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ELEMENTS 
 
 OF 
 
 PHYSICS, 
 
 OR 
 
 NATURAL PHILOSOPHY, 
 
 GENERAL AND MEDICAL, 
 
 EXPLAINED UfDEPEITDEJfTIT OF 
 
 TECHNICAL MATHEMATICS. 
 
 IN TWO VOLUMES. 
 VOJL. II. PART I. 
 
 COMPREHENDING THE SUBJECTS OF 
 
 HEAT AND LIGHT. 
 
 BY NEIL ARNOTT, M. D., 
 
 OF THE HOYAL COLLEGE OP PHYSICIANS. 
 
 FIRST AMERICAN FROM THE FIRST LONDON EDITION. 
 
 CAREY & L.EA. 
 
 1831. 
 

 ADVERTISEMENT 
 
 f ' ^PT / r uf 
 
 TO 
 
 VOL. II. PART I. 
 
 THE second part of volume ii., comprising the sub- 
 jects of ELECTRICITY, MAGNETISM, and ASTRONOMY, and 
 concluding the work, will be put to press after the pub- 
 lication of the present part. The first volume was 
 originally published without the second, although the 
 whole manuscript was prepared, because other works 
 on Natural Philosophy were offered to public notice 
 about the time. The delay of two years with respect 
 to the second volume has occurred, because the au- 
 thor's very little leisure from the duties of his profes- 
 sion (than which perhaps none more interestingly ab- 
 sorbs the time and faculties) was completely taken up 
 by attending to the repeated calls for editions of vol. 
 i. A friend, however, has superintended the printing 
 of the last edition, and has allowed him to proceed 
 with vol. ii. These explanations are given as an apo- 
 logy to the many persons who have honoured the work 
 by expressing disappointment at the tardy appearance 
 of vol. ii. 
 
 The author, while preparing the fourth edition of 
 vol. i., received a copy of a French edition, in which 
 the translator, M. Richard, to fit the work for the ge- 
 neral use of public schools and colleges in France, had 
 
iv ADVERTISEMENT. 
 
 given in notes the common algebraical formulae for the 
 various cases described. The author, at one time, in- 
 tended to have done this himself, but afterwards deter- 
 mined only to add a few remarks on the subject at the 
 end of vol. ii. To this determination he still adheres. 
 In one of the North American English editions of the 
 work, there are also copious notes, but as the author 
 has not yet been able to procure a copy, he cannot re- 
 mark upon them. 
 
 In the present or fourth edition of vol. i., the subject 
 of speech is still farther analyzed, by a complete ex- 
 planation of the hitherto unknown nature of the defect 
 called stuttering or stammering; and the discovery of its 
 nature has suggested to the author an effectual reme- 
 dy, so simple, that sufferers in general will be able at 
 once to adopt it from the description now given. 
 That the purchasers of former editions may not be 
 obliged to procure the last on this account alone, the 
 chief additions to the section in vol. i. are here sub- 
 joined. They occur at page 610 of the fourth edition; 
 and they should be inserted at page 565 of the first 
 edition, at page 589 of the second, and at page 495 of 
 the first American edition. 
 
 London, November, 1829. 
 
APPENDIX 
 
 TO EARLY EDITIONS. 
 
 " The most common case of stuttering, however, is 
 not, as has been almost universally believed, where the 
 individual has a difficulty in respect to some particular 
 letter or articulation, by the disobedience, to the will or 
 power of association, of the parts of the mouth which 
 should form it, but where the spasmodic interruption 
 occurs altogether behind or beyond the mouth, viz. in 
 the glottis, so as to affect all the articulations equally. 
 To a person ignorant of anatomy, and therefore know- 
 ing not what or where the glottis is, it may be sufficient 
 explanation to say, that it is the slit or narrow opening 
 at the top of the wind pipe, by which the air passes to 
 and from the lungs being situated just behind the 
 root of the tongue. It is that which is felt to close 
 suddenly in hiccup, arresting the ingress of air, and 
 that which closes, to prevent the egress of air from the 
 chest of a person lifting a heavy weight or making any 
 straining exertion; it is that also, by the repeated shut- 
 ting of which, a person divides the sound in pronouncing 
 several times, in distinct and rapid succession, any vow- 
 el, as o, o, o, o. Now, the glottis, during common 
 speech need never be closed, and a stutterer is instant- 
 ly cured if, by having his attention properly directed to 
 it, he can keep it open. Had the edges or thin lips of 
 the glottis been visible, like the external lips of the 
 mouth, the nature of stuttering would not so long have 
 
yi ON STUTTERING. 
 
 remained a mystery, and the effort necessary to the 
 cure would have forced itself upon the attention of the 
 most careless observer; but because hidden, and pro- 
 fessional men had not detected in how far they were 
 concerned, and the patient himself had only a vague 
 feeling of some difficulty, which, after straining, gri- 
 mace, gesticulation, and sometimes almost general 
 convulsion of the body, gave way, the uncertainty 
 with respect to the subject has remained. Even many 
 persons who by attention and much labour had over- 
 come the defect in themselves, as Demosthenes did, 
 have not been able to describe to others the nature of 
 their efforts, so as to ensure imitation: and the author 
 doubts much whether the quacks who have succeeded 
 in relieving many cases, but in many also have failed, 
 or have given only temporary relief, really understood 
 what precise end in the action of the organs their im- 
 perfect directions were accomplishing. 
 
 ' 5 <-Now, a stutterer, understanding of anatomy only 
 what is stated above, will comprehend what he is to 
 aim at, by being farther told, that when any sound is 
 continuing, as when he is humming a single note or a 
 tune, the glottis is necessarily open, and therefore, that 
 when he chooses to begin pronouncing or droning any 
 simple sound, as the e of the English word berry (to do 
 which at once no stutterer has difficulty) he thereby 
 opens the glottis, and renders the pronunciation of any 
 other sound easy. If then, in speaking or reading, he 
 joins his words together, as if each phrase formed but 
 one long word, or nearly as a person joins them in 
 singing, (and this may be done without its being at all 
 noted as a peculiarity of speech, for all persons do it 
 more or less in their ordinary conversation,) the voice 
 never stops, the glottis never closes, and there is of 
 
APPENDIX TO EARLY EDITIONS. Vll 
 
 course no stutter. The author has given thitf explana- 
 tion or lesson, with an example to a person, who be- 
 fore would have required half an hour to read a page, 
 but who immediately afterwards read it almost as 
 smoothly as was possible for any one to do; and who 
 then, on transferring the lesson to the speech, by con- 
 tinued practice and attention, obtained the same faci- 
 lity with respect to it. There are many persons not 
 accounted peculiar in their speech, who, in seeking 
 words to express themselves, often rest long between 
 them on the simple sound of e mentioned above, say- 
 ing, for instance, hesitatingly, "el e think e you 
 
 may," the sound never ceasing until the end of the 
 phrase, however long the person may require to pro- 
 nounce it. Now, a stutterer, who to open his glottis 
 at the beginning of a phrase, or to open it in the mid- 
 dle after any interruption, uses such a sound, would 
 not even at first be more remarkable than a drawling 
 speaker, and he would only require to drawl for a little 
 while, until practice facilitated his command of the 
 other sounds. Although producing the simple sound 
 which we call the c of berry, or of the French words de 
 or que, is a means of opening the glottis, which by stut- 
 terers is found very generally to answer, there are many 
 cases in which other means are more suitable, as the 
 intelligent preceptor soon discovers. Were it possible 
 to divide the nerves of the muscles which close the 
 glottis, without at the same time destroying the facul- 
 ty of producing voice, such an operation would be the 
 most immediate and certain cure of stuttering; and 
 the loss of the faculty of closing the glottis would be 
 of no moment. 
 
 " The view given above of the nature of stuttering 
 and its cure, explains the following facts, which to 
 
Vili ON STUTTERING. 
 
 many persons have hitherto appeared extraordinary. 
 Stutterers often can sing well, and without the least in- 
 terruption, for the tune being continued, the glottis 
 does not close. Many stutterers also can read poetry 
 well, or any declamatory composition, in which the 
 uninterrupted tone is almost as remarkable as in sing- 
 ing. The cause of stuttering being so simple as above 
 described, one rule given and explained, may, in cer- 
 tain cases, instantly cure the defect, however aggra- 
 vated, as has been observed in not a few instances; 
 and this explains also why an ignorant pretender may 
 occasionally succeed in curing, by giving a rule of 
 which he knows not the reason, and which he cannot 
 modify to the peculiarities of other cases. The same 
 view of the subject explains why the speech of a stut- 
 terer has been correctly compared to the escape of li- 
 quid from a bottle with a long narrow neck, coming 
 " either as a hurried gush or not at all:" for when the 
 glottis is once opened, and the stutterer feels that he 
 has the power of utterance, he is glad to hurry out as 
 many words as he can, before the interruption again 
 occurs. 
 
 " Should the author's future experience enable him 
 to simplify or render more complete the views of the na- 
 ture and cure of stuttering, which he has given above, 
 so as to facilitate the cure in every variety of case, he 
 will not fail to publish his remarks." 
 
ELEMENTS 
 
 OF 
 
 NATURAL PHILOSOPHY, 
 
 PART FOURTH. 
 
 DOCTRINES OP IMPONDERABLE SUBSTANCE UNDER THE HEADS 
 OF HEAT, LIGHT, ELECTRICITY, AND MAGNETISM. 
 
 To minds beginning this study, it may facilitate the concep- 
 tion of a substance which is without weight, or at least is im- 
 ponderable by human art, to consider the nature of air. Until 
 lately men were so imperfectly acquainted with the constitu- 
 tion of the universe around them, that a person placed in an 
 apartment which offered to view nothing but the naked walls, 
 would have said that it was empty, meaning literally what he 
 said; and even when advertised that there was air in the room, 
 he would still have been far from possessing a clear notion that 
 it was full of aerial fluid just as an open vessel immersed in the 
 sea is full of water, and that if air were not allowed to escape 
 from it, even so small a body as an apple could not be pressed 
 into it additionally by less force than fifty or sixty pounds. 
 This truth however is now clearly understood, and daily ex- 
 emplified in easy pneumatic experiments, and in no way more 
 strikingly than by the recent adoption of the substance of air 
 in place of feathers, as stuffing for beds and pillows. An air- 
 tight bag or sack suspended by its lip in the air, and held quite 
 open by a hoop near its mouth, would appear empty, but if 
 then firmly closed above the hoop, it would have imprisoned 
 its fill of air, just as a bag similarly managed under water would 
 imprison its fill of water; and while in some respects the air 
 would be softer and locally more yielding than feathers, its en- 
 tire mass would be much less compressible. Now this air, 
 
10 IMPONDERABLE 
 
 when weighed by means which modern science has furnished, 
 is found in a cubic foot to contain somewhat more than an 
 ounce, and by strongly pressing it, or by causing it to combine 
 chemically with some other substance, we can reduce it to very 
 small bulk, either with the form of a liquid or of a solid: 
 proving how small a quantity of ponderable matter under cer- 
 tain circumstances will occupy great space. And common air 
 is by no means the lightest known substance, which as power- 
 fully resists the intrusion of other bodies where it exists. Hy- 
 drogen gas, for instance,, of the same space-occupying force, 
 weighs only a fourteenth part as much, and therefore a few 
 drachms of it confined in a bag or bed as broad as the founda- 
 tion of a house, would support a house or a cask as large as a 
 house filled with water to a height of thirty feet, the gas itself 
 being then eighty thousand times lighter than its bulk of gold; 
 and if the pressure on it were diminished,, it would readily 
 expand to a volume a thousand times as great, and would still 
 be exerting a considerable outward elasticity. Again, a mix- 
 ture of oxygen and hydrogen gases, while uniting with explo- 
 sive force to form water, dilates for the time, even under the 
 great pressure of the atmosphere, to a bulk about twenty times 
 greater than the gases have while separate. 
 
 The mind, pursuing the idea of such expansion or occupancy 
 of space by a small quantity of matter, and reflecting on the 
 wonderful divisibility of matter or minuteness of the ulti- 
 mate atoms, as explained in Part I. of this work, might almost 
 admit as a possible reality Newton's hypothetical illustration of 
 that divisibility, viz. that even one ounce of substance uniform- 
 ly distributed over the vast space in which our solar system 
 exists, might leave no quarter of an inch without its particle. 
 Now a fluid in any degree approaching in rarity to this, al- 
 though it might press, resist, communicate motion, and have 
 other influences in common with more ponderable matter, would 
 have neither weight nor inertia discoverable by means at pre- 
 sent known to man. While we are contemplating, then, or 
 modifying the agencies of what causes the phenomena of heat 
 and cold, of light and darkness, of electricity in its forms of 
 thunder and lightning, of galvanism, or of magnetism, in a 
 
SUBSTANCE. 11 
 
 word, the most striking phenomena of nature, we may be deal- 
 ing with matter of the subtile constitution now spoken of. 
 And as in the terrestrial atmosphere there are at least two fluids 
 present, viz. oxygen and nitrogen, of distinct nature, so in a 
 more subtile ether filling all space, there may be various in- 
 gredients. 
 
 A majority of philosophers now incline to the opinion here 
 sketched, that there is at least one such subtile fluid or ether 
 occupying completely the space of the universe, and tend- 
 ing to uniform diffusion by reason of a strong mutual repulsion 
 of its particles, which fluid pervades denser material substances 
 somewhat as water pervades a sponge or a mass of sand, being 
 attracted in a peculiar way by each substance, and which fluid 
 may or may not have weight and inertia. They believe far- 
 ther that the phenomena above alluded to, and which human 
 art can exhibit with highest beauty, or with awful intensity, 
 are produced by the motion of other affections of that fluid, 
 as the sensation of sound in all its varieties is produced in the 
 delicate structure of the ear by a certain motion in the air, or 
 in any other body, having communication with the ear; or as 
 the sensation of jar is perceived by a hand held to one end of 
 a log of wood when a blow is given to the other end. Some 
 philosophers again suppose that the causes of the phenomena 
 are material particles projected through space, somewhat as sand 
 might be scattered by an explosion, and which particles are pre- 
 sent only when the effects are apparent. Some combine these 
 two hypotheses. And some hold all the phenomena of heat 
 to be mere motions in the common matter of the bodies in 
 which the heat exists. 
 
 We mention these hypotheses, not with the view of enter- 
 ing upon a minute examination of their respective merits, or 
 even of asserting that any one of them is true, but merely to 
 make the reader aware of the directions which inquirers' minds 
 have taken in pursuing the investigation. To understand the 
 subjects as far as men yet usefully understand them, and suffi- 
 ciently for a vast number of most useful purposes, it is only 
 necessary, as in other departments of science, to classify im- 
 portant phenomena, so that their nature and resemblances may 
 
12 
 
 IMPONDERABLE SUBSTANCE. 
 
 be clearly perceived. When in treating of the human mind 
 we speak of its retaining an idea, or being depressed, or being 
 heated with passion, &c., we speak of subjects sufficiently de- 
 finite, although we may have no hypothesis as to the intimate 
 nature of the phenomena: and in the same manner may we 
 speak of the accumulation, radiation, or other affections of heat 
 and light. We know nothing of the cause, even of gravity, 
 the grandest influence in nature, but we can calculate its effects 
 with admirable precision. 
 
[ 13 ] 
 
 PART FOURTH. 
 
 SECTION I. ON HEAT. 
 
 ANALYSIS OF THE SECTION. 
 
 Heat (by some called Caloric} may be strikingly referred to as that 
 which causes the difference between winter and summer, between 
 tropical gardens and polar wastes. Its inferior degrees are denoted 
 by the term COLD. It cannot be exhibited apart, nor proved to 
 have weight or inertia, and the change of its quantity in bodies 
 is most conveniently estimated by the concomitant change of their, 
 bulk; any substance so circumstanced as to allow this to be accu- 
 rately measured constituting a THERMOMETER. 
 Heat diffuses itself among neighbouring bodies until all have the 
 same temperature, that is, until all similarly affect a thermometer. 
 It spreads partly through their structure, or by conduction, as it 
 is called, tvith a slow progress, different for each substance, and 
 in fluids modified by the motion of their particles; and it spreads 
 partly also by being shot or radiated like light from one body to 
 another, through transparent media or space, with readiness af- 
 fected by the material and state of the giving and receiving sur- 
 faces. 
 
 Heat, by entering bodies, expands them, and through a range which 
 includes, as three successive stages, the forms of SOLID, LIQUID, 
 and AIR or GAS; becoming thus in nature the grand antagonist and 
 modifier of that attraction which holds corporeal particles together, 
 and which, if acting alone, would reduce the whole material uni- 
 verse to one solid lifeless mass. Each particular substance, accord- 
 ing to the nature, proximity, fyc. of its ultimate particles, takes a 
 certain quantity of heat (said to mark its capacity,) to produce in 
 it a given change of temperature or calorific tension; undergoing 
 expansion then in a degree proper to itself, and changing its form 
 to liquid and air at points of temperature proper to itself; the ex- 
 pansion in bodies generally increasing more rapidly than the tem- 
 perature, because the cohesion of their particles lessens with in- 
 
14 HEAT. 
 
 crease of distance; being remarkably greater therefore in liquids 
 than in solids, and in airs than in liquids; and the rate of expan- 
 sion, moreover, being much quickened as the bodies approach their 
 points of changing form to liquid or air, to produce which changes, 
 a large quantity of heat enters them, but in the new arrangement 
 of particles and increased volume of the mass, it becomes hidden 
 from.the thermometer, and is therefore called LATENT HEAT. For 
 any given substance the changes of form happen so constantly at 
 the same temperature, that they mark fixed points in the general 
 scale of temperature, and enable us to regulate and compare ther- 
 mometers. Heat, by expanding different substances unequally, in- 
 fluences much their chemical combination. Heat influences also 
 the functions of vegetable and animal life. 
 
 The great source of heat is the sun; but electricity, combustion, and 
 other chemical actions, condensation, friction, and the actions of 
 life, are also excitants.* 
 
 " Heat may be strikingly referred to as that which causes 
 the difference between winter and summer, between the 
 gardens of the equator and polar wastes." (See the Ana- 
 lysis, page 13.) 
 
 In the winter of climates, where the temperature is for a 
 time below the freezing point of water, the earth with its 
 waters is bound up in snow and ice, the trees and shrubs are 
 leafless, appearing every where like withered skeletons, count- 
 less multitudes of living creatures, owing either to the bitter 
 cold or deficiency of food, are perishing in the snows na- 
 ture seems dying or dead; but what a change when spring re- 
 turns, that is, when heat returns! The earth is again uncovered 
 and soft, the rivers flow, the lakes are again liquid mirrors, the 
 warm showers come to foster vegetation, which soon covers the 
 
 * It is to be remarked here, that many phenomena in which heat plays an 
 important part, have been already described in preceding chapters of this work; 
 for instance, the action of the steam-engine, the phenomena of winds, many 
 facts in meteorology, &c. under the head of Pneumatics. In a separate trea- 
 tise on heat, these could not with propriety have been omitted; but in a com- 
 prehensive system of science like the present, they find their fit place, where, 
 being surrounded by subjects resembling them in more intricate particulars, 
 they can be more concisely and clearly explained. 
 
CAUSE'OF SEASONS AND CLIMATES. 15 
 
 ground with beauty and plenty. Man, lately inactive, is re- 
 called to many duties; his water-wheels are every where at 
 work, his boats are again on the canals and streams, his busy 
 fleets of industry are along the shores: winged life in new 
 multitudes fills the sky, finny life similarly fills the waters, and 
 every spot of earth teems with vitality and joy. Many persons 
 regard these changes of season as if they came like the succes- 
 sive positions of a turning wheel,, of which one necessarily 
 brings the- next; not adverting that it rs the single circumstance 
 of change of temperature, which does all. But rf the colds of 
 winter arrive too early, they unfailingly produce the wintry 
 scene, and if warmth come before its time in spring, it expands 
 the bud and the blossom, which a return of frost will surely 
 destroy. A seed sown in an ice-house never awakens to life. 
 Again, as regards climates, the earthy matters forming the 
 exterior of our globe, and therefore entering into the composi- 
 tion of soils, are not different for different latitudes, at the 
 equator, for instance, and near the poles. That the aspect of na- 
 ture then in the two situations exhibits a contrast more striking 
 still than between summer and winter, is merely to an inequa- 
 lity of temperature, which is permanent. Were it not for this, 
 in both situations the same vegetables might grow, and the same 
 animals might find their befitting support. But now, in the 
 one, namely, where heat abounds, we seethe magnificent scene 
 of tropical fertility: the earth covered with luxuriant vegeta- 
 tion in endless lovely variety, and even the hard rocks festooned 
 with green, perhaps with the vine, rich in its purple clusters. 
 In the midst of this scene, animal existence is equally abundant, 
 and many of the species are of surpassing beauty the plumage 
 of the birds is as brilliant as the gayest flowers. The warm air 
 is perfume from the spice-beds, the sky and clouds are often 
 dyed in tints as bright as freshest rainbow, and happy human 
 inhabitants call the scene a paradise. Again, where heat is ab- 
 sent, we have the dreary spectacle of polar barrenness, namely, 
 bare rock or mountain, instead of fertile field; water every 
 where hardened to solidity, no rain, nor cloud, nor dew, few 
 motions but drifting snow; vegetable life scarcely existing, 
 and then only in sheltered places turned to the sun and in- 
 
16 HEAT. 
 
 stead of the palms and other trees of India, whose single leaf is 
 almost broad enough to cover a hut, there are bushes and 
 trees, as the furze and fir, having what may be called hairs or 
 bristles, in the room of leaves. In the winter time, during 
 which the sun is not seen for nearly six months, new horrors 
 are added, viz. the darkness and dreadful silence, the cold be- 
 numbing all life, and even freezing mercury a scene into which 
 man may penetrate from happier climes, but where he can only 
 leave his protecting ship and fires for short periods, as he might 
 issue from a diving-bell at the bottom of the ocean. That in 
 these now desolate regions, heat only is wanted to make them 
 like the most favoured countries of the earth, is proved by the 
 recent discoveries under ground of the remnant of animals and 
 vegetables formerly inhabiting them, which now can live only 
 near the equator. While winter then, or the temporary ab- 
 sence of heat, may be called the sleep of nature, the more per- 
 manent torpor about the poles appears like its death; and when 
 we farther reflect, that heat is the great agent in numberless im- 
 portant processes of chemistry and domestic economy, and is 
 the actuating principle of the mighty steam engine which now 
 performs half the work of society, how truly may heat, the sub- 
 ject of our present chapter, be considered as the life or soul of 
 the universe! 
 
 " Heat cannot be exhibited in a separate state, nor proved 
 to have weight or inertia." (Read the Analysis, page 13.) 
 
 Although heat is known to be abundant in the sun-beam, and 
 to radiate around from a blazing fire, we cannot otherwise ar- 
 rest or detect it in its progress than by allowing it to enter, and 
 remain in some ponderable substance. We know hot iron or 
 hot water or hot air, but nature nowhere presents to us, nor 
 has art succeeded in showing us heat alone. 
 
 If we balance a quantity of ice in a delicate weigh-beam, and 
 then leave it to melt, the equilibrium will not be in the slight- 
 est degree disturbed. Or if we substitute for the ice, boiling 
 water or red-hot iron, and leave this to cool, there will be 
 no difference in the result. If we place a pound of mercury in 
 one scale of the weigh-bean and a pound of water in the other, 
 
HAS XO WEIGHT OR INERTIA. 17 
 
 and then either heat or cool both through the same number of 
 thermometrlc degrees, although about thirty times moreheat (as 
 will be explained below (enters or leaves the bulky water than 
 the dense mercury, they will still remain equivalent weights. 
 
 Again, a sun-beam, with its intense light and heat, after being 
 concentrated by a powerful lens or mirror, may be made to fall 
 upon the scale of a most delicate balance, but will produce no 
 depressing effect on the scale, as would follow if what consti- 
 tutes the beam had the least forward motal inertia or momen- 
 tum. 
 
 Such are the facts which have led certain inquirers to deny 
 the material or separate existence of heat, and to hold that it is 
 merely motion of one kind among the material particles of bo- 
 dies generally, as sound is motion of another kind among the 
 same particles. The following facts they consider to have the 
 same bearing in the argument. Heat can be produced without 
 limit by friction, as when savages light their fires by rubbing 
 together two pieces of wood when Count Rum ford made great 
 quantities of water boil, by causing a blunt borer to rub against 
 a mass of metal immersed in the water when Sir Humphrey 
 Davy quickly melted pieces of ice by rubbing them against 
 each other in a room cooled below the freezing point, &c. In- 
 tense heat is produced by the explosion of gunpowder or other 
 fulminating mixture, yet it cannot be conceived to have existed 
 in the small bulk of the powder before the explosion. Other 
 inquirers, on the contrary, have deemed to be proofs of the 
 separate materiality of heat such facts as now follow: that it is 
 radiated through the most perfect vacuum which we can pro- 
 duce, and even more readily than through air; that it radiates 
 in the same place in all directions, without impediment from 
 the crossing rays; that it becomes instantly sensible on the 
 condensation of any material mass, as if then squeezed out from 
 the mass; as when, by compressing air suddenly, we inflame a 
 match immersed in it; or when, on reducing the bulk of iron 
 by hammering, we render it very hot, the warming being 
 greater at the first blow (which most changes the bulk) than 
 afterwards, that when, on mixing bodies which combine so 
 intimately as to occupy less space than when separate, there is 
 
 3 
 
16 HAT- 
 
 a disengagement of heat proportioned to the diminution of vo- 
 lume: that the laws of the spreading of heat in bodies do not 
 resemble those of the spreading of sound, or of any other mo- 
 tion known to us: and that, as to the great and sudden extri- 
 cation of heat by friction or explosion, it may be as truly a rush 
 of the fluid to the part, as in the case of an electrical accumula- 
 tion or discharge. These facts, moreover, they think, square 
 well with their assumption that the phenomena of heat are pro- 
 duced by an exceedingly subtile fluid, or ether, pervading the 
 whole universe, and softening or melting or gasifying bodies, 
 according to the quantity present in each; its own parts being 
 strongly repulsive of each other, and seeking, therefore, widest 
 and most equable diffusion. 
 
 i: The, change of its quantity in bodies is most conveniently 
 estimated by the. concomitant change of their bulk, any 
 substance so circumstanced, as to allow this to be accu- 
 rately measured, constituting a thermometer." (Read 
 the Analysis, page 13.) 
 
 If we heat a wire, it is lengthened; if we heat water in a full 
 vessel, a part runs over; if we heat air in a bladder, the bladder 
 is distended: in a word, if we heat any substance, its volume 
 increases in some proportion to the increase of temperature, 
 and we may measure the increase of volume. The reasons 
 why, in such investigations, a contrivance in which the expan- 
 sion of mercury may be observed, viz. the mercurial thermo- 
 meter, is commonly preferred to others, can only be fully un- 
 derstood by the mind which has considered the whole subject 
 of heat; and we touch upon the matter here, only for the pur- 
 pose of stating that a mercurial thermometer is a small bulb, 
 or bottle of glass filled with mercury, and having a long very 
 narrow stalk or neck, in which the mercury rises when expand- 
 ed by heat, or falls when heat is withdrawn; the stalk between 
 the points at which the mercury stands in freezing and in boil- 
 ing water, being divided into an arbitrary number of degrees, 
 which division appearing on a scale applied to the stalk, is con- 
 tinued similarly above and below these points. 
 
SPKEABIXG BY CO^DUCTIOX. 19 
 
 Heat diffuses itself among neighbouring bodies until all 
 have acquired the same temperature; that is to say, until 
 all will similarly affect a thermometer." (See the Analy- 
 sis, page 13.) 
 
 An iron bolt thrust in among burning coals soon becomes red 
 hot like them. If it be the heater of a tea-urn, it will, when 
 afterwards placed amidst the water, part with its lately acquired 
 heat to the water, until both are of the same temperature. Boil- 
 ing water, again, soon imparts heat to an egg placed in it, and a 
 feverish head yields its heat to a bladder of cold water or ice. 
 A hundred objects enclosed in the same apartment, if tested, 
 after a time, by the thermometer, will all indicate the same 
 temperature. 
 
 '* The inferior degrees of heat are denoted by the term COLD." 
 When the hand touches a body of higher temperature than it- 
 self, it receives heat according to the law now explained, and it 
 experiences a peculiar sensation; when it touches a body of 
 lower temperature than itself, it gives out heat for a like reason, 
 and experiences another and very different sensation. The two 
 are called the sensations of heat and of cold. Now heat and 
 cold, considered as existing in the bodies themselves, although 
 thus appearing opposites, are really degrees of the same object, 
 temperature, contrasted by name for convenience sake, in re- 
 ference to the particular temperature of the individuals speaking 
 of them just as any two nearest mile-stones on a road, although 
 merely marking degrees of the same object, distance, might 
 receive from persons living between them the opposite names of 
 east and west, or of north and south. It is to be remarked, 
 moreover, that the sensation of heat is producible also by a 
 body colder than the hand, provided it be less cold than a body 
 touched immediately before, or than the usual temperature; and 
 the sensation of cold is producible under the opposite circum- 
 stances of touching a comparatively warm body, but which is 
 less warm than something touched just before. This explains 
 the remarkable fact that the same body may appear at the same 
 time, and to the same person, both hot and cold. If a person 
 transfer one hand to common spring-water from touching ice, 
 
that hand will deem the water very warm; while the other 
 hand, transferred to it from a warm bath, would deem it very 
 cold. For a like reason, a person from India, arriving in Eng- 
 land in the spring, deems the air cold, while the inhabitants of 
 the country are diminishing their clothing, because the heat to 
 them is becoming oppressive. Such facts show how necessary 
 it was for men to discover more correct thermometers than their 
 bodily sensations. 
 
 " Spreading partly through their structure, or by conduc- 
 tion, as it is called, with a progress proper to each sub- 
 stance" (Read the Analysis, page 13.) 
 If one end of a rod of iron be held in the fire, a hand grasping 
 the other end soon feels the heat coming through it. Through a 
 similar rod of glass the transmission is much slower, and through 
 one of wood it is slower still. The hand would be burned by 
 the iron, before it felt warmth in the wood, although the inner 
 end were blazing. 
 
 On the fact that different substances are permeable to heat, or 
 have the property of conducting it, in different degrees, depend 
 many interesting phenomena in nature and in the arts: hence 
 it was important to ascertain the degrees exactly, and to classify 
 the substances. Various methods for this purpose have been 
 adopted. For solids similar rods of the different, substances, 
 after being thinly coated with wax, have been placed with their 
 inferior extremities in hot oil, and then the comparative dis- 
 tances to which, in a given time, the wax was melted, furnished 
 one set of indications of the comparative conducting powers: 
 or, equal lengths of the different bare rods being left above the 
 oil, and a small quantity of explosive powder being placed on 
 the top of each, the comparative intervals of time elapsing be- 
 fore the explosions gave another kind of measure: or, equal 
 balls of different substances, with a central cavity in each to re- 
 ceive a thermometer, being heated to the same degree and then 
 suspended in the air to cool, until the thermometer fell to a 
 given point, gave still another list. A modification of the last 
 method was adopted by Count Rumford to ascertain the relative 
 degrees in which furs, feathers, and other materials used for 
 
SPREADING- BY COSDTJCTIO^. 
 
 clothing, conduct heat, or, which is the same thing, resist its 
 passage. He covered tne ball and stem of a thermometer with 
 a certain thickness of the substance to be tried, by placing the 
 thermometer in a larger bulb and stem of glass, and then filling 
 the interval between them with the substance; and, after heat- 
 ing this apparatus to a certain degree, by dipping it in liquid of 
 the desired temperature, he surrounded it by ice, and marked 
 the comparative times required to cool the thermometer a cer- 
 tain number of degrees. The figures following the names of 
 some of the substances in the subjoined list, mark the number 
 of seconds required respectively for cooling it 60. 
 
 These experiments have shown as a general rule, that den- 
 sity in a body favours the passage of heat through it. The 
 best conductors are the metals, and then follow in succession 
 diamond, glass, stones, earths, woods, &c., as here noted: 
 Metals silver, copper, gold, iron, lead. 
 Diamond. 
 Glass. 
 
 Hard stones. 
 Porous earths. 
 Woods. 
 
 Fats or thick oils. 
 Snow. 
 Air ..... ... 57tf 
 
 Sewing silk ------ 917 
 
 Wood ashes ----.. 937 
 
 Charcoal -.-.... 937 
 
 Fine lint ....... 1,032 
 
 Cotton ........ 1,046 
 
 Lamp black ...... 1,117 
 
 Wool ........ i,H8 
 
 Raw silk ....... 1,284 
 
 Beavers' fur ...... 1,296 
 
 Eider down ...... 1,305 
 
 Hares' fur ....... 1,315 
 
 Air appears near the middle of the preceding list, but if its 
 particles are not allowed to move about among themselves, so 
 as to carry heat from one part to another, it conducts (in the 
 
,' 3 HEAT. 
 
 manner of solids) so slowly that Count Rumford doubted whe- 
 ther it conducted at all. It is probably the worst conductor 
 known, that is, the substance which when at rest impedes the 
 passage of heat the most. To this fact seems to be owing in 
 a considerable degree the remarkable non-conducting quality 
 of porous or spongy substances, as feathers, loose filamentous 
 matter, powders, &c., which have much air in their structure, 
 often adherent with a force of attraction which immersion in 
 water, or even being placed in the vacuum of an air-pump, is 
 insufficient to overcome. 
 
 While contemplating the facts recorded in the above table, 
 one cannot but reflect how admirably adapted to their purposes 
 the substances are whieh nature has provided as clothing for 
 the inferior animals; and which man afterwards accommodates 
 with such curious art to his peculiar wants. Animals required 
 to be protected against the chills of night and the biting blasts 
 of winter; and some of them which dwell among eternal ice, 
 could not have lined at all, but for a garment which might shut 
 up within it nearly all the heat which their vital functions pro- 
 duced. Now any covering of a metallic or earthy or woody 
 nature, would have been far from sufficing; but out of a won- 
 drous chemical union of carbon with the soft ingredients of 
 the atmosphere, those beautiful textures are produced called fur 
 and feather, so greatly adorning while they completely protect 
 the wearers: textures, moreover, which grow from the bodies 
 of the animals, in the exact quantity that suits the climate and 
 season, and which are reproduced when by any accident they are 
 partially destroyed. In warm climates the hairy coat of quad- 
 rupeds is comparatively short and thin; as in the elephant, 
 the monkey, the tropical sheep, &c. It is seen to thicken 
 With increasing latitude, furnishing the soft and abundant fleeces 
 of the temperate zones; and towards the poles it is externally 
 shaggy and coarse, as in the arctic bear. In amphibious ani- 
 mals, which have to resist the cold of water as well as of air, 
 the fur grows particularly defensive, as in the otter and bea- 
 ver. Birds, from having very warm blood, required plenteous 
 clothing, but required also to have a smooth surface, that they 
 might pass easily through the air: both objects are securectby 
 
BY CONDUCTION. 
 
 the beautiful structure of feathers, so beautiful and wonderful 
 that writers on natural theology have often particularized it as 
 one of the most striking exemplifications of creative wisdom. 
 Feathers, like fur, appear in kind and quantity suited to parti- 
 cular climates and seasons. The birds of cold regions have 
 covering almost as bulky as their bodies, and if it be warm in 
 those of them which live only in air, in the water-fowl it is 
 warmer still. These last have the interstices of the ordinary 
 plumage filled up by the still more delicate structure called 
 down, particularly on the breast, which in swimming first 
 meets and divides the cold wave. There are animals with 
 warm blood which yet live very constantly immersed in water, 
 as the whale, seal, walrus, &c. Now neither hair nor feathers, 
 however oiled, would have been a fit covering for them; but 
 kind nature has prepared an equal protection in the vast mass 
 of fat or thick oil which surrounds their bodies substances 
 which are scarcely less useful to man than the furs and fea- 
 thers of land animals. 
 
 While speaking of clothing, we may remark, that the bark 
 of trees is also a structure very slowly permeable to heat, 
 and securing therefore the temperature necessary to vegetable 
 life. 
 
 And while we admire what nature has thus done for animals 
 and vegetables, let us not overlook her scarcely less remarkable 
 provision of ice and snow, as winter clothing for the lakes and 
 rivers, for our fields and gardens. Ice, as a protection to wa- 
 ter and its inhabitants, was considered in vol. i. in the explana- 
 tion of why, although solid, it swims on water. We have now 
 to remark that snow, which becomes as a pure white fleece to 
 the earth, is a structure which resists the passage of heat near- 
 ly as much as feathers. It, of course, can defend only from 
 colds below 32 or the freezing point; but it does so most ef- 
 fectually, preserving the roots and seeds and tender plants 
 during the severity of winter. When the green blade of wheat 
 and the beautiful snow-drop flower appear in spring rising 
 through the melting snow, they have recently owed an import- 
 ant shelter to their wintry mantle. Under deep snow, while 
 the thermometer in the air may be far below zero, the tempera- 
 
oi- HEAT. 
 
 ture of the ground rarely remains below the freezing point 
 Now this temperature, to persons some time accustomed to it, 
 is mild and even agreeable. It is much higher than what of- 
 ten prevails for long periods in the atmosphere of the centre 
 and north of Europe. The Laplander, who during his long 
 winter lives under ground, is glad to have additionally over- 
 head a thick covering of snow. Among the hills of the west 
 and north of Britain, during the storms of winter, a house or 
 covering of snow frequently preserves the lives of travellers, 
 and even of whole flocks of sheep, when the keen north wind 
 catching them unprotected, would soon stretch them lifeless 
 along the earth. 
 
 It is because earth conducts heat slowly, that the most intense 
 frosts penetrate but a few inches into it, and that the tempera- 
 ture of the ground a few feet below its surface is nearly the 
 same all the world over. In many mines, even although open 
 to the air, the thermometer does not vary one degree in a 
 twelvemonth. Thus also water in pipes two or three feet un- 
 der ground does not freeze, although it may be frozen in all the 
 smaller branches exposed above. Hence, again, springs never 
 freeze, and therefore become remarkable features in a snow co- 
 vered country. The living water is seen issuing from the bowels 
 of the earth, and running often a considerable way through 
 fringes of green, before the gripe of the frost arrests it; while 
 around it, as is well known to the sportsman, the snipes and wild 
 duck and other birds are wont to congregate. A spring in a fro- 
 zen pond or lake may cause the ice to be so thin over the part 
 where it issues, that a skaiter arriving there will break through 
 and be destroyed. The same spring water which appears warm 
 in winter, is deemed cold in summer, because, although always 
 of the same heat, it is in summer surrounded by warmer atmos- 
 phere and objects. In proportion as buildings are massive, 
 they acquire more of those qualities, which have now been no- 
 ticed of our mother earth. Many of the gothic halls and ca- 
 thedrals are cool in summer and warm in winter as are also old 
 fashioned houses or castles with thick walls and deep cellars. 
 Natural caves in the mountains or sea-shores furnish other ex- 
 amples of a similar kind. 
 
SPItEADINtt BY CONDUCTION. 25 
 
 When in the arts it is desired to prevent the passage of heat 
 out of or into any body or situation, a screen or covering of a 
 slow conducting substance is employed. Thus, to prevent the 
 heat of a smelting or other furnace from being wasted, it is lined 
 with fire bricks, or is covered with clay and sand, or sometimes 
 with powdered charcoal. A furnace so guarded may be touched 
 by the hand, even while containing within it melted gold. To 
 prevent the freezing of water in pipes during the winter, by 
 which occurrence the pipes would be burst, it is common to 
 cover them with straw ropes, or coarse flannel, or to enclose 
 them in a larger outer pipe, with dry charcoal, or saw dust, or 
 chaff, filling up the interval between. If a pipe, on the con- 
 trary, be for the conveyance of steam or other warm fluid, the 
 heat is retained, and therefore saved by the very same means. 
 Ice houses are generally made with double walls, between which 
 dry straw placed, or saw dust, or air, prevents the passage of heat. 
 Pails for carrying ice in summer, or intended to serve as wine 
 coolers, are made on the same principle viz. double vessels, 
 with air or charcoal, filling the interval between them. A flan- 
 nel covering keeps a man warm in winter it is also the best 
 means of keeping ice from melting in summer. Urns for hot 
 water, tea pots, coffee pots, &c. are made with wooden or ivory 
 handles, because if metal were used, it would conduct the heat 
 so readily that the hand could not bear to touch them. 
 
 It is because glass and earthenware are brittle, and do not al- 
 low ready passage to heat, that vessels made of them are so fre- 
 quently broken by sudden change of temperature. On pouring 
 boiling water into such a vessel, the internal part is much heated 
 and expanded (as will be explained more fully in a subsequent 
 page) before the external part has felt the influence, and this IB 
 hence riven or cracked by its connexion with the internal. A 
 chimney mirror is often broken by a lamp or candle placed on 
 the marble shelf too near it. The glass cylinder of an electri- 
 cal machine will sometimes be broken by placing it near the fire, 
 so that one side is heated while the other side receives a cold 
 current of air approaching the fire from a door or window. A 
 red hot rod of iron drawn along a pane of glass will divide it 
 almost like a diamond knife. Even cast iron, as backs of grates, 
 
 4 
 
yti -HEAT. 
 
 iron pots, c., although conducting readily, is often, owing to 
 its brittlcncss, cracked by unequal heating or cooling, as from 
 pouring water on it when hot. Pouring cold water into a heated 
 glass will produce a similar effect. Hence glass vessels intended 
 to be exposed to strong heats and sudden changes, as retorts for 
 distillation, flasks for boiling liquids, &c., are made very thin, 
 that the heat may pervade them almost instantly and with im- 
 punity. 
 
