thermeal atoms," supposed to exercise their functions in the inter COlITERTIBILITY OF HEAT AND MOTION. 323 stices between the material atoms, or other hypotheses of like nature, have not availed to solve the problem. It is, notwithstanding, of no less wonderfully simple a nature than the lawa of the lever, about which the founder of the peripatetic phi losophy cudgelled his brains in vain. After what has gone before, the reader' cannot be in any doubt about what is the course now to be pursued. We must again make quantitative determinations: we must measure and count. If we proceed in this direction and measure the quantity of heat developed by mechanical agency, as well as the amount of force used up in producing it, and compare these quantities with each other, we at once find that they stand to each other in the simplest conceivable relation-that is to say, in an invariable direct proportion, and that the proportion also holds when, inversely, mechanical force is again produced by the aid of heat. Putting these facts into brief and plain language, we may say, HIeat and motion are transformable one into the other. We cannot and ought not, however, to let this suffice us. We require to know how much mechanical force is needed for the production of a given amount of heat, and conversely. In other words, the law of the invariable quantitative relation between motion and heat must be expressed numerically. When we appeal hereupon to experiment, we find that raising the temperature of a given weight of water one degree of the Centigrade scale corresponds to the elevation of an equal weight to the height of about 1,200 [French] feet. This number is THE MECHANICAL EQUIVALENT OF HEAT. The production of heat by friction and other mechanical operations is a fundamental fact of such constant occurrence, that the importance of its establishment on a scientific basis will be recognized by naturalists without any preliminary 324 THE MECHANICAL EQUIVALENT OF HEAT, enumeration of its useful applications; and, for the same reason, a few historical remarks touching the circumstances attending the discovery of the foregoing fundamental law, will not be out of place here. In the summer of 1840, on the occasion of bleeding Europeans newly arrived in Java, I made the observation that the blood drawn from the vein of the arm possessed, almost without exception, a surprisingly bright red colour. This phenomenon riveted my earnest attention. Starting from Lavoisier's theory, according to which animal heat is the result of a process of combustion, I regarded the twofold change of colour which the blood undergoes in the capillaries as a sensible sign —as the visible indication —of an oxidation going on in the blood. In order that the human body may be kept at a uniform temperature, the clevelopment of heat within it must bear a quantitative relation to the heat which it loses-a relation, that is, to the temperature of the surrounding medium; and hence both the production of heat and the process of oxidation, as well as the difference in colour of the two kinds of blood, must be on the whole less in the torrid zones than in colder regions. In accordance with this theory, and having regard to the known physiological facts which bear upon the question, the blood must be regarded as a fermenting liquid undergoing slow combustion, whose most important function-that is, sustaining the process of combustion-is fulfilled without the constituents of the blood (with the exception, that is, of the products of decomposition) leaving the cavities of the bloodvessels or coming into such relation with the organs that an interchange of matter can take place. This may be thus stated in other words: by far the greater part of the assimilated food is burned in the cavities of the blood-vessels themselves, for the purpose of producing a physical effect, and a comparatively small quantity only serves the less important end of ultimately entering the substance of the organs them PROBLEMI OF PHYSIOLOGICAL HEAT, 325 selves, so as to occasion growth and the renewal of the wornout solid parts. If hence it follows that a general balance must be struck in the organism between receipts and expenditure, or between work done and wear and tear, it is unmistakably one of the most important problems with which the physiologist has to deal, to make himself as thoroughly acquainted as it is possible for him to be with the budget of the object of his examination. The wear and tear consists in the amount of matter consumed; the work done is the evolution of heat. This latter effect, however, is of two kinds, inasmuch as the animal body evolves heat on the one hand directly in its own interior, and distributes it by communication to the objects immediately surrounding it; while, on the other hand, it possesses, through its organs of motion, the power of producing heat mechanically by friction or in similar ways, even at distant points. We now require to know Whether the heat directly evolved is ALONE to be laid to the account of the process of combustion, or whether it is the smri of the heat evolved both directly and indirectly that it is to be taken into calculation. This is a question that touches the very foundations of science; and unless it receives a trustworthy answer, the healthy development of the doctrine concerned is not possible. For it has been already shown, by various examples, what are the consequences of neglecting primary quantitative determinations. No wit of man is able to furnish a substitute for what nature offers. The physiological theory of combustion starts from the fundamental proposition, that the quantity of heat which results from the combustion of a given substance is invariablethat is, that its amount is uninfluenced by the circumstances which accompany the combustion; whence we infer, "' in specie," that the chemical effect of combustible matter can undergo no alteration in amount even by the vital process, or 326 THE MiECHANICAL EQUIVALENT OF HEAT. that the living organism, with all its riddles and marvels, cannot create heat out of nothing. But if we hold firm to this physiological axiom, the answer to the question started above is already given. For, unless we wish to attribute again to the organism the power of creating heat which has just been denied to it, it cannot be assumed that the heat which it produces can ever amount to more than the chemical action which takes place. On the combustion-theory there is, then, no alternative, short of sacrificing the theory itself, but to admit that the total amount of heat evolved by the organism, partly directly, and partly indirectly by mechanical action, corresponds quantitatively, or is equal to the amount of combustion. Hence it follows, no less inevitably, that the heat produced mechanically by the oryanism qmust bear an invariable quantitative relation to the iwork expended in producing it. For if, according to the varying construction of the mechanical arrangements which serve for the development of the heat, the same amount of work, and hence the same amount of organic combustion, could produce varying quantib ties of heat, the quantity of heat produced from one and the same expenditure of material would come out smaller at one time and larger at another, which is contrary to our assumption. Further, inasmuch as there is no difference in kind between the mechanical performances of the animal body and those of other inorganic sources of work, it follows that AN INVARIABLE QUANTITATIVE RELATION BETWEEN HEAT AND WORK IS A POSTULATE OF TE OF THE PHYSIOLOGICAL THIEORY OF COMBUSTION. While following in general the direction indicated, it was accordingly needful for me in the end to fix my attention chiefly on the physical connection subsisting between motion and heat; and it was thus impossible for the existence of the mechanical equivalent of heat to remain hidden from me. But, although I have to thank an accident for this discovery, R.ELATION BETWEEN HEAT AND WORK. 32'7 it is none the less my own, and I do not hesitate to assert my right of priority. In order to ensure what had been thus discovered against casualties, I put together the most important points in a short paper which I sent in the spring of 1842 to Liebig, with a request that he would insert it in the Annalenz der Chemice und Phaarmacie, in the forty-second volume of which, page 233, it may be found under the title "' Bemerkungen fiber die Kr[ifte der unbelebten Natur." It was a fortunate circumstance for me that the reception given to my unpretending work by this man, gifted with so deep an insight, at once secured for it an entrance into one of the first scientific organs, and I seize this opportunity of publicly testifying to the great naturalist my gratitude and my esteezm. Liebig himself, however, had about the same time already pointed out, in more general but still unmistakable terms, the connection subsisting between heat and work. In particular, he asserts that the heat produced mechanically by a steamengine is to be attributed solely to the effect of combustion, which can never receive any increase through the fact of its producing mechanical effects, and, through these, again developing heat. From these, and from similar expressions of other scientific men, we may infer that science has recently entered upon a direction in which the existence of the mechanical equivalent of heat could not in any case have remained longer unperceived. In the paper to which reference has been made, the natural law with which we are now concerned is referred back to a few fundamental conceptions of the human mind. The proposition that a magnitude, which does not spring from nothing, cannot be annihilated, is so simple and clear that no valid argument can be urged against its truth, any more than against an axiom of geometry; and until the contrary is 328 THE MECHANICAL EQUIVALENT OF HEAT. proved by some fact established beyond a doubt, we may accept it as true. Now we are taught by experience, that neither motion nor heat ever takes its rise except at the expense of some measurable object, and that in innumerable cases motion disappears without any thing except heat making its appearance. The axiom that we have established leads, then, now to the conclusion that the motion that disappears becomes heat, or, in other words, that both objects bear to each other an invariable quantitative relation. The proof of this conclusion by the method of experiment, the establishment of it in all its details, the tracing of a complete harmony subsisting between the laws of thought and the objective world, is the most interesting, but at the same time the most comprehensive problem that it is possible to find. What I, with feeble powers and without any external support or encouragement, have effected in this direction is truly little enough; but —ultra posse nemo obligatus. In the paper referred to (the first of V[ayer's in the present volume) I have thus expressed myself with regard to the genetic connection of heat and moving force: " If it be now considered as established that in many cases (exceptio confirmat regulam) no other effect of motion can be traced except heat, and that no other cause than motion can be found for the heat that is produced, we prefer the assumption that heat proceeds from motion, to the assumption of a cause without effect and of an effect without a causejust as the chemist, instead of allowing oxygen and hydrogen to disappear without further investigation, and water to be produced in some inexplicable manner, establishes a connection between oxygen and hydrogen on the one hand and water on the other." From this point there is but one step to be made to the goal. At page 257 it is said: "The solution of the equations subsisting between falling-force [that is, the raising of CONNECTION OF HEAT AND MOVING FORCE. 329 weight] and motion requires that the space fallen through in a given time, e. g. the first second, should be experimentally determined; in like manner, the solution of the equations subsisting between falling-force and motion on the one hand and heat on the other, requires an answer to the question, How great is the quantity of heat which corresponds to a given quantity of motion or falling-force? For instance, we must ascertain how high a given weight requires to be raised above the ground in order that its falling-force may be equivalent to the raising of the temperature of an equal weight of water from 0~ to t~ C. The attempt to show that such an equation is the expression of a physical truth may be regarded as the substance of the foregoing remarks.;" By applying the principles that have been set forth to the relations subsisting between the temperature and the volume of gases, we find that the sinking of a mercury column by which a gas is compressed is equivalent to the quantity of heat set fire by the compression; and hence it follows, the ratio between the capacity for heat of air under constant pressure and its capacity under constant volume being taken as =1'421, that the warming of a given weight of water from 0~ to 1~ C. corresponds to the fall of an equal weight from the height of about 365 metres." It is plain that the expression "; equivalent" is here used in quite a different sense from what it bears in chemistry. The difference will be shown most distinctly by an example. When the same weight of potash is neutralized, first, with sulphuric acid, then with nitric acid, the numbers which ex. press the ratio which the absolute weights of these three substances bear to one another are called their equivalents; but there is no thought here either of the quantitative equality or of the transformation of the bodies in question. This peculiar signification which the word "I equivalent' has acquired in chemistry, is doubtless connected with the fact that the chemist has been able to determine the object of 330 THE MECHANICAL EQUIVALENT OF HEAT. his investigation by a common quantitative standard, their ab. solute weights. Let us suppose, however, that we could determine one body, for instance water, only by weight, and another, water-forming or explosive gas, only by volume, and that we had agreed to choose one pound as the unit of weight, and one cubic foot as the unit of volume; we should then have to ascertain how many cubic feet of explosive gas could be obtained from one pound of water, and conversely. This number, without which neither the formation nor the decomposition of water could be made tile subject of calculation, might then be suitably called' the explosive-gas equivalent of water." In this latter sense a raised weight might, in accordance with the known laws of mechanics, be called the "6 equivalent" of the motion resulting from its fall. Now, in order to compare these two objects, the raised and the moving weight, which admit of no common measure, we require that constant number which is generally denoted by g. This number, however, and the mechanical equivalent of heat, whereby the relation subsisting between heat and motion is defined, belong both of them to one and the same category of ideas. In the paper that I have mentioned it is further shown how we may arrive at such a conception of force as admits of being consistently followed to its consequences and is scientifically tenable; and the importance of this subject induces me to return to it again here. The word " force" (Krqft) is used in the higher or scientific mechanics in two distinct senses. I. On the one hand, it denotes every push or pull, every effort of an inert body to change its state of rest or of motion; and this effort, when it is considered alone and apart from the result produced, is called " pushing force," "; pulling force," or shortly " force," and also, in order to distinguish between this and the following conception, A" dead force" (via mortua). MEANING OF THE TEEMI; FORCEE.9 331 II On the other hand, the product of the pressure into the space through which it acts, or, again, the product-or half-product-of the mass into the square of the velocity, is namedl force." In order that motion may actually occur, it is in fact necessary that the mass, whatever it may be, should under the influence of a pressure, and, in the direction of that pressure, traverse a certain space, " the effective space" (Wirkungsrzaum): and in this case a magnitude which is proportional to the "6 pushing force" and to the effective space, likewise receives the name I" force;" but to distinguish it from the mere pushing force, by which alone motion is never actually broughlt about, it is also called the A" vis viva of motion," or; "moving force." With the generic conception of "6 force," the higher mechallics, as an essentially analytic science, is not concerned. In order to arrive at it, we must, according to the general rule, collect together the characters possessed in common by the several species. As is well known, the definition so obtained runs thus-"- Force is every thing which brings about or tends to bring about, alters or tends to alter motion." This definition, however, it is easy to see, is tautological; for the last fourteen words of it might be omitted, and the sense would be still the same. This erroneous solution is occasioned by the nature of the problem, which requires an impossibility. Mere pressure (dead force) and the product of the pressure into the effective space (living force) are magnitudes too thoroughly unlike to be -by possibility combined into a generic conception. Pressure or attraction is, in the theory of motion, what affinity is in chemistry-an abstract conception: living force, like mat-ter, is concrete; and these two kinds of force, however closely connected in the region of the association of ideas, are in reality so widely separated that a frame which should take them both'in must be able to include the whole world. There are several conceivable ways of escaping from the 332 THE MECHANICAL EQUIVALENT OF HEAT. difficulty. For instance, just as we speak of absolute weight, specific weight, and combining weight, without its ever entering any one's head to want to construct a generic idea out of these distinct notions, so two or more meanings may be attached to the word force. This is what is actually done in the higher mechanics, and hence in this branch of science we meet with no mention of a generic conception of "; force." There has been no lack of recommendations to carry, in like manner, the notions of-" dead" and I" living force" as distinct and separate through the other departments of science; it has, however, been found impossible to put in practice such recommendations; for the use of ambiguous expressions, which can in no case contribute any thing to clearness, is altogether inadmissible if confusion can possibly arise. It is true that the mathematician is in no danger of confounding in his calculations a product with one of its factors; but in other departments of knowledge a systematic confusion of ideas exists on this point; and if any thing is to be done toward clearing it up, the source of the error must be stopped; for if we once recognize two meanings of the word "; force," it would be the labour of Sisyphus to try to distinguish between them in each separate case. In order, then, to arrive at any result, we must make up our minds to do without any common denomination of the magnitudes mentioned above, as I. and II., and either to give up the use of the word 6; force" altogether, or to employ it for one only of these two categories. The notion of force was consistently employed in the latter sense by Newton. In solving his problems, he decoms poses the product of the attraction into the eifeclive space into its two factors, and calls the former by the name "; force." As an objection to this mode of proceeding, it must, however, be remarked that in many cases it is not possible thus to decompose the product in question. Let us take, for instance, the following very simple case: a mass M, originally MEANING OF THE TERMI'FORCE." 333 at rest, is caused to move with the (uniform final) velocity c; from the knowledge of the magnitudes M and c it is certainly possible to deduce the value of the product of the force (in Newton's sense) into its effective space, but we are not thereby enabled to conclude as to the magnitude of this force itself. As a matter of fact, the necessity soon made itself felt of treating and naming this product as a whole. It also has been called "I force," and the expressions I,vis viva of motion,";' moving force," " working force," " horse-power " (or force), "c muscular force," &c., have been long naturalized in science. However happy we may, in many respects, think the choice of this word, there is still the objection that a new meaning has been fixed upon an already existing technical expression, without the old one having been called in from circulation at the same time. This formal error has become a Pandora's box, whence has sprung a Babylonian confusion of tongues. Under existing circumstances no choice is left us but to withdraw the term "; force" either from Newton's dead force or from Leibnitz's living force; but in either case we come into conflict with prevailing usage. But if once we have made up our minds to introduce into our science a logically accurate use of terms, even at the cost of existing expressions which have become easy and pleasant to us by long usage, we cannot long hesitate in the choice we have to make between the conceptions I. and II. Let us consider the elementary case of a mass, originally at rest, which receives motion: this happens, as has been already said, by the mass being subjected to a certain push or pull under the influence of which it traverses a certain space, the effective space. Now, however, both the velocity and also the intensity of the push (Newton's force) always vary at every point of the effective space; and in order to multiply these variable magnitudes into effective space, that is, to deduce the quantity of motion from the intensity of the 334 THE MECHANICAL EQUIVALENT OF HEATo pushing force, we must call in the aid of the higher mathe. matics. But hence it follows that, except in statics, where the effective space is nought and the pressure constants the Newtonian conception of force is available only in the higher branches of mechanics; and it is plainly not advisable so to choose our conception of " force" that it cannot be consist ently employed in that branch (namely, the elementary parts of the theory of motion) which of all others is chiefly comn cerned with fundamental notions. It is, however, a totally mistaken method to try to adapt the idea of a force, such as gravity, conceived in Newton's sense, to the elementary parts of science, by leaving out of consideration one of its most important properties, namely its dependence on distance, and to make a I" force" out of Galileo's gravity thus inexactly and in some relations most incorrectly conceived. Some such ideal force (No. III.) seems to hover before the minds of most writers on natural science as the original type of a " force of nature." Such quantitative determinations as hold good only approximately and under certain conditions ought never to be employed to establish definitions. In a calculation, it is true, - e may correctly enough take an arc, which is sufficiently small in comparison with the radius, as equal in size to the sine or to the tangent; but if we attempted to use such a relation in settling first principles, we should lay a foundation for fallacies and errors. The Newtonian idea of force, however, transplanted in the manner that is commonly done into the region of elementary science, is no whit better than the notion of a straight curve. Newton's force, or attraction, in specie gravity, g, is equal to the differential quotient of the velocity by the time; that is, g= — de This expression is quite exact, but in order to understand and apply it a knowledge of the higher mathemat NEWTON'S CONCEPTION OF FORCE. 335 ics is required. On the other hand, it is quite true that, so long as we have to do only with cases in which the space fallen through is so small in comparison with the earth's semidiameter that it may be disregarded, the equation just given may be abbreviated into the very convenient form c= T without any considerable error; but this expression can never be mathematically exact so long as the space fallen through has any calculable magnitude. ]But on the strength of an equation thus radically inaccurate, there are planted in the receptive mind of youth such false notions as-that gravity is a uniformly accelerating (?) force, a moving (?) force whose action is proportional to the time (?); that force is directly proportional (?) to the velocity produced; and many other like errors. It would certainly be a great merit if authors of treatises on physics would help to remedy this state of things, and in framing their definitions would start only from thoroughly exact determinations of magnitudes; for elementary physics in its present form, instead of being a well-grounded science, is only a sort of half-knowledge, such that on passing to the higher and strictly scientific departments the student must try to forget its principles and theorems as quickly as he can. If we have once convinced ourselves by unprejudiced examination that the retention, under that name, of the conception of force distinguished above by I. has nothing but its origin to recommend it, but much to condemn it, the rest follows almost spontaneously. It accords with the laws of thought, as well as with the common usage of language, to connect every production of motion with an expenditure of force. Hence " force" isSomzetzhing which is expended in producing mnotionr; and this something which is expended is to be looked upon as a cause equivalent to the effect, namely, to the motion produced. 