 There is a toy called a Prince Rupert's Drop, which well 
 illustrates our present subject. It is a lump of glass let fall 
 while fused into water, and thereby suddenly cooled and solidi- 
 fied on the outside before the internal part is changed; then as 
 this at last hardens and would contract, it is kept extended by 
 the arch of external crust, to which it coheres. Now if a por- 
 tion of the neck of the lump be broken off, or if other violence 
 be done, which jars its substance, the cohesion is destroyed, and 
 the whole crumbles to dust with a kind of explosion. Any 
 glass cooled suddenly when first made, remains very brittle, for 
 the reason now stated. - What is called the Bologna jar is a 
 very thick small bottle, thus prepared, which bursts by a grain 
 of sand falling into it. The process of annealing, to render 
 glass ware more tough and durable, is merely the allowing it 
 to cool very slowly by placing it in an oven, where the tempe- 
 rature is caused to fall gradually. The tempering of metals by 
 sudden cooling seems to be a process having some relation to 
 that of rendering glass hard and brittle. 
 
 It is the difference of conducting power in bodies which is 
 the cause of a very common error made by persons in esti- 
 mating the temperature of bodies by the touch. In a room 
 without a fire all the articles of furniture soon acquire the same 
 temperature; but if in winter, a person with bare feet were to step 
 from the carpet to the wooden floor, from this to the hearth- 
 stone, and from the stone to the steel fender, his sensation would 
 deem each of these in succession colder than the preceding. 
 Now the truth being that all had the same temperature, only a 
 temperature inferior to that of the living body,the best conductor, 
 when in contact with the body, would carry off heat the fastest, 
 and would therefore be deemed the coldest* Were a similar 
 
.SPREADING BY CONBUOTIOX. 27 
 
 experiment made in a hot house, or in India, while the temper- 
 ature of every thing around were 98, viz. that of the living 
 body, then not the slightest difference would be felt in any of 
 the substances: or lastly, were the experiment made in a room 
 where by any means the general temperature were raised con- 
 siderably above blood heat; then the carpet would be deemed 
 considerably the coolest instead of the warmest, and the other 
 things would appear hotter in the same order in which they ap- 
 peared colder in the winter room. Were a bunch of wool and 
 a piece of iron exposed to the severest cold of Siberia, or of an 
 artificial frigorific mixture, a man might touch the first with 
 impunity (it would merely be felt as rather cold;) but if he 
 grasped the second, his hand would be frost bitten and possibly 
 destroyed: were the two substances on the contrary, transferred 
 to an oven, and heated as far as the wool would bear, he might 
 again touch the wool with impunity (it would then be felt as a 
 little hot,) but the iron would burn his flesh. The author has 
 entered a room where there was no fire, but where the temper- 
 ature from hot air admitted was sufficiently high to boil the 
 fish, &c. of which he afterwards partook at dinner; and he 
 breathed the air with very little uneasiness. He could bear to 
 touch woollen cloth in this room, but no body more solid. 
 
 The foregoing considerations make manifest the error of sup- 
 posing that there is a positive warmth in the materials of cloth- 
 ing. The thick cloak which guards a Spaniard against the cold 
 of winter, is also in summer used by him as protection against 
 the direct rays of the sun: and while in England flannel is our 
 warmest article of dress, yet we cannot more effectually pre- 
 serve ice than by wrapping the vessel containing it in many 
 folds of softest flannel. 
 
 In every case where a substance of different temperature 
 from the living body touches it, a thin surface of the substance 
 immediately shares the heat of the bodily part touched the 
 hand generally; and while in a good conductor, the heat so re- 
 ceived quickly passes inwards, or away from the surface, leaving 
 this in a state to absorb more, in the tardy conductor the heat 
 first received tarries at the surface, which consequently soon ac- 
 quires nearly the same temperature as the hand, and therefore, 
 cold the interior of the substance may be, it does not 
 
HEAT. 
 
 cause the sensation of cold. The hand on a good conductor has 
 to warm it deeply, a slow conductor it warms only superficially. 
 The following cases farther illustrate the same principle. If the 
 ends of an iron poker, and of a piece of wood of the same size, 
 be wrapped in paper and then thrust into a fire, the paper on the 
 wood will begin to burn immediately, while that on the metal 
 will long resist: or if pieces of paper be laid on a wooden 
 plank and on a plate of steel, and then a burning coal be placed 
 on each, the paper on the wood will begin to burn long before 
 that on the plate. The explanation is, that the paper in contact 
 with the good conductor loses to this so rapidly the heat received 
 from the coal, that it remains at too low a temperature to in- 
 flame, and will even cool to blackness the touching part of the 
 coal; while on the tardy conductor the paper becomes almost 
 immediately as hot as the coal. It is because water exposed to 
 the air cannot be heated beyond 212, that it may be made to 
 boil in an egg-shell or a vessel made of paper, held over a lamp, 
 without the containing substance being destroyed; but as soon 
 as it is dried up, the paper will burn and the shell will be cal- 
 cined, as the solder of a common tinned kettle melts under the 
 same circumstances. The reason why the hand judges a cold 
 liquid to be so much colder than a solid of the same tempera- 
 ture is, that from the mobility of the liquid particles among 
 themselves, those in contact with the hand are constantly 
 changing. The impression produced on the hand by very cold 
 mercury is almost insufferable, because mercury is both a ready 
 conductor and a liquid. Again, if a finger held motionless in 
 water feel cold, it will feel colder still when moved about; and 
 a man in the air of a calm frosty morning does not experience a 
 sensation nearly so sharp as if with the same temperature there 
 be wind. A finger held up in the wind discovers the direc- 
 tion in which the wind blows by the greater cold felt on one 
 side, the effect being still more remarkable, if the finger is wet- 
 ted. If a person in a room with a thermometer, were with a 
 fan or bellows to blow the air against it, he would not thereby 
 lower it, because it had already the same temperature as the air, 
 yet the air blown against his own body would appear colder 
 than when at rest, because, being colder than his body, the mo- 
 
SPREADING BV CONDUCTION 
 
 (ion would supply heat-absorbing particles more quickly. In 
 Hke manner, if a fan or bellows were used against a thermome- 
 ter hanging in a furnace or hot house, the thermometer would 
 suffer no change, but the air moved by them against a person 
 would be distressingly hot, like the blasting sirocco of the sandy 
 deserts of Africa. If two similar pieces of ice be placed in a 
 room somewhat warmer than ice, one of them may be made to 
 melt much sooner than the other, by blowing on it with a bel- 
 lows. The reason may here be readily comprehended why a 
 person suffering what is called a cold in the head, or catarrh 
 from the eyes and nose, experiences so much more relief on ap- 
 plying to the face a handkerchief of linen or cambric than one 
 of cotton: it is, that the former by conducting readily absorbs 
 the heat and diminishes the inflammation, while the latter, by 
 refusing to give passage to the heat, increases the temperature 
 and the distress. Popular prejudice has held that there was a 
 poison in cotton. 
 
 " Heat spreading in fluids chiefly by the motion of their 
 particles." (Read the Analysis, page 13.) 
 
 Owing to the mobility among themselves of fluid particles, 
 heat entering a fluid any where below the surface, by dilating 
 and rendering specifically lighter the portion heated, allows 
 the denser fluid around to sink down and force up the rarer; 
 and the continued currents so established, diffuse the heat 
 through the mass much more quickly than heat spreads by con- 
 duction in any solid. 
 
 Count Rumford's experiments led him at first to conclude 
 that liquids, but for this carrying process, by the particles 
 changing their place, were absolutely impassable to heat. A 
 piece of ice will lie very long at the bottom of water which 
 is made to boil at the top by the contact of any hot body; and 
 when it at last melts, Count R. believed that it did so entirely 
 from the heat which passed downwards through the sides of the 
 vessel containing the water. But an ingenious experiment by Dr. 
 Murray decided the question differently. He made a vessel of 
 ice, which, of course, could not carry downwards any heat great- 
 
HEAT. 
 
 er than 32, as ice melts at that degree; and having put into it a 
 quantity of oil at 32, the bulb of a thermometer being a quar- 
 ter of an inch under the surface of the oil, he placed a cup of 
 boiling water in contact with the surface of the oil: in a mi- 
 nute and a-half the thermometer rose nearly a degree, and in 
 seven minutes it rose five degrees, beyond which it did not go. 
 The heat then must have passed downwards through the liquid, 
 proving a conducting power; unless, indeed, it passed by ra- 
 diation, as explained in a subsequent page. 
 
 The internal currents or circulation produced by heat in fluid 
 masses, and of which there are so many important instances in 
 nature, were more fitly explained in the chapter on hydrosta- 
 tics and pneumatics; we shall here therefore allude to them 
 very shortly. 
 
 Perhaps the best experimental illustration of the subject is 
 the placing a tall glass jar, filled with water in which small 
 pieces of amber are floating to show its movements, first in a 
 warm bath, and then in one which is cold. In the first case, 
 the water and amber near the outside of the jar where they are 
 heated, will exhibit a rapid upward current, while in the cen- 
 tre of the jar they will form an opposite or downward current. 
 By afterwards placing the jar in a cold bath, the direction of 
 the currents will be reversed. 
 
 It is, as stated in the former volume, this heating and dilata- 
 tion of the fluid air over a tropical island while acted upon 
 during the middle of the day by the powerful rays of the sun, 
 that allows the colder and heavier air from the face of the 
 ocean around to press inwards upon it and force it upwards in 
 the atmosphere the cold current forming the delightful sea- 
 breeze of the climate. And it is the general heating of tho 
 air over the whole equatorial belt of the earth, which, render- 
 ing it specifically lighter than the air nearer the poles, allows 
 this to assume the form of cool trade winds, constantly blow- 
 ing towards the sun's path, and pressing upwards the hot air, 
 which then spreads away on the top of the atmosphere towards 
 the poles, to mitigate the severity of the northern and south- 
 ern cold. In the watery ocean also there is a circulatory mo- 
 tion of the same kind, although less in degree, tending to dis- 
 
BY CONDUCTION. >\l 
 
 tribute heat and equalize temperature, and contributing to pro- 
 duce some of the great sea currents known to mariners. 
 
 The vertical currents produced by heat, in the ocean, and in 
 great masses of water generally, preserve in and over them a 
 comparatively uniform temperate freshness, while the rocks 
 and soil around may be either parched under a burning sun, or 
 bound up in cold many degrees below freezing. A keen frost 
 chills, and soon hardens in its icy grasp the surface of the 
 ground; but of water similarly exposed, the part first cooled 
 descends to the bottom by its increased density, and forces up 
 a warmer water to take its place; this in its turn is cooled and 
 descends, and a continued circulation is established, so that 
 the surface cannot become ice until the whole mass, of what- 
 ever depth, has been cooled down to its greatest density. The 
 very deep sea, hence, is not frozen in the coldest climates, and 
 in temperate climates the severest winter does not freeze even 
 a deep lake. During this intestine movement in the water, that 
 which ascends to the surface to be cooled, by losing one degree 
 of its heat, warms nearly five hundred times its bulk of air one 
 degree, and thus tempers remarkably the air passing over it. 
 Hence, places in the vicinity of the sea and of lakes are warmer 
 in winter than places farther inland, although nearer the equa* 
 tor. England is much warmer in winter than central Germa- 
 ny, which lies south of England, and the coasts of Scotland 
 and the north of Ireland are warmer than London: snow ne* 
 ver lies long upon these coasts. As continental or inland coun- 
 tries have thus in winter an extreme of cold, so have they ia 
 summer an extreme of heat. Water admits the rays of the 
 sun, and absorbs the heat into the whole thickness of its mass^ 
 (it will be explained afterwards that solar heat penetrates trans- 
 parent fluids as light does,) while earth retains all near its sur- 
 face, and is therefore heated to excess. 
 
 The ventilation of our dwellings and halls of assembly (as 
 explained in vol. i.) is owing to the motion produced by the 
 changed specific gravity of air when heated. The air which 
 is within the house becomes warmer than the external air, and 
 this then presses in at every opening or crevice to displace it. 
 The ventilation of the person by the slow passage of air through 
 
33 BEAT. 
 
 the texture of our clothing is a phenomenon of the same kind; 
 and thicker clothing acts chiefly by diminishing the passage. 
 Hence, an oiled silk or other air tight covering laid on a bed, has 
 greater influence in preserving warmth than an additional blan- 
 ket or more. From the part of bed-clothes immediately over 
 the person there is a constant outward oozing of warm air, 
 and there is an oozing inward of cold air in lower situations 
 around. In many persons the circulation of the blood is so 
 feeble that in winter, they have great difficulty in keeping their 
 feet warm, even in bed, unless with the assistance of a bottle 
 of hot water or some such means, and in consequence they of- 
 ten pass sleepless nights, and suffer in their general health. At 
 the suggestion of the author, in such cases, a long flexible tube 
 has been used, as of spiral wire covered with leather or oiled 
 silk, by which persons can send down to their lower extremi- 
 ties their hot breath, and thus supply to them effectually a na- 
 tural animal warmth, as in a cold day they warm their hands 
 by blowing upon them through their gloves. 
 
 The power of fluids to diffuse heat being due thus to their 
 power of carrying, and not of conducting it, the consequence 
 should follow, that any circumstance which impedes the inter- 
 nal motion of the fluid particles, should diminish the diffusing 
 power. Accordingly, we find that fluids in general transfer 
 heat less readily in proportion as they are more viscid. Wa- 
 ter, for instance, transfers less quickly than spirits; oil than 
 water; molasses or syrup than oil: and water thickened by 
 starch dissolved in it, or which has its internal motion im- 
 peded by feathers or thread immersed in it, less quickly than 
 where it is pure and at liberty. Cooling being merely a mo- 
 tion, the reverse of heating, it is influenced by the same law. 
 Hence, the reason why soups, pies, puddings, and all semifluid 
 masses, retain their heat so long, so much longer than equal bulks 
 of mere fluid. The same law affords explanation of the facts, 
 that very porous masses and powders, as charcoal, metal filings, 
 sawdust, sand, &c. conduct heat more slowly than denser masses; 
 their interstices being filled with air, which scarcely conducts 
 heat, and which, by the structure of the substance, has no free- 
 dom of motion or circulation by which it might carry tho 
 heat 
 
SPREADING BT RADIATION. 33 
 
 " Heat spreads also, partly, by being shot or radiated like 
 light, from one body to another, through transparent me- 
 dia or space, with readiness affected by the material and 
 the state of the giving and receiving surfaces." (Read 
 the Analysis, page 13.) 
 
 If a heated ball of metal be suspended in the air, a hand 
 brought in any direction near to it will experience the sensa- 
 tion of heat: and beneath it the sensation will be as strong as 
 on the side, although the heat has to shoot down through an 
 opposing current of air approaching the heated ball, to rise 
 from it, as explained in a preceding section. A delicate ther- 
 mometer substituted for the hand will equally detect the spread- 
 ing heat, and if held at different distances, will prove it to di- 
 minish in the same ratio as light diminishes in spreading from 
 any luminous centre, viz. to be only a fourth part as intense 
 at a double distance, and in a corresponding proportion for 
 other distances. If the heated body be enclosed in a vacuum, 
 a thermometer placed near it will still be affected in the same 
 manner. If a screen be interposed between the body and the 
 thermometer, the latter will not be affected at all, proving the 
 heat to spread in straight lines. Heat when diffusing itself in 
 this way, to distinguish it from heat passing by contact or com- 
 munication, as described in the last section, is called radiant 
 heat or caloric; that is to say, spreading in rays all round its 
 source as light spreads. 
 
 Radiant heat resembles light, yet, in other respects. It as 
 rapidly permeates certain transparent substances, and its course 
 suffers in them a degree of the bending, termed refraction by 
 opticians. It is reflected from many kinds of polished surfaces, 
 just as light is reflected from a common mirror; and many such 
 surfaces directed to one centre (as when Archimedes made the 
 sun his assistant to burn the Roman ships) or a single concave 
 surface, having its own centre or focus, will concentrate heat 
 just as it does light. Its motion in the sun-beam is so rapid as, 
 for any distance at which men can try the experiment, to ap- 
 pear instantaneous; and the rays of heat from hot iron or burn- 
 ing charcoal, concentrated at great distances by suitable mirrors, 
 a thermometer as quickly a the heat of the setting sun 
 5 
 
BEAT. 
 
 reflected from a distant window. Although light and heat ars 
 united in the sun's ray, they are still separable by our glass 
 prisms or lenses; and the focus of heat behind a burning-glass 
 is not precisely the focus of light. Heat in radiating through 
 air does not warm it, and is not affected by winds or any 
 other motion of the air. These resemblances in the phenomena 
 of light and heat have by some inquirers been held to prove 
 that the two classes of appearances are only different modifica- 
 tions of action in the same subtle substance or ether. 
 
 The diffusion of heat by radiation, as it takes place in an in- 
 stant to any distance, and begins whenever there is any inequa- 
 lity of temperature between bodies exposed to each other, would 
 produce instant balance of temperature throughout nature, but 
 that heat leaves and enters bodies with readiness depending on 
 their internal conducting powers, and on the condition of their 
 surfaces. A black stone-ware tea-pot, for instance, will radiate 
 away 100 degrees of its heat in the same time that a pot of po- 
 lished metal will radiate 12 degrees. 
 
 Professor Leslie was the first to see the importance of inves- 
 tigating this subject, and he had the merit of contriving well- 
 adapted means, and of detecting many of the important facts. 
 As common thermometers are not sufficiently delicate to deter- 
 mine very sudden changes of temperature, where the influence 
 is so slight as in many cases of radiant heat, he used the beau- 
 tiful differential thermometer, contrived by himself, in con- 
 junction with concave mirrors, to concentrate the heat and ac- 
 cumulate its energy. Then taking as his heated body a cubical 
 tin vessel filled with boiling water, and covering it successively 
 with plates or layers of different substances, and with different 
 colours; and exposing the thermometer to it under all the 
 changes, he noted the number of degrees which the thermome- 
 ter rose as seen in the table which here follows, and thus as- 
 certained the radiating power of each sort of covering. 
 
 Lamp black - - 100 
 
 Writing paper - 98 
 
 Crown glass - - 90 
 
 Ice - 87 
 
 Isinglass 75 
 
SPREADING BY RADIATION. 35 
 
 Tarnished lead - 45 
 
 Clean lead . - - 19 
 
 Iron polished - 15 
 
 Tin plate - 12 
 
 Gold, silver, and copper - - 12 
 
 He next reversed the experiments by using his hot water ves- 
 sel always in the same state, and covering the thermometer bulb 
 with the different substances and colours, and thus he ascer- 
 tained that their comparative absorbing powers were very near- 
 ly proportioned to their radiating powers: lamp black, for in- 
 stance, absorbed or was heated 100, while the polished metals 
 absorbed or were heated only 12, and so for the others. And, 
 lastly, the absorbing powers being likewise an indication of the 
 opposite or reflecting powers, (for if a body absorb any given 
 proportion of the heat which falls on it, it must reflect the re- 
 mainder,) he had at the same time ascertained the reflective or 
 mirror powers of the bodies, and therefore all the important 
 points respecting radiant heat in its relation to the substances 
 between which it passes. 
 
 It seems paradoxical that, by putting a clothing of a thin cot- 
 ton or woollen fabric upon the polished tin vessel, the heat 
 should be received by it or dissipated from it much sooner than 
 if the vessel were naked, but such is the fact. And metal with 
 a scratched or roughened surface radiates or receives much more 
 rapidly than if polished. 
 
 The property of absorbing heat depends much upon the co- 
 lour of the substance, and as a general rule the dark colours, 
 viz. those which absorb most light, absorb also most heat.- Dr. 
 Franklin laid pieces of cloth of different colours on snow, and 
 during a given period in which the sun was shining on them, 
 he noted this in the different depths to which, by melting the 
 snow which was under them, they sunk. Hence, appears the 
 importance of having a white dress in summer, that by it, with 
 the sun's light, the heat also may be repelled; and a white dress 
 in winter is good, because it radiates little. Polar animals have 
 generally white furs. White horses are both less heated in the 
 sun, and less chilled in winter, than those of darker hues. 
 
 The rate of cooling in bodies must be influenced by all the 
 
36 
 
 HEAT. 
 
 particulars noted above, viz. substance, surface, colour, and by 
 the excess of heat in the cooling body as compared with those 
 around it. 
 
 The concentrating apparatus used for experiments on the ra- 
 diation of heat consists of two concave tin mirrors, here repre- 
 sented at a and b, so formed and placed in relation to each other, 
 that all the rays of light or heat issuing from the focus of one ; 
 
 C - 
 
 as at c, shall be collected into the focus of the other, d. A 
 stand under one focus, e, is intended to support the body giving 
 out or receiving heat, and a stand under the other, d, is meant 
 to support the thermometer. For farther explanation of the ac- 
 tion of such mirrors, we may refer to what was said of the con- 
 centration of sound in the section on Acoustics, or to what fol- 
 lows in the section on Optics, on the concentration of light. 
 The general rationale of such facts is, that heat, light, sound, an 
 elastic ball, &e. reflected from any point of a surface, returns, 
 if it fall perpendicularly to that point, in the same line by which 
 it approached; but if it fall obliquely, or one side of the perpen- 
 dicular, it returns in a line deviating as much on the other side. 
 Now the surfaces of concave mirrors are so formed, that every 
 ray issuing from the focus shall, when reflected, become paral- 
 lel to every other ray as represented by the dotted lines in the 
 figure; and it is the property of a similar mirror receiving pa- 
 rallel rays to make them all meet in its focus: thus any influ- 
 ence radiating from c, will be again collected at d. The pur- 
 pose and effect of such mirrors in experiments on heat, is mere- 
 ly to concentrate feeble influences, so that they may be more 
 accurately estimated. To show their effect and mode of action, 
 they may be placed exactly facing each other at any convenient 
 distance, and then a hot body of any kind, as a ball of metal or 
 a canister of boiling water, being left in one focus while a ther- 
 
RADIATION OF COLT), 07 
 
 mometer stands in the other, the thermometer will instantly 
 rise; although, if left in any intermediate situation nearer to the 
 hot body, and therefore not in the focus, it will not be affected. 
 If burning charcoal be placed in one focus and a readily com- 
 bustible substance in the other, the latter will be set fire to at 
 the distance of thirty feet or more. 
 
 If in one focus of the mirror apparatus described above, there 
 be placed, instead of the canister of hot water, a piece of ice, 
 the thermometer in the other focus immediately falls. This 
 has been called the radiation of cold, and persons were at one 
 time disposed to think that it proved cold to have a positive ex- 
 istence distinct from heat. The case, however, is merely that 
 the thermometer happens then to be the hotter body in one fo- 
 cus of the mirrors, placed in relation with a colder body, the 
 ice, in the other, and consequently by the law of equable diffu- 
 sion, it must share its heat with the ice, and will fall. The mir- 
 rors in any case have merely the effect, by preventing the spread- 
 ing and dissipation of the radiant heat from either focus, except 
 towards the other, of making two distant bodies act upon each 
 other as if they were very near. All the heat that seeks to ra- 
 diate from the thermometer, d, in the direction of the large sur- 
 face of the mirror b, if not met by an equal tension of force of 
 temperature in the other mirror or focus to which they are di- 
 rected at a and c, will radiate away to c, and become deficient 
 at d. Some inquirers have believed that heat was constantly 
 radiating in exchange from substance to substance (as light ra- 
 diates constantly between opposed bodies,) only more copiously 
 from one side, if the temperature of that was higher: others 
 have held that the movement only took place when the balance 
 of temperature was destroyed; and this is the simplest view. 
 
 There is a remarkable difference in one respect between the 
 heat of the sun and tjiat radiated from any other source, riz. 
 that the first passes through air, glass, water, and transparent 
 bodies generally, very readily; while the latter, although not 
 obstructed by air, is almost totally intercepted or absorbed in 
 passing through any of the other substances named. In our 
 drawing-rooms it is common to have ornamented glass fire- 
 screens, which, while they allow the light to pass, defend the 
 
S3 HEAT. 
 
 face from the heat: but all persons know that the heat of the 
 sun beams, as well as their light, enters our green houses 
 through the glass which covers them. A glass screen inter- 
 posed between the concave mirrors in the apparatus above de- 
 scribed, destroys almost entirely the effect of the heated body 
 in one focus, on the thermometer in the other, and the trifling 
 effect really produced has appeared, to some to be owing to the 
 heat first absorbed by the screen on one of its sides, and then 
 radiated from the other. This conclusion seemed to be sup- 
 ported by the fact that screens of metal or of glass, covered 
 with lamp black, paper, &c., allow transmission nearly in pro- 
 portion to their several absorptive and radiant powers. More 
 careful experiments, however, have been held to prove that a 
 small portion of the heat is suddenly radiated through the glass. 
 A glass mirror reflects the light of a fire, but at first retains all 
 the heat, and only radiates it afterwards as a hot body. 
 
 The doctrines of radiant heat make us aware of the importance 
 of having vessels of polished metal for containing liquids or any 
 thing which we desire to keep warm; hence tea and coffee pots, 
 dishes for soup, &c. should be polished. As a black earthen tea- 
 pot loses heat by radiation nearly in proportion to the number 
 100, while one of silver or other polished metal loses only as 
 12, there will be a corresponding difference in their aptitude for 
 extracting the virtues of any substance infused in them. Pipes 
 for the conveyance of steam or hot air, if left naked, should be 
 of polished metal; but after arriving at the place where they 
 have to give out their heat, their surface should be blackened 
 and rough. A coat of polished mail is not a cold covering. A 
 mirror intended to reflect heat should be of highly polished me- 
 tal: and such is the interior of a screen placed behind roasting 
 meat. A fireman's mask is usually covered with tin foil. It 
 is of advantage that the bottom of a tea kettle or other cooking 
 vessel be black, because the bottom has to absorb heat, but the 
 top should be polished because it has to confine. 
 
 The interesting phenomenon of dew was not at all under- 
 stood until lately, since the laws of radiant heat have been in- 
 vestigated. At sun-rise in particular states of the sky, every 
 blade of grass and leaflet is found, not wetted as if by a shower^ 
 
DEW, G# 
 
 but studded with a row of distinct globules most transparent 
 and beautiful, bending it down by their weight, and falling like 
 pearls when the blade is shaken. These are formed in the 
 course of the night by a gradual deposition on bodies rendered 
 by radiation colder than the air around them, of the moisture 
 which rises invisibly from water surfaces into the air during the 
 heat of the day. In a clear night the objects on the surface of 
 the earth radiate heat upwards through the air which impedes 
 not, while there is nothing nearer than the stars to return the 
 radiation; they consequently soon become colder, and if the air 
 around has its usual load of moisture, part of this will be depo- 
 sited on them, exactly as the invisible moisture in the air of a 
 room is deposited on a cold bottle of wine when first brought 
 from the cellar. Air itself seems not to lose heat by radiation. 
 A thermometer placed upon the earth any time after sun-set until 
 sun-rise next morning, generally stands considerably lower than 
 another suspended in the air a few feet above it; owing to the 
 radiation of heat upwards from the earth, while the air remains 
 nearly in the same state. During the day, while the sun shines, 
 the earth is much warmer than the air. The reason why the 
 dew falls, or forms so much more copiously upon the soft spon- 
 gy surface of leaves and flowers, where it is wanted, than on 
 the hard surface of stones and sand, where it would be of no 
 use, is the difference of their radiating powers. There is no 
 state of the atmosphere in which artificial dew may not be made 
 to form on a body, by sufficiently cooling it, and the degree of 
 heat at which it begins to appear is called the dew point, and is 
 an important particular in the meteorological report of the day. 
 In cloudy nights heat is radiated back from the clouds, and the 
 earth below not being so much cooled, the dew is scanty or de- 
 ficient. On the contrary, when uninformed persons would least 
 expect the dew, viz. in warm very clear nights, and perhaps 
 when the beautiful moon invites to walking, and music adds its 
 charm, as in some of the evenings of autumn with the harvest 
 moon and harvest occupations then is the dew more abundant, 
 and the danger greater to delicate persons of taking harm by 
 walking among the grass. 
 
40 HEAT. 
 
 " Heat by entering bodies expands them, and through a 
 range which includes as three successive stages the forms 
 of solid, liquid, and air, or gas; becoming thus, in nature^ 
 the grand antagonist and modifier of the effects of that 
 attraction which holds corporeal atoms together, and 
 which, if acting alone, ivould reduce the whole material 
 universe to one solid lifeless mass. (Read the Analysis, 
 page 13.) 
 
 If an experimenter take a body which is as free from heat as 
 man can procure a body a bar of solid mercury, for instance, 
 as it exists in a polar winter; and if he then gradually heat such 
 body, to whatever extent, it will acquire an increase of bulk 
 with every increase of temperature: first, there will be simple 
 enlargement or expansion in every direction; then the mass will 
 in addition be softened; then it will be melted or fused, that is 
 to say, in the case supposed, the solid bar will be reduced to 
 the state of liquid mercury, with the cohesive attraction of the 
 atoms nearly overcome; if the mass be still farther heated, the 
 atoms will be repelled from each other to much greater dis- 
 tances, constituting then a very elastic fluid called an air or gas, 
 many hundred times more bulky than the same matter in the 
 solid or liquid state, and capable of forcibly distending an ap- 
 propriate vessel as common air distends a bladder; susceptible, 
 moreover, of dilating indefinitely farther, by farther additions of 
 heat, or by diminution of the atmospheric pressure, against 
 which it had to rise during its formation. A subsequent remo- 
 val of the heat will cause a corresponding progress of contrac- 
 tion, and the various conditions or forms of the substance above 
 enumerated, will be reproduced in a reverse order, until the so- 
 lid mass again appear. 
 
 What is thus true of mercury is proved by modern chemical 
 art to be true also of all the ponderable elements of our globe, 
 and of many of the combinations of these elements, as water, 
 for instance, familiarly known in its three forms of ice, water, 
 and steam; although compound substances generally, by great 
 changes of temperature are decomposed into their elements. 
 
 A student might at 'first have difficulty in believing that the 
 beautiful variety of solid, liquid, and air found among natural 
 
EXPANSION AND CHANGE OP FORM. 41 
 
 bodies, could depend upon the quantities of heat in them, be- 
 cause these forms are all seen existing at the same temperature, 
 but he afterwards learns that each substance has its peculiar re- 
 lation or affinity to heat, and that hence, while at the medium 
 temperature of the earth, some bodies contain so little as to be 
 solids like the metals, stones, earths, &c. ; others have enough 
 to be liquids as mercury, water, oils, &c. ; and others enough 
 to be airs as oxygen, nitrogen, hydrogen, &c. Men, until 
 better informed, are prone to deem the states in which bodies 
 are most frequently observed, to be the natural states of such 
 bodies; and the Indian king but reasoned in a usual way, who 
 held the Dutch navigators, newly arrived on his shores, to be 
 gross imposters, when they said that in their country, at one 
 time of the year, water became so hard that they could walk 
 upon it, and drive their carriages upon it, and shape it into 
 solid blocks. All persons err like this king, who in thinking 
 of the different substances known to them, regard their acci- 
 dental state as to the cohesion of particles which state is 
 really dependent on the temperature of the bodies, and there- 
 fore on the particular planet or situation on the planet where 
 they are found, to be in them an essential natural character. 
 As well might a person who had never seen silk, but as a de- 
 licate gauze or satin enveloping some lovely human form, re- 
 fuse to recognise it in the unsightly coil of the worm which 
 produces it. 
 
 The degrees in a general scale of temperature at which the 
 most important substances in nature change their states from 
 solid to liquid, or from liquid to air, will be noted in a future 
 page. Here we have only to remark, that the differences are 
 very great. Mercury melts at about 80 below the melting 
 point of ice, and porcelain at about 30,000 above. There are 
 some substances which require so high a temperature for their 
 fusion or for their conversion into gas, that human art has dif- 
 ficulty, or even finds it impossible, to produce the changes by 
 simple concentration of heat; but all such are readily soluble 
 in some other substance, possessing already the form of liquid 
 or air: as when gold and platinum are dissolved in nitro-muri- 
 atic acid flint in the fluoric acid, carbon in hydcogen gas. 
 
 6 
 
42 HEAT. 
 
 Now many persons may not have reflected that the dissolving 
 a solid in any fluid menstruum is merely another mode of melt- 
 ing it by heat; yet this is the truth, for the menstruum is itself 
 fluid, only because of the much heat which it contains, and in 
 dissolving the more obdurate substance, it does so merely be- 
 cause its attraction for the substance brings the particles into 
 union with the heat which already exists in itself. Heat then 
 is the only and universal solvent. Its influence is interestingly 
 seen in the fact, that a fluid when heated can dissolve much 
 more of a solid than when cold. Water while hot keeps dis- 
 solved twice as much of many salts as it can when its temper- 
 ature has fallen. -^-There are again in nature many substances 
 having such an affinity for heat, that until lately they have 
 only been known as airs; and even in the present advanced 
 state of art, they cannot by any degree of mere cooling be re- 
 duced to the liquid or solid form; yet all such, when pressure is 
 added to the cooling, OB when the chemical attraction for them 
 of some other substances which already exist in the liquid or 
 solid state, is made to co-operate, may be reduced. An in- 
 stance is afforded by oxygen, when made part of a liquid acid, 
 or of a solid ore. 
 
 Of solids, some on receiving heat become soft before they 
 are liquified, as pitch, glue, iron, &c. ; others change com- 
 pletely at once, as ice in becoming water; and some pass at 
 once to the state of air, without therefore having assumed at 
 all the intermediate state of liquid -they are sublimed, as it 
 is called, and on cooling again may be caught in a powdery 
 state, as is seen in that form of sulphur, or of benzoin, termed 
 the flower of the substance. Of the latter class also are cam- 
 phor, arsenic, corrosive sublimate, and the substance called 
 iodine, which last, from the state of rich ruby crystals, on be- 
 ing heated becomes at once- a dense transparent gas of the same 
 hue, and in cooling resumes its crystalline form. 
 
 The reader having arrived at this place, may peruse again 
 with advantage five pages of vol. i. (between pages 57 and 64 
 in the different editions) which treat of the influence of heat 
 9 n, tlie constitution of 
 
CAPACITY FOR HEAT. 43 
 
 " Each particular substance, according to the nature, prox- 
 imity, fyc. of its ultimate particles, takes a certain quan- 
 tity of heat (said to mark its CAPACITY) to produce in it a 
 given change of temperature or calorific tension." (Read 
 the Analysis, page 13.) 
 
 A pound of water, for instance, to raise its temperature one 
 degree, takes thirty times as much heat as a pound of mercu- 
 ry. This may be proved in various ways. First, if the heat 
 be derived from any uniform source, the water must remain 
 exposed to it thirty times as long as the mercury. Second, if 
 both substances, after being equally heated, be placed in ice 
 until cooled to the freezing point, the heat which escapes again 
 from the water will melt thirty times as much ice as that which 
 escapes from the mercury. Third, when a pound of hot wa- 
 ter is mixed with a pound of cold mercury, instead off the two 
 becoming of a middle temperature, as is the case when equal 
 quantities of hot and cold water are mixed every degree of 
 heat lost by the one becoming just a degree gained by the 
 other the pound of hot water, by giving up one" degree to the 
 pound of cold mercury, raises the temperature o'f the latter 
 thirty degrees; and in the same proportion for other differ- 
 ences: or, on reversing the experiment; a pound of hot mer- 
 cury will be cooled thirty degrees by warming a pound of wa- 
 ter one degree. 
 
 Now each particular substance in nature, just as water or 
 mercury, has its peculiar capacity for heat; and experiments- 
 made by the modes of mixture and of melting ic'e above de- 
 scribed have led to the construction of tables which exhibit the 
 relations. The following short table is an abstract of these, 
 showing the comparative capacities of equal weights of some 
 common substances. Water, for reasons of convenience, has 
 been chosen as the standard of comparison. It appears, then, 
 that a pound of hydrogen gas takes about twenty times more 
 heat to produce in it a given change of temperature than a. 
 pound of water, while a pound of gold takes about twenty 
 times less, and therefore four hundred times less than the hy- 
 drogen. The figures in the table, by marking the comparativ6 
 capacities for heat of various substances, necessarily indicate 
 
44 HEAT. 
 
 also the comparative quantities of ice which would be melted 
 by equal weights of the substances in cooling through an equal 
 number of degrees. A pound of water, the standard, must 
 cool 140 degrees, that is, must give up 140 degrees of its heat 
 to melt one pound of ice. 
 
 Gases. 
 
 Hydrogen - 21$ 
 
 Atmospheric air 1$ 
 
 Carbonic acid gas - - IT 
 
 Common steam - - - - 1$ 
 
 Liquids. 
 Solution of carbonate of ammonia - 2 
 
 ^ To 
 
 Alcohol 1 l 
 
 Water - 
 Milk 
 Olive oil 
 Linseed oil 
 Sulphuric acid 
 Quicksilver 
 
 Solids. 
 
 Ice T 9 ff 
 
 Wheat ..... 
 
 Charcoal . $ 
 
 Chalk - * 
 
 Glass i- 
 
 Iron - 
 
 Zinc T V 
 
 Gold . - - 3 V 
 
 We may remark here that some late researches, by an- 
 other mode of trial, make the capacity of air to be only a 
 quarter that of water, although in the preceding table it ap- 
 pears to be one and three-quarters. Now as the other aeriform 
 fluids have been compared with water through the medium of 
 atmospheric air, if there be an error with respect to this, it 
 
CAPACITY FOR HEAT. 45 
 
 must run through all the figures noting the capacity of other 
 aeriform substances. 
 
 If we seek a reason or reasons why there should be among 
 bodies the differences of capacity here stated, the circumstances 
 chiefly calling attention are the following. First, equal weights 
 of the various substances have very different bulks or volumes, 
 and therefore have different room in which the heat may lodge. 
 Mercury, for instance, is only one-fourteenth part as bulky as 
 water. That the bulk, however, is not the only influencing 
 circumstance appears in the fact, that mercury while having 
 one-fourteenth of the bulk of water, has only one-thirtieth of 
 the capacity. Second, in equal bulks of different substances, 
 the space may be more completely occupied by the particles of 
 one than of another as by the particles of mercury than by 
 those of water. But as the facts are not fully accounted for, 
 even by both of these circumstances, we must seek explana- 
 tion in a third, viz. a difference in the ultimate particles of bo- 
 dies affecting their relations to heat. 
 
 First, The influence of bulk or volume, in determining the 
 capacity for heat, is proved by the facts stated in the preceding 
 table, and by many others. In the table, for instance, it is 
 seen that hydrogen and the gases generally, with their great 
 comparative bulk, have also great capacity; that liquids have 
 less capacity than gases; that solids have less than liquids but 
 the capacity, as already stated, is not in strict proportion to 
 bulk; for hydrogen, which is many thousand times more bulky 
 than an equal weight of water, has only twenty-one times the 
 capacity. Again, if any body whatever be suddenly com- 
 pressed into less bulk, heat will issue from it as if squeezed 
 out. Thus iron or other metal suddenly condensed by the 
 heavy blow of a hammer, is thereby rendered hotter, and the 
 expelled heat will gradually spread from it. Because water 
 and spirit, on being mixed, occupy less space than when sepa- 
 rate, there is from the mixture a corresponding discharge 
 of heat. But the truth is most remarkably exemplified in airs 
 or gases, owing to their great range of elasticity. They may 
 be condensed or dilated a hundred fold or more, and there will 
 be a simultaneous concentration or diffusion of their heat, that 
 
46 HEAT. 
 
 is to say, the production, in the space occupied by them, of in- 
 tense heat or cold. The heat of air just condensed, or the cold 
 of that which has just expanded, is much greater than even the 
 most delicate thermometer can indicate, for there is so little heat 
 altogether even in a considerable volume'of air, that the mass 
 of a mercurial thermometer, although absorbing a great part of 
 it, would be little affected. The extent, however, of th e 
 change of temperature is seen in the facts, that by the sudden 
 condensation of air we may inflame tinder immersed in it, and 
 by allowing air suddenly to expand, we may .convert any wa- 
 tery vapour diffused through it into ice or -snow. Nay, air, 
 containing carbon in perfect solution, as is true of the common 
 coal gas, if first condensed to expel heat, and then allowed sud- 
 denly to expand, will be so cooled -that the carbon will be se- 
 parated like a black cloud, as snow is separated in the case be- 
 fore described. The cold which separates or freezes carbon 
 from a gas holding it in solution, is perhaps the most intense 
 which art can produce. It might be expected that air suddenly 
 compressed into half its previous volume, should become just 
 twice as hot as before, or if suddenly dilated to double volume, 
 should be only half as hot, thus enabling us to ascertain the 
 whole quantity of heat contained in it; but the facts are not 
 so; the temperature changes, near the middle degrees of the 
 scale at least, much less than the density. Air in doubling its 
 volume from a common density, becomes colder only by about 
 50 of Fahrenheit's thermometer. 
 
 The different capacity for heat of air in different states of di- 
 latation, produces effects of great importance in nature as well 
 as in the arts thus, 
 
 On the surface of the earth, near the sea-shore, the air of the 
 atmosphere has a certain density (a cubic foot weighs about one 
 ounce and a quarter) dependent on the weight and pressure of 
 the superincumbent mass; but on a mountain top 15,000 feet 
 high, as half the mass of the atmosphere is below that level, 
 (see " Pneumatics,") the air is bearing but half the pressure 
 and, consequently, has twice the volume of an equal quantity 
 of air at the sea-side, and a temperature, consequently, many 
 degrees inferior; and the air which is at any time on the moun- 
 
CAPACITY FOR HEAT. 47 
 
 tain-top, may have been recently before on an adjoining plain 
 or shore, and in gradually climbing the mountain-side, as a 
 wind, it must have been gradually expanding and cooling in 
 proportion to the diminishing pressure. It is found that air, 
 at first rising from the sea-shore, becomes one degree colder 
 for about 200 feet of perpendicular ascent, and altogether about 
 50 colder in rising 15,000 feet; so that at this latter elevation, 
 water is frozen even near the equator, where the temperature 
 of low plains is at least 30. It thus appears, that if a man 
 could travel with the wind so as to remain always surrounded 
 by the same air, he might begin his journey with it from the 
 summer vineyards of the Rhine, might soon after find it the 
 piercing blast of the Alpine summits; and again, a little after, 
 without any change having occurred in the absolute quantity 
 of its heat, might feel it as the warm breath of the flowers on 
 the plains of Italy. 
 
 The explanation is here ready, of why very elevated moun- 
 tains in all parts of the earth are hooded in perpetual snows. 
 We have just said, that even at the equator, where the average 
 temperature near the sea is 84, water will be frozen when car- 
 ried, to an elevation of 15,000 feet. A line, therefore, traced 
 on a mountain at this level, would divide the portion of it des- 
 tined to sleep under lasting ice and snow from the portion be- 
 low covered with green herbage. This line, wherever found, 
 is called the snow /me, or line of perpetual congelation. At 
 the equator it is high in the atmosphere, because there is a dif- 
 ference of about 50 between the average temperature of the 
 country and the freezing point of water, viz. the difference be- 
 tween 84 and 32, and an elevation of 15,000 feet corresponds 
 to this difference; but in a progress towards the poles, the line 
 is met with gradually nearer to the earth, as the difference in 
 question is less. In Switzerland it is at 6,500 feet above the 
 sea; in Norway, it is below 5,000. With respect to the line 
 of congelation, it is farther to be remarked, that in tropical 
 countries, because the temperature of the air is nearly uniform 
 during the whole year, the line or limit of frost and snow is 
 distinct and unvarying, that is to say, is narrow, particularly 
 where the acclivity is considerable; but, in countries to the 
 
48 HEAT. 
 
 north and south, which experience strong contrast of summer 
 and winter, the line becomes broad and less evident; because 
 in the hot season much snow is melted, or half melted, above 
 what may be called an average line, while in winter much snow 
 and ice are accumulated below this, to be melted again when 
 summer returns. 
 
 In the breadth of the line of congelation for changeable cli- 
 mates, we have the reason of the formation of what are called 
 glaciers around snow-capped mountains situated in such cli- 
 mates, and around such only. The snow near the upper part of 
 the broad line having been only softened or half thawed in the 
 preceding summer, becomes in winter, almost as solid as ice, 
 and in the succeeding summer vast masses of this, detached by 
 the action of the sun, and of the central heat of the earth 
 spreading outwards, and loaded with more recent accumulation 
 of snow, are constantly falling down into the valleys beneath; 
 where, being accumulated, and the crevices filled up with snow 
 or with water, which hardens to ice, they form at last the huge 
 glaciers or seas of ice mers de glace, which render certain 
 regions so remarkable. The falling of such masses (called in 
 Switzerland avalanches,} is what renders the ascent to snow- 
 clad mountains terrific and dangerous. Around Mont Blanc, 
 in the awful solitudes of the elevated valleys, the avalanches 
 are thundering down almost without interruption during the 
 whole summer in which season only the attempt to ascend 
 the mountain can be made, and a pistol-shot, or any consider- 
 able agitation of the air, may suffice to set loose masses that 
 will sweep away a whole convoy. Beneath glaciers there is 
 always going on a melting of that part of the ice which is in 
 contact with the earth, and hence a stream of water constantly 
 issues from the bed of every glacier. These streams, in Swit- 
 zerland, are the beginnings of the magnificent rivers, the Rhine 
 and Rhone. Like the avalanches breaking loose in summer 
 among the mountains, there are in the polar seas vast masses 
 of ice detached from the shores, and which afterwards move 
 into warmer seas to be melted. These often become, to the 
 arctic bear, rafts, on which, to his surprise, he finds himself 
 voyaging into new latitudes, to be left at last adrift in the wide 
 ocean when his ship has vanished from beneath him. 
 
CAPACITY OP BODIES. 49 
 
 Although the proofs are not so immediately apparent, the 
 line of congelation exists as truly every where in the open 
 sky, over sea and plains, as where there are mountain heights 
 to wear its livery; and considerably below the line, the cold, 
 aided by electrical agency, is sufficient to produce, in the form 
 of mist or clouds, a deposition from the air of the watery va- 
 pour contained in it. There is thus in nature an admirable 
 provision to shade the earth at proper times from the too pow- 
 erful rays of the sun, or to supply rain as wanted, without the 
 transparency of the inferior regions of the atmosphere being 
 much affected. As the watery vapour, rising from sea or 
 lake, and invisibly diffused in the atmosphere, can only reach 
 to the height where the cold is great enough to condense it, 
 the clouds may in general be regarded as the top of that at- 
 mosphere of watery vapour, or aeriform water, which is al- 
 ways mixed more or less with the atmosphere of mere air; and 
 as the quantity of watery vapour which can exist invisibly in 
 a given space, depends altogether on the intensity of heat pre- 
 sent, the clouds in a humid atmosphere will be low, and in a 
 dry atmosphere will be high, or there may be none. An aero- 
 naut mounting in his balloon through a clear sky, often ap- 
 proaches a dense cloudy stratum to plunge into it, and for a 
 time to be surrounded with gloom almost of night; the face of 
 earth being hidden from him below, while the heavenly bodies 
 are equally veiled from him above; but rising still higher, he 
 again emerges to brightness, and looks down upon the fleecy 
 ocean rolled in mountain heaps beneath him, as the climber to 
 a lofty peak may look down from the ever pure atmosphere 
 around it on the inferior region of clouds and storms. 
 
 The diminished temperature of air in the higher regions of 
 the atmosphere, often enables the natives of temperate climates, 
 when led by circumstances to reside in tropical countries, 
 where their health may suffer from the heat, to find near at 
 hand, on some mountain height, the congenial temperature 
 of their early homes. The author once, during a visit to the 
 recently inhabited island of Penang, in the strait of Malacca, 
 examined this fact with pleasure not readily forgotten. The 
 centre of the island is occupied by a lofty mountain ridge thick- 
 
 7 
 
50 HEAT. 
 
 ly wooded, on the northern summit of which a few residences, 
 visible from the sea-shore like eagles' nests on a cliff, had just 
 been constructed. Towards these, one morning at sun-rise, on 
 an active little horse of the country, and along a tolerable road, 
 he began to climb from the hot plain below. At first there 
 were around him purely tropical objects, inspiring tropical feel- 
 ings, the latter, modified, indeed, by the reflection that his 
 track lay through a forest, into which until lately the foot of 
 man had never ventured, and where the trees, nursed through 
 ages to their greatest growth, and the stupendous precipices, 
 and the sublime water-fall, &c. had so recently been exposed 
 to human observation; but, as he gradually ascended, the cha- 
 racter of the vegetation was perceived to be changing, and the 
 air was becoming so light and cool as irresistibly to awaken in 
 him thoughts of distant England nay, almost the illusion of 
 his being there. At last, however, the summit being reached, 
 where a clear space opened to view the whole country around, 
 the attention was quickly recalled to the fervid land of the sun. 
 The elevation is so great that at first the eye takes account of 
 only the grander features of the scene, and such nearly as 
 might be met with on a Grecian or Italian shore:- the expanse 
 of sunny water in that beautiful strait, stretching so far north 
 and south, the opposite continental shore with its river wind- 
 ing seaward across the plain, the town and the roadstead near 
 it crowded with ships, which appeared only as specks in a 
 wide-spread map, &c. ; but, on closer inspection, and particu- 
 larly with the aid of the telescope, were descried the rich 
 groves of cocoa nut and banana, the plantations of spice and 
 cotton and sugar cane, the tawny labourers, the bamboo dweK 
 lings, the fanciful canoes or prows, in a word, every object be- 
 speaking the torrid zone. Such then is the scene, which even 
 under the equator, an invalid by climbing a hill may place un- 
 der his eye, and where the thermometer near him stands as in 
 an English month of May. 
 
 The interiors of the islands of Jamaica and Hayti have many 
 situations of great extent, which combine, as above described, 
 the advantages of tropical situation and temperate climate, and 
 English labouring colonists might well inhabit the former. 
 
CAPACITY OP BODIES. 51 
 
 The vast plain of Mexico, and much of the central land of 
 South America, is similarly circumstanced; and it is not un- 
 common, where the ascent to the gigantic Andes is gradual, 
 to find at the bottom of the ridge a town, whose markets are 
 stored only with the productions of the equator, while in a 
 town higher up will be seen only what belongs to the tempe- 
 rate skies of Europe; climates of the earth, naturally distant, 
 thus having met, as it were, in amicable vicinity on the same 
 rising plain. 
 
 Second. It might be anticipated that a dense body, or one 
 in which the constituent particles may be supposed to fill more 
 completely the space occupied by it than the particles do in a 
 rarer body, would have smaller capacity for heat, in proportion 
 to the smaller space left vacant in its mass: and in a general 
 comparison of the capacities of equal bulks of different sub- 
 stances, such anticipation is partly verified, as when a pint 
 of dense mercury is found to have only about half the capaci- 
 ty which a pint of lighter water has. The accordance, howe- 
 ver, is by no means universal, nor at all in proportion to the 
 differences of density. Water, which is denser than oil, and 
 according to the hypothesis should have less capacity, yet has 
 in the same volume nearly double the capacity; and mercury, 
 which being nearly fourteen times denser than water, might be 
 expected to have only a fourteenth of the capacity, has really, 
 for equal volumes, a half, or, as formerly stated, for equal 
 weights, a thirtieth. 
 
 Third. We are at last, therefore^ compelled to admit, that 
 the relation between various substances and heat, which we 
 call capacity for heat, depends much more on the nature of the 
 ultimate atoms of the substances than either on the absolute 
 bulk or comparative density of the masses. Throwing much 
 light on this subject, it has been ascertained in late times, that 
 all material substances are composed of extremely minute un- 
 changeable atoms, and of which, in different substances, the 
 comparative weights have been determined, although not the 
 absolute weights; that is to say, for instance, the atom of gold 
 is known to weigh four times as much as the atom of iron, al- 
 though we do not know ho w many thousands or millions of atoms 
 
52 HEAT. 
 
 are required to form a grain of either. Now, very recent re- 
 searches seem to prove that for each ultimate atom, no matter 
 of what substance, nearly the same quantity of heat is required 
 to produce in a mass of the atoms a given change of tempera- 
 ture. Thus an ounce of iron which has four times as many 
 atoms as an ounce of gold, has four times the capacity for heat. 
 The law seems to hold for all simple substances; and for com- 
 pounds of these, there seems to be another law not yet well 
 made out. 
 
 Instead of the term capacity for heat used in the preceding 
 pages, with respect to particular substances, that of specific 
 heat has* by some authors been preferred ; but as the latter gives 
 to a commencing student the idea rather of specific kinds of 
 heat than of specific quantities, the term capacity has been 
 here retained. 
 
 " Each substance in nature, for a given change of tempe- 
 rature, undergoes expansion in a degree proper to itself, 
 the expansion generally increasing more rapidly than the 
 temperature, as the cohesion of the particles becomes 
 weaker from increased distance, being remarkably great- 
 er, therefore, in liquids than in solids, and in airs than 
 in liquids; the rate being quickened, moreover, near the 
 points of change. (See the Analysis, page 13.) 
 
 The following table, containing the names of some common 
 substances, solid, liquid, and aeriform, shows, by the figures 
 following each name, how much the substance increases in 
 bulk, by having its temperature raised from that of freezing to 
 that of boiling water. A lump of glass, for instance, would 
 gain in the proportion of one cubic inch for every 416 cubic 
 inches contained in it; while a mass of water would gain one 
 inch or part in twenty-three; dilating thus for the same range 
 of temperature nine times more than the glass. 
 
 Solids. 
 
 Glass gains one part in - - 416 
 
 Deal - 416 
 
 Platinum 389 
 
EXPANSION OF BODIES. 53 
 
 Steel - ... 283 
 
 Cast iron - 273 
 
 Iron . 271 
 
 Gold . 221 
 
 Copper - - 194 
 
 Brass - - 177 
 
 Silver - - - 175 
 
 Tin - .170 
 
 Lead - - 117 
 
 Liquids. 
 
 Mercury gains one part in - 55 
 
 Water - - 23 
 
 Oil of turpentine - 14 
 
 Fixed oils - 12 
 
 Alcohol 9 
 
 Common air, i 
 All gases and > gain one part 
 vapours J 
 
 Airs. 
 
 in 
 
 We have to warn readers, here, not to confound the increase 
 hy heat of the general bulk of a solid body with the increase 
 of its length. The latter is only one-third as great as the for- 
 mer. This will be understood by considering that the increase 
 of bulk is divided between the breadth and depth (or thick- 
 ness) in common with the length. If the substance of a me- 
 tallic square rod or wire, for instance, be dilated by heat, the 
 hundredth part of its bulk, it does not gain all that hundredth 
 at its end, becoming perhaps 101 inches long instead of 100; 
 but every part becoming deeper and broader in the same pro- 
 portion as it becomes thicker, (we may suppose it divided 
 into a row of equal little cubes,) the rod gains in length 
 only the third part of an inch. A fluid enclosed in a tube un- 
 changeable by heat (if such tube there were) would show its 
 whole dilatation in an increase of length, because there could 
 be no swelling laterally, and its extremity would, therefore, 
 
54 HEAT. 
 
 have a triple extent of motion from any variation of tempera- 
 ture. A degree of this consequence is obtained in our com- 
 mon thermometers, because the containing glass, although di- 
 latable by heat, is so much less dilatable than the fluid within. 
 As regards solids, we have to inquire so much more frequent- 
 ly respecting the dilatation in length, breadth, &c.; that is to 
 say, the linear dilatation in some direction, than the increase 
 of general bulk, that tables are frequently made stating only 
 the linear dilatation. It may be found at once from the above 
 table, by recollecting that it is one-third of the increase of 
 bulk: thus, if glass, in passing from the freezing to boiling 
 heat of water, dilate one part in 416 of its bulk, it will dilate 
 only the third of a part in length, or a whole part in an extent 
 of three times 416 or 1,248. 
 
 The expansion by heat of solids has been ascertained by 
 bringing microscopic instruments to bear on rods of the vari- 
 ous substances heated to various degrees, in troughs of oil or 
 water. The expansion of fluids, again, is found by filling a 
 glass vessel with a known weight of any fluid, and then ascer- 
 taining how much is made to run over or escape by a given in- 
 crease of heat. This quantity, added to what is required to fill 
 the increased dimensions of the heated glass vessel, (which 
 from the ascertained expansion of glass is known,) forms the 
 whole of the increase. It might be ascertained, also, by put- 
 ting different liquids successively into the thermometer tube, 
 and marking their comparative dilatations from changes of tem- 
 perature examined by another thermometer. 
 
 The general and comparative expansion of solids by heat is ex- 
 emplified in the following cases: 
 
 A cannon ball, when heated, cannot be made to enter an 
 opening, through which, when cold, it passes readily. 
 
 A glass stopper sticking fast in the neck of a bottle often 
 may be released by surrounding the neck with a cloth taken 
 out of warm water or by immersing, the bottle in the water 
 up to the neck: the binding ring is thus heated and expanded 
 sooner than the stopper, and so becomes slack or loose upon it 
 
 Pipes for conveying hot water, steam, hot air, &c. ; if of con- 
 
EXPANSION OP SOLIDS. 55 
 
 siderable length, must have joinings that allow a degree of 
 shortening and lengthening, otherwise a change of temperature 
 may destroy them. An incompetent person undertook to warm 
 a large manufactory hy steam from one boiler. He laid a rigid 
 main pipe along a passage, and opened lateral branches through 
 holes into the several apartments, but on his first admitting the 
 steam, the expansion of the main pipe tore it away from all 
 its branches. 
 
 In an iron railing, a gate which during a cold day may be 
 loose, and easily shut or opened, in a warm day may stick, 
 owing to there being greater expansion of it and of the neigh- 
 bouring railing, than of the earth on which they are placed. 
 Thus, also, the centre of the arch of an iron bridge is higher 
 in warm than in cold w r eather; while, on the contrary, in a sus- 
 pension or chain bridge, the centre is lowered. 
 
 The iron pillars now so much used to support the front walls 
 of houses of which the ground stories serve as shops, with spa- 
 cious windows, in warm weather really lift up the wall which 
 rests upon them, and in cold weather allow it again to sink or 
 subside in a degree considerably greater than if the wall were 
 brick from top to bottom. 
 
 In some situations, (as lately was seen in the beautiful steeple 
 of Bow church, in London,) where the stones of a building are 
 held together by clamps or bars of iron, with their ends bent 
 into them, the expansion in summer of these clamps will force 
 the stones apart sufficiently for dust or sandy particles to lodge 
 between them: and then, on the return of winter, the stones 
 not being at liberty to close as before, will cause the ends of the 
 shortened clamps to be drawn out, and the effect increasing 
 with each revolving year, the structure will at last be loosened, 
 and may fall. 
 
 The pitch of a piano-forte or harp is lowered in a warm day 
 or in a warm room, owing to the expansions of the strings be- 
 ing greater than of the wooden frame-work; and in cold the 
 reverse will happen. A harp or piano, which is well tuned in 
 a morning drawing-room, cannot be perfectly in tune when the 
 crowded evening party has heated the room. 
 
56 HEAT. 
 
 Bell-wires too slack in summer, may be of the proper length 
 in winter. 
 
 One admirable contrivance for keeping the pendulum of a 
 clock always of the same length, by making the greater expan- 
 sion by heat of a middle bar of brass counteract the smaller ex- 
 pansion of two side rods of steel, was explained in vol. i., un- 
 der the head of 6 Pendulum,' as was also the construction of 
 a balance wheel having a corresponding property. A differ- 
 ence of a hundredth of an inch in the length of a common pen- 
 dulum, causes a clock to err ten seconds in twenty-four hours, 
 and a rise or fall of 25 of Fahrenheit's thermometer produces 
 this difference. Another kind of compensation pendulum, dis- 
 tinguished by the name of its inventor, Graham, is obtained 
 by substituting for the solid bob or ball at the bottom, a glass 
 vessel containing mercury. The mercury on expanding by 
 heat has its centre of gravity raised just enough to compensate 
 for the lengthening of the rod of the pendulum. 
 
 Crystals do not expand quite equally in breadth and in 
 length, and the expansion of one part may even cause a con- 
 traction of a part not yet warmed. The same is true of fibrous 
 substances, as wood, which expands and contracts more in 
 breadth than in length. This is proved by the leaking in cold 
 weather of a ship's deck, which in warm weather is tight: an 
 occurrence which the author once, in rounding the Cape of 
 Good Hope, had to regret as the cause of destruction to some 
 valuable specimens of natural history which he had collected 
 among the eastern islands. 
 
 Other interesting examples of expansion and contraction in 
 solids might be mentioned, but the above, in addition to what 
 were given in vol. i. under the head of ' Repulsion, 9 may suf- 
 fice. Bodies expanded by heat, unless when their intimate 
 composition is changed by it, regain exactly their former di- 
 mensions on being cooled. 
 
 As is seen in the preceding table, the expansion of liquids is 
 much greater than of solids. 
 
 A cask quite filled with liquid in winter, must force its plug 
 in summer, or must burst: and a vessel which has been filled 
 
EXPANSION OF AIRS. 57 
 
 to the lip with warm liquid, will not be full when the liquid 
 has cooled. Hence, some cunning dealers in liquids try to 
 make their purchases in very cold weather, and their sales in 
 warm weather. 
 
 There exists a most extraordinary exception, already men- 
 tioned, to the law of expansion by heat, and contraction by cold, 
 producing unspeakable benefits in nature, viz. in the case of wa- 
 ter. Water contracts, according to the law, only down to the 
 temperature of 40, while, from that to 32, which is its freezing 
 point, it again dilates. A very curious consequence of this pe- 
 culiarity is exhibited in the wells of the glaciers of Switzer- 
 land and elsewhere, namely, that when once a pool or shallow 
 well on the ice commences, it goes on quickly deepening itself, 
 until it penetrates to the earth beneath. Supposing the suv- 
 face of the water originally to have nearly the temperature of 
 the melting ice, or 32, but to be afterwards heated by the air 
 and sun, instead of the water being thereby dilated or render- 
 ed specifically lighter, and detained at the surface, it becomes 
 heavier the more nearly it is heated to 40, and therefore sinks 
 down to the bottom of the pit or well; but there, by dissolving 
 some of the ice, and being consequently cooled, it is again ren- 
 dered lighter, and rises to be heated as before, again to de- 
 scend; and this circulation and digging cannot cease until the 
 water has bored its way quite through. 
 
 Airs are expanded by heat still more than liquids. 
 
 The expansion of aeriform bodies by heat produces many 
 important effects in nature. Some of these have already been 
 considered in the preceding parts of this work, as, the rising 
 of heated air in the atmosphere, causing the winds all over the 
 earth; the same in our fires and chimneys supporting combus- 
 tion, and ventilating and purifying our houses; the same, again, 
 from around animal bodies, removing the poisonous or conta- 
 minated air that issues from the lungs, and ensuring a constant 
 supply of fresh air for the support of life, &c. 
 
 It is remarkable, with respect to aeriform bodies, that they 
 are all equally dilated by the same change of temperature, re- 
 ceiving ao. increase of about a third part of their bulk 
 
 S 
 
5S HEAT. 
 
 parts in 100) on being heated from the freezing to the boiling 
 point of water, viz. ISO , and their bulk being, therefore^ dou- 
 bled from the same standard point by about 500. This gene- 
 ral truth holds, not only with respect to the more permanent 
 airs or gases, but also with respect to all steams or vapours in 
 the dry state, that is, when not in contact with the liquid pro- 
 ducing them. The probable reason of this uniformity is, that 
 cohesive attraction, which varies so much in different solids, 
 modifying the effects of heat upon them, in aeriform fluids does 
 not exist at all. 
 
 The extent of this dilatation for airs is so much greater than 
 for liquids or solids, that it forces itself much more strikingly 
 upon the common attention. Thus a bladder containing con- 
 siderably less than its fill of air, becomes tense immediately on 
 being held to the fire. The air in a balloon just escaping from 
 a cloud has been so suddenly expanded by the direct rays of 
 the sun, as to have injured the texture of the balloon; and 
 probably some of the fatal accidents among aeronauts have 
 thus arisen. Burning fuel conveyed into a vessel or case which 
 can be suddenly and strongly closed, will produce an expansion 
 of the air, confined with it, capable of bursting any ordinary 
 material in short, will produce an explosion. 
 
 Now, if not before, at any rate, soon after steam engines 
 began to be used, and had so strikingly shown to what impor- 
 tant purposes the force of an expanding aeriform fluid might be 
 applied, the thought would naturally occur, that the force of com- 
 mon air, dilating by heat, might also be rendered useful. Accord- 
 ingly, a variety of air-expansion engines has been proposed, 
 but as yet no one has been reduced to profitable practice. Had 
 the truth been generally known, which very recent investiga- 
 tions have proved, that any given quantity of heat, when used 
 to dilate air, produces about four times the quantity of expan- 
 sive power that it does when used to form steam, the attempts 
 to bring such an application of heat under control would pro- 
 bably have been much more numerous, and possibly, by this 
 time, in a degree successful. The subject is so interesting that 
 we shall subjoin a few remarks upon it. 
 
 To produce a cubic foot of common steam, from water ori- 
 
EXPANSION OF ATRS. 59 
 
 finally cold, about 1,150 degrees of heat are required, as will 
 be explained a few pages hence. The same quantity of heat 
 would double the volume of about five cubic feet of atmosphe- 
 ric air, as is known from the comparative capacities for heat 
 of the two substances, and the rate of dilatation of air when 
 heated. Now the value for work of the foot of steam passing 
 from the boiler into a working cylinder would be, to press up 
 the piston of the steam engine through a foot, as from c d to a b, 
 with a force all the way of 15 Ibs. per inch of the piston sur- 
 face; while the working value of the five feet of air, in di- 
 lating to double bulk, would be to lift the piston five times as far 
 as the steam, vis. from g h to e f, but 
 with a force gradually diminishing as 
 the expansion went on, from 15 Ibs. 
 per inch at the beginning until the air 
 had dilated to its destined volume, when 
 the force would altogether cease: its 
 whole effect, therefore, would be five 
 feet impulsion of the piston, with a pres- 
 sure, the average between 15 Ibs. and 
 nothing, viz. 7% Ibs. per inch; and the 
 friction in the two cases and the varying intensity of the latter 
 pressure being neglected, the force of the air would be 2k times 
 as great as that of the steam. But it is farther to be considered, 
 that only about half the heat of a fire is applied to use in the 
 steam engine, viz. that part which enters the boiler, while the 
 remainder p;)v es up the chimney; and in an air engine proba- 
 bly the whole might be applied. In an air engine, moreover, 
 there might be great increase of power from the combustion, or 
 semi-explosion of the inflammable gas evolved from the fuel. 
 As it was easy, long before the steam engine was contrived, to 
 determine the expansive force of steam, and to compare it with 
 any other force; by proceeding in a similar way with respect 
 to heated air, we may estimate its expansive power to be four 
 times greater than that of steam from an equal quantity of fuel, 
 and when used at a common or low pressure. We see from 
 this of what importance the discovery would be, of a means 
 enabling us etiectually to apply the force of expanding air. 
 
HEAT. 
 
 If we suppose a fire, , to be placet 
 on a grate near the bottom of a close 
 cylinder, d a, and the cylinder to be 
 full of fresh air recently admitted, and 
 if we then suppose the loose piston, g d, 
 to be pulled upwards, it is evident that 
 all the air in the cylinder above d will be 
 made to pass by the tube e through the 
 fire, and will receive an increased elas- 
 ticity tending to the expansion or increase of volume, which the 
 fire is capable of giving it. If there were only the single close 
 vessel d a, the expansion might be so strong as to burst it; but 
 if another vessel, b c, of equal size were provided, communica- 
 ting with the first through the passage 6, and containing a close- 
 fitting piston, cf 9 like that of a steam engine, the expansion of 
 the air would act to lift the said piston, and by means of it 
 might work water pumps, or do any other service which a steam 
 engine can perform. At the end of the lifting stroke of the pis- 
 ton,/ c, it might be made to open an escape valve for the hot 
 air, placed in any convenient part of the apparatus, and to cause 
 the descent of the blowing piston, d, to expel this, while a new 
 supply of air would enter by another valve into the cylinder 
 above d. The engine would then be ready to repeat its stroke 
 as before, and the working would be continued as in a steam 
 engine. 
 
 The preceding simple conception of an air engine occurred 
 to the author's thoughts while considering the application of a 
 condensed air furnace to some chemical purposes. It appeared 
 to him that the chief difficulties to be surmounted in applying 
 any such engine to use would be, to prevent the very heated air 
 and dust from injuring the valves and other working parts of 
 the engine, and to obviate the inconvenience of the inequality 
 of power at different parts of the stroke. Various expedients 
 occurred to him. The overheating might be prevented by sur- 
 rounding the cylinder, &c. with water; and both cylinder and pis- 
 ton would suffer less from dust if, instead of the common piston, 
 c, represented above, a great hollow plunger, , were used (such 
 as is here represented, and is now common in water pumps for 
 
EXPANSION OP AIRS. 
 
 tfl 
 
 t> 
 
 mines,) embraced by an air-tight neck or col- 
 lar at b c, which neck would be the only part 
 J-l r of the cylinder requiring to be made with 
 nicety. But a more complete security would 
 be obtained by interposing water between the 
 hot air and the piston, as represented in this 
 other sketch, where the working cylinder, d, 
 has a water vessel, b, connected with it, and 
 the heated air is admitted to press upon a 
 float, on the water-surface, to lift the work- 
 ing piston d e. This 
 construction, too, if de- 
 sired, would allow the 
 fire chamber, a, to be 
 made larger than the 
 cylinder, and to be kept 
 constantly filled with 
 highly expansive air, 
 each discharge of which 
 into the space b would be replaced by cold air either from the 
 space above d, driven in through a tube as the piston ascended, 
 or from a distinct blowing cylinder worked by the beam. And 
 if it were wished to apply the same principle to an engine work- 
 ing with double strokes, that is, forcing the piston alternately 
 up and down, as in the double stroke steam engine, the object 
 might be attained, by having a second water vessel, /, commu- 
 nicating with the part of the working cylinder above the piston 
 d; and the air would pass alternately to the one or the other ves- 
 sel b or f 9 by the operation of the cock c, as steam passes in a 
 steam engine: the supply of fresh air to the chamber a would 
 be given by a blowing cylinder worked through a connexion 
 with the engine, as the air pump of a steam engine is worked. 
 The sketch of an air engine, as here given, was included in 
 the specification of a patent for another object engaged in some 
 years ago by a friend of the author's; but he being almost im- 
 mediately called to other business, and the author's professional 
 engagements precluding his attention to the subject, it was not 
 prosecuted. Jn the specification, drawn up by an engineer in 
 
HEAT. 
 
 town, some minor adaptations were described. One experiment 
 has lately been made by a Swedish engineer with the simple 
 form of dry apparatus described at page 59, for the purpose of 
 ascertaining its power, and the effect was found to be several 
 fold greater than of steam from the same quantity of fuel; but 
 the apparatus was rude, and only calculated to prove in a short 
 trial, the existence of the power, but not the fitness of the ma- 
 chine to endure long uninjured, and to be rendered easily obe- 
 dient to control: a complete experiment, therefore, remains still 
 to be made. Could an obedient and durable engine be con- 
 trived, at all approaching in simplicity to the plan given above, 
 its advantages over the steam engine would be very considerable. 
 First, its original cost would be much less, by reason of its 
 small comparative size, its simplicity, and the little nicety of 
 workmanship required. Secondly, it would occupy much less 
 room, and would be very light; hence, its peculiar fitness for 
 purposes of propelling ships and wheel carriages. Thirdly, the 
 quantity of fuel required, being so much less, would not load 
 the ship or carriage, leaving little room, 
 as happens in steam-boats, for any thing 
 else. Fourthly, the expense of fuel and 
 repairing would be little. Fifthly, the 
 engine could be set to work in a few 
 minutes, where a steam engine might 
 require hours. Sixthly, little or no wa- 
 ter would be required for it. 
 
 Another modification of air engine, 
 called a gas vacuum engine, has late- 
 ly been proposed, and many expensive 
 trials have been made of it; but it is in 
 its nature a most wasteful machine, 
 evidently throwing away at least nine- 
 tenths of the power which its princi- 
 ple generates. It was of this nature in 
 an experiment which the author wit- 
 nessed. A little of the common coal 
 gas was admitted by the cock b at the 
 bottom of the cylinder a, and was there 
 
EXPANSION OP AIRS. G3 
 
 inflamed, the lid, c, being at the time raised. The combustion 
 rarified the lower stratum of air, so that the air above was ex- 
 pelled, and about one-fifth of the original contents of the cylin- 
 der was made to occupy the whole. The lid was shut down as 
 nearly as could be judged at the moment of greatest expansion, 
 so that when the small portion of air and vapour remaining was 
 again cooled, the interior of the cylinder approached nearly to 
 the state of vacuum. It, in fact, retained only a fifth of the 
 air. A communication being then opened by the tube e, from 
 the vacuous cylinder to a water reservoir ten feet below, the 
 water was driven up by the atmospheric pressure, and filled 
 more than half of the cylinder. The water so raised was then 
 made to turn a common water wheel, and so to do work. A 
 larger quantity of water, however, could be raised to the same 
 height at less expense by a steam engine. The proposer also 
 hoped, that he would be able to make the atmosphere pressing 
 into his imperfect vacuum, act directly upon a piston as steam 
 does, and with power cheaper than that of steam; but in this 
 anticipation, too, he was completely in error. To produce his 
 imperfect vacuum, cost him very nearly at the same rate as it 
 costs to produce the perfect vacuum in a steam engine, and his 
 vacuum for equal bulks was worth, as a working power, only 
 about one-fourth as much as the steam vacuum. This may be 
 understood by considering that in a perfect .vacuum a piston 
 rises all the way with the same force, which, if common steam 
 be used, is 15 Ibs. per inch (the piston may be supposed to rise 
 from c d to a b,) but if the vacuum were only 
 three-fourths towards being perfect, the pres- 
 sure on the piston would be only three-fourths 
 of 15 Ibs. at the commencement of the stroke, 
 would then rapidly diminish, and would have 
 ceased altogether when the piston had made 
 three-quarters of its journey, or to / The 
 force in the first case would be represented by the whole line 
 c d and the square space cdba, and in the second by the short- 
 ening lines and the triangular space cef. 
 
 On considering the foregoing diagrams we may perceive that 
 in the vacuum engine, by far the greater part of the force pro- 
 
64 HEAT. 
 
 duced by the combustion of the gas is absolutely wasted, or put 
 to no use, viz. the whole expansive force during the sudden 
 combustion or explosion. It is evident that if a tenth part of 
 the aeriform contents of a cylinder acquire elasticity enough, 
 and a fourteenth part in a nice experiment does so to be able 
 afterwards to occupy the whole cylinder, it must begin its ex- 
 pansion with the force of a tenfold atmospheric condensation, 
 that is, a pressure of 150 Ibs on the square inch of a piston 
 withstanding it, which pressure will then gradually diminish as 
 the piston rises, but will amount to an average of five times the 
 atmospheric pressure, or 75 Ibs. per inch all the way; being, 
 therefore, quadruple or more, that of steam against a perfect va- 
 cuum; and, therefore, again, by our former calculation, at least 
 twelve times greater than the force obtained from the imperfect 
 vacuum of the engine under consideration. 
 
 It is a question which the author thinks will one day be an* 
 swered in the affirmative, whether nearly the whole force of ex- 
 ploding gas may not be converted into a calmly working power, 
 producing from a given expenditure, ten times or more the ef- 
 fect obtained in the vacuum engine described above, and there- 
 fore more than from a steam engine incurring the same expense. 
 There are probably various ways in which the object may be 
 attained. The following sketch is offered merely to give the 
 reader an idea of a machine for such a purpose. 
 
 Suppose b to be a very heavy close-fitting 
 piston sliding in the cylinder around it, and 
 suppose the space d open to the cylinder, to be 
 filled with atmospheric air of double or great- 
 er density; then if a mixture of explosive 
 gases, admitted by a cock to the chamber a 
 (formed between the piston and end of the 
 cylinder) be inflamed, the heavy piston will 
 
 TT- be shot forward, like a cannon ball, against 
 
 " & the condensed air in d; and owing to the 
 
 momentum acquired in the first instance, it will advance much 
 beyond the point where the exploded gas and air in d would 
 balance each other at rest: the quantity of gases admitted would 
 be just such as to carry it to the end of the cylinder. The pis- 
 
EXPANSION OF AIRS. 65 
 
 ton rod, e, would then by a catch, or ratchet, he connected with 
 the work to be done, and after the condensation of the exploded 
 gases in the cylinder, would be pressed back again, with the 
 double or greater than atmospheric force in d, as if urged by 
 high pressure steam. The first figure at page 61 represents a 
 form of cylinder which might also answer for this purpose, the 
 heavy plunger being thrown up, to work by its weight in de- 
 scending. 
 
 It is to be remarked, that the first modification of air engine 
 described at page 72, is partly an explosive engine, such as con-* 
 templated above; for, the gas separated from the coal during the 
 moment of slackened combustion, while the lately used air is 
 passing out, becomes an explosive accumulation for the fresh 
 air about to enter. The trial alluded to above proved this to be 
 the fact. 
 
 " The expansion of bodies by heat increases more rapidlyf 
 than the temperature, and particularly near the melting 
 and boiling points; that is, their points of changing into 
 liquid or air, being, however, exactly proportioned to the 
 temperature after the change into air." (.See the Analysis, 
 page 13.) 
 
 If a given quantity of heat, of that, for instance, contained in? 
 some measure of boiling water or of common steam, be added 
 to a mass of cool water, it will produce in this a certain incre^ 
 ment of bulk; and if other equal quantities of heat be afterwards 
 successively added, under the nice management which such art 
 experiment requires, each new addition will produce a greater 
 increment of bulk than the preceding, particularly when the? 
 water approaches to boiling; but after the Water is converted 
 into steam, any farther increase of bulk will be exactly propor- 
 tioned to the increase of temperature. The same truths may 
 be proved by the converse experiment of abstracting successive- 
 ly equal quantities of heat from steam or water (as by making 
 it melt equal quantities of ice,) and noting the rate of contrac- 
 tion. What is thus true of water in relation to heat, is truer 
 also- of bodies generally, each, however, having a rate of ex-* 
 pansion, and temperatures for melting and boiling proper to it* 
 
66 HEAT. 
 
 self. The quickened rate of expansion in solids and liquids 
 might have been anticipated from reflecting, that each successive 
 quantity of heat add,ed to a mass, meets with less resistance to 
 its expanding power than the preceding, owing to the diminish- 
 ing force of cohesion of the partiqles as the mass expands: while 
 in an air or gas, again, as cohesion has altogether ceased, each 
 addition of heat is at liberty to produce its full and equal effect. 
 If the capacity of substances for heat did not increase with their 
 bulk, the terms " increase of heat" and " increase of tempera- 
 ture" would have the same meaning, and this subject would be 
 more simple. 
 
 The reflection will naturally occur here, that as in the com- 
 mon thermometer the mercury must rise or expand more for a 
 given quantity of heat added at a high temperature than at a low 
 temperature, the scale should be divided to correspond with the 
 inequality. Now this reasoning is good, but the difficulty of 
 complying with it in practice is such, that the inconvenience of 
 the slight error arising from an equal division is submitted to. 
 An air thermometer with equal divisions is very correct, but 
 from wanting many of the advantages of the mercurial thermo- 
 meter, is little employed; and fortunately it happens, that in 
 the mercurial thermometer there is such a counterbalancing re- 
 lation between the expansion of the mercury and of the contain- 
 ing glass, as to render the error alluded to, at least for any mid- 
 dle range of temperature, very trifling. The subject of unequal 
 thermometric dilatation in the same liquid, and of differences 
 in different liquids, depending on the proximity to their boiling 
 points, &c., is well illustrated by Du Luc's experiment of filling 
 different thermometer glasses with different liquids, and noting 
 their comparative indications when heated through the same 
 range of temperature. He marked on each the points at which 
 the liquid stood when the glass was placed in freezing and in 
 boiling water, and then divided the intervening spaces into 
 eighty parts or degrees. The discordance of their dilatations 
 is here detailed. 
 
 Mercury. Olive Oil. Alcohol. Water. 
 
 - - - - ----- 
 
 10 - - - - 9.5 ----- 7,9 ----- 0.2 
 
LATENT HEAT. 
 
 Mercury. 
 
 Olive Oil. 
 
 Alcohol. 
 
 Water. 
 
 20 - - 
 
 - - 19.3 - 
 
 - - - - 16.5 - 
 
 - ... 4.1 
 
 30 - - 
 
 - - 29 3 - 
 
 - 2*5 fi - 
 
 Ho 
 
 
 
 
 
 40 - - 
 
 - - 39.2 - 
 
 - - - - 35.1 - 
 
 - - - - 20.5 
 
 50 - - 
 
 - - 49.2 - 
 
 - - - - 45.3 - 
 
 .... 32 
 
 60 - - 
 
 - - 59.3 - 
 
 - - - - 56.2 - 
 
 - - - - 45.8 
 
 70 - - 
 
 - - 69.4 - 
 
 - - - - 67.8 - 
 
 .... 62 
 
 80 - 
 
 - 80 
 
 - 80 
 
 . fin 
 
 The singular discrepancy in the case of water is owing to 
 the peculiarity described in former pages, of its contracting by 
 cold only down to 40 of Fahrenheit, and then dilating again 
 until it freezes. 
 
 Laborious investigations have been made by the French che- 
 mists to discover a comprehensive law determining the rate of 
 expansion in all bodies; but the object is not yet satisfactorily 
 accomplished. 
 
 e ( To melt a solid body, or to vaporize a liquid, a large quan- 
 tity of heat enters it; but in the new arrangement of the 
 particles and generally increased volume of the mass, the 
 heat becomes hidden from the thermometer, and is called 
 LATENT HEAT. It reappears during the contrary changes, 
 of ter whatever interval." (See the Analysis, page 13.) 
 
 THE expansion of bodies by heat, instead of proceeding 
 throughout in some nearly uniform or gradual manner, exhibits 
 in its course two singular transformations of the body, viz. when 
 the solid breaks down into a liquid, and when the liquid swells 
 out into an air or gas. The substance of water, for instance, 
 when at a low temperature, exists in the solid form called ice; 
 but at 32 of Fahrenheit it becomes liquid or water, and at 212, 
 even under the resisting pressure of the atmosphere, it sudden- 
 ly acquires a bulk nearly 2,000 times greater than it had as a li- 
 quid, being then called steam or aeriform water. And other 
 bodies under analogous circumstances undergo similar changes. 
 It is farther remarkable, that although during the changes a large 
 quantity of heat enters the mass, producing in the one case li- 
 quidity, in the other the form of air, the temperature is the very 
 same immediately after as it was before. Water running from 
 
ti8 HEAT. 
 
 inciting ice affects the thermometer but as the ice does, and 
 steam over boiling water appears no hotter than the water. 
 The glory of originally making the discovery of the facts now 
 referred to by the terms latent heat, or caloric of fluidity, 
 belongs to the illustrious Dr. Black. The construction of the 
 modern steam engine was an early result of kindred investiga- 
 tions, made by Dr. Black's friend, James Watt. 
 