336 THE MECHANICAL EQUIVALENT- OF HEAT. This definition not only corresponds perfectly with facts, but it accords as far as possible with that which already exists; for, as I shall show, it contains by implication the conception of force as met with in the higher mechanics, and referred to above by IL. If a mass M, originally at rest, while traversing the effective space s, under the influence and in the direction of the pressure p, acquires the velocity c, we have ps=-Ic2. Since, however, every production of motion implies the existence of a pressure (or of a pull) and an effective space, and also the exhaustion of one at least of these factors, the effective space, it follows that motion can never come into existence except at the cost of this product, ps-=-c2. Anc this it is which for shortness I call "; force." The connection between expenditure and performance (in other words, the exhaustion of force in producing its effect) presents itself in the simplest form in the phenomena of gravitation. The necessary condition of every falling motion is that the centre of gravity of the two masses concerned in it (that is, of the earth and of the falling weight) should approach each other. But in the case of the falling together of the two masses, the approach of their centres of gravity reaches its natural limit, and hence the production of a falling movement is thus bound up with an expenditure, namely, with the exhaustion of the given falling-space, and thereby also of the product of that space into the attraction. The falling down of a weight upon the earth is a process of mechanical combination; and just as in combustion the capacity of performance (that is, the condition of the development of heat) ceases when the act of combination comes to an end, so also the production of motion ceases when the weight has fallen to its lowest position. The weight, when lying on the solid ground, is, like the carbonic acid formed in combustion, nothing but a capuzt nortumun. The affinity, whether mechanical or chemical, is still there after the union just as much as HIGHEST VELOCITY OF A FALLING BODY. 337 before, and opposes a certain resistance to the reduction of the compound; but its power of performance (LeistungsfdChigkeit) is at an end as soon as there is no further available falling-space. Whenever the attraction becomes indefinitely small, or ceases altogether, space is no longer effective space; and thus it follows, from the diminution which gravity undergoes with distance, that falling-space is limited in the centrifugal direction also, and hence that the cause of motion or I" force" is, under all circumstances, a finite magnitude which becomes exhausted in producing its' effect. This fundamental physical truth will be most easily perceived when applied to a special case and reduced to figures. When a pound weight is lifted one foot from the ground, the available force is, as every one knows, — one foot-pound. If the falling-height of this weight amounts to n feet, n not being a large number, the force may be taken as approximately — n foot-pounds. But supposing n, or the original distance of the weight from the earth, to be very considerable, or indeed infinite, the force (that is, the number of foot-pounds) does not by any means thereby become infinite, but, according to Newton's law of gravitation, it becomes at most — r footpounds, where r is the number of feet contained in the earth's semidiameter. Thus how great soever the distance through which a weight falls against the earth, or the time occupied by its fall may be, it. can acquire no higher final velocity than 34,450 Paris feet per second. On the other hand, were the mass of the earth four times as great as it is, its bulk remaining the same, the force would likewise become four times as great, and the maximum velocity would be 68,900 feet. It is one of the essentials of a good terminology that it should put fundamental facts of this kind in a clear light; exactly the opposite, however, is done by the nomenclature at present in use. A few expressions, employed by a very meri15 338 THE MECHANICAL EQUIVALENT OF HEAT. torious naturalist in combating my views, may serve to support this assertion. "6 Although," he says,'" it is quite true that in nature no motion can be annihilated, or that, as it is commonly expressed, the quantity of motion once in existence continues unceasingly and without any lessening, and although in this sense the character of indestructibility belongs to every proximate cause even, every primary cause, that is, every true physical force, possesses the additional characteristic of being inexhaustible. These characteristics will best admit of being unfolded by the closer consideration of gravity, the most active and widely diffused of the natural forces (primary causes), which, as it were the soul of the world, indestructibly and inexhaustibly upholds the life of those great masses on whose motions depends the order of the universe, while requiring no food from without to call forth its ever renewed activity." If these words are intended to contain a material contradiction of the views I have put forward, they must be meant to imply that, by virtue of its being inexhaustible, the attractive power of the earth must be capable of imparting to a falling weight, under certain conceivable circumstances, an infinite velocity. But our author himself in several places lets us see that he has a (quite well-founded) mistrust of any so decided a conclusion: this is shown in the following, among other passages: " If we follow up the chain of causes and effects to its first beginnings, we come at length to the true forces of nature, to those primary causes whose activity does not require that they should be preceded by any others, which ask for no nourishment, but which can ever call forth new motions, as it were, out of an inexhaustible soil, and can uphold and quicken those that are already in being." Again: " If the moon every moment falls, at least virtually, a certain distance toward the earth, what is the force which the next moment pulls it away again, as it were, in OB3JECTIONS CONSIDERED, 339 order to give rise to a new falling force? It is precisely its indestructibility and inexhaustibility, its power at all times and under all circumstances to bring about without ceasing, at least virtually, the same effects, that is the essence of every true force or primary cause." This I" as it were" and "I at least virtually," which always slips in at the critical moment, affords room for the suspicion that our author is himself not quite confident of the power of his "I true natural causes" to give rise to an inexhaustible. amount of motion (of actual exertion of force); and the indefiniteness of these expressions is quite characteristic of the Protean part which the force of gravity plays in writings on natural science. The most arbitrary explanations are given of this word, and then, when facts no longer admit of any thing else, a retreat is sought in the Newtonian conception. Gravity being called a force, and at the same time the term force being connected, in accordance with the common use of language, with the conception of an object capable of producing motion, leads to the false assumption that a mechanical effect (the production of motion) can be produced without a corresponding expenditure of a measurable object; and here is likewise plainly the reason why our author could neither keep clear in his facts nor consistent in his reasoning. If once the production of motion out of nothing is granted, the annihilation of motion must also be admitted as a consequence; and the magnitude of motion must, in accordance with this assumption, be simply proportional to the velocity, or =Mc, and the I" quantity of motion once in existence" must be _ +Mc - lic=:O. But notwithstanding his " inexhaustible forces," the writer ]referred to expressly declares that motion is indestructible; Dut, instead of stating his opinion as to what becomes of motion which disappears by fric-, tion, he says in another place again that it remains "I undecided" whether the effect of a force (the amount of motion produced by it) is.measured by the first or by the second 340 THE MECHANICAL EQUIVALENT OF HEAT. power of the velocity (that is, whether it is or is not destructible): he even appears, from repeated expressions, to hold it possible that a given quantity of heat can produce motion ad infinitum! If such were the case, it would certainly be useless to consider the convertibility of these magnitudes: the ground would rather have been won for the contact theory. The polemics of my respected critic, whoml I have here introduced as the representative and spokesman of prevailing views, and to whom I feel that my sincere thanks are due for his attentive examination of my first publication, appear to me to be necessarily without result, inasmuch as the first problem in combating my assertions, which all revolve about the one point of an invariable quantitative relation between heat and motion, must be to find out that this relation is variable, and in what cases. Formal controversy without a material basis is only beating the air; and as to what relates specially to the questions about force, the first point to consider is, not what sort of thing a: force" is, but to what thing we shall give the name "force." Backwards and forwards talk about gravity is fruitless, since all who understand the matter are agreed as to its nature; for gravity is and remains a differential quotient of the velocity by the time, directly proportional to the attracting mass, and inversely proportional to the square of the distance: on this point a final decision was come to long ago But whether it is expedient to call this magnitude a force is quite another question. Since, whenever an innovation of essential importance is proposed, the public is so ready to misapprehend, I will here state once more, as clearly as I can, my reasons for saying that "' the force of gravity" is an improper expression. It is an unassailable truth that the production of every falling motion is connected with a corresponding expenditure of a measurable magnitude. This magnitude, if it is to be made an object of scientific investigation (and why should it APPLICATION OF THE TERM "FORCE."9 341 not?), must have a name given to it; and in accordance with the logical instinct of roan, as manifested in the genius of language, no other name can be here chosen than the word' force." But since this expression is already used in a quite different sense, we might be tempted to create for the conception which is as yet-in the fundamental parts of science at least-unnamed an entirely new name. Butt before betaking ourselves to this extreme course, which for reasons that are not far to seek would be the one whereby we should be brought most into conflict with existing usage, it is reasonable to inquire whether the word " force," which in itself answers so well to the requirements of the case, is in its right place where it was first put by the schools. According to the common custom of speech, we understand by " force" something moving-a cause of motion; and if, on the one hand, the expression "4moving force" is for this reason, strictly speaking, a pleonasm, the notion of a not moving or II dead" force is, on the other hand, a contradictio in adjecto. If it be said, for instance, that a load which presses with its weight on the ground exerts thereby a forcea force which, though never so great, is unable of itself to bring about the smallest movement-the mode of conception and of expression is quite justified by scholastic usage, but it is so far-fetched that it becomes the source of unnumbered misapprehensions. Between gravity and the force of gravity there is, so far as I know, no difference; and hence I consider the second expression unscientific, inasmuch as it is tautological. Let it not be objected that the IG force " of pressure, the ";force" of gravity, cohesive'"force," &c., are the higher causes of pressure, gravity, and the like. The exact sciences are concerned with phenomena and measurable quantities. The first Cause of things is Deity-a Being ever inscrutable by the intellect of man; while " higher causes," 6' supersensuous forces," and the rest, with all their consequences, be 342 THE MECHANICAL EQUIVALENT OF HEAT. long to the delusive middle region of naturalistic philosophy and mysticism. By a law that is universally true, waste and want go hand in hand. If to the case before us, where this rule likewise meets with confirmation, we apply an equalizing process, and take away the word " force" from the connection in which it is superfluous and hurtful, and bring it to where we are in want of it, we get rid at one time of two important obstacles. The higher mathematics at once cease to be required in order to gain admittance into the theory of motion: nature presents herself in simple beauty before the astonished eye, and even the less gifted may now behold many things which hitherto were concealed from the most learned philosophers. Force and matter are indestructible objects. This lawv, to vwhich individual facts may most simply be referred, and which therefore I might figuratively call the heliocentric standpoint, constitutes a natural basis for physics, chemistry, physiology, and philosophy. Among the facts which, though known, have been hitherto only empirically established and have remained isolated, but which, can be easily referred to this natural law, is the one that electric and magnetic attraction cannot be isolated any more than gravity, or that the strength of this attraction undergoes no alteration, so long as the distance remains the same, by the intervening of indifferent substances (non-conductors). Among facts which have remained unknown up to the most recent times, I will refer only to the influence which the ebb and flow of the tide exerts, in accordance with the known laws of mechanics, on the motion of the earth about its axis. A fact of such importance, standing, as it does, in close relation with the fundamental law just stated, having been able to escape the attention of naturalists, is of itself a proof that the prevailing system has no exclusive title. For the rest, it will not have escaped those who are ac& IOTION AND FALLING-FORCE. 343 quainted with the modern literature of science that a modification of scientific language in the sense of my views is actually beginning to take place. But in matters of this kind the chief part of the work must be left to time. According to what has been said thus far, the vis viva of motion must be called a force. But since the expression vis viva denotes in mechanics, not only a magnitude which is proportional to the mass and to the square of its velocity, but also one which is proportional to the mass and to the height from which it has fallen, force thus conceived naturally divides itself into two very easily distinguished species, each of which requires a distinct technical name, for which the words motion (Bewegung) and falling-force (Fallcraft) seem to me the most appropriate.* Hence, according to this definition, "motion" is always measured by the product of the moved mass into the sqzeare of the velocity, never by the product of the mass into the velocity. By falling-force " we understand a raised weight, or still more generally, a distance in space between two ponderable [a, The distinction here drawn between " motion" and "falling-force" is the same as that made by Helmholtz (Die h~Aalteny cler IKraft, 1847) between "vis viva" (lebendige Kiraft) and "tension" (Spainkraft). The English expressions " dynamical energy " and " statical energy " were used by Prof. W. Thomson (Phil. MIag. S. vol. iv. p. 304, 1852) in the same sense, but were afterwards abandoned by him in favour of the terms " actual energy" and "potential energy" introduced by Prof. Rankine. More recently ("Good Words" for October, 1862) Professors Thomson and Tait have employed the expression "kinetic energy9' in place of "' actual energy." The German word Iieaft in the text has been uniformly translated force, to which term the ambiguity of the German original has thus been transferred. This ambiguity, however, may be avoided in English by allowing the word " force" to retain the meaning which it bears in common language, that is, to denote all resistances which it requires the exertion of a power to overcome (whence the expressions gravitating force, cohesive force, &c.), and by using the word " energy " to denote force as defined by Mayer.-G. C. F.] 344 THE MECHANICAL EQUIVALENT OF HEAT. bodies. In many cases falling-force is measured with suffi cient accuracy by the product of the raised weight into its height; and the expressions "foot-pound," "kilogrammemetre,"' "horse-power," and many others, are conventional units for the measurement of this force, which have of late come into general use, especially in practical mechanics. But in order to find the exact quantitative expression for the mag. nitude in question, we must consider (at least) two masses existing at a determinate distance from each other, which acquire motion by mutually approaching; and we must investigate the relation which exists between the conditions of the motion, namely, the magnitude of the masses and their original and final distance, and the amount of motion produced. It very remarkably happens that this relation is the simplest conceivable; for, according to Newton's lawr of gravitation, the quantity of motion produced is directly proportional to the masses and to the space through which they fall, but inversely proportional to the distances of the centres of gravity of the masses before and after the movement. That is, if A and B are the two masses, c and c' the velocities which they respectively acquire, and h and h' their original and final distances apart, we have Ac- +Bce' = aB (h-to) or in words, the falling-force is eqlua to the 1)roduct of the masses into the space fallen through divided by the tzwo distances. By help of this theorem, which, as will be easily seen, is nothing but a more general and convenient expression of Newton's law of gravitation,@ the laws of the fall of bodies *- Newton's formula relates to the particular case in which the two dis. tances (the initial and the final distance) are equal, so that their product becomes a square. In this case, however, both the space fallen through and the velocity become nought; and hence, when this expression has to CONVERSIONS OF MOTION AND FALLING-FORCE. 345 from cosmical elevations, and also the general laws of central motions, can be developed without its being needful to employ equations of more than the second degree. Having now become acquainted with two species of forcemotion and falling-force-we can arrive at a conception of "1 a force" in general, according to the well-known rule, by collecting together the common characteristics of the two species. To this end, we must consider the properties of these objects somewhat more closely. Their most important property depends on their mutual relation. Whenever a given quantity of falling force clisappears, motion is produced; and by the expenditure of this latter, the falling-force can be reproduced in its original amount. This constant proportion which exists between fallingforce and motion, and is known in the higher mechanics under the name of "the principle of the conservation of vis viva," may be shortly and fitly denoted by the term "6 transformation" ( Unwandlung). For instance, we may say that a planet, in passing from its aphelion to its perihelion, transforms a part of its falling-force into motion, and, as it moves away from the sun again, changes a part of its motion into falling-force. In using the word " transform" in this sense, nothing else can or is intended to be expressed but a constant numerical ratio. But it follows from the axiom mentioned at page 326, that the production of a definite quantity of motion from a given quantity of falling-force, and vice versd, implies that neither falling-force nor motion can be annihilated either totally or in part. We thus obtain the following definition: Forces are transformable, indestructible, and (in contradisbe taken as the starting-point for the calculation of real velocities, mnathe. matical artifices become necessary which are inadmissible in the elementary branches of science. 346 THE MECOHAICAL EQUIVALENT OF HEATL tinction fiom matter) imponderable objects. (Conf. paper already quoted, pp. 328, 329.) It is easy to see that this definition embraces, among other things, the fact that the motion which disappears in mechanical processes of different kinds bears a constant relation to the heat thereby produced, or that motion is convertible, as an indestructible magnitude, into heat. Thus heat is, like motion, a force; and motion, like heat, an imponderable. I have characterized the relation which various forces bear to one another by saying (Phil. Mag. S. 4, vol. xxiv. p. 252) that they are "l different forms under which one and the same object makes its appearance." At the same time I have expressly guarded myself from making the certainly plausible, but unproved, and, as it seems to me, hazardous deduction that thermal phenomena are to be regarded as merely phenomena of motion. The following is what I said upon this point (loc. cit.) p. 376: "' But just as little as the connection between falling-force and motion authorizes the conclusion that the essence of falling-force is motion, can such a conclusion be adopted in the case of heat. We are, on the contrary, rather inclined to infer that before it can become heat, motion-whether simple, or vibratory as in the case of light and radiant heat, &c.must cease to exist as motion." The relation which, as we have seen, subsists between heat and motion has regard to quantity, not to quality; for (to borrow the words of Euclid) things which are equal to one another are not therefore similar. Let us beware of leaving the solid ground of the objective, if we would not entangle ourselves in difficulties of our own making. In the mean time it at least results from the foregoing considerations that the phenomena of heat, electricity, and magnetism do not owe their existence to any particular fluids; and the immateriality of heat, asserted half a century ago by VARIOUS INDS OF HEAT. 347 Rumford, becomes, through the discovery of its mechanical equivalent, a certainty. The form of force denoted by the name "6 heat'" is plainly not single, but includes several distinct, though mutually equivalent, objects, three principal forms of which are distinguished in common language: namely, I. Radiant Heat; II. Free (sensible) 1Heat, Specific Heat; and III. Latent Heat. There can be no doubt that radiant heat must be regarded as a phenomenon of motion, especially since the recent detection of phenomena of interference in the radiation of heat. But whether there really exists, as is commonly assumed, a peculiar aether, of which the vibratory motion is perceived by us as radiant heat, or whether the seat of this motion is the particles of material bodies, is a question that is not yet made out. Still greater obscurity hangs about the essential nature of specific heat, or what goes on in the interior of a heated body. Not only does the unanswered question of the aether enter again here, but, before we canr be in a position to form any clear ideas on this subject, we require to have an exact knowledge of the internal constitution of matter. We are, however, still far from having reached this point; for, in particular, we do not know whether such things as atoms exist-that is, whether matter consists of such constituents as undergo no further change of form in chemical processes. But a span of that time which stretches both backwards and forwards into eternity is meted out to man here on earth, and the space which his foot can tread is narrowly bounded above and below: so also his scientific knowledge finds natural limits in the direction of the infinitely small as well as of the infinitely great. The question of atoms seems to me to lead beyond these limits, and hence I consider it unpractical. An atom in itself can no more become an object of our investigation than a differential, notwithstanding that the ratio which such immensely small auxiliary magnitudes bear to 348 THE MECHANICAL EQUIVALENT OF HEAT. one another may be represented by concrete numbers. In every case, however, the conception of an atom must be regarded as merely relative, and must be considered in connection with some definite process; for, as is well known, the particles of an acid and base may play the part of atoms in the formation and decomposition of a salt, while in another process these atoms may themselves undergo further division. But assuming that, in a chemical sense, atoms have a real existence-an assumption which, among other things, the laws of isomorphism certainly render probable-the further question arises whether, by the continued division of.matter, we can at last arrive at molecules which are atoms in relation to the phenomzena of hect, such that heat cannot penetrate to their interior, and such that, when the whole mass is heated, they for their parts undergo no increase of bulk. But since we are unable to grapple with such preliminary questions as hese, we are forced to confess that, whether the existence of mn ather and of atoms be admitted or not, we are, so far as Pgards the nature of specific heat, in a state of ignorance. The expression " latent heat" has reference to its correctly ecognized property of indestructibility. In all cases in which ilermometrically sensible specific heat disappears, it must be assumed that it eludes our perception only by taking on some other state of existence, and that by an appropriate process of inverse transformation the free heat can be reproduced in its original amount. These are the facts on which the doctrine of latent heat rests; and hence, if we have regard to them only, all the connected phenomena may be claimed as so many confirmations of the principle of the transformation and conservation of force. The conception of latent heat is accordingly nothing else than the conception of something equivalent to free heat, and thus the doctrine of free and specific heat embraces pretty nearly the whole domain of physics. A few examples, chosen from among the abundance of facts, may serve to show HOW LATENT HEAT IS TO BE REGARDED. 349 how, according to my view, the phenomena wherein heat becomes latent are to be regarded. If heat is communicated to a gas retained under constant pressure, the free heat of the gas is increased, and at the same time a calculable quantity of heat becomes latent; the gas is thereby caused to expand, and there is consequently produced an amount of vis viva proportional to the pressure and to the space through which expansion takes place. Therefore as soon as we know how much of the heat that has become latent is to be attributed to the expansion of the gas, we know also the amount of the remainder of the latent heat corresponding to the vis viva produced. Now Gay-Lussac has proved by experiment that the specific heat of a gas undergoes no sensible alteration in flowing from a containing vessel into a vacuum. Hence it follows that a gaseous body opposes no perceptible resistance to the separation of its particles, and that the rarefaction of a gas does not of itself (that is, when it occurs without any evolution of force) cause any heat to become latent. The total quantity of heat which becomes latent by the expansion of a gas is therefore to be taken as the equivalent of the vis viva produced. It results from the principle of the indestructibility of heat-a principle which no one calls in question —that the quantity of heat which has thus become latent must again become free when heat is in any way produced at the expense of the acquired vis viva of motion. Motion is latent heat, and heat is latent motion. The celebrated law of Dulong, that the amount of heat produced by the compression of a gas is dependent on the amount of force expended, and not upon the chemical nature, tension, or temperature of the gas, is a special application of the above general principle. But in the communication so often mentioned I have shown that this law of nature is capable of a very much wider application, and that the heat which becomes latent in the expansion of a gas reappears again in 350 THE MECHANICAL EQUIVALENT OF HEAT. every case, if the vis viva thereby produced is employed to generate heat, whether by the compression of air, by friction, or by the impact of nonelastic bodies; and I have there calculated the mechanical equivalent of heat upon principles of which the accuracy cannot be disputed. I also nmeasured at that time, by way of control, the heat produced in the manufacture of paper in Holland, and compared it with the working force expended, and so found a sufficient degree of concordance between the two quantities. I have recently, moreover, succeeded in constructing, for the purpose of the direct determination of the mechanical equivalent of heat, a very simple thermal dynamometer on a small scale, with which the truth of the principle in question can be demonstrated ad oculos; and I have reason to believe that the efficiency of water-wheels and steam-engines might be easily and advantageously measured by means of3a similar calorimotorial apparatus. It must, however, be left to the future judgment of practical men to decide whether, and to what extent, this method deserves to be preferred to Prony's. Heat further becomes latent in certain changes of the state of aggregation of bodies. Since it is a settled fact that both solid and liquid bodies oppose a certain resistance to the separation of their parts, and since in general an expenditure of vis viva is required for the overcoming of mechanical resistances, we are led to conclude dc priori that whenever the cohesion of a body is diminished or done away with, force or heat must become latent; and this, as is well known, perfectly accords with experience. Starting from this point of view, the French physicist Person has attempted to detect a direct quantitative relation between the latent heat of metals, on which he has made a great number of observations, and their cohesion; but at present determinations of this kind are beset with almost insurmountable difficulties. The heat which becomes latent in the evaporation of water HEAT AND CHANGES OF STATE. 351 has been considered from quite a similar point of view by Holtzmann in his important memoir "' On the Heat and Elasticity of Gases and Vapours." Starting from the principle that elevation of temperature is equivalent to the raising of a weight, this philosopher has likewise calculated the mechanical equivalent of heat from the quantity of heat which becomes latent by the expansion of a gas; and he very rightly conceives of the latent heat of steam as made up of two parts, whereof one, the smaller, is expended in overcoming the opposing pressure of the atmosphere, and can hence be easily calculated by means of the mechanical equivalent of heat, while the remaining part, the amount of which can also be calculated, is what Holtzmann calls the heat required to destroy the cohesion of the water. In all steam-engines this latter portion is wasted, and Holtzmann calculates from these data the superior efficiency of high-pressure compared with low-pressure engines.* If the view here taken of the latent heat of fusion and evaporation is correct, heat must also become latent when hard bodies are reduced to powder; and when such substances pass into the liquid condition from a state of fine division, they must absorb a smaller quantity of heat than when they are liquefied without previous comminution. A few experiments that I have instituted in this direction have not hitherto given any decisive result. It is also worthy of notice that certain solid bodies which are capable of assuming allotropic states, as, for instance, the oxygen-compounds of iron, evolve a considerable quantity of heat on passing from a less to a more hard condition. Such facts, the number of which will doubtless continually increase with time, agree perfectly with the above principle, that diminution of cohesion involves an expenditut~re of heat, and, on the other hand, increase of cohesion a production of heat. * The engines which give the greatest useful effect must be those in which the steaim receives an addition of heat during its expansion. 352 THE MECHANICAL EQUIFVALENT OF HEAT. Customary language, according to which gravity is called a moving force and heat a substance, occasions, on the one hand, the significance of an important natural object, fallingspace, or the space through which a body falls, to be kept as much as possible out of sight, and, on the other hand, heat to be removed to the greatest possible distance from the vis viva of motion. The sciences are thus reduced to an artificial system, over whose fissured surface we can advance in safety only by the powerful aid of the higher analysis. Without doubt the fact that so simple and obvious a matter as the connection between heat and motion could remain unperceived up to the most recent times must also be attributed to the same defect. Nevertheless, as has been already pointed out, the quantitative determination of chemical heating-effects and of galvanic actions, as well as researches into vital phenomena, instituted in the spirit of those of Liebig, must soon have led to the law, not difficult to discover, of the equivalence of heat and motion. In reality this law and its numerical expression, the mechanical equivalent of heat, were published -almost simultaneously in Germany and in England. Starting from the fact that the amount of chemical as well as of galvanic effect is dependent only and solely on the amount of material expenditure, the celebrated English physicist Joule was led to the principle that the phenomena of motion and of heat rest essentially upon one and the same foundation, or, as he expressed himself, in the same way as I have done, heat and motion are transformable one into the other. Not only did this philosopher indisputably make an independent discovery of the natural law in question, but to him belongs the credit of having made numerous and important contributions towards its further establishment and development. Joule has shown that when motion is produced by nmeans of electro-magnetism, the heating effect of the galvanic DISCOVERIES OF JOULE. 353 current is diminished in a corresponding and fixed proportion. Hle has further ascertained that by reversing the poles of a magnetic bar a quantity of heat is produced proportional to the square of the magnetic tension-a fact which was also discovered'by myself, though at a later date. In particular, Joule has likewise demonstrated, by means of numerous experiments, that the heat evolved by friction under various circumstances stands in an unvarying proportion to the amount of force expended. According to his most recent experiments of this kind, he has fixed the mechanical equivalent of heat at 423.* Joule has likewise investigated experimentally, in relation to this question, the thermal behaviour of elastic fluids when expanded, and has thereby confirmed the earlier results of other physicists. The new subject soon began to excite the attention of learned men; but inasmuch as both at home and abroad the subject has been exclusively treated as a foreign discovery, I find myself compelled to make the claims to which priority entitles me; for although the few investigations which I have given to the public, and which have almost disappeared in the flood of communications which every day sends forth without leaving a trace behind, prove, by the very form of their publication, that I am not one who hankers after effect, it is not therefore to be assumed that I am willing to be deprived of intellectual property which documentary evidence proves to be mine. By help of the mechanical equivalent of heat many problems can be solved which, without it, could not be attacked at all: among them, the calculation of the thermal effbect of the falling together of cosmical masses may be especially mentioned. It will not be out of place to indicate here briefly a few results of such calculations.'i That is, 1 thermal unit =423 kilogrammetres. 354 THE MECHANICAL EQUIVALENT OF HEAT. The following is one problem of this kind. It is assumed that a cosmical body enters the atmosphere of our earth with a velocity of four geographical miles per second, and that, in consequence of the resistance which it here encounters, it loses so much of its vis viva of motion that its remaining velocity when it again quits the atmosphere amounts to three miles: the question now arises, How great is the thermal effect which accompanies this process? A simple calculation, based upon the mechanical equivalent of heat, shows that the quantity of heat required is about eight times as great as the heat of combustion of a mass of coal of equal weight with the body in question, one kilogramme of coal being taken as yielding 6,000 thermal units. Hence it follows that the velocity of the motion of shootingstars and fire-balls, which, as is well-known, attains, according to astronomical observations, to from four to eight miles, is a cause fully sufficient to produce the most violent evolution of heat, and an insight into the nature of these remarkable phenomena is thereby afforded to us.* The following is a problem of a similar kind: if two cosmical masses, moving in space about their common centre of gravity, were by any cause whatever, for example by the resistance of the surrounding medium, caused to fall together, the question again arises, How great is the thermal effect corresponding to this process of mechanical combination? Even though the elements of the orbits (that is, their excentricity) may be unknown, we can nevertheless calculate from the given weight and volume of the masses in question the maximum and the minimum of the required effect. Thus let it be supposed, for the sake of an example, that our earth had been divided into two equal globes which had united in * The idea that the meteors here referred to owe their light to a mechanical process-whether friction, or the compression of the air-is not new; but without a knowledge of the mechanical equivalent of heat it could have no scientific foundation. COLLISION OF COSMICAL MlASSES. 35 the manner described: calculation teaches us that the amount of heat which would have been evolved in such a case would considerably exceed that which an equal weight of matter could furfish by the most intense process of chemical action. It is more than probable that the earth has come into ex;istence in some such way, and that in consequence our sun, as seen from the distance of the fixed stars, exhibited at that epoch a transient burst of light. But what took place in our solar system perhaps millions of years ago, still goes on at the present time here and there among the fixed stars; and the transient appearance of stars, which in some cases, like the celebrated star of Tycho Brahe, have at first an extraordinary degree of brilliance, may be satisfactorily explained by assuming the falling together of previously invisible double stars. Contrasting with such explosive bursts of light is the steady radiation, shown continuously through enormous periods, by the greater number of fixed stars, and among them by our sun. Do these appearances, which in so special a manner tempt to higher speculations, constitute a real exception to the exhaustion of a cause in producing its effect, which, in accordance with the foregoing considerations, we have regarded as an established law of Nature? or does the small. sum of human knowledge authorize us in supposing that here also there is an equivalence between performance and expenditure, and in searching for the conditions of that equivalent? To enter further upon this subject would lead us beyond the intended scope of this publication; and I therefore close in the hope that the reader will please to supplement by his own reflection much that in this tract has been left unsaid. SOME THOUGHTS ON THE CONSETRVATION OF FORCE. BY DR. FARADAY. MICTHAEL FARDa Y, son of a smith, was born in London in 17)91. He was taught reading, writing, and arithmetic at a day-school, and in all other things educated himself. At thirteen he was apprenticed to a bookbinder, choosing this vocation in order to be among books. He was early fond of experiment, and averse to trade; and being taken to hear some lectures of Sir Humphrey Davy at the Royal Institution, he resolved to pursue science, and wrote to Davy asking his assistance in obtaining a place. Davy favored his application, and in 1813, at the age of twenty, he was appointed assistant in the laboratory of the Royal Institution. In 1820 he discovered the chloride of carbon, and in 1823 effected the condensation of chlorine and other gases. On this account Davy became jealous of him, and discouraged the idea of recommending him for election to the Royal Society, which, however, took place in 1824. In 1820, Oersted announced his celebrated discovery of electro-magnetism, and Faraday at once entered upon an investigation of the relations of magnetism and electricity. In 1831 he commenced his celebrated series of Experimental Researches in Electricity, which extended to three volumes, published in 1839, 1844, and 1855. In 1827 he published his admirable work on " Chemical Manipulations," and, in 1830, a valuable paper on " The Manufacture of Glass for Optical Purposes." In 1833 he became Professor of Chemistry in the Royal Institution, and he has received numerous honors from the learned societies of Europe. In 1838 he received a pension of ~300 a year, and in 1858 the Queen allotted him a residence in Hampton Court. Dr. Faraday has talents of a high order, both as an original investigator and as a lecturer. Advanced in years, he has now retired to a considerable extent from active duty, but is still in the vigor of his powers, as is shown by his recent lectures to juvenile audiences in the Royal Institution. THE CONSERVATION OF FORCE. AIRIOUS circumstances induce me at the present mo. ment to put forth a consideration regarding the conservation of force. I do not suppose that I can utter any truth respecting it that has not already presented itself to the high and piercing intellects which move within the exalted regions of science; but the course of my own investigations and views makes me think that the consideration may be of service to those persevering labourers (amongst whom I endeavour to class myself) who, occupied in the comparison of physical ideas with fundamental principles, and continually sustaining and aiding themselves by experiment and observa. tion, delight to labour for the advance of natural knowledge, and strive to follow it into undiscovered regions. There is no question which lies closer to the root of all physical knowledge than that which inquires whether force can be destroyed or not. The progress of the strict science of modern times has tended more and more to produce the conviction that " force can neither be created nor destroyed;" and to render daily more manifest the value of the knowledge of that truth in experimental research. To admit, indeed, that force may be destructible or can altogether disappear, would be to admit that matter could be uncreated; for we know.matter only by its forces; and though one of these is 360 THE CONSERVATION OF FORCE. most commonly referred to, namely, gravity, to prove its presence, it is not because gravity has any pretension, or any exemption, amongst the forms of force as regards the principle of conservation, but simply that being, as far as we perceive, inconvertible in its nature and unchangeable in its manifestation, it offers an unchanging test of the matter which we recognize by it. Agreeing with those who admit the conservation of force to be a principle in physics, as large and sure as that of the indestructibility of matter, or the invariability of gravity, I think that no particular idea of force has a right to unlimited or unqualified acceptance that does not include assent to it; and also, to definite amount and definite dis osition of the force, either in one effect or another, for these are necessary consequences; therefore I urge, that the conservation of force ought to be admitted as a physical principle in all our hypotheses, whether partial or general, regarding the actions of matter. I have had doubts in my own mind whether the considerations I am about to advance are not rather metaphysical than physical. I am unable to define what is metaphysical in physical science; and am exceedingly adverse to the easy and unconsidered admission of one supposition upon another, suggested as they often are by very imperfect induction from a small number of facts, or by a very imperfect observation of the facts themselves; but, on the other hand, I think the phi. losopher may be bold in his application of principles which have been developed by close inquiry, have stood through much investigation, and continually increase in force. For instance, time is growing up daily into importance as an element in the exercise of force. The earth moves in its orbit in time; the crust of the earth moves in time; light moves in time; an electro-magnet requires time for its charge by an electric current; to inquire, therefore, whether power, acting either at sensible or insensible distances, always acts in time, is not to be metaphysical; if it acts in time and across space, aCTIO OF FOROEs IN TnIME. 361 it must act by physical lines of force; and our view of the nature of the force may be affected to the extremest degree by the conclusions which experiment and observation on time may supply; being, perhaps, finally determinable only by them. To inquire after the possible time in which gravitating, magnetic, or electric force is exerted, is no more metaphysical than to mark the times of the hands of a clock in their progress; or that of the temple of Serapis and its ascents and descents; or the periods of the occultations of Jupiter's satellites; or that in which the light from them comes to the earth. Again, in some of the known cases of action in time, something happens whilst the tize is passing which did not happen before, and does not continue after; it is, therefore, not metaphysical to expect an effect in every case, or to endeavour to discover its existence and determine its nature. So in regard to the principle of the conservation of force; I do not think that to admit it, and its consequences, whatever they may be, is to be metaphysical; on the contrary, if that word have any application to physics, then I think that any hypothesis, whether of heat, or electricity, or gravitation, or any other form of force, which either willingly or unwillingly dispenses with the principle of conservation, is more liable to the charge than those which, by including it, become so far more strict and precise. Supposing that the truth of the principle of the conservation of force is assented to, I come to its uses. No hypothesis should be admitted, nor any assertion of a fact credited, that denies the principle. No view should be inconsistent or incompatible with it. Many of our hypotheses in the present state of science may not comprehend it, and may be unable to suggest its consequences; but none should oppose or contradict it. If the principle be admitted, we perceive at once that a theory or definition, though it may not contradict the principle, cannot be accepted as sufficient or complete unless the 16 362 THE CONSERVATION OF FORCE. former be contained in it; that however well or perfectly the definition may include and represent the state of things commonly considered under it, that state or result is only partial, and must not be accepted as exhausting the power or being the full equivalent, and therefore cannot be considered as representing its whole nature; that, indeed, it may express only a very small part of the whole, only a residual phenomenon, and hence give us but little indication of the full natural truth. Allowing the principle its force, we ought, in every hypothesis, either to account for its consequences by saying what the changes are when force of a given kind apparently disappears, as when ice thaws, or else should leave space for the idea of the conversion. If any hypothesis, more or less trustworthy on other accounts, is insufficient in expressing it or incompatible with it, the place of deficiency or opposition should be marked as the most important for examination, for there lies the hope of a discovery of new laws or a new condition of force. The deficiency should never be accepted as satisfactory, but be remembered and used as a stimulant to further inquiry; for conversions of force may here be hoped for. Suppositions may be accepted for the time, provided they are not in contradiction with the principle. Even an increased or diminished capacity is better than nothing at all, because such a supposition, if made, must be consistent with the nature of the original hypothesis, and may, therefore, by the application of experiment, be converted into a further test of probable truth. The case of a force simply removed or suspended, without a transferred exertion in some other direction, appears to me to be absolutely impossible. If the principle be accepted as true, we have a right to pursue it to its consequences, no matter what they may be. it is, indeed, a duty to do so. A theory may be perfection, as far as it goes, but a consideration going beyond it, is not for that reason to be shut out. We might as well accept our limited horizon as the limits of the world. 1No magnitude, RELATION OF GRAVITY. 363 either of the phenomena or of the results to be dealt with, should stop our exertions to ascertain, by the use of the principle, that something remains to be discovered, and to trace in what direction that discovery may lie. I will endeavour to illustrate some of the points which have been urged, by reference, in the first instance, to a case of power, which has long had great attractions for me, because of its extreme simplicity, its promising nature, its universal presence, and in its invariability under like circunmstances; on which, though I have experimented* and as yet failed, I think experiment would be well bestowed, I mean the force of gravitation. I believe I represent the received idea of the gravitating force aright in saying that it is a simple attractive force exerted between any two or all the _particles or masses of matter, at every sensible distance, but vwith a strength varying inversely as the square of the distance. The usual idea of the force implies direct action at a distance; and such a view appears to present little difficulty except to Newton, and a few, including myself, who in that respect may be of like mind with him. This idea of gravity appears to me to ignore entirely the principle of the conservation of force; and by the terms of its definition, if taken in an absolute sense, "' varying inversely as the square of the distance," to be in direct opposition to it, and it becomes my duty now to point out where this contradiction occurs, and to use it in illustration of the principle of conservation. Assume two particles of matter, A and B, in free space, and a force in each or in both by which they gravitate towards each other, the force being unalterable for an unchanging distance, but varying inversely as the square of the distance when the latter varies. Then, at the distance of ten, the force may be estimated as one; whilst at the distance of one, that is, one-tenth of the former, the force will be one * Philosophical Transactions, 1851, p. 1. 364 THE CONSERVATION OF FORCE. hundred; and if we suppose an elastic spring to be introduced between the two as a measure of the attractive force, the power compressing it will be a hunlched times as much in the latter case as in the former. But from whence can this enormous increase of power come? If we say that it is the character of this force, and content ourselves with that as a sufficient answer, then it appears to me we admit a crection of power and that to an enormous amount; yet by a change of condition, so small and simple as to fail in leading the least instructed mind to think that it can be a sufficient cause, we should admit a result which would equal the highest act our minds can appreciate of the working of infinite power upon matter; we should let loose the highest law in physical science which our faculties permit us to perceive, namely, the conservation of force. Suppose the two particles, A and B, removed back to the greater distance of ten, then the force of attraction would be only a hundredth part of that they previously possessed; this, according to the statement that the force varies inversely as the square of the distance, would double the strangeness of the above results; it would be an anzihilation of force —an effect equal in its infinity and its consequences with creation, and only within the power of Him who has created. We have a right to view gravitation under every form that either its definition or its effects can suggest to the mind; it is our privilege to do so with every force in nature; and it is only by so doing that we have succeeded, to a large extent, in relating the various forms of power, so as to derive one from another, and thereby obtain confirmatory evidence of the great principle of the conservation of force. Then let us consider the two particles, A and B, as attracting each other by the force of gravitation, under another view. According to the definition, the force depends upon both particles, and if the particle A or B were by itself, it could not gravitate, that is, it could have no attraction, no force of gravity. Suppos THE CASE OF GRAVITATING PARTICLES. 365 ing A to exist in that isolated state and without gravitating force, and then B placed in relation to it, gravitation comes on, as is supposed, on the part of both. Now, without trying to imagine 7how B, which had no gravitating force, can raise up gravitating force in A; and how A, equally without force beforehand, can raise up force in B, still, to imagine it as a fact done, is to admit a creation of force in both particles; and so to bring ourselves within the impossible consequences which have been already referred to. It may be said we cannot have an idea of one particle by itself, and so the reasoning fails. For my part I can comprehend a particle by itself just as easily as many particles; and though I cannot conceive the relation of a lone particle to gravitation, according to the limited view which is at present taken of that force, I can conceive its relation to something which causes gravitation, and with which, whether the particle is alone, or one of a universe of other particles, it is always related. But the reasoning upon a lone particle does not fail; for as the particles can be separated, we can easily conceive of the particle B being removed to an infinite cistance from A, and then the power in A will be infinitely diminished. Such removal of B will be as if it were annihilated in regard to A, and the force in A will be annihilated at the same time; so that the case of a lone particle and that where different instances only are considered become one, being identical with each other in their consequences. And as removal of B to an infinite distance is as regards A annihilation of B, so removal to the smallest degree is, in principle, the same thing with displacement through infinite space; the smallest increase in distance involves annihilation of power; the annihilation of the second particle, so as to have A alone, involves no other consequence in relation to gravity; there is difference in degree, but no difference in the character of the result. It seems hardly necessary to observe, that the same line t66 THE CONSERVATION OF FORCE. of thought grows up in the mind, if we consider the mutua. gravitating action of one particle and many. The particle A will attract the particle B at the. distance of a mile with a certain degree of force; it will attract a particle C at the same distance of a mile with a power equal to that by which it attracts B; if myriads of like particles be placed at the given distance of a mile, A will attract each with equal force and if other particles be accumulated round, it, within and without the sphere of two miles diameter, it will attract them all with a force varying inversely with the square of the distance. How are we to conceive of this force growing up in A to a million-fold or more, and if the surrounding particles be then removed, of its diminution in an equal degree? Or, how are we to look upon the power raised up in all these outer particles by the action of A on them, or by their action one on another, without admitting, according to the limited definition of gravitation, the facile generation and annihilation of force? The assumption which we make for the time with regard to the nature of a power (as gravity, heat, etc.), and the form of words in which we express it, that is, its definition, should be consistent with the fundamental principles of force generally. The conservation of force is a fundamental principle; hence the assumption with regard to a particulamr form of force ought to imply what becomes of the force when its action is increased or diminished, or its direction changed; or else the assumption should admit that it is deficient on that point, being only half competent to represent the force; and, in any case, should not be opposed to the principle of conservation. The usual definition of gravity as can att'ractive force between the particles of matter VAtlING inversely as the square of the distance, whilst it stands as a full definition, of the power, is inconsistent with the principle of the conservation of force. If we accept the principle, such a definition must be an imperfect account of the whole of the force, and is GRAVVIATION BUT PARTIALLY UNDERSTOOD. 367 probably only a description of one exercise of that power, whatever the nature of the force itself may be. If the deft nition be accepted as tacitly including the conservation of force, then it ought to admi that consequences must occur during the suspended or diminished degree in its power as gravitation, equal in importance to the power suspended or hidden; being in fact equivalent to that diminution. It ought also to admit, that it is incompetent to suggest or deal with any of the consequences of that changed part or condition of the force, and cannot tell whether they depend on, or are related to, conditions externac or internal to the gravitating particle; and, as it appears to me, can say neither yes nor no to any of the arguments or probabilities belonging to the subject. -If the definition denies the occurrence of such contingent results, it seems to me to be unphilosophical; if it simply ig~nores them, I think it is imperfect and insufficient; if it admits these things, or any part of them, then it prepares the natural philosopher to look for effects and conditions as yet unknown, and is open to any degree of development of the consequences and relations of power; by denying, it opposes a dogmatic barrier to improvement; by ignoring, it becomes in many respects an inert thing, often much in the way; by admitting, it rises to the dignity of a stimulus to investigation, a pilot to human science. The principle of the conservation of force would lead us to assume, that when A and B attract each other less, because of increasing distance, then some other exertion of power, either within or without them, is proportionately growing up; and again, that when their distance is diminished, as from ten to one, the power of attraction, now increased a hundred-fold, has been produced out of some other form of power which has been equivalently reduced. This enlarged assumption of the nature of gravity is not more metaphysical than the half assumption; and is, I believe, more philosophical and more in accordance with all physical considerations, 368 THE CONSERVATION OF FORCE. The half assumption is, in my view of the matter, more dog, matic and irrational than the whole, because it leaves it to be understood that power can be created and destroyed almost at pleasure. When the equivalents of the various forms of force, as far as they are known, are considered, their differences appear very great; thus, a grain of water is known to have electric relations equivalent to a very powerful flash of lightning. It may therefore be supposed that a very large apparent amount of the force causing the phenomena of gravitation, may be the equivalent of a very small change in some unknown condition of the bodies, whose attraction is varying by change of dcistance. For my own part, miany considerations urge my mind toward the idea of a cause of gravity, which is not resident in the particles of matter merely, but constantly in them, and all space. I have already put forth considerations regarding gravity which partake of this idea,* and it seems to have been unhesitatingly accepted by Newton.t There is one wonderfll condition of matter, perhaps its only true indication, namely, inertia; but in relation to the ordinary definition of gravity, it only adds to the difficulty. For if we consider two particles of matter at a certain distance apart, attracting each other under the power of gravity, and free to approach, they will approach; and when at only half the distance, each will have had stored up in it, because of its inertia, a certain amount of. mechanical force. This must' Proceedings of the Royal Institution, 1855, vol. ii., p. 10, etc. "'That gravity should be innate, inherent, and essential to matter, so that one body: may act upon another at a distance, through a veurm, with. out the mediation of any thing else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an agent, acting constantly according to certain laws; but whether this agent be ma. terial or immaterial I have left to the consideration of my reader."