 We select the following instances as serving to display the sub- 
 ject of latent heat in its various bearings. 
 
 A mass of ice brought into a warm room, and therefore re- 
 ceiving heat from every object around it, will soon reach the 
 temperature of melting, or 32, but afterwards both the ice and 
 the water formed from it will continue at that temperature un- 
 til all be melted: the heat which still continues to enter, ef- 
 fecting a change only in the form of the mass. And in the case 
 supposed, whatever time was required for heating the mass of 
 ice one degree, just one hundred and forty times as much will 
 be required for inciting it; proving that 140 is the latent heat 
 pf water. 
 
 If two similar flasks, one filled with ice at 32, and the other 
 with water at 32, be placed in the same oven, or over the same 
 flame, the water will gain 140 degrees of heat, while the ice is 
 merely dissolving into water at 32: and in the course of the ex- 
 periment, a correspondence will always exist between the phe^ 
 nomena; for instance, when the water has gained 14 of heat, it 
 will be found that just a tenth part of the ice is melted. 
 
 If equal quantities of hot and cold water be mixed together, 
 the whole acquires a middle temperature, each degree lost by the 
 hot water becoming a degree gained by the cold: but if a pound 
 of ice at 32, and a pound of water 140 hotter be mixed toge- 
 ther, the 140 of heat will go merely to melt the ice; for there 
 will result two pounds of water at 32. 
 
 If a flask of water at 32, or its freezing point, and a similar 
 flask of strong brine at 32, but which does not freeze until cooled 
 to near zero, be exposed together in the same cold place, it will 
 be found that when the brine has lost 10 of its heat, the water 
 flask will still exhibit an undiminished temperature, but a four* 
 
LATENT HEAT. 69 
 
 teenth part of its contents will be converted into ice. Now, as 
 in such a case the water flask must continue to radiate away heat 
 just as much as the other, it can maintain its temperature only 
 by absorbing into its general mass the heat which was latent in 
 the portion of water frozen. 
 
 It is possible, by cooling water slowly and in perfect repose, 
 to lower its temperature, while yet liquid, ten degrees below its 
 ordinary freezing point; but then, on the slightest agitation, ice 
 will be formed. It might be expected in such a case, that the 
 whole water would instantly freeze, because all colder than com- 
 mon ice; but no, only a fourteenth part does so, and singularly, 
 both that fourteenth and the remaining liquid are rendered in 
 the moment ten degrees warmer. Here the 140 of latent heat 
 escaping from the fourteenth part which freezes, become 10 of 
 sensible heat for the whole mass, so that the remaining water 
 has the temperature at which it only begins to freeze. 
 
 Strong solutions- in hot water of various neutral salts, if al- 
 lowed to cool while exposed to atmospheric pressure, soon de- 
 posit crystals of the salts; but in a close vessel protecting them 
 from such pressure, they will remain liquid even when cold. 
 Now, at the moment of opening such a bottle, the salt immedi- 
 ately crystallizes, and the latent heat given out by the solidify- 
 ing particles warms very sensibly the remaining liquid and the 
 bottle. 
 
 From the preceding facts it maybe perceived, that the quan- 
 tity of ice formed or melted in any case, becomes a correct mea- 
 sure of the quantity of heat transferred. From this considera- 
 tion, the illustrious Lavoisier constructed his calorimeter, or 
 heat measure. It is a case or vessel lined with ice, and the 
 quantity of heat given out by any body placed in it is indicated 
 by the quantity of water collected from the melted ice. 
 
 Had the latent heat of water been only 1 or 2 instead of 
 140, *the earth, except in its tropical regions, would have been 
 scarcely habitable. The cold of a single night might have frozen 
 an ocean, and the heat of a single day might have converted the 
 accumulated snows of a winter into one sudden and frightful in- 
 undation. As the fact is, however, both changes are beautifully 
 gradual, and easily controlled or prepared for. 
 
70 HEAT. 
 
 The fact of latent heat in other liquids than water is familiar- 
 ly exhibited in the slow melting of the metals; lead or pig-iron 
 for instance of butter or oils of glass, &c. ; and, on the other 
 hand, in the slow solidification of the melted masses when heat 
 is again abstracted. 
 
 The substances below enumerated, while passing from the so- 
 lid to the liquid state, absorb and render latent the quantities of 
 heat here noted: 
 
 Ice - - 140 
 
 Mercury - -142 
 
 Bees'- wax - - 170 
 
 Tin - 442 
 
 Zinc - - 492 
 
 If a pioce of frozen mercury (the temperature of which is at 
 least 40 below zero) be thrown into a little water, the latent 
 heat of the water immediately passes into the mercury and melts 
 it; but then the water is so cooled as to become ice. 
 
 " Latent heat of aeriform fluids" 
 
 Water in a vessel placed over a fire gradually attains the boil- 
 ing temperature, or 212: but its temperature then rises no more, 
 because the farther addition of heat becomes latent in the steam 
 escaping during the ebullition. The quantity of heat which 
 becomes latent in steam is discovered by noting how much more 
 time is required for boiling a quantity of water to dryness, than 
 for merely heating it a certain number of degrees. The expe- 
 riment indicates about 1,000; that is to say, that 1,000 times 
 as much heat is latent in any quantity of water formed into 
 steam as would raise the temperature of the liquid water one 
 degree. Watt had found that water in a vessel placed over a 
 lamp was about six times as long in being completely evapo- 
 rated, as in being originally heated from an ordinary tempera- 
 ture to that of boiling. 
 
 If we place in the same oven or over similar flames two like 
 vessels containing water, one of which vessels is open at top, 
 and the other is strongly elosed, the two will gain heat equally 
 up to the boiling point, but afterwards the open vessel from 
 
LATENT HEAT. 71 
 
 giving out steam will remain at the same temperature, while the 
 other, by confining all the heat that enters, will show the tem- 
 perature, rising as before, until the increasing tendency, of the 
 water to dilate forces the vessel open. Supposing the water in 
 the latter vessel, before vent is given, to have become 100 hot- 
 ter than common boiling water, instead of the whole being im- 
 mediately converted into steam as might be expected, only a 
 tenth part will be so changed, (the same quantity as will be 
 found to have already escaped from the other vessel,) for the 
 tenth part requiring in the form of steam 1,000 of latent heat, 
 will take the excess of 100 from the other nine parts, and will 
 leave them as common boiling water. If water, heated conside- 
 rably beyond the boiling point, be allowed to expand very sud- 
 denly, the whole is blown out of the vessel as a mist, by the 
 steam formed at the same instant through every part of the mass, 
 but the whole mass in such a case is no more converted into 
 true steam, than the whole of very brisk soda water is con- 
 verted into air when similarly thrown out by the sudden extri- 
 cation of the carbonic acid gas, on uncorking the bottle. Mis- 
 conception of this matter has led to most wasteful experiments 
 on steam engines of very high pressure. 
 
 The same indication of the latent heat of steam is obtained 
 by the converse experiment of first converting a quantity of 
 water into steam, and then admitting it to cold water or to ice. 
 A pound of steam will raise the temperature of ten pounds of 
 cold water 100 degress, or will melt about S$ pounds of ice. 
 
 In the great quantity of heat which becomes latent in steam, 
 we perceive the reason why water projected upon a raging fire 
 so powerfully represses it: and hence, again, why Jlre and 
 water are so often adduced, proverbially, as furnishing a striking 
 contrast. 
 
 It was when Watt had discovered how much heat was lost 
 by losing steam, that he contrived the separate condenser for 
 his steam engine, by which he saved three-fourths of the fuel 
 formerly used. 
 
 Substances differ among themselves in .regard to the latent 
 heat of their vapours as much as in their other relations to heat 
 Thus," the latent heat of the vapour or steam of, 
 
72 HEAT. 
 
 Water - is - 1,000 
 
 Vinegar - - 900 
 
 Alcohol - 442 
 
 Ether - 300 
 
 Oil of turpentine - 177 
 
 From the less latent heat in other vapours than in that of wa- 
 ter, we might at first suppose that there would be great advan- 
 tage from using them in steam engines. Accordingly, numerous 
 experiments have been made, and patents secured under this 
 idea; but the fact is, that in the same proportion as the heat is 
 less, the volume of the vapour is less, and therefore no mecha- 
 nical advantage is obtainable. 
 
 The influence of external pressure in keeping the particles of 
 liquids together in opposition to the repulsion of heat seek- 
 ing to render their mass aeriform, was considered in the chap- 
 ter on ' Pneumatics? but to make the present section com- 
 plete, the subject must be here shortly resumed. 
 Because any liquid, water for instance, while receiving heat 
 remains tranquil, and apparently unchanged, until it reaches the 
 boiling point, at which bubbling or conversion into vapour takes 
 place, we might suppose its ordinary boiling temperature neces- 
 sary to enable it, under any circumstances, to assume or to main- 
 tain the form of air. But this is no more true than that a com- 
 mon spring compressed against an obstacle has no tendency to 
 expand or recover itself until the obstacle happen to give way. 
 Liquid water, with its heat, is really a spring much compressed 
 by the weight of the atmosphere, and seeking to expand itself 
 into steam with force proportioned to its temperature. Even 
 at 32, or its freezing point, if placed in a vacuum, it assumes 
 the form of air, unless restrained by a pressure of 1 ounce on 
 each square inch of its surface: and at any higher temperature 
 the restraining force must be greater: at 100, for instance, it 
 must be 13 ounces; at 150, 4lbs.; at 212, 15lbs.; at 250, 
 30lbs., and so on: and whenever the restraining force is much 
 weaker than the expansive tendency, the formation of steam 
 will take place so rapidly as to produce the bubbling and agita- 
 tion called boiling. Now, it is because the atmosphere or ocean 
 
LATENT HEAT. 
 
 73 
 
 of air which surrounds the earth happens to have in it 15 Ibs. 
 weight of air over every square inch of the earth's surface, and 
 presses on all things there accordingly, that 212 is called the 
 hoiling point of water. An atmosphere less heavy would have 
 allowed liquids to burst into vapour at 
 lower temperatures, and one more hea- 
 vy would have had a contrary effect. 
 The exact degree of expansive force for 
 every degree of temperature in water 
 and other liquids, has been ascertained 
 by heating them in vessels furnished 
 either with properly loaded valves, as 
 at f in this figure, or with a tall up- 
 right tube, as d b, into which the liquid, 
 c 9 may force a column of mercury to 
 an elevation marking the expansive ten- 
 dency; the valve and mercury being of 
 course protected from the external at- 
 mospheric pressure, or the necessary allowance being made for 
 that pressure. Boiling at the bottom of a deep vessel is re- 
 sisted by the weight of the liquid in addition to that of the at- 
 mosphere, as already explained, and consequently the tempe- 
 rature at which it occurs is there higher than near the surface 
 of the vessel. Boiling heat is greater also in other cases, as 
 in a deep mine, where of course there is additional depth and 
 weight of atmosphere over any exposed liquid, at times when 
 the 'barometer is unusually high, that is to say, when the at- 
 mosphere is unusually heavy in cases where air or stearn is 
 confined over the boiling surface so as to. press more upon it, 
 as when brewers for a time shut the lid or valve of their great 
 boilers, &c. Water placed on the fire in a strong vessel, from, 
 which steam cannot at all escape, may be rendered even red 
 hot, without a bubble forming or one particle being dissipated; 
 but the tendency to expand into steam is then great enough to 
 burst any known material of moderate thickness. The Mar- 
 quis of Worcester exploded a cannon by shutting up water in 
 it, and then surrounding it with fire. Boiling temperature is 
 lower again when the experiment is made on mountains or in 
 
 10 
 
74 HEAT. 
 
 other situations above the level of the sea, where there is less 
 height of air resting over the boiler. In the city of Mexico, 
 which is 7,000 feet above the sea, water boils before it reaches 
 the heat of 200, instead of, as in places near the sea-level, at 
 212. Wollaston's thermometer, beautifully adapted for de- 
 termining the. height of mountains, balloon ascents, &c., by 
 merely indicating the heat of boiling water in any situation, is 
 a fine illustration of this truth. If, in any place, we take off 
 the atmospheric pressure from a liquid, as by placing it in the 
 receiver of an air pump, it will boil at very low temperatures 
 indeed. Water thus treated boils at 70, which is 20 below 
 the heat of some English summer days, and ether boils when 
 colder than common ice. Generally, in a vacuum, substances 
 boil at a temperature 124 lower than while restrained by the 
 atmospheric pressure. 
 
 Consequences of these truths, respecting the boiling tempera- 
 ture, are the following: 
 
 As water when heated dilates itself in the form of steam, 
 the steam presses on a given extent of surface with the same 
 force as the water itself would do; and in a steam engine, the 
 temperature of the water tells the degree of force which the 
 steam is exerting on the piston. 
 
 Because in the case of steam, the same law holds as for aeri- 
 form fluids generally, viz. that the outward elasticity or spring 
 increases in proportion as the fluid is more condensed high- 
 pressure steam is merely condensed steam, just as high-pres- 
 sure air is condensed air; and to obtain a double or triple pres- 
 sure, we must have twice or thrice the quantity of steam un- 
 der the same volume. 
 
 The reason that high-pressure steam, issuing from a boiler 
 heated perhaps to 300, is not hotter than low-pressure steam 
 from a boiler at 212, is, that in the instant when the high- 
 pressure or condensed steam escapes into the air, it expands 
 until balanced by the pressure of the atmosphere, that is, un- 
 til it become low-pressure steam, and it is cooled by the ex- 
 pansion, as air is cooled on escaping from any condensation. 
 
 The vessel called Papin's Digester, is merely a pot, which 
 
BOILING. 75 
 
 can be kept closed in spite of the force of the steam formed 
 within it; and in such a vessel water can be heated far beyond 
 the ordinary boiling point, enough, for instance, to dissolve 
 and extract all the gluten or jelly of bones, and to form from 
 them a rich soup where common boiling would procure no- 
 thing: or even enough to melt lead. 
 
 The person who urges a fire under a boiling pot with the 
 hope of making the water hotter, is foolishly wasting the fuel; 
 for the water can only boil, and it does so at 212 of the ther- 
 mometer. 
 
 As different substances under a given pressure, become aeri- 
 form at different temperatures, mixtures of such may be de- 
 composed by heat. If a mixture of spirit and water, for in- 
 stance, be placed over a fire, the spirit will boil off long be- 
 fore the water. If the spirituous vapour be caught apart and 
 condensed, it is said to have been distilled. Other distillations 
 are of the same nature. 
 
 The instrument here represented 
 consists of a glass tube blown into 
 k u l DS at tne two ends a and b, and 
 hermetically sealed, after receiving 
 into it some water, but no air. If one of the bulbs be heated 
 more than the other, the steam or vapour in that one will, for 
 the reasons stated above, be denser and stronger than in the 
 other, and will, therefore, force its way into the other; but, 
 owing to the lower temperature there, a part of it will be con- 
 densed, making room for more. Hence, if the difference of 
 temperature between the bulbs be long maintained, the whole 
 water will by a sort of distillation gradually pass into the colder 
 bulb. If the difference of temperature become at any time con- 
 siderable, the liquid will boil in the warmer bulb, even although 
 the source of heat be only the living hand grasping it. 
 
 To the author it appears that by a larger apparatus made on 
 this principle, fresh water might be conveniently obtained from 
 salt water on board ship, or on an island having no fresh springs. 
 Suppose any two air-tight vessels like a and b, communicating 
 by a tube furnished with a stop-cock near b, then if the vessel, 
 , were filled with salt water, and were heated by being ex- 
 
76 HEAT. 
 
 posed to the sun, (its surface being blackened and protected by 
 glass from the cooling effect of the air,) and if the other ves- 
 sel, by after being also filled with water, were made a vacuum 
 by pumping the water out from the bottom, and were then 
 kept as cold as possible by wetted coverings and a current of 
 air,r-on opening the cock at b, vapour would pass over from 
 the heated vessel to be constantly condensed in the colder, and 
 there would be a distillation from the sea water of perfectly 
 sweet water by the natural action of the sun alone. Cases 
 have occurred where a knowledge of this fact would have saved 
 ship-wrecked crews from perishing by thirst; and there are 
 rocky islands in the ocean which would become pleasantly ha- 
 bitable by the adoption of such a means, where now there is no 
 supply of fresh water, but from precarious rain or importation 
 from abroad. 
 
 When a substance has reached the temperature at which it 
 boils, that is to say, at which its vapour becomes a balance to 
 the atmospheric pressure, its dilating force is strong indeed. 
 Persons may not reflect that 15 Ibs. on a square inch is about a 
 ton on a square foot, and such is the power with which the 
 vapour of all boiling substances rises from them sufficient in 
 a single Cornish steam engine to urge the piston with the power 
 of 600 horses! But the tendency to expand at temperatures 
 much below boiling, is still, as already stated, very great, and 
 although not attracting common attention, is silently working 
 many beautiful and important ends in the economy of nature. 
 As into a perfect vacuum, freezing water gives out a steam or 
 vapour that would lift with force of li ounce per inch, or 16 
 Ibs. on a square foot, and even solid ice gives out its vapour of 
 nearly equal strength; so also do other liquids and solids. 
 There is an aeriform mercury, dense in proportion to the tem- 
 perature, in the apparently empty space called the Torricellian 
 vacuum, over the mercury in a barometer tube; and around 
 camphor, all the essential or volatile oils, &c., there is similar- 
 ly an atmosphere of the substance in the form of air. 
 
 It had for a considerable time been known that into a per- 
 fect vacuum bodies emitted, almost instantly, in the form of 
 air, a quantity of their substance proportioned to their tempe- 
 

 EVAPORATION. 77 
 
 rature; but it was reserved for Mr. Dalton to make the ad- 
 mirable discovery, that even into any space filled with air 
 these vapours arise in quantity and density the same as if air 
 were not present the two fluids seeming to be independent of 
 each other, with the exception that in a vacuum the equal dif- 
 fusion of a vapour takes place at once, while in a situation al- 
 ready occupied by air, it proceeds only as the vapour can force 
 its way through the particles of the air, and in general takes 
 place by a tranquil evaporation from the surface instead of the 
 agitation of ebullition. In an apartment with an open vessel 
 of water in it, there is soon, although invisible, a steam of wa- 
 tery vapour mingled with the air, as dense as if the room were 
 a vacuum at the same temperature. 
 
 Consequences of this important truth are the following: 
 
 That it is only an atmosphere of the substance of each body, 
 which by pressing on it can prevent its farther dissipation by 
 heat. Thus we can only save camphor, musk, smelling oils, 
 spirits, water, &c., by placing them in closed bottles or ves- 
 sels, in which, additionally to the air present, an atmosphere 
 of their ewn substance is soon formed, involving the remain- 
 ing masses with pressure proportioned to their temperature and 
 its density. 
 
 The important process of drying things is merely the placing 
 them under an elevated temperature if attainable, and in an at- 
 mosphere not containing so much of the liquid as to be satu- 
 rated at the temperature. The effect of wind or motion of the 
 air in quickening evaporation, is owing to its removing air sa- 
 turated with the moisture, and substituting air which is not 
 thus producing nearly the case of the substance placed in a va- 
 cuum. 
 
 If air, at a certain temperature, contain mixed with it as 
 much water as can be sustained in the form of invisible vapour 
 at that temperature, and if by any cause, as by rising in the at- 
 mosphere, the air be then cooled, it will abstract heat from the 
 vapour, and cause a portion of this to 'be precipitated or visi- 
 bly condensed into a fog or rain. Water rising as invisible 
 vapour from the surface of a lake or river, when it has readied 
 
78 HEAT. 
 
 a certain height, is condensed into the stratum of clouds, which 
 at times usefully protect the fields from the intense meridian 
 sun, and may fall again as refreshing showers over the country. 
 
 It is the tranquil and invisible evaporation of which we are 
 now speaking, which lifts from the surface of the wide ocean 
 all the water which, after condensation, is again returning to 
 it in the myriads of river streams which give life and beauty 
 to the face of nature. 
 
 In warm climates there are inlets of the sea, shut off occa- 
 sionally from the parent ocean, and where, after the sun's rays 
 have drank up all the water, the deposited salt remains to be 
 carried away in loads for the uses of man, as sand is carried 
 from any ordinary shore. There are in the bowels of the 
 earth prodigious accumulations of salt, formed, doubtless, in 
 the same way, during the revolutions of the antediluvian world, 
 and now explored as salt mines. When the Nile overflows its 
 banks with waters, dissolving, although in almost impercepti- 
 ble proportion, mineral substances brought from central Africa, 
 and fills reservoirs afterwards dried up by the sun's he.at, it 
 leaves in these a rich store of crystallized natron or soda. 
 
 The following are other instances of vapour, invisible while 
 at a higher temperature, being thickly precipitated when air, 
 with which it is mixed, is cooled, or when it touches a colder 
 solid body: the steam observed at night and morning hover- 
 ing over brooks and marshes heated by the sun during the 
 day: the frost-smoke, as it is called, which lies on the whole 
 face of the Greenland seas in the beginning of winter, where 
 the water warmed by the long day of the polar summer, con- 
 tinues to emit its vapour for a considerable time after summer 
 is past, into an atmosphere become too cold to preserve it in- 
 visible: the breath or perspiration of animals, of horses in par- 
 ticular after strong exertion, becoming so strikingly visible in 
 cold and damp weather, or even in warm weather, when the 
 air is already charged with moisture: in cities where there 
 are deep drains communicating with kitchens, manufactories, 
 &c., and constantly filled with moist and warm air; the vapour- 
 loaded air, although clear or transparent below, immediately 
 on escaping into a frosty atmosphere, lets go its moisture, with 
 
EVAPORATION. 79 
 
 the appearance of steam issuing from a great subterranean caul- 
 dron. Steam over water in any boiler is transparent or per- 
 fectly aeriform as may be seen when water is made to boil in 
 a vessel of glass, but as soon as it is cooled by contact or ad- 
 mixture of colder air it ceases to be true steam, and is con- 
 densed into small particles of water suspended in the air. 
 Many persons, while thinking of steam, figure it only in this 
 latter state, as particles of water mixed with air nearly as a sub- 
 tle powder might be mixed, and its substance occupying really 
 no more space than the original water did. Now until steam 
 is cooled and condensed, it is of a nature to fill alone any ap- 
 propriate vessel and powerfully distend it, just as air fills and 
 distends a bladder. Steam issuing from the spout of a kettle 
 is hardly seen near the mouth, but as its distance from the 
 spout increases, it is cooled into a thick cloud or vapour. 
 
 In a vessel from which air and atmospheric pressure are ex- 
 cluded, even the temperature of freezing water being sufficient to 
 maintain permanently in the state of gas or air, many substances 
 which exist as liquids under the atmospheric pressure, and 
 the whole mass of such a substance when placed in a vacuum, 
 not being instantly converted into gas, because the portion which 
 first rises becomes an atmosphere weighing upon the remaining 
 mass, and because, moreover, that portion, by absorbing from 
 the mass much heat into the latent state, cools the mass much be- 
 low the freezing point; we see why the liquids now spoken 
 of are so rapidly cooled to at least the freezing point if placed 
 where a vacuum can be maintained, that is. to say, where, after 
 common air has been removed, the aeriform matter rising from 
 them, and absorbing their heat, is promptly and in a continued 
 manner abstracted. It is thus that water placed in the exhausted 
 receiver of an air-pump is so rapidly cooled, and that when 
 there is beside it a vessel of concentrated sulphuric acid, or 
 other substance capable of absorbing the watery vapour as 
 formed, it is soon reduced to the state of ice; or again, that 
 water, or even mercury, surrounded by ether evaporating in a 
 vacuum, is so quickly frozen. It is thus also that if one bulb 
 of the instrument described at page 75 be immersed in a 
 freezing mixture, the water in the other and distant bulb will 
 
SO HEAT. 
 
 soon become ice; for the vapour rising from that water into the 
 vacuum maintained throughout the apparatus by the freezing 
 mixture, is immediately condensed again in the immersed bulb, 
 and leaves the vacuum still free for the ascent of more vapour, 
 to carry away more heat from the water as latent heat, and to 
 make it freeze. 
 
 As we have explained also, that in a liquid there is the same 
 tendency to evaporate whether it be or be not exposed to the 
 air, we see the reason why all evaporation is a very cooling 
 process. The effect however in air, is neither so rapid nor so 
 great as in a vacuum; first, because the presence of the air im- 
 pedes the spreading from the liquid surface of the newly formed 
 vapour, and keeps it where its pressure resists the formation of 
 more vapour; and, secondly, because the air in contact with 
 the liquid, shares its higher temperature with the liquid. Still 
 in India flat dishes of water, placed through the night on beds 
 of twigs and straw kept wet and in a current of air, soon ex- 
 hibit thin cakes of ice and thus ice is procured in India for 
 purposes of luxury. 
 
 The absorption of latent heat in the evaporation which goes 
 on from the sea and earth in all warm climates, greatly tempers 
 the heat of these climates, and the vapour afterwards spreading 
 to the poles, as explained in 'Pneumatics? under the head of 
 winds, carries warmth thither to be given out when it is re- 
 condensed into the form of rain, or is solidified as snow. The 
 formation any where of mist or rain warms the air most sensi- 
 bly, by the liberation of the latent heat from the precipitated 
 vapour. Again, the liquid water which during winter is con- 
 verted into snow or ice had been a reservoir of latent heat 
 stored to temper the frosty air of the commencing cold season; 
 and in the following spring, such ice and snow serves as empty 
 receptacles in which the first violence of the returning sun hides 
 or expends itself; allowing the temperature to change more gra- 
 dually, and for many living beings therefore more safely. The 
 vast stores of ice and snow among high mountains, as among 
 the Alps and Pyrenees, are often stores of mild temperature to 
 regions around; for besides cooling the air near them, they are 
 the never-failing sources of the rivers which run from them 
 
EVAPORATION. 81 
 
 during the whole of summer, carrying freshness through the 
 lands; from the Alps, for instance, proceed the Rhine and 
 Rhone most romantic and beautiful of European streams; and 
 from the Pyrenees, the little Gave, &c., which while channels 
 around from lower regions are almost dried up by the summer 
 heat, flows only the more freshly as the heat is greater^ and the 
 feeding snows are more abundantly dissolved. 
 
 Men in artificially raising temperatures are generally causing 
 the liberation of heat which had been previously latent, and 
 in lowering temperature or producing cold, they almost 
 solely effect their purpose by rendering a quantity of heat 
 latent. 
 
 Lavoisier thought that the heat of all combustion was merely 
 the latent heat of the oxygen gas concerned in the combustion, 
 given out during its combination with the burning body. It ia 
 so in part, but we now know that it depends more on the Inteii* 
 sity of the chemical action between the" combining substances. 
 The water thrown upon quicklime to slakd it, becomes solid 
 in combination with the lime, and gives out its latent heat so 
 remarkably as often to set fire to a Wooden Vessel or ship con- 
 taining it. 
 
 When dwelling-houses, green houses, manufactories, &c. arc 
 warmed, as is now common, by the admission of steam into 
 systems of pipes which branch over them, the heat is chiefly 
 that lately latent in the steam^ and which spreads around as 
 soon as the steamy by touching pipes of lower temperature, is 
 condensed to a state of Water. The modes of most profitably 
 effecting these purposes have to be considered in a future 
 chapter. 
 
 For producing artificial coldy our processes generally involve 
 the circumstance either of a solid changing into a liquid, during 
 which it absorbs^ and hides in its new constitution much of the 
 heat previously sensible in it and in the liquid dissolving it, or 
 of a liquid changing into vapottr, during which heat equally be- 
 comes latent. Thus by dissolving a salt, nitre for instance, in 
 water, we obtain a solution very cold. 
 
 In India the common mode of cooling \vine for table is to 
 
 11 
 
83 HEAT. 
 
 surround the bottles with nitre thus melting; and the water of 
 the solution being evaporated again before next day, the salt is 
 left ready for use as before. Such is the mutual attraction of 
 water and many salts, that they readily combine with form of 
 liquid, even when the water is used in the solid state of ice; 
 and as both the water and the salt then make heat latent, the 
 fall of temperature is very great. Thus common salt and snow 
 mixed, dissolve into liquid brine 37 colder than freezing 
 water, or 5 below the zero of Fahrenheit. 
 
 The following is a short table of easily procured freezing 
 mixtures: 
 
 Frigorific Mixtures. 
 
 Substances mixed. Thermometer sinks. 
 
 Common salt Ipart 7 From any temperature to 5 below 
 
 Snow, or pounded ice 2 5 zero- 
 Common salt ,o~~? From any temperature to 25 below 
 
 Snowonce 12 J> J 
 
 __. . / f zero* 
 
 Nitrate of ammonia 5 j 
 
 Snow. . 3 7 F 320 b t 23 o bdow zero< 
 
 Diluted sulphuric acid 2 5 
 
 Fusedpotass 47 p 32 o ab to 51 o below zero> 
 
 Snow 3 5 
 
 Mtrate of ammoma. ............. 1 - 
 
 Sulphate of soda 8-7 5QO Q0 
 
 Muriatic acid 5 5 
 
 We have already described under other heads the frigorific 
 effect of evaporating in a vacuum or in the air, and of the ope- 
 ration of condensing a gas to squeeze the heat out of it before 
 letting it expand again to a great volume. 
 
 For any given substance, the changes of state from solid to 
 liquid, and from liquid to air, happen under similar cir- 
 cumstances, so precisely at the same temperature, that 
 they mark fixed points in a general scale of temperature, 
 and enable us to regulate and compare our various ther- 
 mometers. (See the Analysis, p. 13.) 
 
 As we can neither weigh heat, nor measure its bulk, nor see 
 it, and as, even if our sense of touch were a correct judge in 
 the matter, which it is not, we dare not touch things that are 
 
EVAPORATION. 
 
 83 
 
 very hot or cold, some other means was wanted for estimating 
 the presence in bodies of this very subtle principle: and a 
 means has been found in the measurement of its most obvious 
 and constant effect, namely, that dilatation or expansion of bo- 
 dies which again ceases when the heat is withdrawn. Any 
 substance so circumstanced as to allow this expansion to be ac- 
 curately measured, becomes to us a thermometer or measure 
 of heat. 
 
 In solid substances, the direct expansion by heat is so small 
 as to be seen or measured with difficulty. In airs, again, the 
 expansion is very extensive, but there is the objection that in 
 any apparatus yet contrived, which will allow their expansion 
 completely to appear, they cannot be protected from the vary- 
 ing pressure of the atmosphere an influence which affects 
 their volume even more than common changes of temperature. 
 But liquids are free from both disadvantages, 
 and when placed in a glass bulb, as a y having 
 a long neck or stalk a b proceeding from it, 
 into which the liquid may rise when expand- 
 ed by heat, to be measured, they form the 
 most generally convenient of thermometers. 
 Then, among liquids, mercury stands, on 
 several accounts, singularly pre-eminent: in 
 it the range of temperature between freezing 
 and boiling reaches a higher point than in 
 
 B- 
 
 F- 
 
 
 any other liquid, and a lower than in all others except alcohol; 
 its little capacity for heat, and ready conducting power, cause 
 it to be very quickly affected by change of temperature; its ex- 
 pansion is singularly equable for equal increase of heat through 
 the important middle part of the scale, which includes the com- 
 mon temperatures on earth, or from the freezing to the boiling 
 heat of water; and it is easy to proportion the bulb and the 
 stalk to each other, so that a small difference of temperature 
 shall cause the mercurial column in the stalk to rise or fall very 
 conspicuously. 
 
 Now, when the important fact was ascertained that ice melts 
 in every case at precisely the same temperature, and that pure 
 water in a metallic vessel; and under a given atmospheric pres- 
 
84 HEAT. 
 
 sure, boils always at the same temperature, it followed, that by 
 placing such a thermometer as above described, first in melting 
 ice, and then in boiling water, and marking upon the stalk the 
 two points at which the mercury stood, viz. F. and B., two 
 fixed or invariable points would be obtained, andthe interval 
 between them might be divided on the glass, or on a suitable 
 scale to be attached to the glass, into any convenient number of 
 parts to be called degrees: it followed farther, that by continuing 
 the divisions to any extent both above and below the fixed 
 points, a general scale of temperature would be obtained, with 
 respect to which all thermometers made on the same principle 
 would perfectly agree, although the size of the divisions on the 
 stalks would vary according to the comparative capacities of the 
 bulb and stalk in the different instruments. Our Newton had 
 the honour first to propose the regulating points of freezing and 
 boiling, and they are now universally adopted; but the inter- 
 val between them has been variously subdivided; >that is to 
 say, there has not been agreement among philosophers as to what 
 should be accounted a degree of heat. In the Centigrade ther- 
 mometer, which is the most simple, the division is into 100 
 equal parts; in Reaumur's, which is commonly used in France, 
 it is into 80 parts; and in Fahrenheit's, which is used in England, 
 it is into 180. In Fahrenheit's, moreover, the freezing point, 
 instead of being called zero, as in the others, is called 32, be- 
 cause the maker chose to begin counting from the lowest heat 
 which he met in Iceland, or 33 below freezing of his scale. 
 To turn the degrees of any one of these thermometers into de- 
 grees of any other, we have only to recollect that 9 of Fahren- 
 heit are equal to 5 of the Centigrade, and to 4 of Reaumur. 
 Therefore, multiplying by 9 and dividing by 5 or 4, or the re- 
 verse, adding or subtracting the 32 of Fahrenheit, gives as the 
 result the degree desired, 
 
 The bulb of a mercurial thermometer is formed by heating in 
 * lamp to fusion the end of a glass tube, which has a very small 
 and equable bore, and then blowing into the tube until the soft- 
 ened end swells like a soap bubble to the size desired. The 
 mercury is forced into such a bulb through its long stalk by the 
 pressure of the atmosphere jit two efforts. First, a portion of 
 
THERMOMETERS. 85 
 
 the air originally in the bulb being expelled by warming the 
 bulb, the open end of the stalk is immersed in mercury, and 
 when the air still remaining in the bulb cools and contracts, a 
 little mercury enters. Secondly, this admitted mercury having 
 been made to boil, so as to fill with its vapour instead of the air, 
 the whole capacity of the bulb and tube, on the open end being 
 again immersed in mercury, and the mercurial vapour within 
 being condensed, the atmosphere presses in fresh mercury to 
 fill the whole vacuum. To complete the making of the ther- 
 mometer, the bulb is again heated to expel so much of the mer- 
 cury as that when cold the tube shall be about one-third full of 
 it, and then before the heated mercury begins to recede, the end 
 or opening is permanently closed by directing upon it the point 
 of a blow-pipe flame. 
 
 Although the direct expansion of any solid body by a mode- 
 rate change of temperature is so inconsiderable as to be with dif- 
 ficulty measured, M. Breguet, of Paris, lately with much in- 
 genuity contrived a thermometer in which the index is moved 
 by the curling or bending of a solid when heated, as when a 
 sheet of damp paper curls on being held before the fire. Having 
 soldered side by side two very small flattened wires of silver and 
 platinum, or of any other metals having different expansibility 
 by heat, he found that all changes of temperature made such com- 
 pound wires bend to a great extent, the metal most shortened 
 or least lengthened acting like a bow-string to pull the other into 
 the arched form: he then, by giving to a compound wire a spiral 
 or cork-screw form, and fixing the upper end of it to a stand, 
 found that an index like the hand of a watch connected with the 
 lower end was turned completely round by a certain change of 
 temperature, and that when a circle of degrees were marked on 
 a plate like a watch face placed below the index, the indications 
 of the instrument perfectly agreed with those of good mercurial 
 thermometers. Other modifications of the same principle have 
 since been successfully tried, so simplified and reduced in bulk as 
 to be introduced into the structure of a pocket watch. 
 
 Air is a substance on several accounts admirably adapted to 
 the formation of a thermometer; for it has great extent of dila- 
 tation from small increase of heat; it quickly receives impression, 
 
86 
 
 HEAT. 
 
 and its dilatation is equal for equal increments of heat at all tem- 
 peratures: but, as already stated, there is the strong objection 
 that the pressure of the atmosphere cannot be excluded, without 
 at the same time confining the air, and affecting its expansion. 
 Mr. Leslie, however, has used for particular purposes an air 
 thermometer, which he calls the differential thermometer. It 
 consists of two bulbs, a and b, filled with 
 air, and connected by a be*nt tube, d c, con- 
 taining liquid, the bulbs being hermetically 
 sealed, so that the atmosphere cannot affect 
 the air within. The difference of heat in one 
 and in the other is marked by the descending 
 of the liquid in one of the tubes, d, which 
 has a scale attached to it. We may observe, 
 that equal divisions or degrees marked on the 
 scale of this thermometer cannot mark equal 
 changes of temperature, as the increasing con- 
 densation and resistance of the air in one 
 bulb requires the force overcoming it progressively to increase. 
 If the resistance, on the contrary, were unvarying, as in an air 
 thermometer, open to a steady atmosphere, equal extent of mo- 
 tion in the fluid would mark equal increments of heat. An air 
 thermometer made of a simple bulb and long stalk of semi-trans- 
 parent porcelain, and containing in its neck melted lead or other 
 fusible metal instead of mercury, and with the mouth down- 
 wards, is well adapted for measuring very high temperatures. 
 
 Temperatures below that of freezing mercury are usually 
 measured by alcohol, as being a substance which has not yet 
 been frozen: and temperatures higher than that of boiling mer- 
 cury are measured by the expansion of air or of metals, as 
 above described, or by the contraction of pieces of baked clay, 
 which when highly heated lose water and become semi-vitri- 
 fied. The use of baked clay was proposed by Wedgewood, 
 and the apparatus has been called Wedgewood's Pyrometer, 
 or fire measure. All contrivances for measuring heat may be 
 graduated so as to correspond with the scale adopted for the 
 mercurial thermometer. 
 
 It is most interesting, while considering the vast number 
 
TABLE OP TEMPERATURE. 87 
 
 and importance of the phenomena produced by heat, to ob- 
 serve the degrees in the general scale of temperature at which 
 they take place. In the following table a selection of the facts 
 are classified, the temperatures being all referred to the scale of 
 Fahrenheit's thermometer. 
 
 Table, of facts connected with the influence of heat corres- 
 ponding to certain temperatures. 
 
 Fahrenheit. Weclgewood. 
 
 Highest temperature measured - - 32,277 ...240 
 
 Chinese porcelain softened - -21,357 ...156 
 
 Cast iron thoroughly melted -20,577 ...150 
 
 Greatest heat of a common smith's forge 17,327 ...125 
 
 Flint glass furnace - 15,897 ...114 
 
 Stone ware baked in - -14,337 ...102 
 
 Welding heat of iron - - 13,427 ... 90 to 95 
 
 Delft ware baked in - 6,407 ... 41 
 
 Fine gold melts - - - 5,237 ... 32 
 
 Settling heat of flint glass - 4,847 ... 29 
 
 Fine silver melts - 4,717 ... 28 
 
 Brass melts - 3,807 ... 21 
 Full red heat (the beginning of Wedge- 
 
 wood's Pyrometer) - - 1,077 ... 
 
 Heat of a common fire 790 
 
 Iron red in the dark - 750 
 
 Quicksilver boils - 660 
 
 Linseed oil boils - 600 
 
 Fahrenheit. 
 
 Lead melts - - 594 
 
 Sulphur melts - - 226 
 
 Water boils - 212 
 
 A compound of three parts of tin, five of lead, and 
 
 eight of bismuth melts - 210 
 Alcohol boils - 174 
 Bees'-wax melts - - 142 
 Ether boils - - 98 
 The present medium temperature of the globe - 50 
 Ice melts 32 
 
88 HEAT. 
 
 Fahrenheit. 
 
 Milk freezes - - 30 
 
 Vinegar freezes -- ... 23 
 
 Strong wine freezes - - - 20 
 
 Weak brine freezes - - zero 
 
 Quicksilver freezes - below zero 40 
 
 Natural temperature observed at Hudson's Bay - 50 
 
 Greatest artificial cold yet measured - - 91 
 
 There is reason for thinking that the higher temperatures 
 noted in this table appear considerably too high, owing to the 
 insufficiency of the thermometer or pyrometer (Wedgewood's) 
 by which they were estimated. 
 
 It is a curious inquiry, suggested by contemplating the pre- 
 ceding table, how much heat may yet remain in bodies at the 
 lowest temperature which We know? No conjecture was ha- 
 zarded on the subject until Dr* Irvine thought it might be elu- 
 cidated by comparing the quantity of heat which becomes la- 
 tent in a body on changing form, with the capacity of the body 
 before and after the change. For instance, with respect to wa- 
 ter, he said, as it requires one-tenth more heat to make a cer- 
 tain change in the temperature of water than in that of an equal 
 quantity of ice, it is probable that ice-cold water just contains 
 altogether one-tenth more heat than an equal quantity of ice 
 at the melting point: then as we know the water to contain 
 exactly 140 more heat than the ice, viz. its latent heat, the 
 whole or absolute quantity of heat in it must be ten times 140, 
 or 1,400. By applying this reasoning, however, to other sub- 
 stances than water, it is proved evidently to be fallacious; and 
 the conclusion follows, that we have as yet no means of solving 
 the question; the thermometer no more telling us the abso- 
 lute quantity of heat in any body than the rising and falling of 
 the water-surface in a well, tells the total depth of the well. 
 