-Seo Newton's Third Letter to Bevrley. INERTIA, GRAVITYN END CONSERVATION. 369 be due to the force exerted, and, if the conservation principle be true, must have consumed an equivalent proportion of the cause of attraction; and yet, according to the definition of gravity, the attractive force is not diminished thereby, but increased four-fold, the force growing up within itself the more rapidly, the more it is occupied in producing other force. On the other hand, if mechanical force from without be used to separate the particles to twice their distance, this force is not stored up in momentum or by inertia, but disappears; and three-fourths of the attractive force at the first distance disappears with it. H-Iow can this be? We know not the physical condition or action from which inertia results; but inertia is always a pure case of the conservation of force. It has a strict relation to gravity, as ap.' pears by the proportionate amount of the force which gravity can communicate to the inert body; but it appears to have the same strict relation to other forces acting at a distance as those of magnetism or electricity, when they are so applied by the tangential balance as to act independent of the gravitating force. It has the like strict relation to force communicated by impact, pull, or in any other way. It enables a body to take np and conserve a given amount of force until that force is transferred to other bodies, or changed into an equivalent of some other form; that is all that we perceive in it; and we cannot find a more striking instance amongst natural, or possible phenomena, of the necessity of the conservation of force as a law of nature; or one more in contrast with the assumed variable condition of the gravitating force supposed to reside in the particles of matter Even gravity itself furnishes the strictest proof of the conservation of force in this, that its power is unchangeable for the same distance; and is by that in striking contrast with the variation which we assume in regard to the cause of gravity, to account for the results at different distances. It will not be imagined for a moment that I am opposed 16 370 TIHE CONSERVATION OF FORCE. to what may be called the law of gravitating action, that Js, the law by which all the known effects of gravity are gov erned; -what I am considering is the definition of the force of gravitation. That the result of one exercise of a power may be inversely as the square of the distance, I believe and admit; and I know that it is so in the case of gravity, and has been verified to an extent that could hardly have been within the conception even of Newton himself when he gave utterance to the law; but that the totality of a force can be elmployed according to that law I do not believe, either in relation to gravitation, or electricity, or magnetism, or any other supposed form of power. I might have drawn reasons for urging a continual recollection of, and reference to, the principle of the conservation of force from other forms of power than that of gravitation; but I think that when founded on gravitating phenomena, they appear in their greatest simplicity; and precisely for this reason, that gravitation has not yet been connected by any degree of convertibility with the other forms of force. If I refer for a few minutes to these other forms, it is only to point in their variations, to the proofs of the value of the principle laid down, the consistency of the known phenomena with it, and the suggestions of research and discovery which arise from it. Heat, for instance, is a mighty form of power, and its effects have been greatly developed; therefore, assump. tions regarding its nature become useful and necessary, and philosophers try to define it. The most probable assumption is, that it is a motion of the particles of matter; but a view, at one time very popular, is, that it consists of a particular fluid of heat. Whether it be viewed in one way or the other, the principle of conservation is admitted, I believe, with all its force'. When transferred from one portion to another portion of like matter, the full amount of heat appears. When transferred to matter of another kind an apparent excess or deficiency often results; the word "1 capacity" is then introduced, USES OF THE PRINCIPLE 371 which, while it acknowledges the principle of conservation, leaves space for research. When employed in changing the state of bodies, the appearance and disappearance of the heat is provided for consistently by the assumption of enlarged or diminished motion, or else space is leit by the term;" capacity" for the partial views which remain to be developed. When converted into mechanical force, in the steam or air engine, and so brought into direct contact with gravity, being then easily placed in relation to it, still the conservation of force is fully respected and wonderfully sustained. The constant amount of heat developed in the whole of a voltaic current described by AIl P. Favre,* and the present state of the knowledge of thermo-electricity, are again fine, partial, or subordinate illustrations of the principles of conservation. Even when rendered radiant, and for the time giving no trace or signs of ordinary heat action, the assumptions regarding its nature have provided for the belief in the conservation of force, by admitting either that it throws the ether into an equivalent state, in sustaining which for the time the power is engaged; or else, that the motion of the particles of heat is employed altogether in their own transit from place to place. It is true that heat often becomes evident or insensible in a manner unknown to us; and we have a right to ask what is happening when the heat disappears in one part, as of the thermo-voltaic current, and appears in another; or when it enlarges or changes the state of bodies; or what would happen, if the heat being presented, such changes were purposely opposed. We have a right to ask these questions, but not to ignore or deny the conservation of force; and one of the highest uses of the principle is to suggest such inquiries. Explications of similar points are continually producedl, and will be most abundant from the hands of those Who, not desiring * Comtes Rendus 1854, vol. xxxix., p. 1212. 372 THE CONSERVATION OF FORCE, to ease their labour by forgetting the principle, are ready to admit it, either tacitly, or, better still, effectively, being then continually guided by it. Such philosophers believe that heat must do its equivalent of work; that if in doing work it seem to disappear, it is still producing its equivalent effect, though often in a manner partially or totally unknown; and that if it give rise to another form of force (as we imperfectly express it), that force is equivalent in power to the heat which has disappeared. What is called chemicac attraction affords equally instructive and suggestive considerations in relation to the principle of the conservation of force. The indestructibility of individual matter is one case, and a most important one, of the conservation of chemical force. A molecule has been endowed with powers which give rise in it to various qualities, and these never change, either in their nature or amount. A partiele of oxygen is ever a particle of oxygen-nothing can in the least wear it. If it enters into combination and disappears as oxygen-if it pass through a thousand combinations, animal, vegetable, mineral-if it lie hid for a thousand years and then be evolved, it is oxygen with its first qualities, neitiler more nor less. It has all its original force, and only that, the amount of force which it disengaged when hiding itself has again to be employed in a reverse direction when it is set at liberty; and if, hereafter, we should decompose oxy~ gen, and find it compounded of other particles, we should only increase the strength of the proof of the conservation of force, for we should have a right to say of these particles, long as they have been hidden, all that we could say of the oxygen itself. Again, the body of facts included in the theory of definite proportions, witnesses to the truth of the conservation of force; and though we know little of the cause of the change of properties of the acting and produced bodies, or how the forces of the former are hid amongst those of the latter, we CHEMICAL ACTION AT A DISTANCE. 373 do not for an instant doubt the conservation, but are moved to look for the manner in which the forces are, for the time, disposed, or if they have taken up another form of force, to search what that form may be. Even chemical action at a distance, which is in such anm tithetical contrast with the ordinary exertion of chemical affinity, since it can produce effects miles away from the particles on which they depend, and which are effectual only by forces acting at insensible distances, still proves the same thing, the conservation of force. Preparations can be made for a chemical action in the simple voltaic circuit, buts until the circuit be complete that action does not occur; yet in completing we can so arrange the circuit, that a distant chemical action, the perfect equivalent of the dominant chemical action, shall be produced; and this result, whilst it establishes the electrochemical equivalent of power, establishes the principle of the conservation of force also, and at the same time suggests many collateral inquiries which have yet to be made and answered, before all that concerns the conservation in this case can be understood. This and other instances of chemical action at a distance carry our inquiring thoughts on from the facts to the physical mode of the exertion of force; for the qualities which seem located and fixed to certain particles of matter appear at a distance in connection* with particles altogether different. They also lead our thoughts to the conversion of one form of power into another; as, for instance, in the heat which the elements of a voltaic pile may either show at the place where they act by their combustion or combination together, or in the distance, where the electric spark may be rendered manifest; or in the wire of fluids of the different parts of the circuit. When we occupy ourselves with the dual forms of power, electricity, and magnetism, we find great latitude of assumption, and necessarily so, for the powers become more and I374 THE CONSERVATION OF FORCEo more complicated in their conditions. But still there is no apparent desire to let loose the force of the principle of conservation, even in those cases where the appearance and disappearance of force may seem most evident and striking. Electricity appears when there is consumption of no other force than that required for friction; we do not know houw, but we search to know, not being willing to admit that the electric force can arise out of nothing~ The two electricities are developed in equal proportions; and having appeared, we may dispose variously of the influence of one upon successive portions of the other, causing many changes in relation, yet never able to make the sum of the force of one kind in the least degree exceed or come short of the suma of the other. In that necessity of equality, we see another direct proof of the conservation of force, in the midst of a thousand changes that require to be developed in their principles before we can consider this part of science as even moderately known to us. One assumption with regard to electricity is, that there is an electric fluid rendered evident by excitement in plus and minus proportions. Another assumption is, that there are two fluids of electricity, each particle of each repelling all particles liEke itself, and attracting all particles of the other kind always, and with a force proportionate to the inverse square of the distance, being so far analogous to the definition of gravity. This hypothesis is antagonistic to the law of the conservation of force, and open to all the objections that have been, or may be, made against the ordinary definition of gravity. Another assumption is, that each particle of the two electricities has a given amount of power, and can only attract contrary particles with the sum of that amount, acting upon each of two-with only half the power it could in like circumstances exert upon one. But various as are the assumptions, the conservation of force (though wanting in the second) is, I think, intended to be included in all. I might OPENING OF NEW PROB @ LEMS. repeat the same observations nearly in regard to magnetismwhether to be assumed as a fluid, or two fluids or electric cur rents-whether the external action be supposed to be action at a distance5 or dependent on an external condition and lines of force-still, all are intended to admit the conservation of power as a principle to which the phenomena are subject. The principles of physical knowledge are now so far developed as to enable us not merely to define or describe the known, but to state reasonable expectations regarding the iunknown; and I think the principle of the conservation of force mray greatly aid experimental philosophers in that duty to science, which consists in the enunciation of problems to be solved. It will lead us, in any case where the force remaining unchanged in form is altered in direction only, to look for the new disposition of the force; as in the cases of magnetism, static electricity, and perhaps gravity, and to ascertain that as a whole it remains unchanged in amount-or, if the original force disappear, either altogether or in part, it will lead us to look for the new condition or form of force which should result, and to develop its equivalency to the force that has disappeared. Likewise, when force is developed, it will cause us to consider the previously-existing equivalent to the force so appearing; and many such cases there are in chemical action. When force disappears, as in the electric or magnetic induction after more or less discharge, or that of gravity with an increasing distance, it will suggest a research as to whether the equivalent change is one witlbin the apparently acting bodies, or one external (in part) to them. It will also raise up inquiry as to the nature of the internal or external state, both before the change and after. If supposed to be external, it will suggest the necessity of a physical process, by which the power is communicated from body to body; and in the case of external action, will lead to the inquiry whether, in any case, there can be truly action 376 THE. CONSERVATIO1N OF FORCE. at a distance, or whether the ether, or some other medium, it not necessarily present. We are not permitted as yet to see the nature of the source of physical power, but we are allowed to see much of the consistency existing amongst the various forms in which it is presented to us. Thus, if, in static electricity, we consider an act of induction, we can perceive the consistency of all other like acts of induction with it. If we then take an electric current, and compare it with this inductive effect, we see their relation and consistency. In the same manner we have arrived at a knowledge of the consistency of magnetism with electricity, and also of chemical action and of heat with all the former; and if we see not the consistency between gravitation with any of these forms of force, I am strongly of the mind that it is because of our ignorance only. How imperfect would our idea of an electric current now be, if we were to leave out of sight its origin, its static and dynamic induetion, its magnetic influence, its chemical and heating effects; or our idea of any one of these results, if we left any of the others unregarded? That there should be a power of gravitation existing by itself, having no relation to the other natural powers, and no respect to the law of the conservation of force, is as little likely as that there should be a principle of levity as well as of gravity. Gravity may be only the residual part of the other forces of nature, as Mlositi has tried to show; but that it should fall out from the law of all other force, and should be outside the reach either of further experiment or philosophical conclusions, is not probable. So we must strive to learn more of this outstanding power, and endeavour to avoid any definition of it which is incompatible with the principles of force generally, for all the phenomena of nature lead us to believe that,the great and governing law is one. I would much rather incline to believe that bodies affecting each other by gravitation act by lines of force of definite amount (somewhat in the manner of magnetic or electric in MENTAL QU2ALIFICATIONS FOR THE INQTiJIY. 37 duction, though with polarity), or by an ether pervading all parts of space, than admit that the conservation of force could be dispensed with. It may be supposed, that one who has little or no mathe matical knowledge should hardly assume a right to judge oi the generality and force of a principle such as that which forms the subject of these remarks. My apology is this: I do not perceive that a mathematical mind, simply as such, has any advantage over an equally acute mind not mathematical, in perceiving the nature and power of a natural principle of action. It cannot of itself introduce the knowledge of any new principle. Dealing with any and every amount of static electricity, the mathematical mind has balanced and adjusted them with wonderful advantage, and has foretold results which the experimentalist can do no more than verify. But it could not discover dynamic-electricity, nor electro-magnetism, nor magneto-electricity, or even suggest them; though when once discovered by the experimentalist, it can take them up with extreme facility. So in respect of the force of gravitation, it has calculated the results of the power in such a wonderful manner as to trace the known planets through their courses and perturbations, and in so doing has discovered a planet before unknown; but there may be results of the gravitating force of other kinds than attraction inversely as the square of the distance, of which it knows nothing, can discover nothing, and can neither assert nor deny their possibility or occurrence. Under these circumstances, a. principle which may be accepted as equally strict with mathematical knowledge, comprehensible without it, applicable by all in their philosophical logic, whatever form that may take, and above all, suggestive, encouraging, and instructive to the. mind of the experimentalist, should be the more earnestly employed and the more frequently resorted to when we are labouring either to discover new regions of science, or to map out and develop those 378 THE CONSErATsTO OF roREE. which are known into one harmonious whole; and if in such strivings, we, whilst applying the principle of conservation, see but imperfectly, still we should endeavour to see, for even an obscure and distorted vision is better than none. Let us, if we can, discover a new thing in any shape; the true appearance and character will be easily developed afterwardso Some are much surprised that I should, as they think, venture to oppose the conclusions of Newton; but here there is a mistake. I do not oppose Newton on any point; it is rather those who sustain the idea of action at a distance, that contradict him. Doubtful as I ought to be of myself, I am certainly very glad to feel that my convictions are in accordance with his conclusions. At the same time, those who occupy themselves with such matters ought not to depend altogether upon authority, but should find reason within themselves, after careful thought and consideration, to use and abide by their own judgment. Newton himself, whilst referring to those who were judging his views, speaks of such as are competent to form an opinion in such matters, and makes a strong distinction between them and those who were incompetent for the case. But after all, the principle of the conservation of force may by some be denied. Well, then, if it be unfounded even in its application to the smallest part of the science of force, the proof must be within our reach, for all physical science isso. In that case, discoveries as large or larger than any yet made, may be anticipated. I do not resist the search for them, for no one can do harm, but only good, who works with an earnest and truthful spirit in such a direction. But let us not admit the destruction or creation of force without clear and constant proof. Just as the chemist owes all the perfection of his science to his dependence on the certainty of gravitation applied by the balance, so may the physical philosopher expect to find the greatest security and the utmost aid in the principle of the conservation of force. All that SUPPLEMNTEARY CONSIDERATIoNS. 379 we have that is good and safe, as the steam-engine, the electric-telegraph, &c., witness to that principle-it would require a perpetual motion, a fire without heat, heat without a source, action without reaction, cause with effect, or effect without a cause, to displace it from its rank as a law of nature. During the year that has passed since the publication of the foregoing views regarding gravitation, &c., I have come to the knowledge of various observations upon them, some adverse, others favourable: these have given me no reason to change my own mode of viewing the subject; but some of them make me think that I have not stated the matter with sufficient precision. The word 1" force" is understood by many to mean simply "; the tendency of a body to pass firom one place to another," which is equivalent, I suppose, to the phrase 6; mechanical force;" those who so restrain its meaning must have found my argument very obscure. WThat I mean by the word "6 force," is the cause of a physical action; the source or sources of all possible changes amongst the particles or materials of the universe. It seems to me that the idea of the conservation of force is absolutely independent of any notion we may form of the nature of force or its varieties, and is as sure and may be as firmly held in thle mind, as, if we, instead of being very ignorant, understood perfectly every point about the cause of force and the varied effects it can produce. There may be perfectly distinct and separate causes of what are called chemical actions, or electrical actions, or gravitating actions, constituting so many forces; but if the a" conservation of force'" is a good and true principle, each of these forces must be subject to it: none can vary in its absolute amount; each must be definite at all times, whether for a particle, or for all the particles in the universe; and the sum also of the tlree 380 THtE CONSERVATION OF FORCE. forces must be equally unchangeable. Or, there may be but one cause for these three sets.of actions, and in place of three forces we may really have but one, convertible in its manifestations; then the proportions between one set of actions and another, as the chemical and the electrical, may become very variable, so as to be utterly inconsistent with the idea of the conservation of two separate- forces (the electrical and the chemical), but perfectly consistent with the conservation of a force, being the common cause of the two or more sets of action. It is perfectly true that we cannot always trace a force by its actions, though we admit its conservation. Oxygen and hydrogen may remain mixed for years without showing any signs of chemical activity; they may be made at any given instant to exhibit active results, and then assume a new state, in which again they appear as passive bodies. Now, though we cannot clearly explain what the chemical force is doing, that is to say, what are its effects during the three periods before, at, and after the active combination, and, only by very vague assumption can approach to a feeble conception of its respective states, yet we do not suppose the creation of a new portion of force for the active moment of time, or the less believe that the forces belonging to the oxygen and hydrogen exist unchanged in their amount at all these periods, though varying in their results. A part may at the active moment be thrown off as mechanical force, a part as radiant force, a part disposed of we know not how; but believing, by the principle of conservation, that it. is not increased or destroyed, our thoughts are directed to search out what at all and every period it is doing, and how it is to be recognized and measured, A problem, founded on the physical truth of nature, is stated, and, being stated, is on the way to its solution. Those who admit the possibility of the common origin of all physical force, and also acknowledge the principle of con SUPtREMACY OF THE PRINCIPLE. 3811 servation, apply that principle to the sum total of the force Though the amount of mechanical force (using habitual language for' convenience sake) may remain unchanged and definite in its character for a long time, yet when, as in the collision of two equal inelastic bodies, it appears to be lost, they find it in the form of heat; and whether they admit that heat to be a continued mechanical action (as is most probable), or assume some other idea, as that of electricity, or action of a heat-fluid, still they hold to the principle of conservation by admitting that the sum of force, i. e. of the 64 cause of action," is the samle, whatever character the effects assume. With them the convertibility of heat, electricity, magnetism, chemical action and motion, is a familiar thought; neither can I perceive any reason why they should be led to exclude, d priori, the cause of gravitation from association with the cause of these other phenomena respectively. All that they are limited by in their various investigations, whatever directions they may take, is the necessity of making no assumption directly contradictory of the conservation of force applied to the sum of all the forces concerned, and to endeavour to discover the different directions in which the various parts of the total force have been exerted. Those who admit separate forces inter-unchangeable, have to show that each of these forces is separately subject to the principle of conservation. If gravitation be such a separate force, and yet its power in the action of two particles be supposed to be diminished fourfold by doubling the distance, surely some new action, having true gravitation character, and that alone, ought to appear, for how else can the totality of the force remain unchanged? To define the force as 6; a simple attractive force exerted between any two or all the particles of matter, with a strength varying inversely as the square of the, distance," is not to answer the question; nor does it indicate or even assume what are the other complementary results which occur; or allow the sup 382 THE CONSERVATION OF FORCE. position that such are necessary: it is simply, as it appears to me, to deny the conservation of force. As to the gravitating force, I do not presume to say that I have the least idea of what occurs in two particles when their power of mutually approaching each other is changed by their being placed at different distances; but I have a strong conviction, through the influence on my mind of the doctrine of conservation, that there is a change; and that the phenomena resulting'from the change will probably appear some day as the result of careful research. If it be said that "'twere to consider too curiously to consider so," then I must dissent: to refrain to consider would be to ignore the principle of the conservation of force, and to stop the inquiry which it suggests-whereas to admit the proper logical force of the principle in our hypotheses and considerations, and to permit its guidance in a cautious yet courageous course of investigation, may give us power to enlarge the generalities we already possess in respect of heat, motion, electricity, magnetism, &c., to associate gravity with them, and perhaps enable us to know whether the essential force of gravitation (and other attractions) is internal or external as respects the attracted bodies. Returning once more to the definition of the gravitating power as " a simple attractive force exerted between any two or all the particTes or masses of matter at every sensible distance, but with a' STRENGTH VARYING inversely as the square of the distance," I ought perhaps to suppose there are many who accept this as a true and sufficient description of the force, and who therefore, in relation to it, deny the principle of conservation. If both are accepted and are thought to be consistent with each other, it cannot be difficult to add words which shall make 6 varying strength" and " conservation" agree together. It cannot be said that the definition merely applies to the effects of gravitation as far as we know theim. So understood, it would form no barrier to progress; for, DEFINITION OF THE GRAVITATING POWER. 383 that particles at different distances are urged toward each other with a power varying inversely as the square of the distance, is a truth: but the definition has not that meaning; and what I object to is the pretence of knowledge which the definition sets up when it assumes to describe, not the partial effects of the fore, but the nature of the force as a whole. ON THE CONINECTION AND EQUIVALENCE OF FORCES BY PROF. J. VON LIEBIG. 17 rJUSTUS voN LIBIG, born at Darmstadt in 1803, after spending telln months in an apothecary's shop, entered the University of Bonn in 1819, and afterwards graduated in medicine in Erlangen. In 1822 he went to Paris, where he studied chemistry two years. In 1824 he read a paper on the Fulminates before the French Institute, which attracted the attention of Humboldt, by whose influence he was appointed adjunct Professor of Chemistry in the University of Giessen. He became professor of this institution in 1826, and established here the first laboratory in Germany for teaching practical chemistry. In 1840 he published his " Chemistry in its applications to Agriculture and Physiology," in the form of a report to the British Association. In 1842 he reported to the same body his work on " Animal Chemistry." About the same time appeared his "Familiar Letters on Chemistry," which has since been rewritten and much extended. He is the author also of various other valuable works. He remained at Giessen till 1852, when he became professor and president of the laboratory in the University of Munich. In 1851 his friends in Europe contributed and presented to him ~1,000, and in 1860 he became President of the Academy of Sciences in Munich. Professor Liebig is a bold and intrepid investigator, and an ardent writer, who has made a profound impression upon his age. While some of his views have not been accepted in the chemical world, and indeed have been abandoned by himself, others have taken their place as valuable additions to the body of scientific truth. The charge that some of his doctrines have proved erroneous does not disturb him; in the true scientific spirit he replies, " Show me the man who makes no mistakes, and I will show you a man who has done nothing." THE CONNECTION AND EQUIVALENCE OF FORCES. T is well known that our machines create no power, but only return what they have received. The motion of a clock is produced by a weight or a spring; but it is the power of the human arm applied to stretch the spring or elevate the weight, which is expended in the movement of the wheels and pendulum in twenty-four hours, or in eight or fourteen days. A water-wheel sets in motion, in a mill, one or more millstones; in a foundry, one or more hammers; in saltkworks or mines it pumps or raises weights to certain heights; in factories, it communicates movements to looms, spinning machines, and rollers. In all these instances, the work performed by the water-wheel is due to the force exerted by the falling water on the buckets, which sets the wheel in motion; and this force must be greater than the resistance presented by the different machines in operation. The performance of the machine is measurable by this force. The work of a steam-engine is executed by the movement of a piston upwards and downwards by the pressure of steam, just as a water-wheel is moved by the pressure of water. The cause of this pressure is heat, which is derived from the chemical process of combustion, and is absorbed by water. By this heat, steam is produced, and the necessary expansion 388 THE CONNECTION AND EQUIVALENCE OF FORCES. obtained for the movement of the piston. It is heat, in this last form, which performs the mechanical work of the machine. Every force acts by producing pressure either from or towards the centre of motion. In every machine in operation the amount of power is always measurable by the resistance overcome; and this again can be expressed by corresponding weights, which that power is capable of raising to a certain height. If one man raises by a pump, in one minute, 150 lbs. of water, and another 200 lbs.; or if one horse draws to a certain distance a load of 20 cwt., and another a load of 30 cwt., it is evident that these numbers express the relative working power of these two men or horses. In mechanics, the working power of every machine is expressed in horse power, that is, a force capable of elevating in each second 75 kilogrammes ( —2' lbs. avoirdupois) to a height of one meter (39.37 inches). The whole power communicated to a machine is not actually available, but is in part lost by fiiction. For if two machines possess the same power, it is found that the greater quantity of work will be executed by the one which has to overcome the smaller amount of friction. In mechanics, friction is always regarded as acting in direct opposition to motion in every machine. It was believed that the working power of a machine could be absolutely annihilated by it. As the proximate cause of the cessation of motion, friction was a palpable fact, and could as such be taken into account; but a fatal error was committed in giving a theoretical view of its mode of action. For if a power could be annihilated, or, in other words, have notizng as its effect, then there would be no contradiction involved in the belief, that out of nothing also power could be created. To this erroneous idea we may partly trace the belief, held for centuries by most able men, in the possibility of discovering a machine which should renew within itself its own power as it was expended, and thus ever STATEMENT OF MAYER S ARGUM1ENT. - 389 continue in motion, without the necessity for any external motive force. The discovery of such a perpetual motion was, indeed, worthy of every effort. It would be as valuable as the bird which lays the golden eggs; for by its means labour would be performed, and money made in abundance without any expenditure. A mass of facts, hitherto unintelligible, have had much light thrown upon them by a more correct view of natural forces, for which we are indebted to a physician, IDr. Mayer, of HIeilbronn, and which, by the investigations of the most eminent natural philosophers and mathematicians, has attained a significance and importance scarcely to be foreseen. According to Dr. ~Mayer, forces are causes, in which full application of the axiom must be found, that every cause must produce an effect which corresponds and is equal to the cause. Causa cequat effectum. Thus if a cause C produces an effect E, then C = E. Should the effect E become the cause of another effect e, then also E = e = C. In such a chain of causes and effects no link, or part of a link, can ever become nothing =- nothing. Should a given cause C have pro duced its corresponding effect E, then C ceases to exist, for it has been converted into E. Consequently, as C passes into E, and the latter into e, it follows that all these causes, as far as relates to their quantity, possess the property of indestr'ucti bility, and to their quality that of cozvertibility. In numberless cases we see a motion cease, without its usual effects being produced, such as lifting a weight or load; but as the force which has caused the motion cannot be reduced to nothing, the question arises, what form has it assumed. Experience gives the answer, by showing that wherever motion is arrested by friction, a blow, or concussion, heat is the result. The motion is the cause of the. heat. The rapid friction of two plates of metal can raise their temperature to redness, and cause the ebullition of water if the friction takes place below its surface. In like manner, 390 THE CONNECTION AND EQUIVALENCE OF FORCES. by rapid motion the iron tires of carriage wheels become frequently so hot that they cannot be touched. In grinding needle-points the steel is heated to redness, and the detached particles burn with sparks. The wooden breaks of railway carriages become frequently so hot by friction that their surface emits an empyreumatic odour. By the friction of an iron grater, particles of white sugar can be melted and heated so far as to acquire the taste of burnt sugar (Caramel). The heat evolved by the friction of two pieces of ice is sufficient to melt them. In the English steel-foundries a bar of steel 10 or 12 inches long, by being heated at, one end in a forge, is welded by hammering to another slender bar, 10 or 12 feet long, without the necessity of further direct application of heaP-a point of great importance for the preservation of the good quality of the steel. Every spot on which the powerful blows of the hammer rapidly descend, becomes red hot, and to the spectators the red glow of heat appears to run up and dcown the bar. This glow is produced by the blows of the hammer, and corresponds to an amount of heat sufficient to raise many pounds of water to the boiling point; whilst the end of the bar heated in the fire would scarcely by itself raise to the same temperature as many ounces of water. According to the preceding views a precise connection exists between the blows of the hammer (the cause) and the heat produced (the effect); and natural philosophers have devised the most ingenious experiments to show this relation. The working power is in this case converted into heat. If the view of Mayer be correct, then should we by this amount of heat thus obtained, be able to reproduce the same amount of work, viz., the same number of blows of the hammer. ]But a closer view of the point shows that we require to elevate the hammer, and that therefore its working power was not inherent, but only lent to it. The hammer was elevated by a water-wheel set in motion by a certain weight of water falling MECHANICAL POWERi CHANGED TO HEAT. 391 on its buckets. Thus to raise a hammer weighing ten pounds to the height of one foot requires at least the fall of ten pounds weight of water from a height of a foot. It was then, properly speaking, this weight of falling water which produced the heat through means of the hammer. By simply altering tile arrangement of the machinery, the same force would have caused a mill-stone to revolve with great rapidity on its axis, or raised by friction two iron disks to a red heat. From experiments instituted to elucidate this point, it has been established that 13,500 blows of a hammer, weighing 10 pounds, falling on a bar of iron from a height of one foot, produce an anmount of heat sufficient to raise one pound of water from the freezing point to that of ebullition. This fact may be represented in another way by saying, that 1,350 cwts. of water, falling from a height of one foot, will raise the temperature of 1 lb. of water from freezing to the boiling point; or 1,350 lbs. of water falling from the same height will raise one pound of water one degree in temperature, or in other words, that this amount of heat corresponds to a working power, capable of elevating 13.- cwt. to the height of one foot. Wherever motion is lost in a machine by friction or by concussion, there is always produced a corresponding amount of heat. When, on the other hand, a certain quantity of work is performed by heat, there disappears, with the mechanical effect obtained, a certain amount of heat, which is expressed by saying that the heat lost by one pound of water in falling one degree in temperature, is equal to the elevation of 138 cwt. to the height of one foot. This quantity of heat becomes then the equivalent or value of the working power expressed by the above numbers. This constant relation between heat and mechanical movement has been confirmed in the most varied manners A rod of metal is extended by a weight, and on its removal resumes its original length, provided certain limits be not exceeded. The same effect is produced by heat; and it is evident that 392 THE CONNECTION AND EQUIVALENCE OF FORCES. an equal force must be exerted by the rod in its extension as in its contraction. lNow, experiment has shown that the relation expressed by the numbers above given, must exist between a given extension of a bar of iron, and the heat or weight which has caused that extension, viz., that a quantity of heat sufficient to raise a pound of water one degree in temperature, will, when communicated to a bar of iron, enable it to elevate a weight of 1,350 lbs. to the height of one foot. An interesting application of this fact was long ago made in the Conservatoire des Arts et Me'tiers, in Paris. In this building, which was formerly a convent, the nave of the church was converted into a museum for industrial products, machines, and implements. In its arch, traversing its length, appeared a crack, which gradually increased to the width of several inches, and permitted the passage of rain and snow. The opening could easily have been closed by stone and lime, but the yielding of the side walls would not have been prevented by these means. The whole building was on the point of being pulled down, when a natural philosopher proposed the following plan, by which the object was accomplished. A number of strong iron rods were firmly fixed at one end to a side wall of the nave, and after passing through the opposite wall were provided on the outside with large nuts, which were screwed up tightly to the wall. By applying burning straw to the rods, they extended in length. The nuts by this extension being now removed several inches finom the wall, were again screwed tight to it. The rods on cooling contracted, with enormous force, and made the side walls ap. proach each other. By repeating the operation the crack entirely disappeared. This building with its retaining rods is still in existence. The working power of a machine, set in motion by elec. tricity, can be expressed by numbers, in the same way as the mechanical effect of heat. An electrical current is generated ELECTRICITY CONVERTED INTO HEAT. 3 93 by a rotating magnet or by solution of zinc in the galvanic battery. Such a current, in circulating through a thick or thin wire, exhibits the same deportment as a fluid flowing through a wide or narrow tube. As a given quantity of fluid requires more time or greater pressure to pass through a narrow tube than through a large, so a thin wire offers a greater resistance than a thick one to the passage of a current of electricity. The current is thus retarded and diminished, one portion only passing through the conductor, the other being converted into heat. According to the amount of heat thus produced by the conversion of the electricity, a conducting wire of platinum can be fused, one of gold fused and converted into vapour, and a considerable quantity of water brought into violent ebullition by passing the current through a thin platinum wire wound round a glass tube in a spiral form. If the electrical current circulates through a wire wound spirally round a bar of iron, the latter is converted into a powerful magnet capable of attracting and carrying several hundred weights of iron. The electrical is converted into the magnetic force, by which a machine may be set in motion. The power of attraction communicated to the iron bar is in exact proportion to the amount of electricity circulating in the surrounding wire, and this current is again dependent on the property of the conductor. That portion of electricity which in the conductor is converted into heat, produces no power of attraction in the iron bar. It follows, from the foregoing, that the quantity of electricity which circulates, of that which produces heat, and the amount of magnetic power convertible into working power, stand in the same relation to each other, as the working power produced in a machine by the pressure of falling water to the heat generated by friction and concussion in the same machine.'The same amount of electricity which, when converted into heat by the resistance of the conductor, raises by one degree the temperature of one -a 394 THE CONNECTION AND EQUIVALENCE OF FORCES. pound of water, generates a magnetic force capable of ele. vating a weight of 138 cwt. to the height of one foot. If the metallic wire through which the electricity is circulating, be cut, and both ends immersed in water, a chemical decomposition of the water into hydrogen and oxygen takes place. The circulating electricity is converted into chemical affinity, and into a power of attraction which causes the separation of the elements of water. With the evolution of the hydrogen and oxygen all traces of the electrical current disappear. The power to produce heat and magnetic force, the usual effects of the electrical current, is apparently in this case annihilated, and in its place we obtain two gases, one of which, hydrogen, when burned in oxygen, reproduces water and evolves heat. Now it has been proved, by careful experiments, that an electrical current of a given strength, which, when converted into heat in a conductor, is capable of raising the temperature of a pound of water by one degree, vill produce by the decomposition of water a quantity of hydrogen, by the combustion of which one pound of water can also be elevated one degree in temperature. The heat and power of attraction which were apparently lost by the decomposition of the water, had only become latent, so to speak, in the elements of water. This heat is again set free on the reunion of these elements, and if converted into working power, would produce the same result (viz., raising a given weight a foot high) as would have been effected by a magnetic power generated by a quantity of electricity circulating round a bar of iron, equal to that which was originally employed in the decomposition of the water. The electrical current is the consequence of a chemical action, and the amount of electricity which circulates can therefore be measured by the quantity of zinc which is dis. solved. The chemical force (affinity) is converted by the solution of the zinc into a corresponding quantity of electricity; and this again in the conductor into its equivalent of THE MOTIVE-FORCE OF PNLAN. 395 heat, or into magnetic force, or, as in the case of the decomposition of water, into chemical force. In no case is there a diminution or increase of force. If, according to the materialist, matter is indestructible, the same holds good with regard to force. It is not extinguished; its apparent annihilation, its disappearance, is only a conversion into some other form. We know now the origin of the heat and light which warm and illuminate our dwellings, of the heat and power generated in our bodies by the vital process. Plants are the one source of all materials used for the production of heat and light, and of that nourishment which must be daily taken to maintain the phenomena of vitality. The elements of plants are earthy in their nature, and are obtained from water, earth, and air. In plants, certain inorganic compounds-carbonic acid, water, and ammonia-are decomposed. The carbon of the carbonic acid, the hydrogen of the water, and the nitrogen of the ammonia, are retained as constituents of their organs, but the oxygen of the carbonic acid and of the water are returned as gas to the air. With out light, however, plants cannot grow. The vital process in plants exhibits itself as directly opposite in its character to the chemical process in the formation of salts. Carbonic acid, water, and zinc, when brought together produce a certain effect on each other.: In virtue of chemical affinity there iss~fbrmed a white powdery compound, containing carbonic acid, zinc, and oxygen from the water, and hydrogen is at the same time evolved. In plants, the living bud or part of the plant takes the place of the zinc. By their growth are formed, from carbonic acid and water, compounds containing carbon and hydrogen, or carbon and the elements of water, and oxygen is at the same time evolved. Sunlight acts in living plants like electricity, which arrests the natural attraction of the elements of water, and separates them from each other. 396 THE CONNITECTION AND EQUIVALENCE OF FORCES. Without the light of the sun plants cannot grow. The living germ, the green leaf, owe to the sun their power of transforming earthy elements into living, vigorous structures. The germ may, indeed, be evolved under ground without the action of light, but only when it breaks through the surface of the soil does it first acquire the power, by the sun's rays, of converting inorganic elements into its own structure. The illuminating and heating rays of the sun, in thus bestowing life, lose their own light and heat. Their power now becomes latent in the new products of the frame, which have been produced under their influence from carbonic acid, water, and ammonia. The light and heat with which our dwellings are illuminated and warmed are but those bestowed by the sun. The food of men and animals consists of two classes of materials, which differ totally in their nature. One class is destined to the production of blood and the maintenance of the structure of the body; the other is similar in composition to ordinary materials for combustion. Sugar, starch, the gum of bread, may be regarded as transformed woody fibre, for we can prepare them from this substance. Fat, in its amount of carbon, resembles closely mineral coal. We heat our bodies as we do stoves, by combustibles which possess the same elements as wood and coal, but which differ essentially from them by being soluble in the fluids of the body. The elements of nutrition from which the temperature of the body. is derived, evidently produce no mechanical power; because power is but converted heat, and the heat which maintains and elevates the temperature of the body does not produce any other effect than that of warmth. All those mechanical operations constantly taking place in the living body, in the movement of organs and limbs, are dependent on an accompanying change in the composition and properties of those highly complex sulphur and nitrogen constituents of the muscles, which, though furnished by the blood, are in the first instance derived directly from the food DERIVATION OF ANIMiAL POWER. 397 of man. The change of position in the elements of these complex bodies, attendant on their rearrangement into new and simpler compounds, necessarily gives rise to motion; and the molecular movement of the particles in a state of change is transferred to the muscular mass. Chemical action is thus evidently the source of mechanical power in bodies. The elements of the food of men and animals which give rise to power and heat, are produced in living plants only by the action of sunlight. The rays of the sun become latent, so to speak, in them in the same way as the current of electricity becomes latent in the hydrogen by decomposition of water. Man, by food, not only maintains the perfect structure of his body, but he daily lays in a store of power and heat, derived in the first instance from the sun. This power and heat, latent f6r a time, reappears and again becomes active when the living structures are resolved by the vital processes into their original elements. The rays of the sun add daily to the store of indestractible forces of our terrestrial body, maintaining life and motion. Thus, from beyond the limits of our earth, the body, the more earthly vessel, derives all that may be called good in it, and of this not a single particle is ever lost. ON THE CORRELATION OF THE PHYSICAL AND VITAL FORCES By DR. WILLIAM B, CARPENTER. WILLIAM BENJAMIN CARPENTER, the eminent English physiologist, was born in the early part of this century, and graduated as doctor of medicine in Edinburgh in 1839. He commenced practice in Bristol, but has been chiefly occupied as a lecthrer and author. His most important works are the "IPrinciples of General and Comparative Physiology" (1839), the "Principles of Human Physiology " (1846), which reached a fifth edition in 1855, and " The Microscope, its Revelations and Uses" (1856). He is the author of various minor, but valuable works, and of many elaborate papers in the cyclopedias and scientific periodicals. For many years he was editor of the " British and Foreign Medico-Chirurgical Review." He was elected a member of the Royal Society in 1844, and is now Professor of Medical Jurisprudence in University College, London; Lecturer on General Anatomy and Physiology at the London Hospital and School of MIedicine, and Examiner in Physiology and Comparative Anatomy in the University of London. In 1850 he published an able paper in the Transactions of the Royal Society on the "i Mutual Relations of the Vital and Physical Forces," and has published upon the same general subject, the essays which close this volume, in the new Quarterly Journal of Science for the present year. Dr. Carpenter is an able and original thinker, as well as a voluminous writer, and has made many valuable contributions to the progress of physiological science. ON THE CORRELATION OF THE PHYS. ICAL AND VITAL FORCES. I.-RELATIONS OF LIGHT AND HEAT TO THE VITAL FORCES OF PLANTS. IN every period of the history of Physiology, attempts have been made to identify all the forces acting in the Living Body with those operating in the Inorganic universe. Because muscular force, when brought to bear on the bones, moves them according to the mechanical laws of lever action, and because the propulsive power of the heart drives the blood through the vessels according to the rules of hydraulics, it has been imagined that the movements of living bodies may be explained on physical principles; the most important consideration of all, namely, the source of that contractile power which the living muscle possesses, but which the dead muscle (though having the same chemical composition) is utterly incapable of exerting, being altogether left out of view. So, again, because the digestive process, whereby food is reduced to a fit state for absorption, as well as the formation of various products of the decomposition that is continually taking place in the living body, may be initiated in the laboratory of the chemist; it has been supposed that the appropriation 402 CORRELATION OF PHYSICAL AND VITAL FORCES. of the nutriment to the production of the living organized tissues of which the several parts of the body are composed, is to be regarded as a chemical action-as if any combination of albumen and gelatine, fat and starch, salt and bone-earth, could make a living Man without the constructive agency inherent in the germ from which his bodily fabric is evolvedo Another class of reasoners have cut the knot which they could not untie, by attributing all the actions of living bodies for which Physics and Chemistry cannot account, to a hypothetical "I Vital principle;" a shadowy agency that does every thing in its own way, but refuses to be made the subject of scientific examination; like the 6 od-force " or the "I spiritual power " to which the lovers of the marvellous are so fond of attributing the mysterious movements of turning and tilting. tables. A more scientific spirit, however, prevails among the best Physiologists of the present day; who, whilst fully recognizing the fact that many of the phenomena of living bodies can be accounted for by the agencies whose operation they trace in the world around, separate into a distinct category-that of vital actions-such as appear to differ altogether in kindcz from the phenomena of Physics and Chemistry, and seek to determine, from the study of the conditions under which these present themselves, the laws of their occurrence. In the prosecution of this inquiry, the Physiologist will find it greatly to his advantage to adopt the method of philosophizing which distinguishes the Physical science of the present from that of the past generation; that, namely, which, whilst fully accepting the logical definition of the cauztse of any phenomenon, as the antecedent, or the concurrence of antecedents on which it is invariably and unconditionally consequent" (Mill), draws a distinction between the dynamical and the material conditions; the former supplying the potwer which does the work, whilst the latter affords the inst'rumental means through which that power operates. DYNAMICAL AND MATERIAL CONDITIONSS 403 Thus if we inspect a Cotton Factory in full action, we find it to contain a vast number of machines, many of them but repetitions of one another, but many, too, presenting the most marked diversities in construction, in operation, and in resultant products. We see, for example, that one is supplied with the raw material, which it cleans and dresses; that another receives the cotton thus prepared, and "i cards" it so as to lay its fibres in such an arrangement as may admit of its being spun; that another series, taking up the Product supplied by the carding machine, twists and draws it out into threads of various degrees of fineness; and that this thread, carried into a fourth set of machines, is woven into a fabric which may be either plain or variously figured according to the construetion of the loom. In every one of these dissimilar operations the force which is immediately concerned in bringing about the results is one and the same; and the variety of its products is dependent solely upon the diversity of the material instruments through which it operates. Yet these arrangements, however skilfully devised, are utterly valueless without the force which brings them into play. All the elaborate mechanism, the triumph of human ingenuity in devising, and of skill in constructing, is as powerless as a corpse without the vis viva which alone can animate it. The giant stroke of the steam-engine, or the majestic revolution of the water-wheel gives the required impulse; and the vast apparatus which was the moment previously in a state of death-like inactivity, is aroused to all the energy of its wondrous life-every part of its complex organization taking upon itself its peculiar mode of activity, and evolving its own special product, in virtue of the share it receives of the one general force distributed through the entire aggregate of machinery. But if we carry back our investigation a stage fmrther, and inquire into the origin of the force supplied by the steamengine or the water-wheel, we soon meet with a new and most significant fact. At our first stage, it is true, we find 404 CORRELATION OF PHYSICAL AND VITAL FORCES. only the same mechanical force acting through a different kind of instrumentality; the strokes of the piston of the steam-engine being dependent upon the elastic force of the vapour of water, whilst the revolution of the water-wheel is maintained by the downward impetus of water en masse. But to what antecedent dynamical agency can we trace these forces? That agency in each case is Heat; a force altogether dissimilar in its ordinary manifestations to the force which produces sensible motion, yet capable of being in turn converted into it and generated by it. For it is from the Heat applied beneath the boiler of the steam-engine that the nonelastic liquid contained in it derives all that potency as elastic vapour, which enables it to overcome the vast mechanical resistance that is set in opposition to it. And, in like manner, it is the heat of the solar rays which pumps up terrestrial waters in the shape of vapour, and thus supplies to ioan a perrenial source of new power in their descent by the force of gravity to the level from which they have been raised.