 From what is said in the last, and in preceding paragraphs, 
 it is evident that the thermometer gives very limited informa- 
 tion with respect to heat: it merely indicates, in fact, what 
 may be called the tension of heat in bodies, or the strength of 
 its tendency to spread from them. Thus i't does not discover 
 that a pound of water takes thirty times as much heat to raise 
 
INFLUENCE ON CHEMICAL UNIONS. 89 
 
 its temperature one degree, as a pound of mercury; nor does 
 it discover the caloric of fluidity absorbed when bodies change 
 their form, and which, indeed, is called latent heat, only be- 
 cause hidden from the thermometer; nor does it tell that there 
 is more heat in a gallon of water than in a pint; and if an ob- 
 server did not make allowance for the increasing rate of ex- 
 pansion in the substance used as a thermometer, as the tempe- 
 rature increases, he would believe the increase of heat to be 
 greater than it is; and, lastly, when a fluid is used as a ther- 
 mometer, the expansion observed is only the excess of the ex- 
 pansion in the fluid over that in the containing, solid, and sub- 
 ject to all the irregularities of expansion in both substances: 
 all proving that the indications of the thermometer, unless in- 
 terpreted by our knowledge of the general laws of heat, no 
 more disclose the true relations of heat to bodies, than the mo- 
 ney accidentally in a man's pocket tells his rank and riches. 
 
 " Heat by its different relation to, different substances has 
 a powerful influence on their chemical combinations." 
 (See the Analysis, page 13.) 
 
 By observations made and recorded through by-gone ages, 
 man has now come to know that the substances constituting the 
 world around him, although appearing to differ in their nature 
 almost to infinity, are yet all made up of a few simple elements 
 variously combined; and he has discovered that the peculiar 
 relations of these elements to heat, as their being unequally 
 expanded by it, and their undergoing fusion and vaporization 
 at different temperatures, furnish him with ready means of se- 
 parating, combining, and new-modifying them to serve to him 
 most useful purposes. Where the primitive savage, looking 
 around on rocks and soils, saw in their diversified aspect almost 
 as little meaning as did the inferior animals which participated 
 with him the shelter of the wood or cave; his son, with pene- 
 tration sharpened by science, descries at once the treasures of 
 the mine, and aided by heat, whose wonderful energies he has 
 learned to control, pursues through all the Protean disguises 
 of ores and salts and solutions, each of the wished-for sub- 
 stances, until he secures it apart. For instance, in what to his 
 
 12 
 
90 ILEAT. 
 
 forefathers for thousands of years appeared but a red dross, he 
 knows that there lies concealed the precious iron- king of me- 
 tals! and soon forcing this in his ardent furnace to assume its 
 metallic form, with implements made of it he afterwards moulds 
 all other bodies to his will: the trees from the forest and the 
 rocks from the quarry, in obedience to these, come to be fa- 
 shioned by him as if they were of soft clay, and at his com- 
 mand rise into the magnificent structures of palaces and ships, 
 with which the earth is now beautified, and the ocean so thick- 
 ly covered. The minute detail of the relations to heat of par- 
 ticular substances forms a great part of the department of sci- 
 ence called chemistry^ (a name taken from an Arabic word 
 signifying fire;) but a general review of the subject belongs 
 to this work. 
 
 The most common ores of metals are combinations of them 
 Avith oxygen, carbonic acid, or sulphur, substances all of which 
 are volatilized at much lower temperatures than the metals. 
 Now, simple roasting, as it is called, or strongly heating the 
 ores, suffices often to drive away great part of these adjuncts^ 
 and where additional assistance is required, it is obtained by 
 mixing with the ore something which when heated attracts the 
 substance to be expelled more strongly than the metal does. 
 Charcoal, for instance, heated with an oxid ore, takes the oxy- 
 gen, and flying off with it as carbonic acid, leaves at the bottom 
 of the furnace or crucible the vivified metal. 
 
 Mercury mixed with the dross of a mine, dissolves any par- 
 ticles of gold or of silver existing in it, and the ingredients of 
 the solution may afterwards be obtained separate by mere heat- 
 ing the mercury passing away as vapour to where it is cooled 
 and again condensed for repeated use, and the more fixed gold 
 or silver remaining pure in its place, -just as in all other dis- 
 tillations, as that of spirit from wine, or of essential oils from 
 water, &c., there is the separation by heat of a more volatile 
 from a less volatile substance. The only difference between 
 what is called drying by heat and distilling is, that in the one 
 case the substance vaporized, being of no use, is allowed to es- 
 cape or be dissipated in the atmosphere; while in the other, be- 
 ing the precious part, it is caught and condensed into the liquid 
 form, 
 
INFLUENCE ON CHEMICAL UNIONS. Hi 
 
 A piece of cold charcoal lies in the air for any length of time 
 without change: but if heated to a certain degree, the mutual 
 cohesion of its particles is so weakened, that is, the particles are 
 so repelled and separated from each other, that their attraction 
 for the oxygen in the air around is allowed to operate, and they 
 combine with that oxygen, so as to produce the phenomenon of 
 combustion. The same is true, under similar circumstances, 
 of almost any dry vegetable or animal substance, and of several 
 of the metals. 
 
 Nitre, sulphur, and charcoal, while cold, may be mixed toge- 
 ther most intimately without any change taking place; but if the 
 mixture, or any part of it, be heated to a certain degree, the 
 whole explodes with extreme violence; for it is gunpowder. 
 By the change of temperature, and the consequently altered re- 
 lative attractions of the different substances, a new chemical ar- 
 rangement of them then takes place with the intense combustion 
 and expansion, which constitute the explosion. 
 
 Sea sand and soda may be mixed, and even ground together, 
 as completely as possible; but if they remain cold, they remain 
 also merely an opaque and useless powder: on heating the mix- 
 ture, however, to diminish the cohesion of the particles of each 
 substance to those of its own kind, so that the mutual attractions 
 of the two substances may come into play, they melt altogether, 
 and unite chemically into the beautiful compound called glass; 
 a product, than which art has formed none more admirable 
 which, in domestic use for instance, is fashioned into the bril- 
 liant chandelier and lustre, into the sparkling furniture of the 
 side board, into the magnificent mirror plate, and which, ex- 
 tended across our window openings, admits the light while it 
 repels the storm: 
 
 Perhaps the influence of temperature on chemical union is 
 nowhere more remarkably exhibited, than in retarding or has- 
 tening the decompositions of dead vegetable and animal sub- 
 stances. The functions of life bring into combination, to form 
 the various textures of organic or living bodies, chiefly four 
 substances, viz. carbon or coal; the ingredients of water, or 
 oxygen and hydrogen; and lastly, nitrogen which substances, 
 when in the proportions found in such bodies, have but slight 
 
1*2 HEAT. 
 
 attraction for each other, and all of which, except the carbon? 
 usually exist as airs. Their connexion, therefore, is easily sub- 
 verted; and particularly by a slight change of temperature, which 
 either so weakens their mutual hold as to allow new arrangements 
 to be formed, or altogether disengages the more volatile of them. 
 At a certain temperature, a solution of sugar (which consists of 
 the three substances first mentioned, carbon, oxygen, and hy- 
 drogen,) undergoes a change into a spirituous wash, from which 
 spirit or alcohol may then be obtained by distillation: but if the 
 heat be continued under certain circumstances, the liquid under- 
 goes a second change, or new arrangement of constituent parti- 
 cles, and becomes vinegar: under still other circumstances it un- 
 dergoes a third change, which is a destructive decomposition, 
 or rotting, as we call it, and the oxygen and hydrogen ascend 
 away as airs. But sugar, and many similar vegetable compounds, 
 preserved at a low temperature, remain unchanged for ages. 
 
 Again, as regards dead animal substances, we find, that al- 
 though at a certain, not very elevated, temperature, they un- 
 dergo that change in the relations of their elements which we 
 call putrefaction, when nearly their whole substance rises again 
 to form part of the atmosphere, still at or below the temperature 
 of freezing water, they remain unaltered for any length of time. 
 In the middle of summer, recently caught salmon, or other fish, 
 packed in boxes with ice, is conveyed fresh from the most re- 
 mote parts of Britain to the capital. In our warmest weather, 
 any meat or game may be long preserved in an ice house. In 
 Russia, Canada, and other northern countries, on the setting in 
 of the hard frosts, when the inferior animals have difficulty in 
 finding food, the inhabitants kill their winter supply, and store 
 their provender of frozen flesh or fowl, as in other countries 
 men store that which is salted or pickled. But the most striking 
 instance of this kind we can adduce is the .fact, that on the shore 
 of Siberia, in 1 SOI, in a vast block or island of ice, of which 
 the surface was then more melted than in the preceding sum- 
 mers, the carcase of an antediluvian elephant was found, per- 
 fectly preserved an elephant differing materially from those 
 now existing on earth, but its skeleton exactly correspond- 
 ing with the specimens found deep buried in various countries, 
 
INFLUENCE ON ANIMATED BEING*. 93 
 
 The creature was soon discovered by the hungry bears of the 
 district, which were seen tearing off its hairy side, and feeding 
 on its flesh, as fresh almost as if it had lived yesterday, although 
 it must have been of an era long anterior to that of any existing 
 monument on earth, of human art, or even of human being. 
 Long after it fell from the ice to the sandy beach, and when its 
 tusks had been carried away for sale by a Tungusian fisherman, 
 and its flesh had been nearly devoured, a naturalist who visited 
 it found an ear still perfect, and its long mane, and part of its 
 upper lip, and eye with the pupil yet distinguishable, which 
 had opened on the glories of a former or younger world! 
 About 30 Ibs. weight of its hair, which had been trodden into 
 the sand by the bears while eating the carcase, was collected, 
 and is now preserved in different museums of natural curiosities; 
 some, for instance, in the museum of the London College of Sur- 
 geons. 
 
 " Heat has a powerful influence also on animated nature^ 
 both vegetable and animal. 79 (Read the Analysis, page 13. 
 
 As the detail of the relations of heat to particular inanimate 
 substances belongs to the province of chemistry, so does the 
 detail of its relations to particular living vegetables and animals 
 belong to the department of Physiology; but a general re- 
 view of the subject is required in a treatise on Natural Philo- 
 sophy. 
 
 The influence which heat, exerts on inanimate nature, is 
 more immediately and completely perceived by the common 
 mind, than its influence on beings which have life. Thus to 
 all it is obvious, that the contrast between a winter and summer 
 landscape, is owing chiefly to the effect of heat on the water of 
 the landscape; that during its absence in winter, there is the 
 dry barren deformity of accumulated ice and snow, covering 
 every thing, the roads impassable, the rivers bound up, per- 
 haps hidden, the air deprived of moisture, and loaded often 
 with powdery drift; and that when warmth comes, the living 
 streams again appear, gliding their way, the cascades pour, the 
 rills murmur, the canals once more offer their bosoms to the 
 boats of commerce, the lake and pool again show their level 
 
94 HEAT. 
 
 face, reflecting the glories of the heavens, and the genial shower 
 falls upon the bosom of the softened earth, become ready to re- 
 ceive the spade or the ploughshare. Now this change is not 
 at all greater than what happens to a winter tree acted upon by 
 the warmth of spring. To take another instance from inani- 
 mate nature, it may be said with truth, that heat applied to the 
 cold boiler of a steam-engine, is the cause of all its succeeding 
 motions; of the heaving of its beam and pumps, the opening 
 and shutting of its valves, the turning of its wheels, and its 
 ultimate performance of work, as of spinning, or weaving, or 
 grinding, or propelling vehicles by land and water; but as truly 
 may it be said, that heat coming to a seed which has lain cold 
 for ages, is the cause of its immediate germination and growth; 
 or coming to a lately frozen tree is the cause of the rising of 
 its sap, the new budding and unfolding of its leaves and blos- 
 soms, the ripening of its fruit. And what is true of one seed 
 or tree, is true of the whole of the vegetable creation. When 
 the warm gales of spring have once breathed on the earth, it 
 soon becomes covered, in field and in forest, with its thick garb 
 of green, and soon opening flowers or blossoms every where 
 breathe back again a fragance to heaven. Among these the he- 
 liotrope is seen always turning its beautiful disc to the sun, 
 and many delicate flowers only open their leaves to catch the 
 direct solar ray, but close them often even when a cloud inter- 
 venes, and certainly when the chills of night approach. On 
 the sunny side of a hill, or in the sheltered crevice of a rock, 
 or on a garden wall with warm exposure, there may be pro- 
 duced grapes, peaches, and other delicious fruits, which will 
 not grow in situations of an opposite character all acknow- 
 ledging heat as the immediate cause, or indispensable condition, 
 of vegetable life. 
 
 But among animals, too, the effects of heat are equally re- 
 markable. The dread silence of winter, for instance, is suc- 
 ceeded in spring by one general cry of joy. Aloft in the air 
 the lark is every where carolling; and in the woods and shrub- 
 beries, a thousand little throats are similarly pouring forth their 
 songs of gladness during the day, the thrush and blackbird 
 near our dwellings, are heard above the rest, and with the even- 
 

 INFLUENCE ON ANIMATED BEINGS. 95 
 
 ing comes the sweet nightingale; for all of which it is the sea- 
 son of love and of exquisite enjoyment. And it is equally so 
 for animal nature generally: in favoured England, for instance, 
 in April and May the whole face of the country resounds with 
 lowings and bleatings and barkings of joy. And even man, the 
 master of the whole, and whose mind embraces all times and 
 place, is far from being insensible to this change of season. His 
 far-seeing reason of course draws delight from the anticipation 
 of autumn, with its fruits; and his benevolence rejoices in the 
 happiness observed among all inferior creatures; but indepen- 
 dently of these considerations, on his own frame the returning 
 warmth exerts a direct influence. In early life, when the na- 
 tural sensibilities are yet fresh and unaltered by the habits of 
 artificial society, spring to man is always a season of delight. 
 The eyes brighten, the whole countenance is animated, and 
 the heart feels as if new life were come, and has longings for 
 fresh objects of endearment. Of those who have passed their 
 early years in the country, or, among the charms of nature, as 
 contrasted with the arts of cities, there are few who, in their 
 morning walks in spring, have not experienced without very 
 definite cause, a kind of tumultuous joy, of which the natural 
 expression would have been, how good the God of nature is 
 to us! Spring is a time when sleeping sensibility is roused to 
 feel that there lies in nature more than the grosser sense per- 
 ceives. The heart is then thrilled with sudden extacy, and 
 wakes to aspiration of sweet acknowledgment. 
 
 Besides these effects of heat, which are comparatively tran- 
 sient as being connected with the seasons, there are other ef- 
 fects on animated nature of a more permanent character. Cer- 
 tain species of vegetables and animals, by their relation to heat y 
 are confined to certain latitudes or climates; as the orange tree 
 and bird of paradise, to warm regions; the fir tree, and arctic 
 bear, to those that are colder; and when individuals of either 
 class can support diversity of climate, they acquire a certain 
 character according to the climate, as seen in the sheep and dogs 
 of the various regions of the earth. In this latter respect there 
 is no instance more interesting than that furnished by the va- 
 rieties of the human race. Assuming that the whole sprung: 
 
U*j HEAT. 
 
 from one stock, what a contrast is there between the native of 
 Central Africa, of temperate Europe, and of the Polar Zone: 
 between the Negro, the Greek, and the Esquimaux: or again, 
 between the dark slender children of Hindostan, the strongly- 
 knit active Roman or Spaniard, and the taller, ruddy, powerful 
 Briton. And in the female sex of the last-named countries, we 
 may remark the gentleness and singular devotedness of fche In- 
 dian woman, the more commanding dark eye and gesture of 
 the graceful nymph of Italy or Spain, and the happily attempered 
 mixture of these qualities in the fair and much-favoured daugh- 
 ters of Britain. 
 
 The very important influence of heat upon the temporary 
 bodily state of animals, becomes an object of much study to the 
 physician: it explains, among many other facts, the connexion 
 of temperature with the rise of fevers and other pestilences, the 
 powerful remedial efficacy of hot and cold bathing, of changes 
 of climate, of regulating the temperature of air breathed by in- 
 valids, the protection from clothes, houses, &c. 
 
 " The great natural source of heat is the sun.-' (See the 
 Analysis, page 13.) 
 
 To be assured of this, it is only necessary to think of the com- 
 parative temperatures of night and day, of climates and of sea- 
 sons, and to reflect that the sun is the sole cause of the differ- 
 ences. We need not wonder, then, that, to many savage nations 
 seeking the source of their life and happiness, the sun has been 
 the object, not only of admiration, but of worship. 
 
 The heat comes from the sun with his light. If a sun-beam 
 enter by a small opening an apartment otherwise closed and 
 dark, it illuminates intensely the spot or object on which it first 
 falls, and its light being then scattered around, all the objects in 
 the room" become feebly visible. Again, a cold thermometer, 
 held to receive the direct ray, rises much; while in any other si- 
 tuation it is less affected: proving the heat to be like the light, 
 widely diffused, and so to lose proportionately of its intensity. 
 Light passes from the sun to the earth in about eight minutes of 
 time, as will be fully .explained in a future chapter; and there is 
 every reason to conclude that heat travels at the same rate, 
 
THE SUN THE GREAT SOURCE. 97 
 
 Human art can gather the sun-beams together, and by the 
 intense heat produced in the focus of their meeting, furnishes 
 another proof that the sun is the great source of heat. A pane 
 of glass in a window, or a small mirror, will reflect the sun's 
 ray so as to offend an eye receiving it at a distance of miles as 
 may be observed soon after the rising, or before the setting of 
 the sun, when his ray is nearly horizontal, and the heat ac- 
 companies the ray; for by many such mirrors directed towards 
 one point, a combustible object placed there would be inflamed. 
 Archimedes set fire to the Roman ships by sun-beams, returned 
 from many points to one, his god-like genius thus rivalling by 
 natural means, the supposed feats of fabled Jupiter with his 
 thunderbolts. Again, when the light of a broad sun-beam is 
 made by a convex glass or lens to converge to one point or fo- 
 cus, the concentrated heat is also there for a piece of metal 
 held in the focus drops like melting wax: and if the glass be 
 purposely moved, its focus will pierce through the most obdu- 
 rate substances, as red hot wire pierces through paper or wood. 
 A hunter on his hill, and travelling hordes on the plains, often 
 conveniently light their fires at the sun himself, by directing his 
 energies through a burning-glass. 
 
 The direct ray of the sun, simply received into a box which 
 is covered with glass to exclude the cold air, and is lined with 
 charcoal or burned cork to absorb heat, and to prevent the es- 
 cape of heat once received, will raise a thermometer in the box 
 to the temperature of 230 degrees of Fahrenheit, a temperature 
 considerably above that of boiling water. And the experiment 
 succeeds in any part of the earth where there is a clear atmos- 
 phere, and where the sun attains considerable apparent altitude. 
 We see therefore that a solar oven might in some cases be used. 
 In operating with the apparatus suggested by the author, and 
 described at page 75, for distilling water by the heat of the sun, 
 the vessel intended to absorb the heat, and to act as the still, 
 should be enclosed in a case lined and covered as above de- 
 scribed. 
 
 Reflecting on such facts as now recorded, and on the globu- 
 lar form and the motions of our earth, we have a measure of 
 the differences of climate and of season that should be found 
 
 13 
 
98 HEAT. 
 
 upon the earth. It is evident that the part of the globe turned 
 directly to the sun, receives his rays as abundantly as if it were 
 a perfect plane, similarly facing him, while on parts, which, as 
 viewed from the sun, would be called the sides of the globe, 
 with the increasing obliquity of aspect, an equal breadth or 
 quantity of rays is spread over a larger and a larger surface; 
 and at the very edge the light passes level with the surface, 
 and altogether without touching. The sunny side of many a 
 steep hill, in England, receives the sun's rays in summer as 
 perpendicularly as the plains about the equator; and such hill- 
 side is not heated like these plains, only because the air over 
 it is colder just as mountain-tops, even at the equator, owing 
 to the rarified and therefore cold air around them, remain for 
 ever hooded in snow. In England, at the time of the equi- 
 noxes, a level plain receives only about half as much of the 
 sun's light and heat as an equal extent of level surface near the 
 equator; and in the short days of winter it receives conside- 
 rably less than a third of its summer allowance. 
 
 There are few contrasts in nature more striking than some 
 of the consequences of different intensity of the sun's influ- 
 ence: that, for instance, of the inhabitants of India, at mid- 
 day, in the hot season, with the thermometer at 120, running to 
 the shade of their bungalows, darkening their windows, hanging 
 wetted mats upon the walls and roofs, and sprinkling the floors, 
 fanning themselves with ever-moving punkas, and feeling the 
 slightest covering or exertion too much while, on the other 
 hand, the dwellers in Greenland, with the thermometer below 
 zero, are loaded with furs, and are seeking the direct sunshine 
 or heat from a fire, as their life and comfort. Again, there is 
 the contrast observed on passing, as the author once did, in ten 
 days, from such a paradise as Rio de Janeiro, with all its ve- 
 getable riches, to Tristan da Cunha, and the Isle of Desolation 
 in the Southern Ocean, which exhibit only cold and naked 
 rocks; but yet where the scene was swarming with its appro- 
 priate inhabitants the sea with seals, and the air with clouds 
 of sea fowl, playing over the never-resting waves like flakes 
 of eddying snow. Were a person for a moment to doubt whe- 
 ther the sun be the real cause of such differences, and of cer- 
 
THE SUN THE GREAT SOURCE. 99 
 
 tain creatures being found only in certain zones of the earth, 
 let him reflect on the extraordinary migration of animals, which 
 have their home not in any fixed region, hut wherever the sun 
 has for a time a particular degree of influence, and which ac- 
 cordingly follow the sun in the changes of season. We have 
 the swallow in such numbers, coming to visit the British isles 
 in the spring, to play over our woods and waters, in pursuit of 
 the insects which the heat then breeds in the air, welcome 
 harbingers of the coming summer and its riches; and in au- 
 tumn the same creatures are seen congregating on our shores, 
 to wing their flight back in united multitudes to more southern 
 countries, where, in turn, there is a temperate influence of the 
 sun. The same season brings to England the nightingale, and 
 makes our woodlands resound with the note of the cuckoo. In 
 the waters of our bays and coasts, again, there appear with the 
 seasons the vast shoals of fish, as the herring and mackarel, 
 which prove such abundant food for millions of human beings; 
 and the salmon, at stated times, penetrates from the ocean far 
 up the mountain-streams, to deposite its spawn for future sup- 
 ply; all, by their movements, contributing to the harmonious 
 and beneficent system of the universe. 
 
 With respect to the sun as a source of heat, there have been 
 two opinions among philosophers; one class believing that the 
 sun is an intensely heated mass, which radiates its heat and 
 light around, like a mass of intensely heated iron: and another 
 class holding that heat is merely an affection or state of an 
 ethereal fluid, which occupies all space, as sound is an affection 
 or motion of air, and that the sun may produce the phenomena 
 of light and heat without waste of its temperature or substance, 
 as a bell may without waste continue to produce sound: hold- 
 ing farther, that the sun below its luminous atmosphere may 
 be habitable even by such animals as live on this earth. Those 
 who take the first view, are awakened to the dread contempla- 
 tion of a universe carrying in itself, if its laws remain constant, 
 the seeds of its certain decay, or at least of great periodical re- 
 volutions: the others may view the universe as destined to last 
 nearly unchanged, until a new act of the will of its Creator 
 shall again alter or destroy it. 
 
100 HEAT. 
 
 Of one fact there can be no doubt, viz. that the present tem- 
 perature of the earth is much lower than the temperature in 
 remote past time. The rocks, called primitive, as granite and 
 gneiss, constituting the interiors of our great mountain masses, 
 and the substrata of our plains, bear evident marks of having 
 been at one period in a molten state, from which they have 
 been solidified by a very gradual cooling; and even the whole 
 mass of the earth at some time must have been so fluid or soft, 
 as, in obedience to gravity, to have assumed its rounded form, 
 and in obedience to the centrifugal force of its whirling, to 
 have bulged out, at its great circumference or equator, the se- 
 venteen miles which its equatorial diameter exceeds the polar; 
 the same, by the by, in degrees corresponding to the various 
 speed of rotation, being true of all the other planets belonging 
 to the solar system. Again, while in excavating below the 
 surface of the globe, or in examining its structure as exposed 
 to view by volcanic or other convulsions, men encounter in 
 very many situations a thickness of more than a mile, of the 
 wreck and remains of former states of the world as on dig- 
 ging eighty feet under vineyards near Mount Vesuvius, they 
 encounter the buried cities of Herculaneum and Pompeii they 
 farther discover that the animal and vegetable remains buried, 
 without number, in the present cold climates of the earth, and 
 evidently resting near where the creatures lived, are all of kinds 
 now inhabiting only the warmer or tropical regions. Lastly, 
 in the operations of mining, the deeper men go, the higher 
 they find the temperature to be, at the rate of a degree for 
 about 200 feet of descent; which fact, as heat tends to equable 
 diffusion, proves both that the central heat of our earth must 
 have had another source than a radiation from the sun of the 
 present intensity; and that the surface of the earth is now ra- 
 diating away more heat than it receives from the sun. The 
 conclusion then follows, that the temperature of the world is 
 still falling, although, perhaps, so slowly that a change may 
 not be detected even within centuries. Possibly in very re- 
 mote antiquity that may have been true which the early Greeks 
 erroneously thought true in their day, viz. that the equator of 
 
COMBUSTION. 101 
 
 the earth, by reason of its great heat, was a barrier impassable 
 by man between the northern and southern hemispheres. 
 
 " Electricity a source of heat." (See the Analysis.) 
 
 This subject can only be satisfactorily entered upon in the 
 chapter devoted exclusively to electricity, and is therefore de- 
 ferred. Suffice it here to say, that while an electrical discharge 
 or current passes from one situation to another, the substance 
 serving as a conductor is often heated, melted, or dissipated, in 
 such a manner as to make it doubtful whether we possess any 
 more powerful means of producing these effects. We may re- 
 mark, too, that in certain cases of the electrical current, the 
 heat is accompanied by as intense a light as art can exhibit. 
 
 " Combustion and other chemical actions as sources of 
 heat." (See the Analysis, page 13.) 
 
 Of the phenomena of nature there is perhaps none, which, 
 to the uninstructed, appears so inexplicable and so wonderful 
 as that of fire or combustion whether contemplated in its 
 beauty or in its terrors. Fire is seen in its beauty when used 
 by man for his domestic purposes, as when it blazes cheerfully 
 over his parlour hearth, or beams around its steady light from 
 his lamps and chandeliers. It is seen again in its terrors, when 
 spreading by accident from some focus, it envelopes in sudden 
 flame the draperies and other furniture of an apartment; or 
 when breaking from a first apartment it rages through a whole 
 habitation, consuming as its food and carrying in its long flames 
 to the sky, almost every thing save the stone walls, left as a 
 blackened skeleton; or, again, when still wider spread, it is at 
 the same dread moment, devouring with deafening uproar a 
 whole town or a forest: nay, it is terrible fire, labouring with- 
 in the bowels of the earth, which first prepares and then urges 
 up to heaven the volcanic eruption of flame and red-hot rocks, 
 during which the region around often quakes and is uptorn, 
 with demolition of its cities into sudden tombs of the inhabi- 
 tants, with change in the course of its rivers, with conversion 
 of its plains into lakes, or of its lake beds into dry land. Fire 
 appears terrible also in the meteors of night; and worse than 
 
102 HEAT. 
 
 terrible when, intentionally lighted by human hands, it bursts 
 from the cannon's womb to produce the carnage of the battle. 
 Fire among many nations of antiquity was regarded with awe 
 and holy reverence, the sun himself being honoured chiefly as 
 its concentration or supposed abode. Then there were sacred 
 fires in many of the temples, and fire was used to complete the 
 splendour of the most august ceremonies. But, more remark- 
 able still, Moses, a worshipper of the one true God, has re- 
 corded of the Burning Bush, and of burnt offerings made to 
 that God: and at the present day, in many Christian churches, 
 there are ever-burning lamps and frequent magnificent illumi- 
 nations. Now, this wondrous principle of fire, which when 
 the savage man first saw it spreading, perhaps, after the thun- 
 der clap or the rubbing of forest branches in a storm, so as to 
 threaten universal destruction, he so naturally accounted the 
 demon, if not the God of nature, this principle man's art has 
 now tamed to be a most obedient, and by far the most useful 
 of all his servants. Fire, being, in truth, but a concentration 
 of the element of heat, which, in its tranquil and invisible 
 diffusion, we have already contemplated as the beneficent life 
 or soul of the universe the cause of seasons and climates, and 
 of all the changes or activity which distinguish a living world 
 from a dead and frozen mass; man, by acquiring command 
 over it, can command heat when and where he wills, and thus 
 truly becomes in a second degree the ruler of nature. Fire, in 
 man's service, may be figured as a legion of spirits to whom 
 no labour is difficult, and who, in any particular case, have 
 power or magnitude exactly proportioned to the quantity of 
 food or fuel afforded; of whom, moreover, man can at any mo- 
 ment conjure up one or many by the magic stroke of his flint 
 and steel. In every private dwelling he has of these fiery spi- 
 rits as domestic servants in the kitchen and in the parlour. 
 In his manufactories they are melting glass for him, and re- 
 ducing ores, and boiling and evaporating for a hundred pur- 
 poses. But it is chiefly while chained to the steam engine, 
 that they show their miraculous powers: as when, putting 
 forth a giant's strength, they heave a river from the bottom of 
 a mine, or urge a vast ship through the winter storm ; or when 
 
COMBUSTIOIT. 103 
 
 in nice dexterity equalling, if not surpassing, what human 
 hands can effect, they twist the silken or cotton threads, and 
 weave them into most delicate fabrics. Men, now grown fa- 
 miliar with such prodigies, have almost ceased to be moved by 
 them; but few persons can resist a feeling of wonder and admi- 
 ration when chemistry, in its progress of discovery, every now 
 and then calls forth the hidden spirit of combustion in some 
 new or less familiar guise: for instance, when a piece of iron 
 wire, lighted as a taper in oxygen gas, burns with such resplen- 
 dent brilliancy; or when phosphorus similarly placed, throws 
 around its overpowering flood of flame; or when small por- 
 tions of the metal called potassium, being cast upon the sur- 
 face of water, become as beads of most intense light running 
 about there, and crossing as in a merry dance; or, lastly, 
 when flames produced from particular substances are seen rising 
 deep-tinged with most vivid and beautiful colours. 
 
 Singularly interesting then, to philosophers, as in such par- 
 ticulars the phenomenon of combustion must always have ap- 
 peared, one may wonder that its true nature could remain to them 
 so long a mystery; but until the admirable researches of Davy, 
 made only a few years ago, their conjectures had scarcely ap- 
 proached the truth. An opinion long prevailed, that in every 
 combustible substance there was present a certain quantity of 
 a something denominated phlogiston, which, on being disen- 
 gaged or separated, became obvious to human sense as light and 
 heat. The white oxid of zinc, for instance, named the flowers 
 of zinc, and into which the metal is changed by burning, was 
 supposed to be the metal deprived of its phlogiston; and when 
 the metal again appeared, on this oxid being heated with char- 
 coal, it was supposed simply to have recovered phlogiston from 
 the charcoal. The illustrious Lavoisier had the merit of mosi 
 clearly disproving this hypothesis, by showing, for instance,, 
 that the flowers of zinc were heavier than the piece of- metal 
 from which they were produced, by the exact weight of the ox- 
 ygen gas, which disappeared in the combustion, &c. ; and he 
 showed further, that in this and many other cases, combustion 
 was merely the act of two substances combining chemically; but 
 he fell into an error almost as great as that which he overthrew, 
 
104 HEAT. 
 
 by supposing that oxygen had always to be one of the com- 
 bining substances, and that the heat and light given out in every 
 case had been previously latent in that oxygen. 
 
 When Sir Humphrey Davy began his labours on the subject, 
 than which labours there is not perhaps on record a more per- 
 fect specimen of truly scientific research, it was already known 
 that bodies when compressed, or by any means reduced in bulk, 
 generally gave out a part of their heat, as when air condensed 
 in the match syringe lights tinder, or when water and sul- 
 phuric acid, uniting into a compound of smaller volume than the 
 separate ingredients, become very hot, or when water poured 
 upon quicklime to slake it, and becoming solid with it, produces 
 heat sufficient to inflame wood, as has been fatally proved by the 
 burning of many lime-loaded ships; it being evident, moreover, 
 that the heat produced during the chemical unions depended 
 more upon the energy of the action which united the substances 
 than upon the change of volume produced. 
 
 Farther, it was known that any substance having its tempera- 
 ture raised, by whatever means, to 800 or more of Fahrenheit's 
 thermometer, became incandescent or luminous, as when iron, 
 or stone, or any substance not dissipated by heat, is placed in a 
 common fire; in the first degree the substance being said to be 
 red-hot, and at higher temperatures to be white-hot. 
 
 Now, out of these two truths Davy constructed his explanation. 
 He asserted, that in any case, combustion is merely the ap- 
 pearance produced when substances, which have perhaps still 
 stronger attraction for each other than quicklime and water, are 
 combining chemically, so as to become heated at least to the de- 
 gree of incandescence. During the phenomenon there is not, as 
 was formerly supposed, something altogether consumed or de- 
 stroyed, or something called phlogiston escaping; the substances 
 concerned are but assuming a new form or arrangement. Thus 
 if a piece of charcoal be enclosed in a glass vessel filled with air, 
 and of which the mouth dips into a liquid to confine the air, and 
 if the charcoal be then heated to a certain degree, by means of a 
 burning-glass or otherwise, the cohesion of its particles gives 
 way to their attraction for the oxygen of the air around them, 
 and they immediately begin to combine with the air so energeti- 
 
COMBUSTION. 105 
 
 cally as to produce a heat still much greater, accompanied by 
 the light or incandescence of combustion. The charcoal, under 
 these circumstances, soon entirely disappears, or is dissolved in 
 the air, as sugar may be dissolved in water; but if the air be af- 
 terwards weighed, it is found to have gained in its weight the 
 exact weight of the charcoal which has disappeared; and a che- 
 mist can again separate the charcoal from the air, and use either 
 for any purpose as before. In like manner, if a piece of iron 
 wire be heated at one end, which is then plunged into a jar of 
 oxygen gas, it will burn as a most brilliant taper, and will gra- 
 dually fall in the form of oxidized drops, or scales of iron, to 
 the bottom of the vessel. Now during this process the quantity 
 of oxygen will be diminished, but if the scales mentioned be 
 collected, they will be found to weigh just as much more than 
 the original wire expended, as there is of oxygen lost or com- 
 bined with them. A chemist can separate this iron and oxygen, 
 and exhibit them apart as before, without change. Again, if 
 iron and sulphur in certain proportions be heated together, they 
 unite with vivid combustion, but the product weighs exactly as 
 much as the original ingredients. 
 
 While every instance of combustion is thus only a case of 
 chemical union, going on with such intensity of action as to pro- 
 duce incandescence, still, according to the nature of the sub- 
 stances combining, the appearance produced varies much. It 
 may be, for instance, with flame or without flame. The great 
 combining substance in nature, that is to say, the most univer- 
 sally distributed, is oxygen, of which the name is now become 
 familiar even to the ears of the unlearned. It forms four-fifths 
 of the substance of water and one-fifth of our atmosphere, being 
 on the latter account present every where, and ready to unite 
 itself with any matter exposed to it at the necessary temperature. 
 Now of substances burning in air, those which are originally 
 aeriform, as coal gas, or which on being heated are vaporized 
 or rendered aeriform before the union takes place, as oil or wax, 
 assume the appearance of flame; viz. the aeriform particles 
 usually invisible are raised to the incandescent temperature; but 
 when the substance combining with the oxygen remains solid, 
 while its particles are gradually lifted away by the oxygen act- 
 
 14 
 
106 HEAT. 
 
 ing only at the surface of their mass, it appears during the whole 
 time only as a red-hot stone. The latter is the case of charcoal, 
 coke, Welch stone coal, &c., while in the case of wood, com- 
 mon coal, &c., a greater or less portion of the inflammable mat- 
 ter is by the heat of the combustion converted into vapour, and 
 produces the beautiful appearance of flame. 
 
 Of the substances called combustible, and thus called because 
 they combine with oxygen so energetically as to become incan- 
 descent, there are only a few which will begin to unite or burn 
 at the common temperature of our globe, the others requiring 
 to be at some higher and peculiar temperature. Thus phospho- 
 rus begins to burn at 150, sulphur at 550, charcoal at 750, 
 hydrogen at 800, &c.; it appearing that up to these temperatures 
 the attraction of the atoms of the substances among themselves 
 is sufficient to resist the other attraction, or that of oxygen. But 
 when the combustion once begins, the temperature, from the 
 effect of the combustion itself, rises instantly much beyond the 
 degree necessary for the commencement of the process. Oxy- 
 gen and hydrogen, which begin to burn or combine at 800, 
 produce a flame of as intense heat as human art can excite. 
 
 On the circumstance that bodies require to have a certain pre- 
 paratory temperature before beginning thus to combine with ox- 
 ygen, depend many important facts in nature and art. Hence the 
 safety with which most combustibles may be exposed at ordinary 
 temperatures to the contact of atmospheric air: otherwise coal, 
 wood, &c. in the moment of being exposed to the air would 
 catch fire, as really happens to phosphorated hydrogen gas; or 
 to the metal called potassium, even when thrown into cold water, 
 the metal attracting the oxygen from the water instantly, and 
 with intense combustion. If a fire or flame be so small that it 
 does not produce heat enough to maintain the inflaming tempe- 
 rature of the substance, the combustion will soon be extinguished. 
 Thus a common coal fire, if not watched by gathering together 
 the remnants to reduce the surface of wasteful radiation, will be 
 extinguished long before the fuel is all expended: but not so 
 with a fire of wood or of paper, which substances burn more 
 readily than coal. The Welch stone coal can only be made to 
 burn in very largo masses, or when mixed with a more inflam- 
 
COMBUSTION. 107 
 
 rnablo coal or other fuel. A substance placed in pure oxygen 
 gas burns with much greater intensity, and will begin burning 
 at a lower temperature than if placed in atmospheric air, which 
 contains only one-fifth of oxygen and four-fifths of another sub- 
 stance, nitrogen, which does not aid the combustion, because 
 the nitrogen, by absorbing much of the heat of the combustion, 
 lowers the temperature. Iron wire will burn as a taper in oxy- 
 gen, but not in common air; and a common taper or flaming 
 piece of wood just extinguished by blowing on it, will imme- 
 diately be rekindled if placed in oxygen. Again, a lamp with 
 a very small wick, as of one thread, and producing therefore 
 very little heat, will not burn in cold weather, and at any time will 
 be extinguished by a foreign body brought near it so as to cool it; 
 a small metallic nob, for instance, presented to it on the end of a 
 wire, or a metallic ring let down over it; but if the ball or ring be 
 hot, the effect will not follow. By more powerful refrigerating 
 processes, even a considerable lamp or candle may be put out. 
 These discoveries led Davy to the construction of his miners' 
 safety lamp, which is merely a lamp surrounded by a wire 
 gauze, of which the meshes are of such size that a flame of the 
 gas attempting to pass through is so cooled by the heat-absorb- 
 ing and heat-conducting power of the metal, as to be extin- 
 guished. A wire gauze gradually let down upon any common 
 flame, annihilates the part of it which should appear above the 
 gauze; but the combustible vapour passing invisibly through the 
 gauze may be lighted afresh on its upper side. Oxygen and 
 hydrogen, which are the constituents of water, when uniting, 
 produce such intense heat that the momentary expansion of the 
 newly formed water then in the state of steam, is such as to 
 constitute a violent explosion; and when meeting jets of the two 
 gases at a certain point allow a continued flame to be formed, 
 the most refractory substances melt in it like wax in a common 
 taper, yet these gases may be kept mixed together in the cold 
 reservoir of a condensed air blow-pipe without combining, and 
 when they are set on fire issuing as a jet from a small opening, 
 the flame docs not travel inwards through the opening as 
 might be feared, because it is cooled by the metal of the ori- 
 fice. 
 
108 HEAT. 
 
 While solid bodies become very visible or incandescent at 
 about 1,000 of Fahrenheit, airs, owing to their tenuity of con- 
 dition, require to be heated much farther before they take on 
 the vivid appearance of flame; and airs of light atoms, like hy- 
 drogen, require to be heated still more than heavier airs. Thus 
 a wire held in the pale blue flame of pure hydrogen, becomes 
 much more luminous than the flame itself; and the flame of 
 mixed oxygen and hydrogen escaping from a very minute ori- 
 fice in a glass tube, may itself be scarcely visible, while the ex- 
 tremity of the tube heated by it becomes like a brilliant star. 
 Hence the light of many flames may be increased by placing a 
 wire gauze or other solid body in the flame. Consideration of 
 this subject enables us to explain why common coal gas, which 
 consists of hydrogen holding a quantity of carbon in solution, 
 gives in burning a stronger light than pure hydrogen, and why 
 oil gas, which contains about twice as much carbon as the 
 coal gas, gives also about twice as much light: for it appears 
 that the atmospheric air, which first mixes with these gases 
 as they issue to burn, is sufficient to combine with all their hy- 
 drogen (which is most strongly attracts,) but not at the same 
 time with all their carbon; the particles of the carbon therefore 
 are separated or precipitated in the flame, and become so many 
 solid particles most intensely heated and luminous; and after- 
 wards when they have ascended a little higher, they meet with 
 new oxygen and burn in their turn, giving a second dose of light. 
 That this decomposition of the gas really occurs is proved by 
 placing a wire gauze in the flame, when we find that if held 
 near the middle of the flame, it is immediately loaded with the 
 particles of charcoal separated there, and cooled by it so as to 
 cohere; while if held at the bottom of the flame where the car- 
 bon is not yet separated, it retains none, and if held at the top 
 of the flame, where they are already burned, it similarly retains 
 none. A candle or lamp is said to smoke when the heat pro- 
 duced by it is not sufficient to effect the total combustion of the 
 carbon which rises in its flame. 
 
 When oxygen mixed with certain of the inflammable gases 
 or vapours is raised to a temperature even considerably below 
 
FUEL. 109 
 
 that of common burning or explosion, a union still takes place, 
 but very slowly, so that the temperature never rises to that ne- 
 cessary to exhibit flame. This phenomenon has been called in- 
 visible combustion. It is remarkably exemplified on plunging 
 platinum or gold wire moderately heated into such a mixture: 
 the combination then goes on in the immediate vicinity of the 
 hot wire; and although without flame, still with sufficient dis- 
 engagement of heat to maintain the wire in an incandescent or 
 luminous state, as long as there are gases left to combine. Thus 
 the vapour always arising at a common temperature from the 
 mouth of a phial of ether (ether consists chiefly of hydrogen 
 and carbon,) if made to pass through a coil of heated platinum 
 wire, will, while by this slow combustion, combining with the 
 oxygen of the air around it, give out heat enough to keep the 
 wire so luminous as to serve as a little lamp by which to read 
 from the dial plate of a watch through the night. A beautiful 
 modification of this principle has been adopted in the miners' 
 safety lamp; and when the air of the mine is too impure to 
 maintain the flame, it still suffices thus to produce a continued 
 light from the incandescent metal. 
 
 "Fuel." 
 
 Heat being, in the sense already explained, the life of the 
 universe, and man having command over nature chiefly by his 
 power of controlling heat, which power again comes to him 
 with the ability to produce combustion, it is of great interest 
 to inquire what substances he can most easily procure as food 
 for combustion, or fuel, as it is called, and how these may be 
 most advantageously employed. To speak on this subject at 
 all, fully in reference to the various arts of life, would be to 
 compose an extensive work, but an interesting sketch may be 
 comprised within narrow limits. 
 
 Although there are a great number of substances, which, in 
 the act of their chemical union, occasion the heat and light which 
 constitute combustion, still, by far the greater part of these, 
 in an uncombined state, are so sparingly distributed in nature, 
 and are, therefore, procurable with such difficulty, that heat 
 obtained by sacrificing them would be much too expensive to 
 
110 HEAT. 
 
 be within common means. Providence, however, has willed 
 that the elementary substance in nature which has the most en- 
 ergetic attraction for almost all other substances, and which, 
 therefore, produces in uniting with them the most intense heat, 
 is also the most universally distributed of all. This substance 
 is oxygen. It forms part of our atmosphere, and therefore 
 penetrates, and is present wherever man can exist or breathe, 
 offering itself at once to his service. . Then, for the. purpose of 
 combining with the oxygen, there are chiefly two other sub- 
 stances also very widely scattered, and therefore easily pro- 
 curable and cheap. These are carbon and hydrogen, the great 
 materials of all vegetable bodies, and therefore of our forest 
 trees, and of coal beds, which seem to be the remains of ante- 
 diluvian forests. Carbon is found nearly alone in hard coal, 
 but "it is united with a large proportion of hydrogen in caking 
 coal, wood, wax, resins, tallow, and oils. The gases used for 
 illumination are merely hydrogen, holding certain quantities 
 of carbon in solution; and all bodies which burn with flame 
 give out such gases in the act of combustion. In the great 
 mass of the earth, as known to man, the stones, earths, and 
 water, forming its surface, are already combinations of oxygen 
 with other substances, and are therefore not in a state to pro- 
 duce fresh combustion; but carbon and hydrogen, by various 
 processes of vegetable and animal life, are in numberless situa- 
 tions becoming accumulated, so as to be fit for fuel: as by 
 other processes the atmosphere is always preserved with its 
 due proportion of oxygen. 
 
 The name fuel is given only to the substances which com- 
 bine with oxygen, and not to the oxygen itself, probably be- 
 cause the former being solid or liquid, and therefore more ob- 
 vious to sense, were known as producers of combustion long 
 before the existence of the aeriform ingredient was even sus- 
 pected. 
 
 Oils, fat, wax, &c. being or becoming in their combustion 
 aeriform, exhibit the appearance of flame, as already explained, 
 and, hence, are chiefly used for the purpose of giving light. 
 Wood, again, and coal, are more frequently used for mere heat- 
 ing. But the chemist's lamp for distilling and evaporating, his 
 
FUEL. Ill 
 
 common blow-pipe for directing the point of a (lame upon any 
 substance to melt it, and his condensed-air blow-pipe, whose 
 flame of oxygen and hydrogen is capable of melting the most 
 refractory substances, prove that it is chiefly the expense of 
 the former kinds of fuel which has nearly limited them to the 
 office of light-giving. Lately an important application of oil 
 or fat as heat-giving fuel has been made in a general cooking 
 apparatus, which promises to effect a considerable diminution 
 of house-keeping expense. 
 
 Wood was the common fuel of the early world when coal 
 mines were not yet known, and still in many countries it is so 
 abundant as to be the cheapest fuel. Charcoal is the name 
 given to what remains of wood after it has been heated in a 
 close place, during which operation the hydrogen and other mi- 
 nor ingredients are driven away in the form o'f vapour. Char- 
 coal is nearly pure carbon. Coke, again, is the carbon obtained 
 by a similar preparation of coal. The wood and coal, if simi- 
 larly heated in the air, would burn or combine with the Oxy- 
 gen of the air; but heated in a vessel or place which excludes 
 air, they merely give out their more volatile parts. 
 
 Good coal, where it abounds, is now for ordinary purposes 
 by much the cheapest kind of fuel; and since, within a few 
 years, men have learned to obtain from it separately, and to 
 use, instead of oil and wax, its illuminating gas, viz. its hy- 
 drogen, holding in solution a little carbon, it has become doubly 
 precious to them. A person reflecting that heat is the magic 
 power which vivifies nature, and that coal is what best gives 
 heat for the endless purposes of human society, cannot without 
 admiration think of the rich stores of coal which exist treasured 
 up in the bowels of the earth for man's use. And Britain, in this 
 respect, is singularly favoured. Her coal mines are in effect 
 mines of labour or power vastly more precious than the gold 
 and silver mines of Peru, for they may be said to produce 
 abundantly every thing which labour and ingenuity can pro- 
 duce, and they have essentially contributed to make her mis- 
 tress of the industry and commerce of the earth. Britain has 
 become, to the civilized world around, nearly what an ordinary 
 town is to the rural district in which it stands, and of this vast 
 
112 HEAT. 
 
 and glorious city the mines in question are the coal cellars, 
 stored at the present rate of consumption for about 1,000 years; 
 a supply which, as coming improvements in the arts of life 
 will naturally bring economy of fuel, or substitution of other 
 means to effect similar purposes, may be regarded as exhaust- 
 less. 
 
 Coal, we can scarcely doubt, is the remains of antediluvian 
 forests, swept together during the convulsions of nature into 
 deep valleys, and there afterwards compressed and solidified 
 by superincumbent deposites of earthy matters, these deposites 
 being probably aided in their operation by heat. In many 
 coal beds the trees of former times yet retain their form, so 
 that their species, can be easily distinguished, and there are bu- 
 ried among them other vegetable and animal remains of con- 
 temporaneous inhabitants of the earth. Coal is found of diffe- 
 rent qualities. In some places it is almost unmixed carbon, 
 and exceedingly solid, as if it had been coked by subterranean 
 heat. Such is the stone coal of Wales, which in 100 parts 
 contains 97 of pure carbon, and only three of hydrogen and 
 earthy matter. In other places the coal contains hydrogen in 
 nearly as large proportion as wood does, and so combined with 
 part of the carbon as to form the oily or pitchy substances ex- 
 isting in the coal, and which, when burning, produce flame, 
 and, when rising unburned, constitute smoke. 
 
 The comparative values, as fuel, of different kinds of carbo- 
 naceous matter, have been found on experiment to be as in the 
 following tables. 
 
 1 lb. of Melts of ice. 
 
 Good coal - - 90 Ibs. 
 
 Coke - - 84 
 
 Charcoal of wood - 95 
 
 Wood - 32 
 
 Peat - - - 19 
 
 Lavoisier, in making experiments on combustibles generally, 
 to ascertain the quantities of oxygen expanded, and of heat 
 given out during the combustion of a given quantity of each, 
 obtained the following results: 
 
IT-EL. 11.; 
 
 1 Ib. of Melts of ice. Takes of oxygen. 
 
 Hydrogen gas - - 370 Ibs. - - - - 7 Ibs. 
 
 Carburetted hydrogen - 85 4 
 
 Olive oil - - 120 ;; 
 
 Wax - - 110 ;; 
 
 Tallow - 105 3 
 
 Charcoal - - 95 2-J 
 
 Phosphorus - 100 1$ 
 
 Sulphur - - 25 1 
 
 There are some remarks with respect to the using of common 
 fuel, which seem to demand a place here. 
 
 A pound of coke produces nearly as much heat as a pound 
 of coal; but we must remember that a pound of coal gives only 
 three-quarters of a pound of coke, although the latter is more 
 bulky than the former. 
 
 It is wasteful to wet fuel, because the moisture, on being eva- 
 porated, carries off with it as latent, and therefore useless heat, 
 a considerable proportion of what the combustion produces. 
 It is a very common prejudice, that the wetting of coal, by 
 making it last longer, is effecting a great saving; but while, iu 
 truth, it restrains the combustion, and for a time makes a bad 
 fire, it also wastes the heat. 
 
 Coal containing much hydrogen, as all flaming coal does, is 
 used wastefully when any of the hydrogen escapes without 
 burning; for, first, the great heat Which the combustion of such 
 hydrogen would produce is not obtained; and, secondly, the 
 hydrogen, while becoming gas, absorbs still more heat into the 
 latent state than an equal weight of water would. Now, the 
 smoke of a fire is the hydrogen of the coal rising in combina- 
 tion with a portion of carbon. We see, therefore, that by de- 
 stroying or burning smoke, we not only prevent a nuisance, 
 but effect a great saving. The reason that common fires give 
 out so much smoke is, either that the smoke, or what we shall 
 call the vaporized pitch, is not sufficiently heated to burn, or 
 that the air mixed with it as it ascends in the chimney, has al- 
 ready, while passing through the fire, been deprived of its 
 free oxygen. If the pitch be very much heated, its ingredi- 
 
 15 
 
114 HEAT. 
 
 ents assume a new arrangement, becoming transparent, and 
 constituting the common coal gas of our lamps; but, at lower 
 temperatures, the pitch is seen jetting as a dense smoke, from 
 cracks or openings in the coal a smoke, however, which im- 
 mediately becomes a brilliant flame if lighted by a piece of 
 burning paper or the approximation of the combustion. The 
 alternate bursting out and extinction of these burning jets of 
 pitchy vapour, contribute to render a common fire an object so 
 lively, and of such agreeable contemplation in the winter even- 
 ings. When coal is first thrown upon a fire, a great quantity 
 of vaporized pitch escapes as a dense cold smoke, too cold to 
 burn, and for a time the flame is smothered, or there is none; 
 but as the fresh coal is heated its vapour reproduces the flame 
 as before. In close fire places, those, viz. of great boilers, as 
 of steam engines, brewing, and dyeing apparatus, &c., all the 
 air which enters after the furnace door is shut, must pass 
 through the grate and the burning fuel lying on it, and there 
 its oxygen is consumed by the red hot coal before it ascends 
 to where the smoke is. The smoke, therefore, however hot, 
 passes away unburnt, unless sometimes, as over foundery fur- 
 naces, where the heat is very great indeed, and it burns as a 
 flame or great lamp at the chimney-top on reaching the oxy- 
 gen of the open atmosphere. . 
 
 There have been many modes proposed of destroying smoke: 
 one has been to admit, by a suitable opening, a certain quantity 
 of fresh air to the space above the fire, the oxygen of which 
 air may inflame the smoke. At a certain point of time after 
 the addition of fresh fuel, this plan succeeds, and for the mo- 
 ment effects a saving of fuel; but the difficulty of admitting 
 just the quantity of air required to suit the varying demand for 
 it, has not been overcome, and hence from there being no 
 saving on the whole, the plan has been abandoned. When just 
 enough air entered, the flame produced gave so intense a heat 
 as in several cases to have burned or destroyed the parts of va- 
 luable boilers exposed to it; and when, on the contrary, too 
 much air entered, it injuriously cooled the boiler. The con- 
 trivance at present most commonly adopted for burning smoke 
 is that of Mr. Brunton, viz. a circular fire grate, kept turning 
 
i;i.. 115 
 
 like a horizontal wheel, and on which coal is by machinery 
 made to fall in a gradual manner, so as to be uniformly spread 
 over it. The coal falls so gradually, that although there is ge- 
 nerally a little smoke from it, there is never much, the oxy- 
 gen which finds entrance, through, and around the grate, being 
 always in quantity the vsame, and nearly sufficient. A smoke- 
 consuming fire would be constructed on a perfect principle, in 
 which the fuel were made to burn only at the upper surface of 
 its mass, and so that the pitch and gas disengaged from it, as 
 the heat spread downwards, might have to pass through the 
 burning coals where fresh air was mixing with them: thus the 
 gas and smoke, being the most inflammable parts, would burn 
 first and be all consumed. This was the principle proposed in 
 a fire-place suggested by the author for the great brewery of 
 Mr. Meux in his neighbourhood, and tried at the time when 
 attempts were extensively made to abate the nuisance of smoke 
 in towns. The experiment proved the theoretical perfection 
 of the method, and that it would produce a saving of 15 or 20 
 per cent, on the expenditure of coal; but before a durable 
 grate of the kind was completed, the Welch stone-coal was in- 
 troduced, which has 97 per cent, of pure carbon, and therefore 
 no pitch to evaporate, and no smoke, and it was at once adop- 
 ted there and in many other places. Coal in a deep narrow 
 trough, as a b c d, if lighted at its sur- 
 face a b, burns with a lofty flame as if 
 it were the wick of a large lamp; for 
 all the gas given out from the coal be- 
 low, as that is gradually heated, passes 
 through the burning fuel and becomes 
 a flame. Now, if we suppose nhany 
 such troughs placed together, with intervals between them, in 
 place of the fire bars of a common grate or furnace, there would 
 be a perfect unsmoking fire place. Such was that made on the 
 occasion mentioned; and although flimsy and imperfect, as a 
 mere experimental apparatus, it put beyond a doubt the possi- 
 bility of accomplishing its object. The reason of the vast 
 saving of fuel by such a grate is, that the smoke, instead of 
 stealing away latent heatbeing yet itself the most combustible 
 
116 HEAT. 
 
 and precious part of the fuel, gives all its powers and worth to 
 the purpose of the combustion. The coal rested on moveable 
 bottoms in the troughs, and was moved up like the wick of a 
 lamp, by its screw : the bottoms might be lifted in many 
 ways. The author believes that this construction, simplified as 
 much as possible, will still be adopted for the Newcastle or 
 flaming coal, the consequences would be so important. The 
 principle has been already extensively introduced for common 
 parlour fires by Mr. Cutler in his stove, which is merely a 
 common grate, having instead of bottom bars a deep box to 
 hold the coal for a whole day, with a moveable bottom, which 
 lifts the coal up as wanted. From such a fire there is alwa}-s 
 ascending a long beautiful flame; and much more heat is given 
 out, than from the same quantity of coal burned in the common 
 way: the chimney never requires sweeping, for there is abso- 
 lutely no smoke, and therefore no soot. 
 
 It is evident that if a house or apartment with the air in it, 
 were once warmed to a certain degree, it would for ever retain 
 its temperature, but for the escape of heat through the walls 
 and windows, or with the air from within, whether passing 
 away as necessary ventilation or as waste. A perfect system 
 of heating, therefore, would consist in diminishing as much as 
 possible these causes of loss, with reference both to the expense 
 of the means and the salubrity of the dwelling, and in produ* 
 cing and distributing the heat judiciously. It may be asserted 
 that a fourth part of the fuel generally expended in English 
 houses, if more skilfully used, would better secure comfort and 
 health than all that is now expended. But it does not accord 
 with the character of this general work to enter into minute 
 detail on the subject. Remarks were made upon it in vol. i. 
 in the chapter on " Pneumatics^ under the head of " warm- 
 ing and ventilating," and more minute information may be ob- 
 tained from Mr. Tredgold's work, expressly devoted to it. 
 
 The consideration of furnaces, blow-pipes, &c. may appear 
 to some so closely connected with our present subject as to de- 
 mand a place here, but by treating of them we should be en- 
 croaching on the province of the chemist, &c. We may state 
 generally, that furnaces are merely arrangements of parts by 
 
PROM FRICTION. 117 
 
 \vhich coal or other fuel heated to the degree at which it com- 
 bines rapidly with the oxygen of the atmospheric air, is placed 
 in circumstances favourable to the rapid renewal of the air, 
 and a common blow pipe is merely a jet of air thrown from a 
 minute opening into any flame, so as with great precision to 
 direct the point of the flame upon the body to be heated. The 
 sand bath and water bath of the chemist are merely means of 
 ensuring a more uniform or steady temperature: a vessel im- 
 bedded in sand, so that heat can reach it only through the sand, 
 cannot be very suddenly heated or cooled, because sand is a 
 slow conductor; and a vessel immersed in boiling water, can 
 never have greater heat than 212, or the boiling heat of water. 
 For certain purposes, hotter baths, as of high pressure steam, 
 or of vapour of oil of turpentine, or of boiling whale-oil, have 
 been used. On such subjects, readers may consult works on 
 " chemistry applied to the arts." 
 
 " Condensation and Friction as causes of heat." (Read the 
 Analysis, p. 13.) 
 
 A soft iron nail laid upon an anvil, and receiving in rapid 
 succession three or four powerful blows of a hammer, becomes 
 hot enough to light a match, and if longer hammered, will be- 
 come incandescent or red hot, partly from the diminished vo- 
 lume or condensation of the iron, on the principle already ex- 
 plained, and partly from the percussion or friction, in a way 
 not yet well understood, but probably electrical. 
 
 In the familiar case of the mutual percussion of flint and 
 steel, small portions of one or both are struck off by the vio- 
 lence of the collision, in a state of white heat, and the particles 
 of the iron burn in passing through the air: in a vacuum the 
 heated particles are equally produced, but are scarcely visible 
 from this combustion not occurring. In both cases, they suffice 
 to inflame gunpowder, or to light tinder. When the materials 
 are good, the shower of sparks from the sudden blow is most 
 copious and brilliant. 
 
 The heat produced by friction alone, without perceivable con- 
 densation of the bodies concerned, is exemplified in many facts. 
 Two dry branches kept strongly rubbing against each other by 
 
118 HEAT. 
 
 the wind, have sometimes set a wood on fire. Savages light 
 their fires by analogous means. Men warm their cold hands in 
 winter, by rubbing them against each other, or against their 
 coat sleeves. Again, the axletree of a heavily laden wagon or 
 other carriage, if left without oil, often inflames. The line 
 attached to a whale harpoon, as it runs over the side of the 
 boat when the huge monster dives after the harpoon has entered 
 his flesh, requires water to be constantly thrown on it to prevent 
 its setting fire to the boat. A cable drawn very rapidly through 
 the hawse hole by the falling anchor, produces great heat there 
 and smoke. When a magnificent ship is launched from the 
 builder's yard into the deep, and glides along the sloping beams, 
 a dense smoke rises from the points of rubbing contact. 
 
 (i The Functions of Animal Life a source of Heat." (Read 
 the Analysis, p. 13.) 
 
 It is one of the remarkable facts in nature, that living animal 
 bodies, and to a certain degree living vegetables also, have the 
 property of maintaining in themselves a peculiar temperature, 
 whether surrounded by bodies that are hotter or colder than 
 they. Captain Parry's sailors, during the polar winter, where 
 they were breathing air that could freeze mercury, still had 
 the natural warmth in them of 98 of Fahrenheit; and the inha- 
 bitants of India, where the same thermometer stands sometimes 
 at 115 in the shade, have their blood at no higher a tempera- 
 ture. 
 
 It was at one time the favourite explanation of this, that ani- 
 mal heat was produced in the lungs, during respiration from the 
 oxygen then admitted. This oxygen combines with carbon from 
 the blood, and becomes carbonic acid as in combustion, and it 
 was supposed to give out a portion of its latent heat, as in ac- 
 tual combustion; which heat being then spread over the body 
 by the circulating blood, maintained the temperature. We now 
 know, that if such a process assist, which it probably does, 
 for the animal heat has generally a relation to the quantity of 
 oxygen expended in any particular case, and when an animal 
 being already much heated needs no increase, very little oxy- 
 gen disappears, still much of the effect is dependent on the 
 
ANIMAL FUNCTIONS A SOURCE. 119 
 
 influence of the nerves, either directly or indirectly, through 
 the vital functions governed by them. Mr. Brodie, in his im- 
 portant experiments upon the subject, found that although in 
 animals apparently dead from injury done to the nervous sys- 
 tem, he could artificially continue the action of respiration, with 
 the usual formation of carbonic acid, still the temperature fell 
 very quickly. The maintenance of low temperature in an ani- 
 mal immersed in an air hotter than itself, is partly attributable 
 to the copious perspiration and evaporation which then take 
 place, and which absorb into the latent form the excess of heat 
 then existing. Perspiration, both from the skin and internal 
 surface of the lungs, occurs generally in proportion to the ex- 
 cess of heat. Dogs and other animals, when much heated, as 
 they cannot throw off or diminish their natural covering, in- 
 crease the evaporating surface by protruding a long humid 
 tongue. 
 
 The power in animals of preserving their peculiar tempera- 
 ture has its limits. Intense cold coming suddenly upon a man 
 who has not sufficient protection, first causes a sensation of pain, 
 and then brings on an almost irresistible sleepiness, which if 
 indulged would be fatal. Sir Joseph Banks having gone on 
 shore one day near the cold Cape Horn, and being fatigued, 
 was so overcome by the feeling mentioned, that he entreated his 
 companions to let him sleep for a little while. His prayer 
 granted, might have allowed that sleep to come upon him which 
 ends not the sleep of death! as, under similar circumstances, 
 it came upon so many thousands of the army which Buonaparte 
 led into Russia, and lost there during the disastrous retreat 
 through the snows. Buonaparte's celebrated bulletin allowed, 
 that in one night, when the thermometer of Reaumur stood at 
 19 below zero, 30,000 horses died! Cold in inferior degrees, 
 and longer continued, acting on persons imperfectly protected 
 by clothing, &c., induces a variety of diseases, which destroy 
 more slowly. A great excess of heat, again, may at once ex- 
 cite a fatal apoplexy, and heat in inferior degrees, but long con- 
 tinued, may cause those fevers, &c., which prevail in warm 
 climates, and which are so destructive to strangers in these cli- 
 mates. 
 
120 HEAT. 
 
 Each species of animal has a peculiar temperature natural to 
 it, and in the diversity are found creatures fitted to live in all 
 parts of the earth, what is wanting in internal bodily constitu- 
 tion being found in the admirable protecting covering which na- 
 ture has provided for them covering which grows from their 
 bodies, with form of fur or feather, in the exact degree required, 
 and even so as in the same animal to vary with climate and sea- 
 son. Such covering, however, has been denied to man; but the 
 denial is not one of unkindness: it is the indication of his su- 
 perior nature and destinies. Godlike reason was bestowed on 
 man, by which he subjects all nature to his use, and he was 
 left to clothe himself. 
 
 The human race is naturally the inhabitant of a warm climate, 
 and the paradise described as Adam's first abode, may be said 
 still to exist over vast regions about the equator. There the 
 sun's influence is strong and uniform, producing a rich and warm 
 garden, in which human beings, however ignorant of the world 
 which they had come to inhabit, would have their necessities 
 supplied almost by wishing. The ripe fruit is there always 
 hanging from the branches; of clothing there is required only 
 what moral feelings may dictate, or what may be supposed to 
 add grace to the form; and as a shelter from the weather, a few 
 broad leaves spread on connected reeds will complete an Indian 
 hut. The human family, in multiplying and spreading in all 
 directions from such a centre, would find, to the east and west, 
 only the lengthened paradise, with slightly varying features of 
 beauty; but to the north and south, the changes of season, which 
 make the bee of high latitudes lay up its winter store of honey, 
 and send migrating birds from country to country in search of 
 warmth and food, would also rouse man's energies to protect 
 himself. His faculties of foresight and contrivance would come 
 into play, awakening industry; and, as their fruits, he would 
 soon possess the knowledge and the arts which secure a happy 
 existence in all climates, from the equator almost to the pole. 
 It is chiefly because man lias learned to produce at will, and to 
 control, the wonder-working principle of heat, that in the rude 
 winter, which seems the death of nature, he, and other tropical 
 animals and plants which he protects, do not in reality perish 
 
COMBUSTION. 121 
 
 even as a canary bird escaped from its cage, or an infant ex- 
 posed among the snow-hills. By producing heat from his fire, 
 he obtains a novel and most pleasurable sort of existence; and 
 in the night while the dark and freezing winds are howling over 
 his roof, he basks in the presence of his mimic sun, surrounded 
 by his friends and all the delights of society, while in his store- 
 rooms, or in those of merchants at his command, he has the 
 treasured delicacies of every season and clime. He soon be- 
 comes aware, too, that the dreary winter, instead of being a 
 curse, is really in many respects a blessing, by arousing from the 
 apathy to which the eternal serenity of a tropical sky so much 
 disposes. In climates where labour and ingenuity must pre- 
 cede enjoyment, every faculty of mind and body is invigorated; 
 and hence the sterner climates form the perfect man. It is in 
 them that the arts and sciences have reached their present ad- 
 vancement, and that the brightest examples have appeared of 
 intellectual and moral excellence. 
 
( 133 ) 
 
 PART FOURTH. 
 
 (Continued.] 
 SECTION II. ON LIGHT, OR OPTICS. 
 
 ANALYSIS OF THE SECTION. 
 
 Light is 'an emanation from the sun and other luminous bodies be- 
 coming less intense as it spreads, and which, by falling on other 
 bodies, andbeing reflected from them to the eye, renders them visible. 
 It moves with great velocity, and in straight lines where there is 
 no obstacle leaving shadows where it cannot fall. It passes rea- 
 dily through some bodies which are therefore called transparent; 
 but when it enters or leaves their surfaces obliquely, it suffers at 
 them a degree of bending or refrattion proportioned to the obliqui- 
 ty. And a beam of white light thus refracted or bent, under cer- 
 tain circumstances, is resolved into beams of all the elementary co- 
 lours, ivhich, however, on being again blended, become the white 
 light as before. 
 
 Transparent bodies, as glass, may be made of such form as to cause 
 all the rays which pass through them from any given point to bend 
 and meet again in another point beyond them; the body, then, 
 because usually in form somewhat resembling aflat bean or lentil, 
 being called a LENS. And when the light thus proceeding from 
 every point of an object placed before a lens is collected in corres- 
 ponding points behind it, a perfect image of the object is there 
 produced, to be seen on a white screen placed to receive it, or in the 
 air, by looking towards it in a certain direction. Now, the most 
 important optical instruments, and even the living eye, are merely 
 arrangements of parts for producing and viewing such an image 
 under variety of circumstances. Wlicn this image is received upon a 
 suitable white surface or screen in a dark room, the arrangement 
 
LIGHT. 
 
 2.5 called, according to -minor circumstances, a CAMERA OBSCURA, 
 
 a MAGIC LANTEUX, OT SOLAR MICROSCOPE. Jind tilt EYE itself I S 
 
 in fact, but a small camera obscura, of which the pupil is the 
 round opening or window before the lens, enabling the mind to 
 judge of external objects by the size, brightness, colour, fyc., of 
 the very minute but most perfect images or pictures formed at the 
 back of the eye, on the smooth screen of nerve called the retina. 
 The art of painting aims at producing on a larger scale such 
 a picture, and tvhich tvhen afterwards held before the eye, and 
 reproducing itself in miniature upon the retina, may excite the 
 same impression as the original objects. When the image beyond 
 a lens, formed as above described, is viewed in the air by looking 
 at it in a particular direction, then there is exhibited the arrange- 
 ment of parts constituting the TELESCOPE, or COMMON MICROS- 
 COPE. 
 
 Light falling on very smooth or polished surfaces, is reflected so 
 nearly in the order in which it falls, as to appear to the eye as if 
 coming directly from the objects originally emitting it, and sue] L 
 surfaces are called mirrors. Mirrors may be plane, convex, or con- 
 cave; and certain forms will produce images by reflection, just as 
 lenses produce them by refraction; so that there are reflecting tele- 
 scopes, microscopes, $*c., as there are refracting instruments of 
 the same kind. Light, again, falling on bodies of rougher or irre- 
 gular surface, or which have other peculiarities, is so modified as 
 to produce all those phenomena of colour and varied brightness seen 
 among natural bodies, and giving them their distinctive characters 
 and beauty. 
 
 " Light." (See the Analysis.) 
 
 The phenomena of light and vision have always been held 
 to constitute a most interesting branch of natural science; whe- 
 ther in regard to the beauty of light, or its utility. The beauty 
 is seen spread over a varied landscape among the beds of the 
 flower-gardens, on the spangled meads, in the plumage of birds, 
 in the clouds around the rising and setting sun, in the circles 
 of the rainbow. And the utility may be judged of by the re- 
 flection, that had man been compelled to supply his wants by 
 groping in utter and unchangeable darkness, even if originally 
 created with all the knowledge now existing in the world, he 
 
124 LIGHT. 
 
 could scarcely have secured his existence for one day. Indeed, 
 the earth without light would have been an unfit abode even 
 for grubs, generated and living always amidst their food. Eter- 
 nal night would have been universal death. Light, then, while 
 the beauteous garb of nature, clothing the garden and the mea- 
 dow, glowing in the ruby sparkling in the diamond, is 
 also the absolutely necessary medium of communication between 
 living creatures and the universe around them. The rising sun 
 is what converts the wilderness of darkness which night covered, 
 and which to the young mind, not ye.t aware of the regularity 
 of nature's changes, is so full of horror, into a visible and lovely 
 paradise. No wonder, then, if, in early ages of the world, 
 man has often been seen bending the knee before the glorious 
 luminary, and worshipping it as the God of Nature. When 
 a mariner who has been toiling in midnight gloom and tempest, 
 at last perceives the dawn of day, or even the rising of the 
 rnoon, the waves seem to him less lofty, the wind is only half 
 as fierce, sweet hope beams on him with the light of heaven, 
 and brings gladness to his heart. A man, wherever placed in 
 light, receives by the eye from every object around, from hill and 
 tree, and even a single leaf, nay, from every point in every 
 object, and at every moment of time, a messenger of light to tell 
 him what is there, and in what condition. Were he omnipre- 
 sent, or had he the power of flitting from place to place with 
 the speed of the wind, he could scarcely be more promptly in- 
 formed. And even in many cases where distance intervenes 
 not, light can impart at once, knowledge which, by any other 
 conceivable means, could come only tediously, or not at all. 
 For example, when the illuminated countenance is revealing 
 the secret workings of the heart, the tongue would in vain try 
 to speak, even in long phrases, what one smile of friendship or 
 affection can in an instant convey; and had there been no light, 
 man never could have been aware of the miniature worlds of 
 life and activity which, even in a drop of water, the microscope 
 discovers to him; nor could he have formed any idea of the 
 admirable structure belonging to many minute objects. It is 
 light, again, which gives the telegraph, by which men converse 
 from hill to hill, or across an extent of raging sea, and which 
 
EMANATION PROM THE SUN, &C. 125 
 
 pouring upon the eye through the optic tube, brings intelli- 
 gence of events passing in the remotest regions of space. 
 
 "Emanation from the sun," $c. (See the Analysis, page 
 
 122.) 
 
 The relation of the sun to light is most strikingly marked 
 in the contrast between night and day; as the relation between 
 combustion and light is seen in the brilliancy of an illuminated 
 hall or theatre, as compared with the perfect darkness when 
 the chandeliers are extinguished. In tropical countries, where 
 the sun rises almost perpendicularly, and allows not the long 
 dawn and twilight of temperate latitudes, the change from 
 perfect darkness to the overpowering effulgence of day, is so 
 sudden as to be most impressive. An eye turned to the east 
 has scarcely noted a commencing brightness there, when that 
 brightness has already become a glow; and if clouds be floating 
 near to meet the upward rays, they appear as masses of golden 
 fleece suspended in the sky: a little after the whole atmosphere 
 is bright, and the stream of direct light bending round, makes 
 the lofty mountain-tops shine like burnished pinnacles; then as 
 the stream reaches to still lower and lower levels, the inhabi- 
 tants of these in succession see the radiant circle first rising 
 above the horizon like a tip of flame, but soon displaying, as in 
 days of Pagan worship, all its breadth and glory, too bright 
 for the eye to dwell upon. With evening the same appear- 
 ances recur in a reversed order, ending, as in the morning they 
 began, in complete darkness. 
 
 Light emanates also from the stars, but they are so distant as 
 in that respect to be of little importance to this earth. And 
 there are still other transient sources in animal and vegetable 
 nature, and among solar phosphori, but they do not merit par- 
 ticular attention here. 
 
 There have been two opinions respecting the nature of light: 
 one, that it consists of extremely minute particles darting all 
 around from the luminous body; the other, that the phenome- 
 non is altogether dependent on an undulation among the par- 
 ticles of a very subtile elastic fluid diffused through space, as 
 sound is dependent on an undulation among air particles. Now, 
 
126 LIGHT. 
 
 if light be particles darting around, their minuteness must be 
 wonderful, as a taper can fill with them for hours a space of 
 four miles in diameter; and with the extreme velocity of light, 
 if its particles possessed at all the property of matter called in- 
 ertia, their momentum should be very remarkable, it being 
 found, however, that even a large sun-beam collected by a 
 burning-glass, and thrown upon the scale of a most delicate ba- 
 lance, has not the slightest effect upon the equilibrium. Such, 
 and many other facts to be treated of in subsequent parts of 
 this work, lead to the opinion that there is an undulation of an 
 elastic fluid concerned in producing the phenomenon of light. 
 
 " Becoming less intense as it spreads." (See the Analysis^ 
 
 page 122.) 
 
 Any emanation from a central point, in spreading through 
 wider space, becomes proportionally thinner or less intense. 
 Thus, if a taper be placed in the centre of a box, each side of 
 which is a foot square, all the light must fall on the sides of the 
 box, and will have a certain intensity there; if the taper be 
 then placed in a box with sides of two feet square, there will 
 be only the same quantity of light, but that will be spread over 
 four times the surface (a square of two feet is made up of four 
 squares of one foot,) and will therefore on any part of that 
 surface be only one-fourth part as strong or intense as in the 
 first box: and so for any other size of box or space, the inten- 
 sity diminishing as the square of the distance increases. 
 
 Hence four times as much light and heat fall upon a foot of 
 this earth's surface as upon a foot of the surface of the planet 
 Mars, which is twice as distant from the sun: as four times 
 as much light and heat fall on a man who is at one yard from 
 the fire, as on another who is at two yards. 
 
 " Falling on other bodies makes them visible. 7 ' (Read the 
 Analysis, page 122.) 
 
 If the window shutter of an apartment be perfectly closed, 
 an eye there turns upon an absolute blank: it perceives nothing. 
 If a ray of the sun be then admitted, and made to fall upon any 
 object, that object becomes bright, and affects the eye as if it 
 
MAKES BODIES VISIBLE. 127 
 
 were itself luminous. It returns a part of the light which falls 
 upon it, and it is visible in all directions, proving that it scat- 
 ters the received light all around. This scattered light, again, 
 falling on other objects, and reflected from and among them un- 
 til absorbed, like echo repeated many times and lost between 
 perpendicular rocks, makes all of them also visible, although in 
 a less degree, and the whole apartment is said to be lighted. 
 If the sun's ray be made to fall upon a thing which from its na- 
 ture reflects much of the light, as a sheet of white paper, the 
 apartment will be well lighted: if, on the contrary, it be re- 
 ceived on black velvet, which returns hardly any light, the 
 apartment will remain dark; and, again, if received on a po- 
 lished mirror, which returns nearly the whole light, but in one 
 direction only, and therefore throws it upon some other single 
 object, the effect will be according to the nature of that object^ 
 and nearly as if the ray had fallen directly upon it. 
 
 Now, all bodies on earth, and very remarkably the mass of 
 atmosphere surrounding the earth, retain and diffuse among 
 themselves for a time the light received directly from the sun, 
 and by so doing, maintain that milder radiance so agreeable to 
 the sight, which renders objects visible when the sun's direct 
 ray does not fall upon them. But for this fact, indeed, all bo- 
 dies shadowed from the sun, whether by intervening clouds or 
 by any other more opaque masses on earth, would be perfect- 
 ly black or dark; that is, totally invisible. And without an 
 atmosphere, the sun would appear a red hot orb in a black sky. 
 On lofty summits, where half the atmosphere is below the le- 
 vel, the direct rays of the sun are painfully intense, and the 
 sky is of darkest blue. 
 
 A shadow is the name given to the comparative darkness of 
 places or objects, prevented by intervening obstacles from re- 
 ceiving the direct rays of some luminous body shining on the 
 things around. The apparent darkness of a shadow, however, 
 i is not proportioned to its real darkness, but to the intensity of 
 the surrounding lights. A landscape may be very bright, CVCR 
 when the sun is veiled by clouds, and then little or no shadow 
 is perceived; but, as soon as the clouds pass away, deep sha- 
 dows are cast behind every projecting object. Yet the objects 
 
128 LIGHT. 
 
 and places then appearing so dark, are, in reality, more illumi- 
 nated than before the shadow existed; for they are receiving, 
 and again scattering new light from all the more intensely il- 
 luminated objects around Ihem. A finger held between a can- 
 dle and the wall, casts a shadow of a certain intensity; if ano- 
 ther candle be then placed in the same line from the wall, the 
 shadow will appear doubly dark, although, in fact, more light 
 will be reaching the eye from it than before. If the candles 
 be separated laterally, so as to produce two shadows of the finger, 
 but which coincide or overlap in one part, that part will be of 
 double darkness, as compared with the remainders. The most 
 accurate mode of comparing lights is to place them at different 
 distances from a screen or wall, so as to make them at the same 
 time throw equally dark shadows; and then, according to the 
 law of decreasing intensity explained above, to calculate the 
 intensities of the sources of light by the difference of their 
 distances from the wall. The eye judges very easily of the 
 equal intensity of compared shadows. 
 
 The real darkness of a shadow depends on the number and 
 nature of the light-reflecting objects around it. Thus, sha- 
 dows are less remarkable opposite to any white surface, as that 
 of a recently painted wall. The reason why the moon when 
 eclipsed, that is, as will be afterwards explained, when passing 
 through the shadow cast by the earth on the side away from 
 the sun, is almost quite invisible, is, that there are no similar 
 bodies bearing laterally on the moon to share their light with 
 it. And the reason why our nights on earth are darker than 
 the shadows behind a house or rock in the sun-shine of day, 
 is merely that there are not other earths near us to reflect light 
 into the great night-shadow of the earth, as there are other 
 houses and rocks to illuminate the day-shadow of these. The 
 moon is the only light-reflecting body which the earth has near 
 it; and we perceive how much less dark the night-shadow is 
 when the moon is so placed as to bear upon it. The eclipsed 
 moon, again, is invisible, because facing the shadowed part of 
 the earth; but when the moon is in the situation called new 
 moon,, the bright crescent, or part directly illuminated by the 
 sun, is always seen to be surrounding the shaded part, as if 
 
MOVES WITH GREAT VELOCITY. 
 
 holding the old moon in its arms: that is, the shaded side of 
 the moon is then, in a degree, visible to us, because facing the 
 enlightened side of the earth. 
 
 Many persons have doubted whether the light of the moon 
 could be altogether reflected light of the sun; the moon ap- 
 pearing to them more luminous than any opaque body on earth 
 merely exposed to the sun's rays. Their error has arisen from 
 their contrasting the moon while returning direct sunshine 
 with the shadows of night on the earth around them. But 
 could they then see on a hill near them, a white tower or other 
 object scattering light as when receiving the rays of a meridian 
 sun, that object would appear to them to be on fire, and there- 
 fore much brighter than the moon. The moon, when above 
 the horizon in the day-time, is perfectly visible on earth, and 
 is then throwing towards the earth as much light as during the 
 night; but the day moon does not appear more luminous than 
 any small white cloud, and although visible every day except 
 near the change, many persons have passed their lives without 
 ever observing it. The full moon gives to the earth only about 
 a one-hundred-thousandth part as much light as the sun. 
 
 " Light moves with great velocity." (See the Analysis, 
 page 122.) 
 
 The extraordinary precision with which the astronomical 
 skill of modern times enables men to foretell the times of re- 
 markable appearances or changes among the heavenly bodies, 
 has served for the detection of the fact, that light is not an in- 
 stantaneous communication between distant objects and the 
 eye, as was formerly believed, but a messenger which requires 
 time to travel: and the rate of travelling has been ascertained 
 in the same way. 
 