-*The power of the steam-engine, indeed, is itself derived more remotely from those same rays; for the Heat applied to its boilers is but the expression of the chemical change involved in combustion; that combustion is sustained either by the wood which is the product of the vegetative activity of the present day, or by the coal which represents the vegetative life of a remote geological epoch; and that vegetative activity, whether present or past, represents an equivalent amount of Solar Light and Heat, used up in the decomposition of the carbonic acid of the atmosphere, by the instrumentality of the growing plant.t Thus in either case we come, directly or indirectly, to Solar Radiationz as the main* See on this subject the recent admirable address of Sir William Armstrong at the Meeting of the British Association at Newcastle. t This was discerned by the genius of George Stephenson, before the general doctrine of the Correlation of Forces had been given to the world by Mayer and Grove. ESTABLISHMENT OF THE GENERAL PRINCIPLE. 405 spring of our mechanical power; the vis viva of our whole microcosm. Modern physical inquiry ventures even one step further, and seeks the source of Light and Heat of the Sun itself. Are these, as formerly supposed, the result of combustion, or are they, as surmised by Mayer and Thomson, the expression of the motive power continually generated in the fall of aerolites towards the Sun, and as continually annihilated by their impact on its surface? Leaving the discussion of this question to Physical Philosophers, I proceed now to my own proper subject. It is now about twenty years since Dr. ~Mayer first broadly announced, in all its generality, the great principle now known as that of "I Conservation of Force;" as a necessary cieduction from two axioms or essential truths-ex nihilo ni fit, and nil fit ad nihiluMn-the validity of which no true philosopher would ever have theoretically questioned, but of which he was (in my judgment) the first to appreciate the full practical bearing. Thanks to the labours of Faraday, Grove, Joule, Thomson, and Tyndall, to say nothing of those of Helmholtz and other distinguished Continental savans, the great doctrine expressed by the term " Conservation of Force" is now amongst the best-established generalizations of Physical Science; and every thoughtful Physiologist must desire to see the same course of inquiry thoroughly pursued in regard to the phenomena of living bodies. This ground was first broken by Dr. Mayer in his remarkable treatise, "; Die Organische Bewegung in ihrem Zusammenhange mit dem Stoffwechsel" (;' On Organic Movement in its relation to Material Changes," IH-eilbronn, 1845); in which he distinctly set forth the principle that the source of all changes in the living Organism, animal as well as vegetable, lies in the forces acting upon it from without; whilst the changes in its own composition brought about by these agencies, he considers to be the immediate source of the forces which are generated by it. In treating of these forces, however, he dwells chiefly on 406 CORRELATION OF PHYSICAL AND VITAL FORCES. the production of ~IMotion, Heat, Light, and Electricity by living bodies; touching more slightly upon the phenomena of Growth and Development, which constitute, in the eye of the physiologist, the distinct province of vitality. In a memoir of my own, G" On the Mutual Relations of the Vital and Physical Forces," published in the Philosophical Transactions for 1850,* I aimed to show that the general doctrine of the;" Cor' relation of the Physical Forces" propounded by BMr. Grove, was equally applicable to those Vital forces which must be assumed as the moving powers in the production of purely physiological phenomena; these forces being generated in living bodies by thile transformation of the Light, Heat, and Chemical Action supplied by the world around, and being given back to it again, either during their life, or after its cessation, chiefly in Motion and Heat, but also to a less degree in Light and Electricity. This memoir attracted but little attention at the time, being regarded, I believe, as too speculative; but I have since had abundant evidence that the minds of thoughtful Physiologists, as well as Physicists, are moving in the same direction; and as' the progress of science since the publication of my former memoir, would lead me to present some parts of my scheme of doctrine in a different form,t j-I venture to bring it again before the public in the form of a sketch (I claim for it no other title), of the aspect in which the application of the principle of the "6 Conservation of Force " to Physiology now presents itself to my mind. At this date the labours of Dr. Mayer were not known either to myself' or (so far as I am aware) to any one else in this country, save the late Dr. Baly, who a few months after the publication of my Memoir, placed in my hands the pamphlet, " Die Organische Bewegunig; " to which I took the earliest opportunity in my power of drawing public attention in "The British and Foreign Medico-Chirurgical Review " for July, 1851, p. 237. { I have especially profited by a Memoir on the Correlation of Physical, Chemical, and Vital Force, and the Conservation of Force in Vital Phenomena, by Prof. Le Conte (of South Carolina College), in Silliman's American Journal for Nov., 1859, reprinted in the Philosophical Magazine for 1860. CHAIACT-ERICTICS OF VITAL ACTIVITY. 407 If, in the first place, we inquire what it is that essentially distinguishes Vital from every kind of Physical activity, we find this distinction most characteristically expressed in the fact that a germ endowed with Life, developes itself into an organism of a type resembling that of its parent; that this organism is the subject of incessant changes, which all tend in the first place to the evolution of its typical form, and subsequently to its maintenance in that form, notwithstanding the antagonism of Chemical and Physical agencies, which are contin'ually tending to produce its disintegration; but that, as its term of existence is prolonged, its conservative power declines so as to become less and less able to resist these disintegrating forces, to which it finally succumbs, leaving the organism to be resolved by their agency into the components from which its materials were originally drawn. The history of a living organism, then, is one of incessant change; and the conditions of this change are to be found partly in the organism itself, and partly in the external agencies to which it is subjected. That condition which is inherent in the organism, being derived hereditarily from its progenitors, may be conveniently termed its germitnal capacity; its parallel in the inorganic world being that fundamental difference in properties which constitutes the distinction between one substance, whether elementary or compound, and another; in virtue of which each II behaves" in its own characteristic manner when subjected to new conditions. Thus, although there may be nothing in the aspect or sensible properties of the germ of a Polype, to distinguish from that of a iMan, we find that each develops itself, if the requisite conditions be supplied, into its typical form, and ino other; if the developmental conditions required by either be not supplied we do not find a different type evolved, but no evolution at all takes place.*' It is quite true that among certain of the lower tribes, both of Plants 408 CORRELATION OF PHYSICAL AND- VITAL FORCES. Now the difference between a being of high and a being of lowo organization essentially consists in this: that in the latter the constituent parts of the fabric evolved by the process of growth from the original germ, are similar to each other in structure and endowments, whilst in the former they are progressively differentiated with the advance of development, so that the fabric comes at last to consist of a number of organs, or instruments, more or less dissimilar in structure, composition, and endowments. Thus in the lowest forms of Vegetable life, the primordial germ multiplies itself by duplicative subdivision into an apparently unlimited number of cells, each of them similar to every other, and capable of maintaining its existence independently of them. And in that lowest Rhizopod type. of Animal life, the knowledge of, which is among the most remarkable fruits of modern biological research, " the Physiologist has a case in which those vital operations which he is elsewhere accustomed to see carried on by an elaborate apparatus, are performed without any special instruments whatever; a little particle of apparently homogeneous jelly changing itself into a greater variety of forms than the fabled Proteus, laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, appropriating its nutritious material without absorbent vessels or a circulating system, moving from place to place without muscles, feeling (if it has any power to do so) without nerves, propagating itself without genital apparatus, and and Animals, especially the Pungi and Enezozoa —similar germs may develop themselves into very dissimilar forms, according to the conditions under which they are evolved; but such diversities are only the same kind as those which manifest themselves among individuals in the higher Plants and Animals, and only show that in the types in question there is a less close conformity to one pattern. Neither in these groups, nor in that group of Foramzinifera, in which I have been led to regard the range of variation as peculiarly great, does any tendency ever show itself to the agsumptiou of the characters of any group fundamentally dissimilar. THE rLOWEST GRADE OF LIFE. 409 not only this, but in many instances forming shelly coverings of a symmetry and complexity not surpassed by those of any testaceous animals,"' whilst the mere separation of a fragment of this jelly is sufficient to originate a new and independentt organism, so that any number of these beings may be produced by the successive detachment of such particles from a single Rhizopod, each of them retaining (so far as wve have at present the means of knowing) the characteristic endowments of the stock from which it was an offset. When, on, the other hand, we watch the evolution of any of the higher types of Organization, whether vegetable or animal, we observe that although in the first instance the primordial cell multiplies itself by duplicative subdivision into an aggregation of cells, which are apparently but repetitions of itself and of each other, this homogeneous extension has in each case a definite limit, speedily giving place -to a structural differentiation, which becomes more and more decided with the progress of development, until in that most heterogeneous of all types —the lHuman Organism —no two parts are precisely identical, except those which correspond to each other on the opposite sides of the body. With this structural differentiation is associated a corresponding differentiation of function; for whilst in the life of the most highly developed and complex organism we witness no act which is not foreshadowed, however vaguely, in that of the lowest and simplest, yet we observe in it that same I' division of labour" which constitutes the essential characteristic of the highest grade of civilization. For, in what may be termed the elementary form of Humnan Society, in which every individual relies upon himself alone for the supply of all his wants, no greater result can be obtained by the aggregate action of the entire community than its mere maintenance; but as each in-: See the Author's Introduction to the Study of the Foraminifera, published by the Ray Society, 1862: Preface, p. vii. 18 410 CORRELATION OF PHYSICAL ANID VITAL FORCES. dividual selects a special mode of activity for himself2 and aims at improvement in that specialty, he finds himself attaining a higher and still higher degree of aptitude for it; and this specialization tends to increase as opportunities arise for new modes of activity, until that complex fabric is evolved which constitutes the most developed form of the Social State wherein every individual finds the workl-mental or bodilyfor which he is best fitted, and in which he may reach the highest attainable perfection; while the mutual dependence of the whole (which is the necessary result of this specialization, of parts) is such that every individual works for the benefit of all his fellows, as well as for his ow-n. As it is only in such a state of society that the greatest triumphs of human ability become possible, so is it only in the most differentiated types of Organization that Vital Activity can present its highest manifestations. In the one case, as in the other, does the result depend upon a process of gradual development, in which, under the influence of agencies whose nature constitutes a proper object of scientific inquiry, that most general form in which the fabric-whether corporeal or social -originates, evolves itself into that most special in which its development culminates. But notwithstanding the wonderful diversity of structure and of endowments which we meet with in the study of any complex Organism, we encounter i harmonious unity or cobrdination in its entire aggregate of actions, which is yet more wonderful. It is in this harmony or coirdination, whose tendency is to the conservation of the organism, that the state of Health or Normal life essentially consists. And the more profound our investigations of its conditions, the more definite becomes the conclusion to which we are led by the study of them-that it is fundamentally based on the common origin of all these diversified parts in the same germ, the vital endowments of which, equally diffused throughout the entire fabric in those lowest forms of organization in which every THiE FtUNDAMENTAL CHARACTERISTIC OF LIFE, 411 part is but a repetition of every other, are differentiated in the highest amongst a variety of organs, acquiring in virtue of this differentiation a much greater intensity. Thus, then, we may take that mode of Vital Activity which manifests itself in the evolution of the germ into the complete organism repeating the type of its parent, and the subsequent maintenance of that organism in its integrity, in the one case as in the other, at the expense of materials derived from external sources-as the most universal and most fundamental characteristic of Life; and we have now to consider the nature and source of the Force or Powerl by which that evolution is brought about. The prevalent opinion has until lately been, that this power is inherent in the germ; which has been supposed to derive from its parent not merely its material substance, but a nisus formantivus, bildungstrieb, or germv force, in virtue of which it builds itself up into the likeness of its parent, and maintains itself in thalt likeness until the force is exhausted, at the same time imparting a fraction of it to each of its progeny. In this mode of viewing the subject, all the organizing force required to build up an Oak or a Palm, an Elephant or a Whale, must be concentrated in a minute particle, only discernible by microscopic aid, and the aggregate of all the germ-forces appertaining to the descendants, however numerous, of a common parentage, must have existed in their original progenitors. Thus in the case of the successive viviparous broods of Aphides, a germfol ce capable of organizing a mass of living structure, which would amount (it has been calculated)* in the tenth brood to the bulk of five hundred millions of stout men, must have been shut up in the single individual, weighing perhaps the 1-1000th of a grain, from which the first brood was evolved. And in like manner, the germ-force which has organized the S- ee Prof. Huxley on the "Agamic Reproduction of Aphis," in Linnm. Trans., vol. xxii., p. 215. 412 CORRELATION OF PHYSICAL AND VITAL FORCES. bodies of all the individual men that have lived from Adam to the present day, must have been concentrated in the body of their common.ancestor. A more complete redctuio cad absurdum can scarcely be brought against any hypothesis; and we may consider it proved that in som e way or other, fresh organizing force is constantly being supplied fromn vithout during the whole period of the exercise of its activity. When we look carefully into the question, however, we find that what the germ really supplies is not the force, but the directive cagency; thus rather resembling the control exercised by the superintendent builder, who is charged with working out the design of the architect, than the bodily force of the workmen who labour under his guidance in the construction of the fabric.' The actual constructive force, as we learn from an extensive survey of the phenomena of life, is supplied by HIeat, the influence of which upon the rate of growth and development, both animal and vegetable, is so mnarked as to have universally attracted tihe attention of Physiologists, who, however, have for the most part only recognized in it a vitca stizazdus that calls forth the latent power of the germ, instead of looking upon it as itself furnishing the power that does the work. It has been from the narrow limitation of the area over which physiological research has been commonly prosecuted. that the intimacy of this relationship between Heat- and the Organizing force has not sooner become apparent. Whilst the vital phenomena of VWarm-blooded animals, which possess within themselves the means of maintaining a constant temperature, were made the sole, or at any rate the chief objects of study, it was not likely that the inquirer would recognize the full influence of external heat in accelerating, or of cold in retarding their functional activity. It is only when the survey is extended to Cold-blooded anic - mals and to Plants, that the immediate and direct relation bes tween Heat and Vital Activity, as manifested in the rate of growth and development, or of other changes peculiar to the DYI~NAMICS OF G ERiMINWATION. 413 living body, is unmistakably manifested. To some of those phenomena, which afford the best illustrations of the mode in which Heat acts upon the living organism, attention will now be directed. Conmmencing with the Vegetable kingdom, we find that the operation of Heat as the motive power or dynamical agency, to which the phenomena of growth and development are to be referred: is peculiarly well seen in the process of Germination. The seed consists of an embryo which has already advanced to a certain stage of development, and of a store of nutriment laid up as the material for its further evolution; and in the fact that this evolution is carried on at the expense of organic compounds already prepared by extrinsic agency, until (the store of these being exhausted) the young plant is sufficiently far advanced in its development to be able to elaborate them for itself, the condition of the germinating embryo resembles thlat of an Animal. Now, the seed may remain (under favourable circumstances) in a state of absolute inaction durinog an unlimited period. If secluded from the free access of air and moisture, and kept at a low temperature, it is removed from all influences that would on the one hand occasion its disintegration, or on the other, would call it into active life. But when again exposed to air and moisture, and subjected to a higher temperature, it either germinates or decays, according as the embryo it contains has or has not preserved its vital endowments-a question which only experiment can resolve. The process of germination is by no means a simple one. The nutriment stored up in the seed is in great part in the condition of insoluble starch; and this must be brought into a soluble form before it can be appropriated by the enmbryo. The metamorphosis is effected by the agency of a ferment termed cdiastase, which is laid up in the immediate neighbourhood of the embryo, and which, when brought to act on starch, converts it in the first instance into soluble dextrine, and then (if its action be continued) into sugar. The 414 COIRIEL2ATION OF PHYSICAL AND VITAL FORCES. dextrine and sugar, combined with the albuminous and oily compounds also stored up in the seed9 form the 6 protoplasm," which is the substance immediately supplied to the young plant as the material of its tissues; and the conversion of this protoplasm into various forms of organized tissue, which become more and more differentiated as development advances, is obviously referable to the vital activity of the germ. Now it can be very easily shown experimentally that the rate of growtth in the germinating embryo is so closely related (within certain limits) to the amount of heat supplied, as to place its dependence on that agency beyond reasonable question: so that we seem fully entitled to say thlat Heat, acting through the germ, supplies the constructive force or power by which the Vegetable fabric is built up.`'t But there appears to be another source of that power in the seed itself. In the conversion of the insoluble starch of the seed into sugar, and probably also in a further metamorphosis of a part of that sugar, a, large quantity of carbon is eliminated from the seed by combining with the oxygen of the air, so as to form carbonic acid; this combination is necessarily attended with a disengagement of heat, which becomes very sensible when (as in molting) a large number of germinating seeds are aggregated together; and it cannot but be regarded as probable that the heat thus evolved within the seed concurs with that derived from without, in supplying to the germ the force that promotes its evolution. - The effect of Heat is doubtless manifested very differently by different seeds; such variations being partly specific, partly niviclzvdcal. But these are no greater than we see in the inorganic -world: the increment of temperature and the augmentation of bulk exhibited by different substances when subjected to the same absolute measure of heat, being as diverse as the substances themselves. The whole process of " Malting," it may be remarked, is based on the regularity with which the seeds of a particular species may be at any time forced to a definite rate of germination by a definite increment of temperature. DYNAMIC FUNCTIONS OF PLANTS. 415 The condition of the Plant which has attained a more advanced stage of its development, differs from that of the germinating embryo essentially in this particular, that the organic compounds which it requires as the materials of the extension of the fabric are formed by itself, instead of being supplied to it from without. The tissues of the coloured surfaces of the leaves and stems, when acted on by light, have the peculiar power of generating-at the expense of carbonic acid, water, and ammonia-various ternary and quaternary organic compounds, such as chlorophyll, starch, oil, and albumen; and of the compounds thus generated, some are appropriated by the constructive force of the plant (derived from the heat with which it is supplied) to the formation of new tissues; whilst others are stored up in the cavities of those tissues, where they ultimately serve either for the evolution of parts subsequently developed, or for the nutrition of animals which employ them as food. Of the source of those peculiar affinities by which the components of the starch, albumen, &aC, are brought together, we have no right to speak confidently; but looking to the fact that these compounds are not produced in any case by the direct union of their elements, and that a decomposition of binary compounds seems to be a necessary antecedent of their formation, it is scarcely improbable that, as suggested by Prof. Le Conte (op. cit.), that source is to be found in the chemical forces set free in the preliminary act of decomposition, in which the elements would be liberated in that "G nascent condition" which is well known to be one of peculiar energy. The influence of Light, then, upon Vegetable organism appears to be essentially exerted in bringing about what may be considered a higher mode of chemical combination between oxygen, hydrogen, and carbon, with the addition of nitrogen in certain cases; and there is no evidence that it extends beyond this. That the appropriation of the materials thus prepared, and their conversion into organized tissue in the opera 4[16 COBRELATION OF PHYSICAL AND VITAL FORCES. tions of growth and development, are dependent on the agency of Heat, is just as evident in the stage of maturity as in that of germination. And there is reason to believe, further, that an additional source of organizing force is to be found in the retrograde metamorphosis of organic compounds that goes on during the whole life of the plant; of which metamorphosis the expression is furnished by the production of carbonic acid. This is peculiarly remarkable in the case of the Fuiigi, ywhich, being incapable of forming new compounds under the influ ence of light, are entirely supported by the organic matters they absord, and which in this respect correspond on the one hand with the germinating embryo, and on the other with Animals. Such a decompqsition of a portion of the absorbed material is the only conceivable source of the large quantity of carbonic acid they are constantly giving out; and it would not seem unlikely that the force supplied by this retrograde metamorphosis of the superfluous components of their food, which fall down (so to speak) from the elevated plane of I proximate principles," to the lower level of comparatively simple binary compounds, supplies a force which raises another portion to the rank of living tissue, thus accounting in some de. gree for the very rapid growth for which this tribe of Plants is so remarkable. This exhalation of carbonic acid, however, is not peculiar to Fungi and germinating embryos, for it takes place during the whole life of flowering plants, both by day and by night, in sunshine andcl in shade, and from their green as well as from their dark surfaces; and it is not improbable that, as in the case of the Fungi, its source lies partly in the organic matters absorbed, recent ~ivestigations'- having rendered it probable that Plants really take up and assimilate soluble 7hiuUS, which, being a more highly carbonized substance than starch, dextrine, or cellulose, can only be con-' See the Memoir of M. Risler, "' On the Absorption of Humus," in the "Bibliothelque Universelle," N. S., 1858, tom. i., p. 305. PLATT-PRODUCTS STORES OF FORCE. 