 The eclipses of the satellites or moons of the planet Jupiter 
 had been carefully observed for some time, and a rule was ob- 
 tained which foretold the instants in all future time when the 
 satellites were to glide into the shadow of the planet, and dis- 
 appear, or again to emerge into view. Now, it was found, that 
 these appearances took place 16 minutes sooner when Jupiter 
 was near the earth, or on the same side of the sun with the 
 
 17 
 
130 LIGHT. 
 
 earth, than when it was on the other side; that is to say, more 
 distant from the earth by one diameter of the earth's orbit, and 
 at all intermediate stations the difference diminished from the 
 16i minutes, in exact proportion to the less distance from the 
 earth. This proves then that light takes 16^ minutes to travel 
 across the earth's orbit, and 8$ minutes for half that distance, 
 or to come down to us from the sun. 
 
 The velocity of light, ascertained in this way, is such, that 
 in one second of time, viz. during a single vibration of a com- 
 mon clock pendulum, it would go from London io Edinburgh 
 and back 200 times, and the distance between these is 400 
 miles. This velocity is so surprising that the philosophic Dr. 
 Hooke, when it was first asserted that light was thus progres- 
 sive, said he could more easily believe the passage to be abso- 
 lutely instantaneous, even for any distance, than that there 
 should be a progressive movement so inconceivably swift. The 
 truth, however, is now put quite beyond a doubt by many col- 
 lateral facts bearing upon it. 
 
 As regards all phenomena upon earth, they may be regarded 
 as happening at the very instant when the eye perceives them; 
 the difference of time being too small to be appreciated: for, 
 as shown in the preceding paragraph, if our sight could reach 
 from London to Edinburgh, we should perceive a phenome- 
 non there in the four-hundredth part of a second after its oc- 
 currence. 
 
 It is, hence, usual, and not sensibly incorrect, when we are 
 measuring the velocity of sound, as when a cannon is fired, by 
 observing the time between the flash and the report, to suppose 
 that the event takes place at the very moment when it is per- 
 ceived by the eye. 
 
 In using a telegraph, no sensible time is lost on account of 
 light requiring time to travel. A message can be sent from 
 London to Portsmouth in a minute and a half; and at the same 
 rate a communication might pass to Rome in about half an 
 hour, to Constantinople in forty minutes, to Calcutta in a few 
 hours, and so on. A telegraph is any object which can be 
 made to assume different forms or appearances at the will of 
 an attendant, and so that the changes may be distinguished at 
 a distance. A pole with moveable arms is the common con- 
 
i 
 
 PROCEEDS IN STRAIGHT LINES. 131 
 
 struction, each position standing for a letter, or cipher, or 
 word, or sentence, as may be agreed upon. Telegraphic sig- 
 nals between ships at sea are generally made by a few flags, the 
 meanings of each being varied by the mast on which it is hoist- 
 ed, and by its combination with others. 
 
 " Light proceeds in straight lines," $c. (Read the Analy- 
 sis, page 122.) 
 
 Our very notion of a straight line is taken from the direc- 
 tion in which light moves: but we can verify a line so ob- 
 tained by other means, as by stretching a cord between the two 
 extremes, or by suspending a weight by a cord, and making a 
 moveable solid measure to correspond with this, which mea- 
 sure may be used in any other case. 
 
 We can see through a straight tube, but not through a crook- 
 ed one. The vista through a long straight tunnel is striking 
 as an illustration of this fact, and of the diminution in the ap- 
 parent size of objects as they are more distant. If a person 
 enter one end of the canal tunnel, two miles long, cut through 
 the chalk hills near Rochester, to join the Thames and Med- 
 way rivers, the opening at the distant end is seen as a minute 
 luminous speck, having the form of the general arch, and ap- 
 pearing in the centre of the shade to an eye placed in the cen- 
 tre; and a person who has advanced half way through the tun- 
 nel, may see the luminous speck, at each end, then appearing 
 a little larger than in the former case. 
 
 In taking aim with gun or arrow, we are merely trying to 
 make the projectile go to the desired objects nearly by the path 
 along which the light comes from the object to the eye. 
 
 A carpenter looks along the edge of a plank, &c. to see whe- 
 ther it be straight. 
 
 Because light moves in straight lines, if a number of similar 
 objects be placed in a row from the eye, the nearest one hides 
 the others. In a wood or a city, a person sees only the trees 
 or houses that are near. 
 
 Some ignorant people believe that a squinting person can see 
 round a corner, as they believe that a crooked gun can shoot 
 round a corner. 
 
!;"!:> LIGHT 
 
 All astronomical and trigonometrical observations are made 
 on the faith of this property of light, the observer holding that 
 any object is situated from him in the direction in which the 
 light comes to him from it. When the mariner, after watching 
 for hours in cloudy weather, has caught a glimpse of the sun or 
 a star through his sextant glass, he has ascertained his place 
 among the trackless waves, and boldly advances through the 
 midst of hidden dangers. And the beam from the light house 
 looking from the rocky height over the sea, would be useless if 
 the light from it came not in a straight line. 
 
 " Leaving shadows where it cannot fall." (See the Ana- 
 lysis, page 122.) 
 
 The form of shadows proves that light moves in straight 
 lines, for the outline of the shadow is always correctly that of 
 the object as seen from the luminous body. If the light bent 
 round the body, this could not be. 
 
 The shadow of a face on the wall is a correct profile. 
 
 As a wheel presented edgeways to the eye appears only like 
 a broad line, but becomes oval or round as it is more turned, so 
 a wheel presented edgeways to the sun or other light, casts a 
 linear shadow on the wall behind it, the shadow becoming oval 
 or round as the position is changed. 
 
 A globe, a cylinder, a cone, and a flat circle, will all throw 
 the same form of shadow if held with their axes pointing to the 
 luminous body, and therefore by the shadow only, these objects 
 could not be distinguished. 
 
 The figure of a rabbit cut in pasteboard, will throw the same 
 sbadow on the wall as the animal itself ; and, again, that sha- 
 dow may be perfectly imitated by a certain position of the two 
 hands joined, as is known to those who find pleasure in wit- 
 nessing the surprise and delight of infancy made to behold such 
 a shadow mimicking the actions of life. 
 
 A man under the vertical sun stands upon his little round 
 shadow; but as the sun declines in the afternoon, the shadow 
 juts out on the opposite side, and at last may extend over a 
 whole field. 
 
 A distant cloud which appears to the eye of an observer only 
 
FORM OP SHADOWS. 133 
 
 as a line in the sky, maybe shadowing a whole region; for 
 clouds generally form in level strata, and when viewed at a 
 distance are seen edgeways. 
 
 The velocity of the wind may be ascertained by marking the 
 time which the shadow of a cloud takes to pass over a plain or 
 other space of known dimension. 
 
 All the heavenly bodies of the solar system cast a shadow 
 beyond them on the side opposite to the sun, as is seen when 
 any body previously visible passes where that shadow is. The 
 satellites or moons of Jupiter, when they suddenly disappear to 
 our glasses, have generally only plunged into the shadow of the 
 planet, and are not hidden behind its body, as many suppose. 
 When our own moon is the subject of that phenomenon so aw- 
 ful in the early ages of the world, an eclipse, she is only pass- 
 ing through the round shadow which the earth casts beyond it. 
 
 When the luminous centre is larger than the body which 
 casts the shadow, the shadow will he less than the body. This 
 is true of the shadows of all the planets and of the earth, be- 
 cause they are less than the sun. 
 
 On the contrary, if the light-giving surface is smaller than 
 the opaque body, the shadow will be larger than the body. 
 The shadow of a hand held between a candle and the wall is 
 gigantic; and a small pasteboard figure of a man placed near a 
 narrow centre of light, throws a shadow as big as a real man. 
 The latter fact has been amusingly illustrated by the art of 
 making phantasmagoric shadows. 
 
 When the surface which receives a shadow is not directly ex- 
 posed to the light, the shadow may be much larger than the ob- 
 ject, even although the sun himself be throwing the light; as is 
 seen when a slightly projecting roof shadows from the high 
 sun of summer noon the whole front of a house, or as is proved 
 by the long evening shadows of all countries. 
 
 "Light passes readily through some bodies which are 
 therefore called transparent; but when it enters or leaves 
 their surfaces obliquely, its course is bent." (Read the 
 Analysis, page 122.) 
 
 It may well excite the surprise of inquirers that light, of 
 
134 LIGHT. 
 
 which the constituent particles are so inconceivably minute, 
 should still be able to dart readily and in every direction through 
 great masses of solid matter, but such is the truth. Thick 
 plates of solid glass, blocks of rock crystal, mountains of ice, 
 &c., are instantly pervaded by the beam of the sun. 
 
 What it is in the constitution of one mass as compared with 
 another, which fits the former to transmit light, and the latter 
 to obstruct it, we cannot clearly explain, but we perceive that 
 the arrangement of the particles has more influence than their 
 peculiar nature. Nothing is more opaque than thick masses of 
 the metals, but nothing is more transparent than equally thick 
 masses of the same metals in solution, nor than the glasses of 
 which a metal forms a large proportion. The thousand salts 
 formed by the union of the metals or earths with the diluted 
 acids, are all transparent, when, in cooling from the fluid to the 
 solid state, their particles have been allowed to arrange them- 
 selves according to the laws of their mutual attraction, that is 
 to say, to form crystals; but the same substances in other states, 
 as when reduced to powder, are opaque. Even the pure me- 
 tals themselves, when reduced to leaves of great thinness, are 
 transparent, as may be perceived by looking at a lamp through 
 fine gold leaf. It is to be remarked, however, that even the 
 most transparent bodies intercept a considerable part of the 
 light which enters them: a depth of seven feet of pure water 
 intercepts about one half, so that the bottom of the sea is very 
 dark. And of the sun's light, when passing obliquely, through 
 the atmosphere towards the earth, only a small part arrives. 
 
 Light having once entered a transparent mass of uniform na- 
 ture, passes forward in it as straightly as in a vacuum; but at 
 the surface, whether on entering or leaving it, if the passage be 
 oblique, and if the mass be of a different density from the 
 transparent medium around it, a very curious and most impor- 
 tant phenomenon occurs, viz. the light suffers a degree of bend- 
 ing from its antecedent- direction, or a refraction, proportioned 
 to the obliquity. 
 
 But for this fact, which to many persons might at first ap- 
 pear a subject of regret, as preventing the distinct vision of ob- 
 jects through all transparent media, light could have been of 
 
REFRACTION. 135 
 
 little utility to man. There could have been neither lenses as 
 now, nor any optical instruments, as telescopes and microscopes, 
 of which lenses form a part; nor even the eye itself. 
 
 Light falling from the air directly or perpendicularly upon a 
 surface of water, glass, or any such transparent body, passes 
 through without suffering the least bending; -a ray for instance 
 
 shot from a to the point c, in the sur- 
 face of a piece of glass g h, would 
 reach directly across to b; but if the 
 , ray fell obliquely, as from d to c, 
 then, instead of continuing in its first 
 direction, and going on to i and k, it 
 would at the moment of its entrance 
 be bent downwards into the path c e, 
 nearer to a line c o, called the per- 
 pendicular to the surface at the point 
 
 of entrance, and then moving straightly while in the substance 
 of the glass, it would, when it passed out again at e, in the op- 
 posite surface, be bent just as much as at first, but in the con- 
 trary direction, or away from a similar perpendicular at that 
 surface, viz. into the line e f instead of e n. A ray therefore 
 passing obliquely through a transparent body of parallel sur- 
 faces, has its course shifted a little to one side of the original 
 course, but still proceeds in the same direction, or in a line pa- 
 rallel to the first as here shown in the line e f, parallel and 
 near to the line i k. 
 
 The degree of bending or refraction of light in traversing a 
 transparent surface is ascertained by comparing the obliquity of 
 its approach to the surface with the obliquity of its departure 
 after passing; and for this purpose a line is supposed to be 
 drawn perpendicularly through the surface at the point where 
 the ray passes (as a b in the above figure drawn through c y 
 where the ray d c passes,) and the relative positions of the ray 
 to this line on both sides of the surface, are easily ascertained. 
 Thus the line a d, drawn from any point of such perpendicular 
 to the ray before passing, is a measure of the original obliquity 
 or angular distance of the ray, and is called the sine of the 
 angle of incidence, and the other line o e drawn from a cor- 
 
136 LIGHT. 
 
 responding point of the perpendicular to the ray after the pass- 
 ing, is a measure of the obliquity after refraction, and is called 
 the sine of the angle of refraction: by comparing these 
 two lines in any case, the problem is solved. 
 
 When light passes obliquely from air into water, the refrac- 
 tion or bending produced is such, that the line a d measuring 
 the obliquity before refraction, is always longer than the line 
 o e measuring it after refraction, by nearly one-third of the latter, 
 and the refractive power of water is therefore signified by the 
 index. 1 or 1.33; as in like manner the greater refractive power 
 of common glass has the index !, that of diamond the index 
 2|, and so on. And it is important to remark, that for the 
 same substance, whatever relation holds between the obliquity 
 of a ray and the refraction in any one case, the same holds for 
 all cases. If, for instance, where the obliquity, as measured by 
 its sine, is 40, and the refraction is half, or 20, then in the 
 same substance an obliquity of 10 will occasion a refraction of 
 5, and an obliquity of 4 will occasion a refraction of 2; and 
 so on. 
 
 As a general rule, the refractive power of transparent sub- 
 stances or media is proportioned to their densities. It increases, 
 for instance, through the list of air, water, salt, glass, &c. 
 But Newton, while engaged in his experiments upon the sub- 
 ject, observed that inflammable bodies had greater refractive 
 power than others, and he then hazarded the conjecture, almost 
 of inspired sagacity, and which chemistry has since so remark- 
 ably verified, that diamond and water contained inflammable 
 ingredients. We now know that diamond is merely crystal- 
 lized carbon, and that water consists altogether of hydrogen or 
 inflammable air and oxygen. Diamond has nearly the greatest 
 light-bending power of any known substances, and hence comes 
 in part its brilliancy as a jewel. 
 
 No good explanation, has been given of the singular fact of 
 refraction; but to facilitate the conception and remembrance of 
 it, we say that it happens as if it were owing to an attraction be- 
 tween the light and the refracting body or medium. The light 
 approaching from d to c, for instance, may be supposed to be 
 attracted by the solid body below it, so as at the surface to be 
 
(7 
 
 REFRACTION. 137 
 
 bent into the direction c e; and, again, on leaving the body to 
 be still equally attracted and bent back, so as to take the direc- 
 tion ef, instead of e n. 
 
 The following are familiar examples of this bending of light in 
 passing from one medium to another. 
 
 If an empty basin or other vessel, b cf, be placed in the sun's 
 light, so that the rays falling within it may reach low on the 
 side, as to d, but not to the bottom, 
 $. then, on filling the vessel with water, 
 rx^& the sun will be found to be shining on 
 the bottom or down to e, as well as on 
 the side. The reason of this pheno- 
 menon is, that water being a denser 
 medium than air, the light, on enter- 
 ing it at c, is bent towards the perpendicular at the point of in- 
 cidence, or cf, and so reaches the bottom. Again, if a coin 
 or medal were laid on the bottom of such a vessel at e, it would 
 not, while the vessel were empty, be seen by an eye at a, but 
 would be visible there immediately on the vessel being filled 
 with water; because then, the light leaving it in the direction 
 e c, towards the edge of the vessel, would at c, on passing from 
 the water into air, be bent away from the perpendicular c h, 
 and instead of going to g would reach the eye at a. The coin 
 moreover would appear to the eye to be in the direction c dy 
 instead of in the true direction c e: for the eye not being able to 
 discover that the light had been Bent in its course, would ju*dge 
 the object to be in the line by which the light came from it. 
 
 It is thus because objects at the bottom of water, when viewed 
 obliquely, do not appear so low as they really are, that a per- 
 son examining a river or pond, or any clear water, from its 
 bank, naturally judges its depth to be less than the truth. Many 
 a young life has been sacrificed to this error. A person look- 
 ing from a boat directly down upon objects at the bottom of 
 water, sees them in their true places and at their true distances, 
 but if he view them more and more obliquely, the appearance 
 is more and more deceiving, until at last it represents them as 
 at less than half of their true depth. 
 
 18 
 
1 38 LIGHT. 
 
 The ship in which the author sailed once in the China sea, 
 before danger was apprehended, had entered by a narrow pas- 
 sage into a horse-shoe enclosure of coral rocks. When the 
 alarm was given, the predicament had become truly terrific. 
 On every side, in water most singularly transparent, as the 
 wave swelled, the rocks appeared to be almost at the surface of 
 the water, and the anchor, which in the first moments had been 
 let go to limit the danger, appeared to be lifted with them- It 
 was judged that if the ship, then drawing 24 feet, or the depth 
 of a two-storied house, had moved but a little way in almost 
 any direction, she must have met her certain destruction. On 
 sending boats around to sound and to search, the place of en- 
 trance was again discovered, and was safely traversed a second 
 time as an outlet from that terrible prison. 
 
 On account of this bending of light from' objects under water, 
 there is more difficulty in hitting them with a bullet or spear. 
 The aim by a person not directly over a fish must be made at 
 ;a point apparently below it, otherwise the weapon will miss it 
 by flying too high. The spear is sometimes used in this coun- 
 try for killing salmon, but is a common weapon among the isl- 
 ?inders of the Atlantic and Pacific Oceans for killing the alba- 
 core; the use of it, like that of the fly hook in England, afford- 
 ing, to the fishermen, good sport as well as profit. The author 
 once with much interest witnessed at St. Helena this employ- 
 ment of the spear. A small fish previously half-killed, that it 
 might not try to escape, was every minute or two thrown upon 
 the.water as a bait, in the sight of perhaps a hundred great al- 
 bacores, greedily waiting for it at one side below, and know- 
 ing the danger to which they exposed themselves by darting 
 across to seize it. Some albacore bold enough, soon made at 
 the mouthful, apparently with the speed of lightning, but yet 
 with speed which did not save him, for every now and then 
 the thrown spear met the adventurer, and held him writhing 
 there in a cloud of his death-blood. After a victim so de- 
 stroyed, the scene of action was changed. 
 
 The bending of light when passing obliquely from water, is 
 also the reason of the following facts. A straight rod or stick, 
 of which a portion is immersed in water, appears crooked or 
 
REFRACTION. 139 
 
 broken at the surface of the water, the portion immersed seem- 
 ing to be bent upwards. That part of a ship or boat visible 
 under water, appears much flatter or shallower than it really 
 is. A deep-bodied fish seen near the surface of water, appears 
 almost a flat fish. A round body there appears oval. A gold 
 fish in a vase may appear as two fishes, being seen as well by 
 light bent through the upper surface of the water, as by straight 
 rays passing through the side of the glass. To see bodies un- 
 der water, in their true places and of their true proportions, 
 the eye must view them through a tube, of which the distant 
 end, closed with plate glass, is held in the water. 
 
 As light is bent on entering from air into water, glass, or 
 other substance denser than air, so is it also bent on coming 
 from void space into the ocean of our atmosphere. Hence none 
 of the heavenly bodies, except when directly over our heads, 
 
 are seen by us in their true 
 situations. They all appear 
 a little higher than they real- 
 ly are, as- when to a specta- 
 tor at d, supposed on the sur- 
 face of the earth, a star real- 
 ly at A appears to be at a i 
 because its ray on reaching 
 the atmosphere at c is bent downwards. In astronomical books 
 there is always introduced a table of refraction, as it is called, 
 showing what correction mast be made on this account for dif- 
 ferent apparent altitudes. This effect of our atmosphere so- 
 bends the rays of the sun that we see him in the morning be- 
 fore he is really above the horizon, and we see him in the 
 evening after he is really below it, viz. the ray coming hori- 
 zontally from 'e to d, appears to come from b, although in truth 
 it comes from the lower situation B, and is bent into the level 
 line only at e'. Oar atmosphere thus, by the bending of light 
 as well as by itself becoming luminous, lengthens at dawn and 
 twilight the duration of the lovely day. As the atmosphere is 
 denser near the surface of the earth than higher up, the light 
 is more and more bent as it descends, and hence describes a 
 
140 LIGHT. 
 
 course which is a little curved, and therefore unlike the course 
 of light in water. 
 
 Certain states of the atmosphere depending upon its humi- 
 dity, warmth, &c., change very considerably its ordinary re- 
 fractive power, hence in one state of it, a certain hill or island 
 may appear low and scarcely rising above the intervening 
 heights or ocean, while in another state, the same object shall 
 be seen towering above: and from a certain station, a city in a 
 neighbouring valley may be either entirely visible, or it may 
 show only the tops of its steeples, as if the bed on which it 
 rested had sunk deeper into the earth. In days of ignorance 
 and superstition, such appearances have sometimes excited a 
 strange interest. 
 
 A beautiful phenomenon is observable in a day of warm sun- 
 shine, owing to the bending of light in passing through media 
 of different densities. Black or dark-coloured substances, by 
 absorbing much light and heat from the sun's rays, and warming* 
 the air in contact with them, until it dilates and rises in the 
 surrounding air, as oil rises in water, cause the light, from 
 more distant objects, reaching the eye through the rarefied me- 
 dium, to be bent a little; and owing to the heated air rising ir- 
 regularly under the influence of the wind and other causes, 
 these objects acquire the appearance of having a tremulous or a 
 dancing motion. In a warm clear day, the whole landscape at 
 last appears to be thus dancing. 
 
 The same phenomenon is to be observed at any time, by 
 looking at an object beyond the top of a chimney from which 
 hot air is rising. An illicit distillery was once discovered by 
 the exciseman happening thus to look across a hole used as the 
 chimney, although charcoal was the fuel, and there was no 
 vestige of smoke. 
 
 This bending of light by the varying states of the atmosphere 
 makes precaution necessary in making very nice geometrical 
 observations: as in measuring base lines for the construction 
 of maps or charts. 
 
 As it is the obliquity between the passing ray and the sur- 
 face, which in any case of refraction determines the degree of 
 
REFRACTION. 
 
 141 
 
 bending, a body seen through a medium of irregular surface 
 appears distorted according to the nature of that surface. It is 
 because the two surfaces of common window-glass are not per- 
 fect planes, and not perfectly parallel to each other, as in the 
 case of plate glass, that objects seen through the former appear 
 generally more or less out of shape; and hence comes the ele- 
 gance and beauty of plate glass windows: and hence the singu- 
 lar distortion of things viewed through that swelling or lump 
 of glass which remains where the glass-blower's instrument 
 was attached, and which appears at the centre of certain very 
 coarse panes. 
 
 The refraction or bending of light is interestingly exempli- 
 fied in the effect of the glass called a prism, viz. a wedge or 
 three-sided rod of glass, such as that of which the end is here 
 
 represented at b c. A 
 ray from , falling on 
 the surface at b, is bent 
 towards the internal per- 
 pendicular, and therefore 
 reaches c, but, on escaping again at c, it is bent away from the 
 external perpendicular, and thus, with its original deviation 
 doubled, goes on to d. 
 
 The law of light's bending, according to the obliquity with 
 which it traverses the surfaces of a transparent body, is well 
 elucidated by the effect of what is called a multiplying glass; 
 that is to say, a piece of glass like .a bee, 
 having many distinct faces cut upon it at an- 
 gles with each other. If a small object, a 
 coloured bead for instance, be placed at d r 
 an eye at e will see as many beads as there 
 are distinct surfaces or faces on the glass; for, 
 first, the ray, d a, passing perpendicularly, 
 and therefore straight through, will form an 
 image as if no glass intervened, then, the 
 rays from d to the surface b will be bent by 
 the oblique surface, and will show the object 
 as if it were in the direction e b; then the 
 light falling on the still more oblique sur- 
 face, c, will be still more bent, and will reach 
 
142 LIGHT. 
 
 the eye in the direction c e, exhibiting a similar object also in 
 that direction and so of all the other surfaces. If the rela- 
 tive places of the eye and object be changed, the result will still 
 be the same. A plate of glass, roughened or cut into cross fur- 
 rows, becomes a very good screen or window blind, by so dis- 
 turbing the passage of light through it, that objects beyond it 
 are not distinguishable. 
 
 " <ftnd a beam of white, light thus made to bend, is resolved 
 into beams of the various primary colours; which beams, 
 however, on being again blended, become white light as 
 before. 7 ' (Read the Analysis, page 122.) 
 
 The most extraordinary fact connected with the bending of 
 light is, that a pure ray of white light from the sun admitted 
 into a darkened room by a hole in the window shutter, and 
 made to bend by passing through transparent surfaces which it 
 
 meets very obliquely (as 
 the ray a, admitted and 
 made to bend by passing 
 through the prism of 
 glass, b c, to fall upon 
 the wall at d,) instead of bending all together and appearing 
 still as the same white ray, is divided into several rays, which 
 falling on the white wall, are seen to be of different most vi- 
 vid colours. The original white ray is said thus to be ana- 
 lyzed or divided into elements. 
 
 This solar spectrum, as it is called, formed upon the wall, 
 consists, when the light is admitted by a narrow horizontal 
 slit, of four coloured patches corresponding to the slit, and ap- 
 pearing in the order, from the bottom, of red, green, blue, and 
 violet. If the slit be then made a little wider, the patches at 
 their edges overlap each other, and, as a painter would say, 
 produce by the mixture of their elementary colours various 
 new tints. Then the spectrum consists of the seven colours 
 commonly enumerated and seen in the rainbow, viz. red, yel- 
 low, orange, green, blue, indigo, and violet. Had red, yel- 
 low, blue, and violet been the four colours obtained in the first 
 experiment, the occurrence of the others, viz. of the orange, 
 
COLOURED REFRACTION. 143 
 
 from the mixing edges of the red and yellow of the green, 
 from the mixture of the yellow and blue, and of the indigo, 
 from the mixture of blue and violet, would have been antici- 
 pated. But the true facts of the case not being such, prove that 
 they are not yet well understood. When Newton first made 
 known the phenomenon of the many-coloured spectrum, and 
 the extraordinary conclusions to which it led, he excited uni- 
 versal astonishment; for the common idea of purity, the most 
 unmixed, was that of white light. In farther corroboration of 
 the notion of the compound nature of light, he mentioned, that 
 if the colours which appear on the spectrum be painted sepa- 
 rately round the rim of a wheel, and the wheel be then turned 
 rapidly, the individual colours cease to be distinguished, and a 
 white band only appears where they are whirling: also, that if 
 the rays of the spectrum, produced by a prism, be again ga- 
 thered together by a lens, they reproduce white light. The 
 red is the kind of light which is least bent in refraction, and 
 the violet that which is most bent. It was at one time said, 
 as an explanation, that the differently coloured particles in 
 light had different degrees of gravity or inertia, and were, . 
 therefore, not all equally bent. It is farther remarkable, with 
 respect to the solar spectrum, that much of the heat in the ray 
 is still less refracted than even the red light, for a thermome- 
 ter held below the red light rises higher than in any part of 
 the visible spectrum; and that there is an influence more re- 
 frangible than even the violet rays, producing powerful che- 
 mical and magnetical effects. The different spots of colour 
 are not all of the same size, and there is a difference in this re- 
 spect according to the refracting substance. 
 
 All transparent substances, in bending light, produce more or 
 less of the separation of colour; but it is important to remark, 
 that the quality of merely bending a beam, or of refraction, 
 and that of dividing it into coloured beams, or of dispersion, 
 are distinct qualities, and by no means proportioned to each 
 other in the same substances. Newton, from not discovering 
 this, despaired that a perfect telescope of refraction could ever 
 be made: he supposed that the bent light would always become 
 coloured, and so render the objects indistinct. We now know, 
 
144 LIGHT. 
 
 however, that by combining two or more media, we may ob- 
 tain bending of light without dispersion, thus, by opposing 
 a glass which bends five degrees and disperses one degree, to 
 another glass which bends three degrees and disperses one, the 
 opposing dispersions will just counterbalance or neutralize each 
 other, while the two degrees of excess of bending will remain 
 to be applied to use. 
 
 The diversified colours of the substances around us depend 
 merely upon the fitness of these, from texture or other cause, to 
 reflect or transmit certain modifications of common light, and 
 the colour is not a part or property of the body itself. We 
 shall soon find that the vivid colours of the rainbow are mere- 
 ly the white light of the sun: reflected to us after being bent 
 and modified by the colourless drops of falling rain; and that 
 the sparkling with appearance of rubies and emeralds, which 
 we see in the cut-glass lustre, is a phenomenon of the same 
 kind: and that by scratching the surface of a piece of metal 
 so as to have a given number of lines in a given space, we can 
 cause the same substance to appear of any colour we please. 
 
 " Transparent bodies, as glass , may be made of such form 
 as to cause all the rays of light which pass through them 
 from any one point, to bend so as to meet again in ano- 
 ther point beyond them, the body itself, from the re- 
 quired form generally resembling that of a flat bean or 
 lentil, being then called a "LENS." (Read the Analysis, 
 page 122.) 
 
 The innumerable rays of light, of which five only are here 
 represented, issuing from any point as c, towards any surface 
 in the situation a b, are said to form a cone or pencil of di- 
 verging light. Now, it is evident that to make all such rays 
 converge or meet again in one place, as f, beyond a transpa- 
 rent body placed at a b, it would be necessary, while the mid- 
 dle ray or axis of the pencil, c d, did not bend at all, for the 
 others to be bent more and more, in proportion as they fell 
 upon the body farther and farther from the centre d. Recol- 
 lecting then the law of refraction, that light entering from air 
 through the surface of any denser medium, as glass, is bent 
 
REFRACTION BY LENSES. 145 
 
 there towards the perpendicular at the internal surface, in pro- 
 portion to the obliquity of incidence, and on leaving the op- 
 posite surface, is correspondingly bent away from its external 
 perpendicular, (see the case of the prism at p. 141,) we see 
 that if a piece of glass were placed at b, of such form that 
 the rays falling upon it from c should meet and leave its sur- 
 faces with greater and greater obliquity in some regular pro- 
 portion, as the points of incidence were more distant from the 
 centre d, the purpose would be obtained. And we have the 
 pleasure of knowing that a glass, of which the surface is 
 ground which it easily may be to have a regular convexity 
 or bulging, as if it were a portion cut off from the surface of 
 a globe, can be shown to answer very correctly the required 
 condition. Such a glass, similarly ground on both sides, is 
 here represented edgeways, between and &, where the ray, c d, 
 falling on its middle, or perpendicularly, and similarly leaving 
 it, is seen going straight through to f; but the ray, c e, meet- 
 ing the surface with a certain degree of obliquity, is bent down 
 a little, first on entering the surface at e, and then as much 
 more on leaving the opposite surface with equal obliquity, and 
 "so arrives at f; then the ray c a, for corresponding reasons, is 
 still more bent, and equally arrives at f; and the case would 
 be similar of any other rays that might be examined. The 
 point f is usually called a focus, (meaning a fire place,) be- 
 cause when the light of the sun is thus gathered, the heat con 
 centrated with it is powerful enough to make combustibles in- 
 flame. We have here to remark farther, that, in accordance 
 both with calculation and experiment, the direction in which 
 
 19 
 
146 LIGHT. 
 
 s 
 
 a pencil of rays falls upon a lens does not affect the result of 
 the convergence to a focus, only the focus is always in the di- 
 rection of the central ray of the pencil or beam; it will be at 
 p, for instance, for light issuing from o, and at z for light is- 
 suing from x. 
 
 The lens represented at a b above, or at fig. 1, in the annexed 
 diagram, having both sides convex, is called a double convex 
 lens. A glass convex only on one side, and plane or flat on 
 Fig. 1234 5 6 the other, as shown 
 
 at (fig 2,) would as 
 effectually gather 
 the rays, but with 
 half the power, 
 and the point of 
 meeting or focus would be therefore so much the more distant. 
 Such a glass is called a plano-convex lens. Then the gather- 
 ing or converging power of any glass, whether doubly or singly 
 convex, is in proportion to the degree of its convexity or 
 bulging of surfaces; for the less it bulges, the more nearly does 
 it approach to a plane glass, and the more it bulges, the more ob- 
 liquely will the rays at any distance from the centre fall upon its 
 surface, and the sooner, therefore, in consequence of their being 
 more bent, will they all meet the axis-ray; hence fig. 1 would 
 converge much more quickly than fig. 3, which represents near- 
 ly a common spectacle glass; and a very minute globe is the 
 form most powerfully converging of all. The surfaces of fig. 
 1, are portions of a comparatively small globe; those of fig. 3 
 are comparatively smaller portions, but of a globe much larger. 
 Concave lenses as fig. 4, a double concave, and fig. 5, a plano- 
 concave lens, in obedience to the same law of refraction, spread 
 rays, or bend them away from the axis of the pencil, in the 
 same degree that similarly convex lenses gather them. A con- 
 cave lens, therefore, receiving the converging pencil of rays 
 from a convex lens, might restore them to their former direc- 
 tion. Very useful purposes, as will be afterwards explained, 
 are served in optics, by certain combinations of differently 
 formed lenses. A lens may be convex on one side and con- 
 cave on the other, as at fig. 6, a meniscus lens (so called be- 
 
IMAGES BY REFRACTION. 147 
 
 cause it resembles the crescent moon,) and its effect will be ac- 
 cording to the form which predominates. 
 
 A person recollecting the case of the <f multiplying glass," 
 described a few pages back, might say, but is not a convex 
 lens merely a multiplying glass of a much greater number of 
 faces, and why then, instead of one image, does it not make 
 thousands? The answer is, that the multiplying glass, by 
 every face, bends a set of rays, capable of forming a distinct 
 image: but the lens has no surface large enough to bend more 
 than a single ray, and it concentrates all the single rays into 
 one place, to form there one image of great vividness and 
 beauty. 
 
 " Jlnd when] the light proceeding from every point of an 
 object placed before a lens is collected in corresponding 
 points behind it, a perfect image of the object is there 
 produced. When the image is received upon a suitable 
 white surface, in a dark place, the arrangement is called 
 according to minor circumstances, the CAMERA OBSCURA, 
 SOLAR MICROSCOPE, or MAGIC LANTERN.'* (Read the Ana- 
 lysis, page 122.) 
 
 Words are wanting to express the admirable consequences to 
 man of the curious property of a lens, that it can bring together 
 to a focal point all the rays of light which traverse it from any 
 one point of an object placed before it. The following instance 
 
 will lead to the consideration 
 and understanding of others. 
 
 I If a lens, as a, be placed so as 
 
 -V; .- ... to fill up an opening made in 
 
 the window shutter of a dark- 
 ened room, then from any object before that opening, as the 
 cross here represented, all the light which each point of the 
 cross emits towards the lens will be concentrated or gathered 
 together in a corresponding focal point behind the lens or with- 
 in the room, and if a sheet of paper be held there at the dis- 
 tance of the focal points, a beautiful image of the object will 
 be seen upon the paper. 
 
 In these few words, we have described the interesting contri- 
 
148 LIGHT. 
 
 vance called the camera obscura or dark chamber; and when 
 a glass is chosen of proper size and focal distance, and a screen 
 or the wall of the chamber is properly prepared to receive the 
 light, the most enchanting portraiture is instantly produced of 
 the whole scene which the window commands. With what 
 rapture does the school boy first view this lovely picture drawn 
 by nature's own pencil, and with colours borrowed directly 
 from the sun's bright ray with what rapture, as his eyes search 
 over it, does he recognise his playmates there, and perhaps the 
 river in which he bathes, and where he sails his boat, and the 
 wood in whose solitudes he loves to wander, and the mountain 
 heights which he climbs to meet the fresh breeze, and at a dis- 
 tance from the world, to allow play to the workings of his 
 young fancy, beginning to shoot far into time and space. The 
 great peculiarity of such a picture is, that it does not like others 
 portray still nature, but every thing with appropriate motion 
 or changes: there the playmates are all in action; the leafy trees 
 may wave in the wind, the clouds may sail along, the sun may 
 rise or may set, and even the lightning's gleam may dart across: 
 or, again, commenced enterprises may be brought to a close, 
 the traveller may climb the distant hill and disappear, the fish- 
 erman may draw his net and store his gains, the well contested 
 race may be won or lost. A Malayan chief in the island of 
 Sumatra, was so surprised and pleased by a small portable ca- 
 mera obscura which the author had among his apparatus, that 
 he seemed disposed to give for it almost any thing he pos- 
 sessed. 
 
 It appears in the last diagram that the image formed beyond 
 a lens by the gathered light, is in a contrary position to the ob- 
 ject itself, that is, inverted, because, the light from the top 
 of the object, darts through the opening or glass in a descend- 
 ing direction, and that from the bottom rises to the opening, 
 and in the same direction passes beyond it. It is usual, there- 
 fore, in a camera obscura to place a small mirror immediately 
 behind the lens, so as to throw all the light which enters, 
 downwards to a whitened table, upon which the picture may 
 be conveniently contemplated. 
 
 The camera obscura often gives very useful assistance to 
 
CAMERA OBSCURA. 149 
 
 young painters, by enabling them to trace correctly the outlines 
 of the objects placed before it, and also to study effects of light, 
 shade, and colour, more profitably than they at first can, by 
 looking at the objects themselves. The laws of perspective 
 are most intelligibly illustrated in this most true picture. 
 
 An effect approaching in a degree to that of the complete 
 camera obscura now described, is produced by merely making 
 a small hole in the shutter of a dark room, and letting the light 
 which enters by it fall on any white surface beyond. The 
 whole landscape is then dimly portrayed upon the surface. If 
 a cross be held before the opening as in the last figure, it is evi- 
 dent that from every point of the cross light will enter by the 
 opening, and will fall on corresponding parts of a sheet of pa- 
 per held behind, but as the light from each point spreads as 
 it departs, becoming a pencil or cone of light instead of a ray, 
 it will fall on a surface of the paper at least as large as the 
 opening, and thus the light from adjoining spots will mix at 
 the edges, and will render the images misty and indistinct, 
 somewhat like those on the back of tapestry. If the opening 
 be very small, the picture will be well defined, but very feebly 
 illuminated; and if the opening be of considerable size, the 
 mixing of the pencils will be so great as to leave no particular 
 object distinguishable. In the latter case, and supposing the 
 opening to be larger than the pupils of a thousand eyes, if a 
 lens be introduced, it will converge every pencil of light to an 
 exact point, and the picture will instantly be rendered perfectly 
 clear. 
 
 The distance from a lens at which an image is formed or the 
 rays of the light meet, depends, first, upon the refractive or 
 bending power of the lens, and therefore on the nature of its 
 substance, and the form of the lens; and, secondly, upon the 
 direction of the rays of light when they reach the lens, viz. 
 whether they are divergent, parallel, or convergent. We have 
 already explained that glass refracts about twice as much as wa- 
 ter, and that diamond refracts about twice as much as glass; 
 and we have considered the effect of different degrees of con- 
 vexity in lenses arising equally whether the lens be of water 
 enclosed between glasses like watch glasses, or of solid glass, 
 
150 LIGHT 
 
 or of rock crystal, or of diamond itself. We now proceed to 
 consider the joint effect of the refractive power, and of the di- 
 rection of the incident rays. 
 
 Rays falling from a on a comparatively flat or weak lens at 
 L, might meet only at d, or even farther off; while with a 
 
 stronger or more convex lens, they might meet at c or at b: a 
 lens weaker still might only destroy the divergence of the rays, 
 without being able to give them any convergence, or to bend 
 them enough to bring them to a point at all, and then they 
 would proceed all parallel to each other, as seen at e and f: 
 and if the lens were yet weaker, it might only destroy a part 
 of the divergence, causing the rays from a to go to g and A, 
 after passing through, instead of to, i and k, in their original 
 direction. 
 
 In an analogous manner, light coming to the lens, in the con- 
 trary directions from b c d^ &c., might, according to the strength 
 of the lens, be all made to come to a focus at a or at /, or in 
 some more distant point; or the rays might become parallel, 
 as m and n, and therefore never come to a focus, or they might 
 remain divergent. 
 
 It may be observed in the figure above, that the farther aa 
 object is from the lens, the less divergent are the rays darting 
 from it towards the lens; or the more nearly do they approach 
 to being parallel. From b there is much divergence, from c 
 less, from d less still, and rays from a great distance, as those 
 cut off at efj appear quite parallel. If the distance of the ra- 
 diant point be very great, they really are so nearly parallel that 
 a very nice test is required to detect the non-accordance. Rays, 
 for instance, coming to the earth from the sun, do not diverge 
 the millionth of an inch in a thousand miles. Hence where we 
 
CAMERA OBSCURA. 151 
 
 wish to make experiments with parallel rays, we take those of 
 the sun. 
 
 Any two points so situated on the opposite sides of a lens, 
 as that wheh either becomes the radiant point of light, the other 
 is the focus of such light, are called conjugate foci. An ob- 
 ject and its image formed by a lens must always be in conju- 
 gate foci, and when the one is nearer the lens, the other will 
 be in a certain proportion more distant. 
 
 What is called the principal focus of a lens, and by the dis- 
 tance of which from the glass we compare or classify lenses 
 among themselves, is the point at which the sun's rays are made 
 to meet; and thus, by holding the glass in the sun, and noting 
 at what distance behind it the little luminous spot or image of 
 the sun is formed, we can at once ascertain the focus of a glass* 
 as at a for the rays e, and f. 
 
 It is remarkable that the bending power of the common glass 
 used for lenses should be such, that the focus of a double lens of 
 glass is just where the centre of the sphere would be,, of which 
 the surface of the lens is a portion. This gives us another fact 
 with which to associate the recollection that the focus is near as 
 the convexity of the lens is greater, that is to say, as the surface 
 is a portion of a smaller sphere. And such being the law, it may 
 be proved by calculation, as well as by the fact, that if a candle 
 be held from a lens at twice the principal focal distance, suppose 
 at c for a lens with the focus at cr, the image of the candle will 
 be formed at /just as far on the other side. Thus, then, by try- 
 ing with a lens until the image of a candle is formed at the same 
 distance from it as the object is, we have a second mode of as- 
 certaining the focal distance of a lens. Other kinds of glass and 
 Other substances refract with different power; but the facts now 
 stated should be retained in the memory as standards of com- 
 parison. 
 