417 verted into compounds of the latter kind by parting with some of its carbon. But it may also take place at the expense of compounds previously generated by the plant itself, and stored up in its tissues; of which we seem to have an example in the unusual production of carbonic acid which takes place at the period of flowering, especially in such plants as have a fleshy disk, or receptacle containing a large quantity of starch; and thus, it may be surmised, an extra supply of force is provided for the maturation of those generative products whose preparation seems to be the highest expression of the vital power of the vegetable organism. The entire aggregate of organic compounds contained in the vegetable tissues, then, may be considered as the expres. sion, not merely of a certain amount of the material elements, oxygen, lydrogen, carbon, and nitrogen, derived (directly or indirectly) from the water, carbonic acid, and ammonia of the atmosphere, but also of a certain amount of force which has been exerted in raising these from the lower plane of simple binary compounds, to the higher level of complex, "' proximate principles;" whilst the portion of these actually converted into organized tissue may be considered as the expression of a further measure of force, which, acting under the directive agency of the germ, has served to build up the fabric in its characteristic type. This constructive action goes on during the whole Life of the Plant, which essentially manifests itself either in the extension of the original fabric (to which in many instances there seems no determinate limit), or in the production of the germs of new and independent organisms. It is interesting to remark that the development of the more permanent parts involves the successional decay and renewal of parts whose existence is temporary. The'" fall of the leaf" is the effect, not the cause, of the cessation of that peculiar functional activity of its tissues, which consists in the elaboration of the nutritive material required for the production of wood. And it would seem as if the duration of their 18*t 418 CORRELATION OF PHYSICAL AND VITAL FORE ES. existence stands in an inverse ratio to the energy of their ace tion; the leaves of I" evergreens," which are not cast off until the appearance of a new succession, effecting their functional changes at a much less rapid rate than do those of "6 decin cluous " trees, whose term of life is far more brief. Thus, the final cause or purpose of the whole Vital Activity of the Plant, so far as the individual is concerned, is to produce an indefinite extension of the dense, woody, almost inert, but permanent portions of the fabric, by the successional development, decay, and renewal of the soft, active, and tramn sitory cellular parenchyma; and according to the principles already stated, the descent of a portion of the materials of the latter to the condition of binary compounds, which is manifested in the largely-increased exhalation of carbonic acid that takes place from the leaves in the later part of the season, comes to the aid of external Heat in supplying the force by which another portion of those materials is raised to the condition of organized tissue. The vital activity of the Plant, however, is further manifested in the provision made for the propagation of its race, by the production of the germs of new individuals; and here, again, we observe that whilst a higher temperature is usually required for the development of the flower, and the maturation of the seed, than that which suffices to sustain the ordinary processes of vegetation, a special provision appears to be made in some instances for the evolution of force in the sexual apparatus itself, by the retrograde metamorphosis of a portion of the organic compounds prepared by the previous nutritive operations. This seems the nearest approach presented in the Vegetable organism, to what we shall find to be an ordinary mode of activity in the Animal. That the performance of the generative act involves an extraordinary expenditure of vital force appears from this remarkable fact, that blossoms which wither and die as soon as the ovules have been fertilized, may be kept fresh for a long period if fertilization be prevented. RMECONVERSION OF ORGANIC FORCES. 419 The decay which is continually going on during the life of a Plant restores to the inorganic world, in the form of car bonic acid, water, and ammonia, a part of the materials drawn from it in the act of vegetation; and a reservation being made of those vegetable products which are consumed as food by Animals, or which are preserved (like timber, flax, cotton, &c.) in a state of permanence, the various forms of decomposition which take place after death complete that restoration. But in returning, however slowly, to the condition of water, carbonic acid, ammonia, &c., the constituents of Plants give forth an amount of heat equivalent to that which they would generate by the process of ordinary combustion; and thus they restore to the inorganic world, not only the mzaterials, but the forces, at the expense of which the vegetable fabric was constructed. It is for the most part only in the humblest Plants, and in a particular phase of their lives, that such a restoration takes place in the form of mzotion, this motion being, like growth and development, an expression of the vital activity of the'i Zoospores" of Algce, and being obviously intended for their dispersion. Hence we seem justified in affirming that the Correlation between heat and the organizing force of Plants is not less intimate than that which exists between heat and motion. The special attribute of- the vegetable germ is its power of utilizing, after its own particular fashion, the heat which it receives, and of applying it as a constructive power to the building-up of its fabric afer its characteristic type. 420 COiRa]ELATION OF PHIYSICAL AND VITAL FORCTES, IL-RELATIONS OF LIGHT AND HEAT TO THE VITAL FORCES OF ANIMALS. T.IosE of our readers who accompanied us through the first part of our inquiry are aware that it was our object to show, that as force is never lost in the inorganic world5 so force is never created in the organic; but that those various operations of vegetable life which are solmetimes vaguely attributed to the agency of an occult "vital principle'," and are referred by more exact thinkers to certain Vital Forces inherent in the organism of the plant, are really sustained by solar light and heat. These, we have a-rgned, supply to each germ the w]hole power by which it builclds itself up, at the expense of the materials it draws from the Inorganic Universe, into the complete organism; while the mode in which that power is exerted (generally as vital force, specially as the determining cause of the form peculiar to each type) depencds upon the "germinal capacity" or directive agency inherent in each particular germ. The first stage in this constructive operation consists in the production of certain organic compounds of a purely chemical nature-sueh as gum, starch, sugar, chlorophyll, oil, and albumen-at the expense of the oxygen, hydrogen, carbon, and nitrogen derived from the water, carboiic acid, and ammonia of the at.mosphere; whilst the secozcd consists in the further elevation of a portion of these organic compounds to the rank of organized tissue possessing attributes distinctively vital. Of thle whole amount of organic compounds generated by the plant, it is but a comparatively small part (a) that undergoes this p2~rogressive metamorphosis into living tissue. Another small proportion (b) undergoes a retrograde metamorphosis, by )which the original binary components are reproduced; and in this descent of organic compounds to the lower plane, the power con GRADES OF ORG NSIC ASCGEN1T 423 sumed in their elevation is given forth in the form of heat and organizing force (as is specially seen in germination), which help to raise the portion a to a higher level, But by far the larger part (c) of the organic compounds generated by plants remains stored up in their fabric, without undergoing any further elevation; and it is at the expense of these, rather than of the actual tissues of plants, that the life of animals is sustained. When, instead of yielding up any portion of its substance for the sustenance of animals, the entire vegetable organism undergoes retrograde metamorphosis, it not only gives back to the inorganic world the binary compounds from -which it derived its own constituents, but in the descent of the several components of its fabric to that simple condition-whether by ordinary combustion (as in the burning of coal) or by slow decay —it gives out the equivalents of the light and heat by'which they were elevated in the first instance. In applying these views to the interpretation of the phe nomena of animal life, we find ourselves, at the commencemlent of our inquiry, on a higher platform (so to speak) than that from which we had to ascend in watching the construce tive processes of the plant. For, whilst the plant had first to prepare the pabumuh for its developmental operations, the animal has this already provided for it, not only at the earliest phase of its development, but during the whole period of its existence; and all its -manifestations of vital activity are dependent upon a constant and adequate supply of the same pabdurun. The first of these manifestations is, as in the plant, the building-up of the organism by the appropriation of material supplied from external sources under the directive agency of the germ. The ovum of the animal, like the seed of the plant, contains a store of appropriate nutriment previously elaborated by the parent; and this store suffices for the development of the embryo, up to the period at which it can obtain and digest alimentary materials for itself. That ~22 COERELAION OF PHYSICAL AND VITAL FORCES. period occurs, in the different tribes of animals, at very dissimilar stages of the entire developmental process. In many of the lower classes, the embryo comes forth from the egg, and commences its independent existence, in a condition which, as compared with the adult form, would be as if a, human embryo were to be thrown upon the world to obtain its own subsistence only a few weeks after conception; and its whole subsequent growth and development takes place at the expense of the nutriment which it ingests for itself. We have examples of this in the class of insects, many of which come forth from the egg in the state of extremely simple and minute worms, having scarcely any power of movement, but an extraordinary voracity. The eggs having been deposited in situations fitted to afford an ample supply of appropriate nutriment (those of the fiesh-fly, for example, being laid in carcases, and those of the cabbage-butterfly upon a cabbageleaf), each larva on its emersion is as well provided with alimentary material as if it had been furnished with a large supplemental yolk of its own; and by availing itself of this, it speedily grows to many hundred or even many thousand times its original size, without making any considerable advance in development. But having thus laid up in its tissues a large additional store of material, it passes into a state which, so far as the external manifestations of life are concerned, is one of torpor, but which is really one of great developmental activity: for it is during the ypFpa state that those new parts are evolved, which are characteristic of the perfect insect, and of which scarcely a trace was discoverable in the larva; so that the assumption of this state may be likened in many respects to a reentrance of the larva into the ovum. On its termination, the imago or perfect insect comes forth complete in all its parts, and soon manifests the locomotive and sensorial powers by which it is specially distinguished, and of which the extraordinary predominance seems to justify our regarding insects as the types of purely cznimal life. DEVELOPMIENT OF INSECTS. 423 There are some insects whose imago-life has but a very short duration, the performance of the generative act being appai ently the only object of this state of their existence: and such for the most part take no food whatever after their final emersion, their vital activity being maintained, for the short period it endures, by the material assimilated during their larva state.@ But those whose period of activity is prolonged, and upon whose energy there are extraordinary demands, are scarcely less voracious in their innago than in their larvacondition; the food they consume not being applied to the increase of their bodies, which grow very little after the assumption of the imago-state, but chiefly to their maintenance; no inconsiderable portion of it, however, being appropriated in the female to the production of ova, the entire mass of which deposited by a single individual is sometimes enormous. That the performance of the generative act involves not merely a consumption of material, but a special expenditure of force, appears from a fact to be presently stated, corresponding to that already noticed in regard to plants. Now if we look for the source of the various forms of vital force —which may be distinguished as constructive, sensori-motor, and generative-that are manifested in the different stages of the life of an insect, we find them to lie, on the one hand, in the heat with which the organism is supplied from external sources, and, on the other, in the food provided for it. The agency of heat, as the moving power of the constructive operations, is even more distinctly shown in the development of tie larva within the egg, and in the development of the imago within its ptpa-case, than it is in -* It is not a little curious that in the tribe of Eotiferet, or Wheel-animalcules, all the males yet discovered are entirely destitute of digestive apparatus, and are thus incapable of taking any food whatever; so that not only the whole of their development within the egg, but the whole of their active life after their emersion from it, is carried on at the expense of the store of yolk provided by the parent. 424 coRRELATION OF PiHYSICAL AND VITAL FORCES. the germinating seed; the rate of each of these processes being strictly regulated by the temperature to which the organism is subjected. Thus ova which are ordinarily not hatched until the leaves suitable for the food of their larva have been put forth, may be made, by artificial heat, to produce a brood in the winter; whilst, on the other hand, i- they be kept at a low temperature, their hatching may be retarded almost indefinitely without the destruction of their vitality. The same is true of the pupa-state; and it it remarkable that duiring the latter part of that state, in which the developmental process goes on with extraordinary rapidity, there is in certain insects a special provision for an elevation of the temperature of the embryo by a process resembling incubation. Whether, in addition to the heat imparted from. without, there is any addition of force developed within (as in the germinating seed) by the return of a part of the organic constituents of the food to the condition of binary compounds, cannot at present be stated with confidence: the probability is, however, that such a retrograde metamorphosis does take place, adequate evidence of its occurrence during the incubation of the bird's egg being afforded by the liberation of carbonic acid, which is there found to be an essential condition of the developmental process. During the larva-state there is very little powver of maintaining an independent temperature, so that the sustenance of vital activity is still mainly due to tile heat supplied fiom without. But in the active state of the perfect insect there is a _rpoduction of heat quite comparable to that of warml-blooded animals; and this is effected by the retrograde smetamorphosis of certain organic constituents of the food, of which we find the expression in the exhllalation of carbonic acid and water. Thus the food of aninmals becomes an internal source of heat, which may render -them independent of external temperature. Further, a like retrograde metamorphosis of certain constituents of the food is the source of that sensori-mrotor powter VITAL FORCES OF INSECTS. 425 which is the peculiar characteristic of the animal organism; for on the one hand the demand for food, on the other the amount of metamorphosis indicated by the quantity of carbonic acid exhaled, bear a very close relation to the quantity of that power which is put forth. This relation is peculiarly manifest in insects, since their conditions of activity and repose present a greater contrast in their respective rates of metamorphosis, than do those of any other animals. Of the exercise of generative force we have no similar measure; but that it is only a special modification of ordinary vital activity appears from this circumstance, that the life of those insects which ordinarily die very soon after sexual congress and the deposition of the ova, may be considerably prolonged if the sexes be kept apart so that congress cannot take place. 1ioreover, it has been shown by recent inquiries into the agamic reproduction. of insects and other animals, that the process of generation differs far less from those reproductive acts which must be referred to the category of the ordinary nutritive processes, tShan had been previously supposed. Thus, then, we find that in the animal organism the demand for food has reference not merely to its use as a aznte-'riaJl for the construction of the fabric; food serves also as a generator of force; and this force may be of various kindsheat and motor-power being the principal but by no means the only modes under which it manifests itself. We shall now inquire what there is peculiar in the sources of the vital force which animates the organisms of the higher animals at different stages of life. That the developmental force which occasions the evolution of the germ in the higher vertebrata is really supplied by the heat to which the ovum is subjected, may be regarded as a fact established beyond all question. In frogs and other amphibia, which have no special means of imparting a high temperature to their eggs, the rate of development (which in the early stages can be readily determined with great exact. 426 CORRELATION OF PHYSICAL AND VITAL FORCES. ness) is entirely governed by the degree of warmth to which the ovum is subjected. But in serpents there is a peculiar provision for s1upplying heat; the felnale performing a kind of incubation upon her eggs, and generating in her own bodly a temperature much above that of- the surrounding air.*' In birds, the developmental process can only be maintained by the steady application of external warmth, and this to a degree'much higher than that which is needed in the case of cold-blooded animals; and we may notice two results of this application as very significant of the dynamical relation between heat and developmental force-first, that the period required for the evolution of the germ into the nmature embryo is nearly constant, each species having a definite period of incubation-and second, that the grade of development attained by thile embryo before its emersion is relatively much higher than it is in cold-blooded vertebrata generally; the only instances in which any thing like the same stage is attained without a special incubation, being those in which (as in the turtle and crocodile) the eggs are hatched under the influence of a high external temperature. This higher development is attained at the expense of a much greater consumption of nutrient material; the store laid up in the 6 food yolk" and "6 albumen" of the bird's egg, being many times greater in proportion to the size of the animal which laid it, than that contained in the whole egg of a frog or a fish. There is evidence in that liberation of carbonic acid which has been ascertained to go on in the egg (as in tile germinating seed) during the whole of the developmental process, that the return of a portion of the organic substances provided for the suste-' In the Viper the eggs are usually retained within the oviduct until they are hatched. In the Python, which recently went through the process of incubation in the Zoological Gardens, the eggs were imbedded in the coils of the body; the temperature to which they were subjected (as ascertained by a thermometer placed in the midst of them) averaging 90~ F., whilst that of the cage averaged 600 F. DYNTA ICS OF EMBRYONIC DELELOPMENT. 427 nance of the embryo, to the condition of simple binary compounds, is an essential condition of the process; and since it can scarcely be supposed that the object of this metamorphosis can be to furnish heat (an ample supply of that force being afforded by the body of the parent), it seems not unlikely that its purpose is to supply a force that concurs with the heat received from without in maintaining the process of organization. The development of the embryo within the body, in the mammalia, imparts to it a steady temperature equivalent to that of the parent itself; and in all save the irn)lacentca orders of this 2lass, that development is carried still further than in birds, the -new-born mammal being yet more complete in all its parts, and its size bearing a larger proportion to that of its parent, than even in birds. It is doubtless owing in great part to the constancy of the temperature to which the embryo is subjected, that its rate of developlment (as shown by the fixed term of utero-gestation) is so uniform. The supply of organizable material here afforded by the ovum itself is very small, and suffices only for the very earliest stage of the constructive process; but a special provision is very soon made for the nutrition of the embryo by materials cdirectly supplied by the parent; and the imbibition of these takes the place, during the whole remainder of fcetal life, of the appropriation of the materials supplied in the bird's egg by the "1 food yolk" and "d albumen.", To what extent a retrograde metamorphosis of nutrient material takes place in the fetal mammal, we have no precise means of determining; since the products of that metamorphosis are probably for the most part imparted (through the placental circulation) to the blood of the mother, and got rid of through her excretory apparatus. But sufficient evidence of such a metamorphosis is afforded by the presence of urea in the amniotic fluid and of biliary matter in the intestines, to make it probable that it takes place not less actively (to say the least) in the fcetal 428 CORREL-ATION OF PHYSICiAL AND VITAL FOItCES. mammal than it does in the chick in ovo. Indeed, it is impossible to study the growth of any of the higher organismswhich not merely consists in the formation of new parts, but also involves a vast amount of interstitial change-without perceiving that in the remodelling which is incessantly going on, the parts first formed must be removed to make way for those which have to take their place. And such removal can scarcely be accomplished without a retrograde mletamorphosis, which, as in the numerous cases already referred to, may be considered with great probability as setting free constructive force to be applied in the production of new tissue. If, now, we pass on from the intra-uterine life of the mammalian organism to that period of its existence which intervenes between birth and maturity, we see that a temporary provision is made in the acts of lactation and nursing for affording both food and warmth to the young creature, which is at first incapable of adequately providing itself with aliment, or of resisting external cold without fostering aid. And we notice that the offspring of man remains longer dependent upon parental care than that of any other mammal, in accordance with the higher grade of development to be ultimately attained. But when the period of infancy has passed, the child, adequately supplied with food, and protected by the clothing which makes up for the deficiency of other tegumentary covering, ought to be able to maintain its own heat, save in an extremely depressed temperature; and this it does by the metamorphosis of organic substances, partly derived from its own fabric, and partly supplied directly by the food, into binary compounds. During the whole period of growth and development, we find the producing power at its highest point;9 the circulation of blood being more rapid, and the amount of carbonic acid generated and thrown off being much greater in proportion to the bulk of the body, than at any subsequent period of life. We find, too, in the large amount of other excretions, the evidence of a rapid 1o CONSTRUCTIVE FORCES OF ANIMALS. 429 metamorphosis of tissue; and it can hardly be questioned (if our general doctrines be well founded) that the constructive force that operates in the completion of the fabric will be derived in part from the heat so largely generated by chemical change, and'in part from the descent which a portion of the fabric itself is continually maling from the higher plane of organized tissue to the lower plane of dead nmatter. This high measure of vital activity can only be sustained by an ample supply of food; which thus supplies both materiac for the construction of the organism, and the force by whose agency that construction is accomplished. HI-ow completely dependent the constructive process still is upon heat, is shown by the phenomena of reparation in coldblooded animals; since not only can the rate at which they take place be experimentally shown to bear a direct relation to the temperature to which these animals are subjected, but it has been ascertained that any extraordinary act of reparation (such as the reproduction of a limb in the salamander) will only be performed under the influence of a temperature much higher than that required for the maintenance of the ordinary vital activity. After the maturity of the organism has been attained, there is no longer any call for a larger measure of constructive force than is required for the mnaintenaxnce of its integrity; but there seems evidence that even then the required force has to be supplied by a retrograde Ietamorphosis of a ponrtion of the constituents of the food, over and above that which serves to generate animal heat. For it has been experimentally found that, in the ordinary life of an adult nianammal, the quantity of food necessary to keep the body in its normal condition is nearly twice that iwhich would be required to supply the' waste " of the organism, as measured by the total amount of excreta when food is withheld; and hence it seems almost certain that the descent of a portion of the organic constituents of this food to the lower level of simple binary compounds is a necessary condition of the ele 430 CORRELATION OF PHYSICL AND VITAL' FORCES. vation of another portion to the state of living organized tissue. The conditions of animal existence, moreover, involve a constant expenditure of rotor force through the instrumentality of the nervo-rmuscular apparatus; and the exercise of the purely psychical powers, through the instrumentality of the brain, constitutes a further expenditure of force, even when no bodily exertion is made as its result. TWe have now to consider the conditions under which these forces are developed, and the sources from which they are derived. The doctrine at present commonly received among physiologists upon these points may be stated as follows:-The functional activity of the nervous and muscular apparatuses involves, as its necessary condition, the disintegration of their tissues; the components of which, uniting with the oxygen of the blood, enter into new and simpler combinations, which are ultimately eliminated from the body by the excretory operations. In such a retrograde metamorphosis of tissue, we have two sources of the liberation of force:-first, its descent from the condition of living, to that of dead matter, involving a liberation of that force which was originally concerned in its organization;*-and second, the further descent of its complex organic components to the lower plane of simple binary compounds. If we trace back these forces to their proximate source, we find both of them in the food at the a It was by Liebig (" Animal Chemistry," 1842) that the doctrine was first distinctly promulgated which had been already more vaguely affirmed by various Physiologists, that every production of motion by an Animal involves a proportional disintegration of muscular substance. But he seems to have regarded the motor force produced as the expression only of the vital force by which the tissue was previously animated; and to have looked upon its disintegration by oxygenation as simply a consequence of its death. The doctrine of the " Correlation of Forces " being at that time undeveloped, he was not prepared to recognize a source of motor power in the ulterior chemical changes which the substance of the muscle undergoes; but seems to have regarded them as only concerned in the production of heat. TEST OF ANIMAL MOTOR FOCtOE. 431 expense of which the animal organism is constructed; for besides supplying the material of the tissues, a portion of that food (as already shown) becomes the source, in its retrograde metamorphosis, of the production of the heat which supplies the constructive power, whilst another portion may afford, by a like descent, a yet more direct supply of organizing force. And thus we find in the action of solar light and heat upon plants —whereby they are enabled not merely to extend themselves almost without limit, but also to accumulate in their substance a store of organic compounds for the consumption of animals-the ultimate source not only of the materials required by animals for their nutrition, but also of the forces of various kinds which these exert. Recent investigations have rendered it doubtful, however, whether the doctrine that every exertion of the functional power of the nervo-muscular apparatus involves the disintegration of a certain equivalent amount of tissue, really expresses the whole truth. It has been maintained, on the basis of carefully-conducted experiments, in the first place, that the amount of work done by an animal may be greater than can be accounted for by the ultimate metamorphosis of the azotized constituents of its food, their mechanical equivalent being estimated by the heat producible by the combustion of the carbon and oxygen which they contain;* and secondly, that whilst there is not a constant relation (as affirmed by Liebig) between the amount of motor force producecl and the amount of disintegration of muscular tissue represented by the appearance of urea in the urine, such a constant relation does exist between the development of motor force and the increase of carbonic acid in the expired air, as shows thati between these two phenomena there is a most intimate -elae This view has been expressed to the author by two ver) high authorities, Prof. Helmholtz and Prof. William Thomson, independently of each other, as an almost necessary inference frolnm thle data furnished by the experiments of Dr. Joule. 432 COREELATION OF PHYSICAL AND VITAL FORCES. tionship.* And the concurrence of these independent indications seems to justify the inference that nzotor force may be developed, like heat, by the metamorphosis of constituents of food which are not converted into living tissue; —an inference which so fully harmonizes withl the doctrine of the direct convertibility of these two forces, now established as one of the surest results of physical investigation, as to have in itself no inherent improbability. Of the conditions which determine the generation of motor force, on the one hand, from the disintegration of muscular tissue, on the other ifrom the metamorphosis of the components of the food, nothing definite can at present be stated; but we seenm to have a typical example of the former in the parturient action of the uterus, whose muscular substance, built up for this one effort, forthwith undergoes a rapid retrograde metamorphosis; -whilst it can scarcely be regarded as improbable that the constant activity of the heart and of the respiratory muscles, which gives them no opportunity of renovation by rest, is sustained not so much by the continual renewal of their substance (of which renewal there is no histological evidence -whatever) as by a metamorphosis of matters external to themselves, supplying a force which is manifested through their instrumentality. To sum up: The life of man, or of any of the higher animals, essentially consists in the manifestation of forces of various kinds, of which the organism is the instrument; and these forces are developed by the retrograde metamorphosis of the organic compounds generated by the instrumentality of the plant, -, whereby they ultimately rieturn to the simple binary forms (water, carbonic acid, and ammonia), which serve as the essential food of vegetables. Of these organic compounds, one portion (a) is converted into the: On these last points reference is especially made to the recent experi. ments of Dr. Edward Smith. SUMMARY OF THE ARGUMENT. 433 substance of the living body, by a constructive force which (in so far as it is not supplied by the direct agency of external heat) is developed by the retrograde metamorphosis of another portion (b) of the food. And whilst the ultimate descent of the first-named portion (a) to the simple condition from which it was originally drawn, becomes one source of the peculiarly animal powers-the _psychical and the mzotor —exerted by the organism, another source of these may be found in a like metamorphosis of a further portion (c) of the food which has never been converted into living tissue. Thus, during the whole life of the animal, the organism is restoring to the world around both the materials and the forces which it draws from it; and after its death this restoration is completed, as in plants, by the final decomposition of its substance. But there is this marked contrast between the two kingdoms of organic nature in their material and dynamical relations to the inorganic world-that whilst the vegetable is constantly engaged (so to speak) in raising its component materials from a lower plane to the higher, by means of the power which it draws from the solar rays, the animal, whilst raising one portion of these to a still higher level'by the descent of another portion to a lower, ultimately lets down the whole of what the plant had raised; in so doing, however, giving back to the universe, in the form of heat and motion, the equivalent of the light and heat which the plant had taken from it. 19 INDEX. A Carbonic acid the index of animal powv, 431. Air-gun, action of the, 219, 220. Carnot, 224. Ancient method of inquiry, 317. - on the transfer of heat, 70. Ancients, philosophic aims of the, 13. Carpenter, Dr., xxxi., xxxiv., 173. - their mode of explaining phenomena, - biographical notice of, 400. 103. Catalysis, 6, 169. Andrews, Dr., 135, 162. Chemical action, what is it? 153. Animal force, derivation of, 423. - - a cause of light, 162. - heat, source of, 324. - - produces magnetism, 162. - nutrition, 421. - - the source of mechanical power, 897, - system, early conception of, 212. - affinity as a cause of motion, 152.Anomaly in expansion and contraction, 52. - - a cause of heat, 159. Aphides, multiplication of, 411. - force converted into electrical, 15-4. Aristotle, 317. Chemistry of plants, 395. Astronomy, tendency of its progress, xi. Clarke,Mkit. Latimer, 129, 181. - difficulties of, 319. Clausius, Prof., xxviii., 104, 228. - early development of, 320. Cohesive attraction, 171. Atmosphere, limit of, 136. Cold regarded dynamically, 48. - pressure of, 319., Colding, Pirof., 224. Atomic theory, objections to, 164. Coleridge, 108. Atoms, question of existence of, 347. Collision, effects of, 29. Authority, value of, 11. Combustion, key to the phenomena of, 821, Automatic machines, 211. Comets, Mayer on, 270. Compression, heat resulting from, 82. Conservation of force, importance of the B problem, 356. Constancy of the sun's mass, 282. Bacon, Francis, philosophy of, 14. Constructive action of plants, 417. Becquerel, 34., 102,125. Cooling of the earth, 80. Beclard,!M., 175. Cordier, M., 307, 313. Bergmann on electrical effects, 35, Correlation of physical forces, meaning of Bessel, 241. the phrase, 8, 383. Brodie, Sir B., 175. --— the problem to be solved, 189. Buffon, 314. -- -- - difficulties of the investigation, 194. C Cosmogony, Grove on, 81. Cotton factory as a distributor of force, 403. Causation, Grove on, 15. - secondary, 18. Cause and effect, 251. D -- - simultaneity of, 17. Cause, nature of, 402. Daguerre, 3., 112. Caoutchouc, anomaly of, 53. Dalton, 157, 159. INDEX, 435 Darkness converted into light, 119. F Davy, Sir HI., xxvii., 4,134, 245. Day, variation in length of the, 303. Falling bodies, highest velocity of, 337. De Candolle, 57. — phenomena of, 318. Decay restores heat to the universe, 419. - force, 253, 348. D)e la PFive, 57. Faraday, Dr., discoveries of, 6, 85, 14, Despritz, Al., 76. 145. Democritus, 127. - biogvaphical notice of, 358. Descartes, xi. Favre, M., 8371. Desormes, Clement, 75. Faye's theory of comets, 41. Development of the embryo; derivation Flower figures in ice, 50. of its force, 427. Fizeau, MI., 129. Diathermancy, 278. Forbes, Jas. D., xix. Differentiation in animal development, Force, indestructibility of, xii. 409. - use of the doctrine, xiii. Diminmtion of the earth's volume, 308. - importance of new views of, xxx. Disiscovery, conditions of, 104. - Grove's definition of, 19. Distinctions among the forces imperfect, - imparted to machines, 218. TS78. - what is it? 251. Dove, HT1., 126. - definition of, 335. Dlufour, M[., 96. - meaning of the term, 830, 3179 Dulong and Petit, 189. - living and dead, 333. Dynamic function of the germ, 412. - animal, derived from decomposition of food, 4032. Foucault, M., 129. E: Fourier, A1., oO9. Franklin, Dr,, xvii. Earth, age of, 245. Fresnel on heat repellency, 41. - form of, 297. Friction, definition of, 30. Earth's interior heat, 236, 300. - producing electricity, 83. - - - proofs of, 302. -of homogeneous substances, 38. - -- reasonableness of, 303. - heat of, 389. Effect of electrical discharge upon gases, 93. Electrical induction, 84. Electricity, limitation of the term, 35. Gassiot, Mr., 59, 135. - from caoutchouc, 35. Gay Lnussac, I., 167. - the link among the other forces, 37. Germ-force, 411. - as initiating other forces, 83. Germination, dynamics of, 413. - what is it? 88. Generative force in insects, 425. -produces chemical decomposition, 84. Gravity implies a plenum, 36S8. - atmospheric, 87. - in relation to other forces, 170, 253. -intensity of, 89. - is it a force? 3889, 340, 341. - effect upon the terminals, 90. - but half understood, 3866. - molecular changes of conductors, 95. - relation to conservation of force, 863, - hypothesis of a fluid unnecessary, 98, Grove, aim of his essay, 19. 101. - biographical notice of, 2. - animal, 99. - claims of, 5. -in what it consists, 101. - electrical images on glass, 86. -initiates motion, 104. - discoveries on the relations of heat to - and heat, 106. the gases of water, 65. and light, 107. Grotthus, 163. - produced by blowpipe flame, 155. Growth in the mammalia, 428. Electro-chemical equivalence, 894. Guillemin, HT., 151. - - equivalents of power, 313. Electro-magnetic equivalence, 393. Emotions, correlations of, xxxiv. H Ethereal medium, 123, 242, 271, 347. - - objections to, 133. Heat, material hypothesis of, annihilated Equivalent, meaning applied to the term, by Rumford, xxiii., xxiv. 329. - definition of, 89. Ericsson, HIr., 78. - dynamical effects of, 40. Erman on friction of homogeneous sub- - latent, 41, 348. stances, 33. - influence of structure over the colln Essences, the search for, 14. dition of, 57. Euler, Leonard, 123. - produces the various forces, 57. Events can never be repeated, 193 - reflection of, 61. Expansion, inequality of,'. - converted into light, 63. 436 IDEXo Heat (comitilsdecd). Light (coniszebecl). - chemical and magnetic influence of, 64. - produces the other forces, 116. - as producing motive-power, 68. - influence of the recipient surface upon, - practical application of, 77. 120. - produced by chemical combination, - and sound, analogies of, 126. cause of, 161. - and molecular changes, 180. - definition of, 226. - a motion of ordinary matter, 133. - force only available in the cooling pro- - its loss or absorption, 139. cess, 228. - function of, in organization, 415. - universal law of, 201. Life of the higher animals, in what it con- sources of, 261. sists, 432. - solar, its chemical origin, 266. Living force, 218. - developed by friction, 277. Littrow, 271. - upon high mountains, 282. -lost by volcanoes, 310. M - lost by the ocean, 311. -and motion, convertibility of, 323. Machines compared with the living sys- and work, 826. term, 79. - converted into organizing force, 412. - driving force of, 214, 387. Helmholtz, 175. Muggi, Dr., 148. - biographical notice of, 210. Magnetism a directive force, 142. - claims of, 224. - influence of, upon light and heat, 145. Henry, Prof., xxviii., 290. - as an initiating force, 147. Herschel, Sir John, 119, 264. - static and dynamic. 149. Herschel, Sir WV., 118, 136. Magneto-electric machines, 222. - his hypothesis of the scm's action, 260. Marrion, Mir. 151. Hipparchus, 243. Masson, M., 1835. History of scientific discovery, xv. Mathematics, function of, in investigation, Holtzmann, 851. 877. Horner, rM., 812. Matter, ultimate structure of may never Hume on causation, 16. be known, 187. Huxley, Prof., 411. Matteucci, 79, 84, 97, 151, 172, 175. Hydraulic ram, 4. Mayer, Dr., 8. - Tyndall's estimate of, xxx. - biographical sketch of, 230. -physiological commencement of his Indestructibility of matter, xii. inquiries, 824. Inertia, relation to forces, 869. - secures his discovery against rival Intellectual and physical forces, xxxv. claimants, 224. Intensity of the sum's heat, 270. - various references to, xxix., 241, 827, Interaction of natural forces, 211. 8350, 389, 465. Insects, development of, 422. Measure of the sun's heat, 264. Mechanical problem, the modern, 212. - force and heat, 262. J - origin of solar heat, 2T6. - equivalent of heat, 816, 891. Joule, Dr. J. P., xxiv.,, 833, 151, 224, 812. -- -- -- applications of, 353. Melloni, researches of, 59. Mental and vital forces, xxxii. {K Metamorphosis retrograde in animals, 421. Kant, 23. Meteorites, 270. Karsten, 85. - heat generated by, 284. Kirchoff's researches, 62. Metaphysics in science, 860. Knoblauch, M., 56, 118. Modern science, alleged materializing tenKnowledge, slow growth of, 12. dency of, xi. Molecules, how the term is employed by Grove, 89. L Moon, effect of collision with the ealth, 804. Laplace, 231, 242, 248, 291. Mongolfier, Mi., 4. Latent heat, 41, 348. Moral forces, correlation of, xxxvii. Lavoisier, xii. Morgan, Mr., 185. Leconte, Prof., xxviii. Morichini, 117. Liebig, vi., 174, 175, 238, 480. Mossoti, 171. - biographical notice of, 386. Motion, as an affection of matter, 25. Light, polarization of, 110. - never destroyed, 27. - chemical action of, 111. - produces heat, 28. - affects all forms of matter, 115. - produces various forces, 87. INDEX. 437 Motion (conetinmecf). Respiration in cold-blooded animals, 429. - an affection of matters 186. Roget, Dr., 4. - vast effects of, 217. Rotation of the earth, diminution of the, Mountain tops, heat of, 282. 299. Royal Institution, foundation of the, xxx. N Rule for the investigation of nature, 316, 818. Nebular hypothesis, 230. Rumford, Count, 4. Nerve-power, source of, 480. - summary of his claims, xv. New views, public acceptance of, 9. - sketch of, xvii. New doctrine of forces, how far accepted, - researches of, xx., 347. xiv. Newton, xiii., xviii., 241, 276, 282, 368, 3178. - speculations on light, 137. S Niepee do St. Victor, 3I., 125. Numbers the highest aim of investigation, Seebeck, Dr., 118. 320. Seguin, M., 4, 76. Sensations of heat disturb the judgment, O 49. Sensations, misleading, 174. Oersted's discoveries, 5. Science, tendency of, toward immaterialOptic axis of crystals, 171. ism, xii. Organic beings, source of their motions, - the great event in the general progress 237. of, xvi. -- kingdoms of nature, relations to each -the true scope of, xxxi. other, 4833. - discovery of its fundamental principle, - co-ordination, 410. 8316. - correlations, Grove on, 172. Simultaneous discovery, examples of, xv. Organization, distinction between high and Social forces, correlation of, xxxvi. loiw grades of, 408. Solar radiation, effects of, 236. Origin of the smn's heat, 276. - - origin of organic power, 240. Origin of terrestrial power, 236, 340. - heat, amount of, 243, 264. - spots, 286. P -- atmosphere, 2S8. - radiation, dynamical effects of, 403, 404. Page, Mr., 151. Sondhauss, Mir., 126. Particles, in what sense the term is used Sound and light, 259. by Grove, 389. Specific heat, dynamic view of, 53. Pasteur, MI., 132. Spectrum, analysis of, 62. Peltier, Ml,, 96. Spencer, Herbert, on the persistence of Perpetual motion, 191, 192, 212, 219, 221, force, xxxix. 228, 388. Steam engine, action of, 220. Persistence of force, xxxix. Stephenson, George, 115. Photography, chemistry of, 111. Stewart, B3alfour, 61. Physical science of the present distin- Store of force in the universe, 232. guished from that of the past, 402. - - - in the planetary system at present, Planetary atmospheres, 137. 234. - motions, 267. Structure of bodies as affecting heat- system, structure of, 280. motion, 55. Planets, temperatures of, 121. Sun, cooling of, 265. Plants, chemistry of, 420. Plenum, a universal, 134. T Pliicker, 171. Plurality of worlds, 122. Talbot, Mr., 112. Poisson, MI., 81. Tension, a static force, 22. Pouillet, MI., 244, 264, 280. Theories, immature, 110. Powell, Baden, on Newton's rings, 41. - new, how they are to be judged, 196. Practical arts after the middle ages, 211. - the test of, 276. Ptolemaic system, 11. Theorizing, what is it? 198. Thermography, 58. Q.E Thilorier's experiment on carbonic acid, 48. Thompson, Prof., 52, 227, 229. Qualities of matter, transference of, 15. Tidal wave, 241, 291. Time as an element of dynamic changes, 860. R — n element in the sequence of pheRankine, Mr., xxviii. nomena, 17. Regnault, MI., 225. - necessity of its early measurement, 320. 438 INDEX, Torricelli, 103. W Transmutation of force chemically illustrated, 872. Wartmann, Mr., 146. Tyndall, xxx., 57. Water, expansion of, as it approaches freezing, 50. Water-power, force produced by, 215. U Watt, James, 75. Wortheim, 96, 150. Units of heat producedcin combustion, 261. Whewell, Dr., xxvi., 136. Wollaston, Dr., 136. Wilson, Dr., 136. V Words, misguiding influence of, 94. - their social import, 180. Vacuum, not yet obtained, 184. Work performed by chemical force, 226. Vital and physical forces, correlation of, Wood, Dr., 54,160. xxxii. - effects, crude explanations of, 401. y - activity, characteristics of, 407. - - of plants, purpose of, 418. Young, Dr. Thomas, xxvii,, 124, 125, 128, -- principle, 402, 420. 129, 138. Vis viva, 218, 263. Volcanoes, loss of heat by, 310. Z Volta, 153. Voltaic arc, cause of light of, 88. Zodiacal light, 272. ~1E EA ND. D. Appleton ct Co., New York, have nzow ready, A NEW IN \WHIC1t THE LATEST FACTS AN7D PRINCIPLES OF TI-E SCIENCE ARE EXPLAINED AND APPLIED TO THE ARTS OF LIFE AND THE PHEENOMENA OF NATUIRE. A EiW EDMITSOM ENTI nELY E Wv I TTE N AND M U C EN LARG-ED. WITH BY EDWbARD L. YOUMIANS, M. D. 12mo. 460 pages. Price $1.25. The special attention of Educators is solicited to this work, on the fole lowing grounds: I. It brings up the science to the present date, incorporating the new discoveries, the corrected views and more comprehensive principles which have resulted from recent inquiry. Among these may be mentioned the discoveries in Spectremz Analysis, the doctrines of the Conservation and Correlation of Forces, the researches of Berthelot on the Artificial Productions of Org7anic S'ubstatsces, the interesting researches of Graham on the Crpstalloid and Colloid condition of lllatter, with many other results of recent investigation not found in contemporary text-books. II. Avoiding excess of technicalities, it presents the subject in a lucid, forcible, and attractive style. III. It is profusely illustrated with cuts of objects, apparatus, and experiments, which enable the student to pursue the subject alone or in schools without ap. ps'atus. IV. Directions for experimental operations are much condensed, anld descip, tions of unimportant chemical substances are made very brief, or altogethler omit, ted, thus obtaining space to treat with lunusal fulness the " chemistry of common life," and the later revelations of this beautiful science. V. It presents just such a view of the leading principles and mnore important facts of the science as is demanded for the purposes of general education. VI. The work is arranged tupon a natural method, the topics beilig so presented as to unfold the true order of Nature's activities. -Part I treats of the natural forces by which matter is transformed. Part II, of the application of these forces to the lower or mineral world. Part III, of the organic kingdom, which rises out of the preceding; while Part IV, or Physiological Chemistry, completes the scheme iis the world of life. VII. It presents the science not only as a brasnch but as a amsens of educationa valuable instrument of intellectual culture and discipline. VIII. It gives a clear exposition of the origin and nature of scientific knowledge and the value of scientific studies for purposes of education. 2 A Speciment Copy for examination will be sezt, post paid, on receipt of 62 cents. Works psublis/eed by D. Appletoib &W Co. CONSIDERED AS A 3ODE OF MOTION, Being a Course of Twelve Lectures delivered before the Royal Institution of Great Britain,l BY JOIIN TYNDALL, F. R. S., PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION-lAUTOnR OF TMIE "GLACIERS OF THE ALPS," ETC. With One Hundred Illustrations. Svo, 480 pages. Price, $2.00. From the American Journal of Soience.-With all the skill which has made Faraday the master of experimental science in Great Britain, Professor Tyndall enjoys the advantage of a superior general culture, and is thus enabled to set forth his philosophy with all the graces of eloquence and the finish of superior diction. With a simplicity, and absence of technicalities, which render his explanations lucid to uuscientific minds, and at the same time a thoroughness and originality by twhich he instructs the most learned, he unfolds all the modern philosophy of heat. His work takes rank at once as a classic upon the subject. New York Times.-Professor Tyndall's course of lectures on heat is one of the most beautiful illustrations of a mode of handling scientific subjects, which is comparatively new, and which promises the best results, both to science and to literatune generally; we mean the treatment of subjects in a style at once ps3ojfoundz and f2oieslarC. The title of Professor Tyndall's work indicates the theory of heat held by him, and indeed the only one now held by scientific men-it is a szocfe of noione. Boston Journal.-He exhibits the curious and beautiful workings of natmue in a most delightful manner. Before the reader particles of water lock themselves or fly asunder with a movement regulated like a dance. They form themselves intio liquid flowers with fine serrated petals, or into rosettes of frozen gauze; they bolmd upward in boiling fountains, or creep slowly onward in stupendous glaciers. Flames burst into music and sing, or cease to sing, as the experimenter pleases, and metals paint them.. selves upon a screen in dazzling hues as the painter touches his canvas. I ew York Tribune.-The most original and important contribution that has yet been made to the theory and literature of thermotics. Scientific American.-The work is written in a charming style, and is the most valuable contribution to scientific literature that b as been published in many years. It is the most popular exposition of the dynamical theory of heat that has yet appeared. The old material theory of heat may be said to be deifunct. Louisville Demoorat.-This is one of the most delightful scientific works we have ever met. The lectures are so full of life and spirit that we can almost imagine the lecturer before us, and see his brilliant experiments in every stage of their progress. The theory is so carefully and thoroughly explained that no one can fail to understand it. Such books as these create a love for science. Independent.-Professor Tyndall's expositions and experiments are remarkably thoughtful, ingenious, clear, and convincing; portions of the book have almost the interest of a romance, so startling are the descriptions and elucidations. Works of Herbert g oeScer publihaTed by JD. Apletonz & Co. (NOW IN PRESS,) 01ORAL POLITICAL, AYD -ESTHE TIC. In one Volume. Large 12mo. CONTENTS: I. The Philosophy of Style. If. Over-Legislation. HIt. Morals of Trade. IV. Personal Beauty. V. Representative Governiment. VI. Prison-Ethics. VII. Railway Morals and Railway Policy. VIII. Gracefulness. IX. State Tamperings with Money and Banks. X. Reform; the Dangers and the Safeguards. ALSO, SOCIAL STATICS; OR) *, co THE CONDITIONS ESSENTIAL TO HUMAN HAPPINESS SPECIFIED, AND THE FIRST OF THEM DEVELOPED. hn one Volume. Large l2mo. All these works are rich in materials for forming intelligent opinions, even where we are unable to agree with those put forth by the author. Much may be learned from them in departments in which our common Educational system is very deficient. Tho active citizen may derive from them accurate systematized information concerning his highest duties to society, and the principles on which they are based. He may gain clearer notions of the value and bearing of evidence, and be better able to distinguish between facts and inferences. He may find common things suggestive of wsiser thought -nay, we will venture to say of truer eniotion-than before. By giving us fuller realizations of liberty and justice his writings will tend to increase our self-reliance in the great emergency of civilization to which we have been sunamoned.-Atla2 tic Jfrontlig. o?,rks oj' Herbervst,irfccer peblsglhed by _). _i2pplcton & Co. A NEW SYSTEMI OF PHILOSOPHY. PRINt'NIPLES OF BIOLOGY. T: is work is now in course of publication in quart;erly numbes, (from 80 to iLi0 pages each), by subscription, at $2 per annum. It is to form two volunies, of which the first is nearly completed, four numbers having been issued. While it comprises a statement of those general principles and laws of life to whichl science has attained, it is stamped with a mlarked originality, both in the views propounded and in the method of treating the subject. It will be a standard and invaluable work. Some idea of the discussion milay be formed by glancing over a few of the first chapter headings. PART PFiIRST.-DATA or BsIOLOG~. I. Organic Matter; II. The actions of Forces on Organic MIatter; IIL The Reactions of Organic Mlatter on Forces; IV. Proximate Definition of Life; V. The Correspondence between Life and its Circumstances; VI. The Degree of Life Varies with the Degree of Correspondence; VII Scope of Biology. PAnT SECOND-, —INDUCTIONS OF BIOLOGY. I. Growth; 11. Development; III. Function; IV. Waste and Repair; V. Adaptation; VI. Individuality; VII. Genesis; VIII. Heredity; IX. Variation; X. Genesis, Heredity, and Variation; XI. Classification; XII. Distribution. Ir. Spencer is equally remarkable for his search afier first principles; for his acute attempts to decompose mental phenomena into their primary elements; and for his broad generalizations of mental activity, mind in conneetion with instinct, and all the analogies presented by life in its universal st:pects.-Mtcincc-C7co- Oeirmp~icccl Review. Works of Jerbert Spencer publishecd by D.A. AppletGon & Co. A NEW SYSTEMI OF PHILOSOPHY. PIl' SST P IXXINC.IPLESo. Lol. Large 12mo. 515 Pages. P:rice $2 00. CONTENTS: PART FIRST.-I/The Uheknowccble.,:,aptes i. Religion and Science; II. Ultimate Religious Ideas; III. Ultimate Scientific Ideas; IV. The Relativity of all Knowledge; V. The Reconciliation. PART SECOND. —iaC8 of the JtiowCable. I. Laws in General; II. The Law of Evolution; III. The same continued; IV. The Causes of Evolution; V. Space, Time, Batter, Motion, and Force; VI. The Indestructibility of Matter; VII. The Continuity of Motion; VIIL The Persistence of Force; IX. The Correlation and Equivalence of Forces; X. The Direction of Motion; XI. The Rhythm of -M'otion; XII. The Conditions Essential to Evolution; XIII. The Instability of the Homogeneous; XIV. The lMultiplication of Effects; XV. Differentiation and Integration; XVI. Equilibration; XVII. Summary and Conclusion. In the first part of this work Mr. Spencer defines the province, limits, and relations of religion and science, and determines the legitimate scope of philosophy. In part second he unfolds those fundamental principles which have been arrived at within the sphere of the knowable; which are true of all orders of phenonema, and thus constitute the foundation of all philosophy. The law of Evolution, Mr. Spencer maintains to be universal, and he has here worked it out as the basis of his system. These First Principles are the foundation of a system of Philosophy bolder, more elaborate, and comprehensive perhaps, than any other which nas been hitherto designed in England.-Britishl Quarterly.eview. A work lofty in aim and remarkable in execution.- Coronzill l.cagazine. In the works of Herbert Spencer we have the rudiments of a positive Theology, and an immense step toward the perfection of the science of Psychology.-C lzristiaz Excmizer. If we mistake not, in spite of the very negative character of his own results, he has foreshadowed some strong arguments for the doctrine of a positive Christian Theology.-Neew Einglander. As far as the frontiers of knowledge, where the intellect may go, there is no living man whose guidance may more safely be trusted.-lAtteiw Afonthiy. Works of Herbert S~penceerpublished Zy:D. by Apikon & Co. ILLUSTRATIONS OF UNIVERSAL PROGRESS, A SERIES OF DISCUSSIONS. 1 Vol. Large 12mo. 47i0 Pages. Price $2 00. CONTENTS: American Notice of Spencer's New System of Philosophy. L Progress: its Law and Cause. II. 1M1anners and Fashion. III. The Genesis of Science. IV. The Physiology of Laughter. V. The Origin and Function of Miusic. VI.. The Nebular Hypothesis. VII. Bain on the Emotions and the Will. VIII. Illogical Geology. TX. The Development Hypothesis. X. The Social Organism. XI. Use and Beauty. XII. The Sources of Architectural Types. XIII. The Use of Anthropomorphism. These Essays constitute a body of massive and original thought upon a large variety of important topics, and will be read with pleasure by all who appreciate a bold and powerful treatment Mf fundamental themes. The general thought which pervades this hbok is beyond doubt the most impor. tant that the human mind has yet reached. —N. E. Inlejpendent. Those who have read the work on Education, will remember the ana. lytic tendency of the author's mind-his clear perception and admirable ex. position of first principles-his wide grasp of facts-his lucid and vigorous style, and the constant and controlling bearing of the discussion on practical results. These traits characterize all Mr. Spencer's writings, and mark, in an eminent degree, the present volume.-L. Y. ibunbse. We regard the distinguishing feature of this work to be the peculiarly interesting character of its matter to the general reader. This is a great literary as well as philosophic triumph. In the evolution of a system of Philosophy which demands serious attention, and a keen exercise of the intellect to fathom and appreciate, he has mingled much that is really popular a.nd entertaining.-Rochester Democrat.