 Because the focal point of light passing through a lens is at the 
 same distance from the centre of the lens, in whatever direction 
 the light passes through, a surface placed to receive the image 
 of any object, should really be concave; that is to say, all parts 
 of it should be at the same distance from the centre of the lens, 
 otherwise the image will be more perfect either at its middle than 
 
152 LIGHT. 
 
 towards its edges, or vice versa but it is not found necessary 
 to attend to this in common practice. 
 
 The size of an image formed behind a lens is always propor- 
 tioned to its distance from the lens, and the image is as much 
 larger or smaller than the object as it is farther from or nearer to 
 the lens than the object. This will be evident from considering 
 
 the annexed figure, c repre- 
 sents a lens, which, according 
 to its power, will form an image 
 of the cross a b in some situ- 
 ation, as at d, e, g, &c. Now 
 wherever the image is formed, 
 
 and by whatever lens, one end of it must be in contact with the 
 line a g, and the other end with the line b h; and as these lines 
 cross each other at c, and widen regularly afterwards, a line join- 
 ing them (and the image is really such a line,) must always 
 be shorter the nearer it is to c; that is to say, shorter in propor- 
 tion to the converging power of the lens. 
 
 Many persons may not have reflected, that the little luminous 
 circle called the focus of a burning glass, is really but the image 
 or picture of the sun formed by that glass or lens. The inten- 
 sity of the heat and of the light is of course in proportion as 
 the image is smaller than the glass which forms it, and the near- 
 er that the image is formed to the lens, or the more powerfully 
 convergent that the lens is, the smaller will the image be. Mr. 
 Parker's famous burning lens, which cost ;700, and is now the 
 property of the emperor of China, was three feet in diameter, 
 and the diameter of the sun's image formed by it was one inch: it 
 concentrated, therefore, about 1,300 times to render the effect 
 still more powerful, a smaller lens was placed behind the larger, 
 farther reducing the size of the image to one-sixth. Very sur- 
 prising effects were produced by this lens, in the melting of me- 
 tals, inflaming of combustibles, &c. The size of burning lenses, 
 until lately, was limited by the difficulty of obtaining the great 
 pieces of glass required to form them: but they are now built 
 up of many pieces suitably united together. Some large lenses 
 have been made of water, that is, of water enclosed between 
 meniscus glasses, like watch glasses. A common goblet of wa- 
 
SOLAR MICROSCOPE. 153 
 
 ter, or a vase holding gold fishes, has acted as a burning glass in 
 some cases, where, in consequence of its being left in a sunny 
 window near the curtains, a house has been burned. 
 
 And the nearer that an object is brought to a lens, the more 
 distant, and therefore the larger will its image be: for, as the 
 rays towards a lens diverge in proportion to the nearness of the 
 object, and therefore with the same power of lens, must meet 
 farther behind, as seen in the figure at page 201, then the axis 
 of the rays, as the lines c a and c b in the last figure, will have 
 separated far before the rays meet, and will have made the image 
 proportionally larger. If we suppose little d in the same dia- 
 gram to be the object, its image would be a b. The sun is ex- 
 actly as much larger than his image formed by a burning glass, 
 as he is more distant from it than the image; and if we had a 
 canvas of sufficient size hung up in a distant space, a very bright 
 object of a quarter of an inch diameter might be made to form 
 an image as broad as the sun. 
 
 From all these considerations, we see that, in a camera obscu- 
 ra, the screen should be from the lens, at the distance of its prin- 
 cipal focus for distant objects, and a little farther than this for near 
 objects. Accordingly, the lens is generally fixed in a sliding 
 piece, which allows the distance from the screen to be adjusted to 
 circumstances. If therepresentatipn is wished to be large, the lens 
 must be of a long focus; if to be small, the lens must be of a short 
 focus. Again, when by the reversed use of the lens, a small object, 
 as d, is to be magnified to such a size as a b, then the object must 
 be placed a little beyond the focus of the glass; for if placed near- 
 er, the pencils of rays from it would never be gathered to fo- 
 cal points, and no image would be formed at any distance. 
 
 When, as alluded to in the last sentence, a small object is 
 placed very near a lens, and the image of it is thrown upon the 
 wall of a dark room, perhaps a hundred times farther from. the 
 lens than the object is, the image is a greatly magnified repre- 
 sentation of the object, viz. it is a hundred times longer and a 
 hundred times broader, and therefore has ten thousand times as 
 much surface as the object; but if in this experiment the object 
 be illuminated only in an ordinary degree, the light from it is 
 so scattered as not to suffice for distinct vision. Hence, to at- 
 
 20 
 
154 LIGHT. 
 
 tain fully in this manner the purpose of a microscope, a very 
 strong light, concentrated by a suitable mirror or glass, must be 
 directed upon the object. When the light of the sun is used 
 in such a case, the complete apparatus is called the solar micro- 
 scope, and serves beautifully to display the structure of many 
 minute objects. When artificial light is used, as of a lamp, the 
 apparatus is called the lucernal microscope or magic lantern. 
 A good solar microscope becomes one of the most interest- 
 ing presents which science has made to man, for aiding him in 
 his researches into the secrets of nature. With the late im- 
 provements in the construction of lenses, by which the disper- 
 sion of light, or the rainbow fringe, is prevented (as will be 
 explained under the head of " Tejescopes/') objects maybe 
 magnified two or three hundred thousand times, and still be so 
 luminous as to be beautifully distinct: thus a cheese-mite will 
 appear of the dimensions of a little pig, and creatures altogether 
 invisible to the naked eye, or perceived by it only as minute 
 white points, are discovered to be animated beings, having the 
 perfect proportions, and often the great beauty of larger ani- 
 mals, and endowed with similar appetites, passions, and appa- 
 rent ingenuity, but with an activity far surpassing that met 
 with in the more bulky creation. A judicious selection of ob- 
 jects, for the solar microscope, is calculated exceedingly to 
 surprise the mind on first attending to them, and to fill it with 
 high conceptions of the infinity of God's creation. With the 
 common microscope only one person at a time can feast his 
 wonder; but with the solar, a whole roomful of company may 
 at once contemplate the same objects and witness the same ac- 
 tions, and thus have their admiration increased by the con- 
 sciousness of a sympathy. 
 
 The magic lantern, we have said, consists of a powerful lens, 
 with objects, highly illuminated by lamp light, placed so near 
 it that their imnges are formed far off, and are therefore pro- 
 portionally larger. For the magic lantern the objects are ge- 
 nerally paintings on glass made with transparent colours; and 
 the glass is formed to slide through a slit or passage behind the 
 lens. The lens itself, or what may be called half of it, for 
 there are often two lenses joined to give greater power, is 
 
MAGIC LANTERN. 155 
 
 moveable with the tube which is seen projecting from the lan- 
 tern, so that its distance from the object may be varied, and 
 thus a corresponding -approach to or receding from the screen 
 may be allowed, which will produce an increase or lessening 
 of the magnitude of the visible picture on the wall. 
 
 Some public lecturers on astronomy and branches of natural 
 history prefer having the drawings and paintings, required for 
 the elucidation of their subjects made in miniature upon glass, 
 to be magnified afterwards to the degree desired, and shown 
 upon any part of the lecture room by the magic lantern. 
 
 A thick fog or mist at night will sometimes reflect the images 
 of a magic lantern so as to make them distinctly visible; and 
 there are several cases on record, where persons, wickedly in- 
 genious in this way, have terrified ignorant individuals almost 
 to death, by throwing spectres from a concealed lantern. Some 
 years ago a sentinel in St. James' Park was thus persuaded that 
 he had seen supernatural beings near him among the trees. 
 
 A very charming illusion is produced by a magic lantern 
 manoeuvred on one side of a thin screen, let down like the cur- 
 tain of a theatre, while the spectators, not aware of the exist- 
 ence of the screen, are sitting on the other side. The image 
 may be first thrown upon the screen with the lantern very near, 
 and then it will be small, and exceedingly bright if desired, 
 because the light is much concentrated. By the exhibitor 
 then gradually receding from the screen, and at the same time 
 adjusting the distance of the lens from the picture, the image, 
 which may be that of a genius flying in the air, becomes larger, 
 and to the spectators who do not even know that there is a 
 screen there, it will appear to be soaring and approaching un- 
 til at last the expanded wings and limbs may seem hovering 
 almost over their heads. An endless variety of most ingenious 
 and beautiful exhibitions of this kind have been made, under 
 the name of the phantasmagoria, or raising of spectres. 
 
 " And the EYE itself is, in fact, but a small camera obscu- 
 ra, of which the pupil is the round opening or window 
 before the lens." (Read the Analysis, page 122.) 
 
 The Eye. And who could at first believe that in describing 
 
156 LIGHT. 
 
 the camera obscura, as we have now done, we had in reality 
 been describing that most interesting of the objects of creation, 
 the living eye itself, the great inlet of man's knowledge, that 
 which may be called the visible dwelling of the soul, or at least 
 the window of that dwelling that from which all the fire of pas- 
 sion darts, through which the languor of exhaustion is perceived, 
 in which. life and thought seem concentrated! Yet the eye is 
 nothing but a simple camera obscura, formed of the parts de- 
 scribed above as essential to the camera obscura: but in its 
 simplicity it is so perfect, so unspeakably perfect, that the 
 searchers after tangible evidences of the existence of an all- 
 wise and good Creator, have declared their willingness to be 
 limited to it alone in the midst of millions, as their one tri- 
 umphant proof. Yv'e shall now describe it and its actions. 
 Keeping present to us the idea of the camera obscura, as alrea- 
 dy treated of, the use of the various parts of the eye will be 
 declared by merely enumerating them. This paragraph should 
 be perused while the reader has the opportunity of observing 
 either his own eye reflected in a glass, or the eye of some com- 
 panion near him. 
 
 The human eye, then, is a globular chamber of the size of 
 a large walnut, formed externally by a very tough membrane 
 called, from its hardness, the sclerotic coat, in the front of 
 which there is one round opening or window, named, because 
 of its horny texture, the coniea. The chamber is lined with 
 a finer membrane or web the choroid, which, to ensure the 
 internal darkness of the place, is covered with a black paint, 
 the pigment-um nigrum. This lining at the edge of the 
 round window is bordered by a folded drapery the ciliary 
 processes, hidden from without by being behind the curious 
 contractile window curtain, the iris, through the central open- 
 ing of which, or pupil, the light enters. Immediately behind 
 the pupil is suspended by attachments among the ciliary pro- 
 cesses, the crystalline lens, a double convex most transparent 
 body of considerable hardness, which so influences the light 
 passing through it from external objects, as to form most per- 
 fect images of these objects in the way already described, on 
 the back wall of the eye, over which the optic nerve, then 
 
THE EYE STIIUCTURE. 
 
 157 
 
 called the retina, is spread as a second lining. The eye is 
 maintained in its globular condition by a watery liquid, which 
 distends its external coverings, and which in the compartment 
 before the lens, or the anterior chamber of the eye, being per- 
 fectly limpid, is called the aqueous humour, and in the re- 
 mainder or larger posterior chamber, being enclosed in a 
 transparent spongy structure, so as to acquire somewhat of the 
 appearance of melted glass, is called the vitreous humour. 
 
 The annexed figure represents an eye of the common dimen- 
 sions, supposed to be cut through the middle, from above down- 
 
 wards. C is the outer or sclerotic coat, known popularly, 
 where most exposed in front, as the white of the eye. A is 
 the transparent cornea joined to the edge of the round open- 
 ing of the sclerotic: it is more bulging than the sclerotic, or 
 forms a portion of a smaller sphere than the general eye-ball, 
 so that while it may be truly called a bow window, it, or rather 
 the convex surface of its contained water, is also a powerful 
 lens for acting on the pencils of entering light. At 13, and si- 
 milarly all round the edge of the cornea, is attached the win- 
 dow curtain or iris, shown here edgeways, immersed in the 
 aqueous humour, and hanging inwards from above and below 
 towards its central opening or jmpil, through which the rays 
 of light are passing to the lens. The iris has in its structure 
 two sets of fibres, the circular and "the radiating, which cross 
 and act in opposition to each other. When the circular fibres 
 contract, the pupil is lessened, when the radiating contract, it 
 is enlarged: and the changes happen according to the intensity 
 of light and the state of sensibility of the retina, as may at 
 
158 LIGHT. 
 
 any time be proved by closing the eye-lids for a moment to 
 make the pupil dilate, and then opening them towards a strong 
 light, to make it contract. Behind the pupil is seen the lens 
 D with its circumference attached to the ciliary processes E: 
 it is more convex behind than before. The disease of the eye, 
 called cataract, (from a Greek word implying obstruction,} is 
 the circumstance of the lens becoming opaque, and the cure is to 
 extract the lens entirely, or to depress it to the bottom of the eye, 
 and then to substitute for it externally a powerful artificial lens 
 or spectacle-glass. The three lines, forming here the bounda- 
 ry of the eye, stand for its three coats, as they have been 
 called, the strong sclerotic, and the double lining of the cho- 
 roid and retina. The figure of a cross is represented upon 
 the retina as formed by the light entering from the cross with- 
 
 J O O 
 
 out, which cross has to appear here small and near, although 
 supposed to be large and distant. The image of the cross is 
 inverted, as explained for the camera obscura: but we shall 
 learn below that the perception of an object may be equally 
 distinct in whatever position the image be on the retina. It 
 has been explained above, that a lens can form a perfect image 
 of considerable extent only on a concave surface, and the retina 
 is such a surface. The present diagram farther explains what 
 is meant by the anterior and posterior chambers of the eye, 
 viz. the compartments which are before and behind the crys- 
 talline lens D. 
 
 The nature of the eye as a camera obscura is beautifully ex- 
 hibited by taking the eye of a recently killed bullock, and after 
 carefully cutting away or thinning the outer coat of it be- 
 hind, by going with it to a dark place and directing the pupil 
 towards any brightly illuminated objects; then, through the se- 
 mi-transparent retina left at the back of the eye may be seen a 
 minute but perfect picture of all such objects a picture, there- 
 fore, formed on the back of the little apartment or camera ob- 
 scura, by the agency of the convex cornea and lens in front. 
 
 Understanding from all this, that when a man is engaged in 
 Tvhat is called looking at an object, his mind is in truth only 
 taking cognizance of the picture or impression made on his re- 
 tina, it excites admiration in us to think of the exquisite deli- 
 
THE EYE INVERTED IMAGE. 159 
 
 cacy of texture arid of sensibility which the retina must pos- 
 sess, that there may be the perfect perception which really oc- 
 curs of even the separate parts of the minute images there 
 formed. A whole printed sheet of newspaper, for instance, 
 may be represented on the retina on less surface than that of a 
 finger nail, and yet not only shall every word and letter he se- 
 parately perceivable, but even any imperfection of a single let- 
 ter. Or, more wonderful still, when at night an eye is turned 
 up to the blue vault of heaven, there is portrayed on the little 
 concave of the retina the boundless concave of the sky, with 
 every object in its just proportions. There a moon in beauti- 
 ful miniature may be sailing among her white edged clouds, 
 and surrounded by a thousand twinkling stars, so that to an 
 animalcule supposed to be within and near the people, the reti- 
 na might appear another starry firmament with all its glory. 
 If the images in the human eye be thus minute, what must 
 they be in the little eye of a canary bird, or of another animal 
 smaller still ! How wonderful are the works of nature ! 
 
 Because the images formed on the retina are always inverted 
 as respects the position of the objects producing them just as 
 happens in a simple camera obscura, persons have wondered 
 that things should appear upright, or in their true situations. The 
 explanation is not difficult. It is known that a man with wry 
 neck judges as correctly of the position of the objects around 
 him as any other person never deeming them, for instance, 
 inclined or crooked, because their images are inclined as re- 
 gards the natural perpendicular of his retina; and that a bed- 
 ridden person obliged to keep the head upon the pillow, soon 
 acquires the faculty of the person with wry neck: and that an 
 afiected girl inclining her head while trying her attitudes, from 
 much practice judges of the manoeuvres of a beau as conve- 
 niently in that way as in any other; and that boys who at play 
 bend down to look backwards through their legs, although a 
 little puzzled at first, because the usual position of the images 
 on the retina is reversed, soon see as well in that way as in 
 any other. It appears, therefore, that while the mind studies 
 the form, colour, &c. of external objects in their images project- 
 ed on the retina, it judges of their position by the direction iu 
 
160 LIGHT. 
 
 which the light comes from them towards the eye no more 
 deeming an object to be placed low because its image may be 
 low in the eye, than a man in a room into which a sun-beam 
 enters by a hole in the window shutter, deems the sun low be- 
 cause its image is on the floor. A candle carried past a key- 
 hole, throws its light through to the opposite wall, so as to 
 cause the luminous spot there to move in a direction the oppo- 
 site of that in which the candle is carried ; but a child is very 
 young who has not learned to judge at once in such a case, of 
 the true motion of the candle by the opposite apparent motion 
 of the image. A boatman, who, being accustomed to his oar, 
 can direct its point against any object with great certainty, has 
 Ion" 1 ceased to reflect, that to move the point of the oar in some 
 one direction, his hand must move in the contrary direction. 
 Now the seeing things upright, by images which are inverted, 
 is a phenomenon akin to those which we have reviewed. 
 
 Another question somewhat allied to the last is, why, as we 
 have two eyes, and there is an image of any object placed before 
 them formed in each why the object does not appear to us to 
 be double. In answer to this, again, we need only to state the 
 simple facts of the case. In the two eyes there are correspond- 
 ing points, such that when a similar impression is made on both, 
 the sensation or vision is single: but if the least disturbance of 
 the position occur, the vision becomes double. And the eyes 
 are so wonderfully associated, that from earliest infancy they 
 constantly move in perfect unison. By slightly pressing a fin- 
 ger on the ball of either eye, so as to prevent its following the 
 motion of the other, there is immediately produced the double 
 vision; and tumours about the eye often have the same effect. 
 Persons who squint have always double vision: but they ac- 
 quire the power of attending to the sensation in one eye at a 
 time. Animals which have the eyes placed on opposite sides 
 of the head, so that the two can never be directed to the same 
 point, must have in a more remarkable degree the faculty of 
 thus attending to one eye at a time. 
 
 The corresponding points in the two eyes are equidistant and 
 in similar directions from the centres of the retinae, called the 
 points of distinct vision, at which centres the imaginary lines 
 
THE EYE DOUBLE VISIOX. 161 
 
 named the axes of the eyes terminate; and it is worthy of re- 
 mark that these points, in being both to the right or both to 
 the left of the centres, must be one of them on the inside of 
 the centre as regards the nose, and the other on the outside. 
 When the two eyes are directed to any object, their axes meet 
 at it, and the centres of the two retinae are opposite to it, and 
 all the other points of the eyes have perfect mutual correspon- 
 dence as regards that object, giving the sensation of single vi- 
 sion; but the images formed at the same time, of an object 
 nearer to or farther from the eye than the first supposed, can- 
 not fall on corresponding points; for an object nearer than 
 where the axes meet would have its images on the outsides of 
 the eyes, and an object more distant would have its images on 
 the insides of the eyes, and in either case the vision would be 
 double. Thus, if a person hold the two fore-fingers in a line 
 from his eyes, so that one may be more distant than the other, 
 by then looking at the nearest, the more distant will appear 
 double, and by looking at the more distant, the nearer will ap- 
 pear double. 
 
 The reason of the term " point of distinct vision," applied 
 to the centre of the retina, is discovered at once by looking 
 at a printed page, and observing that only the one letter to 
 which the axis of the eye is directed, is distinctly seen; so that 
 although the whole page be depicted on the retina at once, the 
 eye, in reading, directs its centre successively to every part. 
 
 On examining a dead eye, the point of distinct vision is dis- 
 tinguishable from the retina around by being more transparent. 
 Now it might have been expected that this point would have 
 been the spot where the optic nerve enters the eye: but in 
 truth the optic nerve enters considerably nearer to the nose than 
 the centre of the retina; and, very singularly, where it enters, 
 the part is altogether blind or insensible. Had the two optic 
 nerves then entered at corresponding points of the retina, (in 
 the sense explained above,) there would have appeared a black 
 spot on every object opposite to the insensible points; but as 
 the case really stands, the part of any object from which the 
 light passes to the insensible part of one eye must be opposite 
 
 21 
 
162 LIGHT. 
 
 to a sensible part of the other. The existence of the insensible 
 or blind spot, where the nerve of the eye enters, is discovera- 
 ble by placing in a row three objects wafers, for instance 
 on a table, with intervals of about two inches between them, 
 and then looking with one eye from a distance of about 
 eight inches at the wafer which is towards the nose; the 
 middle wafer will then be invisible, although the eye sees that 
 on each side of it; and if the eye be moved still farther away, 
 the middle wafer will come into view, and the external will 
 disappear. Or, again, the fact may be proved by shutting one 
 eye and looking with the other at the nail of a finger held be- 
 fore it, while another finger is gradually moved away laterally: 
 the point of the moving finger when at a certain distance from 
 the other will disappear, but will be seen again when moved 
 away still a little farther. 
 
 It appearing then, from the explanations now given, that 
 there cannot be perfect sight unless where a perfect image is 
 formed on the retina; and the truth having been formerly ex- 
 plained, that images behind any lens will be at different dis- 
 tances from it, according to the various distances of the objects 
 in front, that is to say, according as the pencils of light which 
 fall upon it have more or less of divergence in them, it follows, 
 that the eye in being able, as it is, to see distinctly objects at 
 any distance beyond about five inches, possesses a power of 
 altering the relation of its parts to accommodate itself to the 
 circumstances. We do not yet perfectly know whether it does 
 this by lengthening or changing the form of the ball through 
 the action of the surrounding muscles, or by changing the place 
 or the form of the lens, but that one or more of these events 
 occurs there can be no doubt. 
 
 Among the eyes of the myriads of human creatures, howe- 
 ver, it happens that all do not originally possess these powers 
 exactly in the requisite degree, and that many lose them, as 
 life advances, from a natural or usual decay. 
 
 Persons are called short-sighted whose eyes from too great 
 convexity of the cornea or lens, have so strong a bending or 
 converging power, that the rays of light entering them are 
 
THE EYE SHORT SIGHT LONG SIGHT. 163 
 
 brought to a focus before 
 reaching the retina at cr, 
 for instance, instead of at 
 bf so that the rays, by 
 spreading again beyond 
 the focus, produce on the retina that sort of indistinct image 
 which is seen in a camera obscura of which the screen is too 
 distant from the lens. This defect of sight obliges the indivi- 
 dual, when using the naked eye, to hold objects very near it, 
 that the consequent greater divergence of the rays may be pro- 
 portioned to the unusual refractive power of the eye; or the 
 person may find a remedy in placing concave lenses between 
 the object and the eyes, which lenses, by rendering light from 
 objects at a usual distance more divergent, (as explained at p. 
 146,) cause the perfect images in the eye to be formed farther 
 from the lens, and thereby on the retina itself. Without con- 
 cave spectacles as the lenses are called when fixed together 
 in a frame persons with the defect now under consideration 
 cannot see distinctly any object that is distant, and from which 
 the rays, because coming nearly parallel, are quickly gathered 
 to a focus. This defect often diminishes with years, and the 
 person who in youth needed spectacles, in old age sees well 
 without them. 
 
 There is an opposite defect of deficient converging power in 
 the eye, dependent on a too great flatness of the cornea or lens, 
 which defect is much more common than the last-mentioned, 
 for the great majority of persons, after middle age, sooner or 
 later, begin to experience it. In this case the rays of light 
 are not yet collected into a focus when they reach the retina; 
 they would only meet at b, for instance, instead of at c, and, 
 
 hence, the image is indis- 
 tinct, in the same manner 
 as in a camera obscura of 
 which the screen is held 
 too near the lens. Per- 
 sons suffering this defect cannot, when using the naked eye, see 
 distinctly any object very near to it, because the gathering or 
 converging power of the eye cannot conquer the great diver- 
 
164 LIGHT. 
 
 gence of rays coming from a near point; and, hence, such per- 
 sons always remove objects under examination to a considera- 
 ble distance, often to that of arm's length, so as to receive 
 from them only the rays nearly parallel. These persons, in 
 contradistinction to the last described, are called long-sighted 
 persons. Their defect is remedied by the common convex 
 spectacles, which do part of the converging work, so to ex- 
 press ourselves, before the light enters the eye, leaving undone 
 only that which the eye can easily accomplish. As this ail- 
 ment, like the last, is met with in all degrees, it becomes re- 
 quisite to choose spectacles accordingly: certain curvatures or 
 strengths have been numbered as naturally belonging to diffe- 
 rent ages or periods of life, but each person should choose, un- 
 der the direction of an experienced judge, until that strength 
 is found which enables him to read, without any straining of 
 the eye ? at the common distance of from twelve to eighteen 
 inches. We cannot apply the mind to this part of our subject 
 without feeling admiration at what science has accomplished 
 for man in guarding and improving his sight. Now, that in 
 civilized society the most common employments and enjoy- 
 ments of life are such as to require visual power capable of dis- 
 tinguishing minute objects letters, for instance to deprive 
 old men of their spectacles, would be to condemn many of 
 them to useless inactivity and a listless blank of mind for the 
 remainder of their lives. 
 
 An eye much accustomed to examine near and minute ob- 
 jects, often loses something of its pliancy, and becomes defec- 
 tive when tried at distant things, as the watch-maker's eye, 
 the engraver's, &c. On the other hand, the old seaman, whose 
 eye has so often and uninterruptedly been bent on the distant 
 horizon, straining to catch the view of an expected sail, or of 
 land, has a power of discovering distant things which is won- 
 derful; but he often experiences some deficiency in regard to 
 near things. 
 
 A man who uses his eyes under water sees very indistinct- 
 ly, because the difference of density between the two transpa- 
 rent media water and the eye, from the first of which, in the 
 case supposed, the light passes to the second, is not so great as 
 
THE EYE DURATION OF IMPRESSIONS. 165 
 
 between air and the eye, the bending or refraction of the light 
 is consequently not so great. A man, to see well under wa- 
 ter, therefore, requires to aid the usual power of his eyes, by 
 strong convex spectacles. It is for the reason now explained, 
 that the lens of a fish's eye is extremely convex: indeed it is 
 almost round, as is every day seen in the white round head 
 which issues from the eye of a boiled fish that little globe 
 being the crystalline lens of the fish coagulated or hardened 
 like white of egg during the cooking. 
 
 There are many important considerations connected with the 
 sensibility of the retina, which regard rather the laws of life 
 than of light, but we must here glance at a few of them. 
 
 Any impression of light made upon the retina lasts for about 
 the sixth of a second. Hence, when the burning end of a 
 stick is made to describe any line or curve, its path becomes a 
 line of light; and if it revolve in a circle six times in a second, 
 that circle will appear to the eye a complete circle of fire. 
 The polished end of an elastic wire, fixed by its other end in 
 a block of wood, being made to vibrate, similarly forms a line 
 or curve of light. A harp string while vihrating as it sounds, 
 appears like a flat riband. Lightning or other meteor, darting 
 across the sky, although, in fact, but a moving luminous point, 
 is generally thought of as a long line of light: the term forked- 
 lightning has reference to this prejudice. The same remark 
 applies, in a degree, to a sky-rocket in its rapid ascent. Two 
 or more colours painted separately on the rim of a wheel which 
 is made to turn rapidly, appear to the eye to be as completely 
 united as if they were really mixed: it has been already ex- 
 plained how patches of the various colours of the rainbow 
 mixed in this way form white light. If on one side of a card 
 a little bird be painted, and on a corresponding part of the other 
 side a cage, then, on making the card turn rapidly by twisting 
 between the fingers two threads fixed to its opposite edges, the 
 little bird will appear to be imprisoned in the cage: or, again, 
 if a pensive Juliet sitting in her bower occupy one side of the 
 card, and a longing Romeo the other, by the magic turn of the 
 threads the passionate lovers may instantly be brought toge- 
 
166 LIGHT. 
 
 ther. Dr. Paris displayed taste and an amiable ingenuity, in 
 designing this toy with great variety of subjects. 
 
 A certain intensity of light is necessary to distinct vision, 
 but the degree varies with the previous state of the organ. A 
 person passing from the bright day into a shaded room, for a 
 time may fancy himself in total darkness; and to persons sitting 
 in the room and become accustomed to the less light so as to 
 see well with it, he will appear to be almost blind. The dawn 
 of morning after the darkness of night appears much brighter 
 than an equal degree of light in the evening. When, as the 
 night falls, our lamps or candles are first' introduced, the glare 
 is often for a time offensive: and the same feeling is still 
 stronger on opening, in the morning, bed-room window shut- 
 ters or close-drawn curtains. After the repose of night, the 
 sensibility of the eye is such that the globules of blood in the 
 capillary vessels of the retina produce the impression on it of 
 little globes of light crossing among each other as the tortuous 
 vessels do. To a prisoner after long confinement in a dark 
 dungeon, the light of the sun is almost insupportable. And a 
 dungeon, which to common eyes is utterly dark, still to its 
 long-held inmate has ceased to be so. There are various in- 
 stances in the records of the barbarous ages, of prisoners con- 
 fined for years in utter darkness, who at last could see and 
 make companions of the mice which frequented their cells. 
 The darkness of a total eclipse after bright sunshine appears 
 much more deep than that of midnight, because of the sudden 
 contrast. The long polar night of mouths ceases to appear 
 very dark to the polar inhabitants, [f an eye be directed for 
 a time to a black wafer laid on a sheet of white paper, and af- 
 terwards to another part of the sheet, a portion of the paper 
 of the size of the wafer will appear brilliantly illuminated: for 
 the ordinary degree of light from it appears intense to the part 
 of the eye lately receiving almost none. An eye directed long 
 and intensely upon any minute object as when a sailor watches 
 a speck in the distant horizon, supposed to be a ship, or when 
 a hunter on the brown heath keeps his eye fixed on some game 
 nearly of the colour of the heath, or when an astronomer gazes 
 long at a little star has the sensibility of its centre at last ex- 
 
THE EYE PERCEPTION OF COLOUR. 167 
 
 haustod, and ceases to perceive the object; hut on directing the 
 axis of the eye a little to one side of the object, so that an 
 image may be formed only near the centre, the object may be 
 again perceived, and the centre in the mean time enjoying re- 
 posp, will recover its power. 
 
 But the most extraordinary fact connected with the sensibili- 
 ty of the retina is, that if part of it be strongly exercised by 
 looking for a time at an object of any bright colour, on then 
 turning the eye away or altogether shutting it, an impression 
 or spectrum will remain of the same form as the object lately 
 contemplated, but of a perfectly different colour. Thus if an 
 eye be directed for a time to a red wafer laid on white paper, 
 and be then shut or turned to another part of the paper, a beau- 
 tifully bright green wafer will be seen; and vice versa, a green 
 wafer will produce a red spectrum, an orange wafer will similar- 
 ly produce a blue spectrum, a yellow one a violet spectrum, 
 &c. ; and a cluster of wafers will produce a similar cluster of 
 opposite colours. If the hand be th"n held over the eyelids to 
 darken the eyes and prevent entirely the approach of light, the 
 spectrum of the bright parts will be luminous surrounded by a 
 dark ground, and when the hand is again removed the contra- 
 trary will be true. Again, if the eye be in a degree fatigued 
 by looking at the setting sun, or even at a window with a 
 bright sky beyond it, or at any very bright object, on then shut- 
 ting it, the lately contemplated forms will be perceived, first 
 of one vivid colour, and then of another, until perhaps all the 
 primary colours have passed in review. These extraordinary 
 facts prove that the sensation of light and colour, although ex- 
 citable by light, is also producible without it. This truth gave 
 occasion to Darwin's ingenious theory, that the sensation of 
 any particular colour, of red for instance, is dependent upon a 
 certain state of contraction of the minute fibres of the retina, 
 as the sensation of a particular tone depends on a certain fre- 
 quency of vibration of some part of the ear, and that the fibres, 
 when fatigued in that condition, seek relief when at liberty, by 
 throwing themselves in an opposite state, as a man whose 
 back is fatigued by bending forward, relieves himself not by 
 merely standing erect, but by bending the spine backwards* 
 
168 LIGHT. 
 
 which new condition, whether produced by light or by any 
 other cause, gives the sensition of green. He applied his ex- 
 planation similarly to all other cases of colour. It is remarka- 
 ble that the colours which thus appear opposite to each other 
 in kmd are those which when the solar spectrum produced by 
 a prism, as described a few pages back, is painted round a wheel 
 or circle, are opposite to each other in place. 
 
 There are persons who, although having distinct perceptions 
 of form and of light and shade, have not the power of distin- 
 guishing colours. It is common for such persons to deem pink 
 and pea-green (naturally opposites) the same colour, and there- 
 fore not to distinguish difference of colour in a red berry and 
 the leaves around it. A man with this defect, trusting to his 
 own judgment, might, without knowing it, dress himself like a 
 parrot. 
 
 " The mind judges of external objects by the relative size, 
 brightness, colour, fyc., of the minute but perfect images 
 of them formed at the back of the eye on the expansion of 
 nerve called the retina; and the art of painting is success- 
 ful in proportion as it produces on a larger scale a pic- 
 ture* tvhich, when afterwards held before the eye to repro- 
 duce itself in miniature upon the retina, may excite the 
 same impression as the original object." (Read the Ana- 
 lysis, page 162.) 
 
 We now understand how an admirable miniature resemblance 
 of the objects before us is produced upon the retina of the eye, 
 by the light from them refracted in passing through the different 
 parts of the eye; but after all, this is only a picture, and the in- 
 quiry remains which many persons would suppose so simple 
 as to be trifling, but which is in reality most curious and im- 
 portant how we are thereby enabled to judge of the magnitudes, 
 distances, and other particulars respecting the things examined. 
 Here it will be found, to the surprise of persons first entering 
 upon the subject, that we learn the meaning of a scene or picto- 
 rial signs only gradually, as we do of any other system of signs, 
 and that a person no more sees, in the complete sense of the 
 word, that is to say, no more understands any scene or prospect 
 

 THE EYE VISUAL ANGLE. 169 
 
 when he first opens his eyes upon it, and has a perfect picture 
 of it on his retina, than he understands or can read a printed 
 page, on first looking into a book before he has learned his let- 
 ters. Most interesting information has been obtained on this 
 subject, by observing the facts where blindness from birth has 
 been, by surgical operation, suddenly cured in persons arrived 
 at maturity. 
 
 If a man were placed from infancy in an apartment fitted up 
 as a camera obscura, and had no means of becoming acquainted 
 with external nature, but by watching the images appearing 
 upon the screen, he could learn almost nothing of the universe 
 around him; but if after a time he were allowed to walk out, and 
 to examine by the touch and by measurement the different objects 
 portrayed there, and to ascertain what size, shape, and distance 
 of an object corresponded with a certain magnitude, form, po- 
 sition, and brightness of image, the imagery might at last be to 
 him a very clear indication of such particulars, and through them 
 of nearly all else that he desired to know; making him in ima- 
 gination present to the objects around, almost as if he went and 
 examined them with his hands like a blind man, or in any other 
 way. In the same manner, nearly, the soul may be considered 
 as if originally placed in the little camera obscura of the eye, 
 where it has to acquire experience of external nature by com- 
 manding the services of the bodily limbs or members. The 
 judging of things by sight, then, is merely the interpreting one 
 set of signs, as judging by sounds or language is interpreting 
 another, and judging by hieroglyphics or any written characters 
 is interpreting a third. The common visual signs on the retina, 
 however, are those most readily learned or understood, from 
 having certain relations in form, &c., to the things signified. 
 
 Bodies differ and are distinguished among themselves chiefly 
 by their comparative dimensions, that is, their size and shape; and 
 to ascertain these and the relative distances, are the great objects 
 which by the eye the mind seeks to accomplish. Now, it ef- 
 fects its ends by considering collectively, 
 
 1st. The space and place occupied by objects in the field of 
 view, measured by what is called the visual angle. 
 
 2d. The intensity of light, shade, and colour. 
 
 22 
 
170 
 
 LIGHT. 
 
 3d. The divergence, of the rays of light. 
 4th. The convergence of the axes of the eyes. 
 We shall treat of these particulars separately in the order now 
 mentioned. 
 
 1 st. The space and place occupied in the field of vieiv, mea- 
 sured by the visual angle. 
 
 The field of view is that open or visible space before the eyes, 
 in which objects are seen; and the term may mean either the 
 small field visible in one position of the eyes, or that which is 
 perceived on directing them all around. As the eye may be 
 turned in every direction, it may be considered as placed in the 
 centre of a hollow sphere, where it sees the several objects around 
 occupying certain situations and certain proportions of the cir- 
 cumference: and if 
 a man were really 
 surrounded by a 
 large globe or 
 sphere of glass, as 
 a, through which 
 he might view ob- 
 jects, which sphere 
 had any equal di- 
 visions or degrees 
 marked upon it all 
 around, he would 
 be able at once to 
 say exactly what 
 portion of his 
 sphere or field of view was-sfcadowed or occupied by any sin- 
 gle object, as the cross here shown at z, and thus to describe 
 very intelligibly, either for his own recollection, or to inform 
 others, its relative magnitude and situation as then appearing to 
 him, just as he might say, on looking at a tree in the garden 
 through a common window (which is a portion of the field of 
 view really divided by the cross bars,) whether he saw the whole 
 tree through one pane or through several, and through which 
 pane or panes he saw it. It may be remarked farther, that whe- 
 
THE EYE APPARENT SIZE OF OBJECTS. 171 
 
 thcr the supposed sphere of glass were large or small, viz. were 
 as a or as b or c, the part of its surface apparently occupied by 
 any object beyond or within it, woujd bear the same proportion 
 to the whole surface. Now, as men have found it convenient 
 to consider a circle (and every circle) as divisible into 360 de- 
 grees (which are smaller therefore in a small than in a large cir- 
 cle, although in each having the safhe relation to the whole,) the 
 ready mode of comparing the apparent magnitude of objects is 
 to say how many of these degrees of the field of view supposed 
 a portion of a hollow sphere surrounding a man each object 
 occupied: and this is really what is meant by the apparent size 
 of an object. And because the most convenient way of mea- 
 suring a portion of a circle, of which the whole is not seen, is 
 to measure the angle formed at its centre by lines drawn from 
 the extremities of the portion, as here the angle at e formed 
 by the lines c e and g e, the object is said to occupy a certain 
 number of degrees of the circumference of the circle, or to sub- 
 tend an angle of the same number of degrees at its centre, and 
 this angle is called the visual angle, the subject of our present 
 disquisition. 
 
 The visual angle then, in regard to any object, is that in- 
 cluded between the lines or rays, as a u and d i, which from 
 the extreme points of the object, as a d, cross in the lens of 
 the eye, and go afterwards to form the extremes of the image 
 on the retina, and, as formerly explained, the angle is the same 
 on either side of the lens, viz. towards the object or towards 
 the image. Now, if all bodies were at the same distance from 
 the eye, the magnitudes of their images formed on the retina, 
 or, in other words, of the visual angles subtended by them, 
 would be an exact measure of their comparative real magni- 
 tudes, as is 
 seen in i u, 
 the image of 
 the great cross 
 a d, and in i o, 
 the image of 
 the small cross 
 b d: but it is evident here, that the cross c c, which is twice as 
 
173 
 
 LIGHT. 
 
 large as b d, makes, because twice as far off, an image of only 
 the same size as b d, and an image, therefore, only half as large 
 as that of a cross, a d, equal in size with itself: and the same 
 rule of proportion holds for all other comparative distances 
 at a hundred times the distance, an object appearing only the 
 hundredth part as tall, and so forth. To judge, therefore, by 
 the eye of the true size of an object, we must know its dis- 
 tance as well as its apparent size or visual angle. 
 
 Many familiar facts receive their explanation from the law of 
 the visual angle or apparent size, being less always in pro- 
 portion as the distance of an object is greater. 
 
 A man (or a cross) at d, standing near the outside of a win- 
 dow, as b c (shown here edgeways,) may to a spectator seated 
 within the window at h, subtend the same visual angle, or ap- 
 pear as tall as the window, the light from his head passing 
 through the top of the window, and that from his feet passing 
 
 through the bottom: 
 but if the man then, 
 move away from the 
 window, the specta- 
 tor will be able to 
 see his whole body 
 through a smaller and a smaller extent of the window, as 
 through half its height or a c, when he is twice as distant from 
 the eye, or at /, and through the third or o c, when he shall 
 be three times as distant, or at g, and so forth, for any other 
 distance; so that soon a small figure of a man cut in paper, if 
 laid upon the glass, would exactly cover the part of it through 
 which the light from him entered to the spectator's eye, and 
 would then, by completely hiding him from view, be an exact 
 measure of his apparent size: and, at last, a fly passing over 
 the pane might equally hide him, and the fly then would sub- 
 tend a larger visual angle than he, that is to say, would be 
 forming on the retina a larger image than the man. Thus, it 
 often happens in reality, thaf a person sitting near a window, 
 and intent upon some subject of study or of conversation, mis- 
 takes a fly on the glass for a man at a distance \ or, on the con- 
 
THE EYE APPARENT SIZE OP OBJECTS. 173 
 
 trary, a man for a fly. It is ascertained that the eye, with an 
 ordinary degree of light, can see an ohject which in the field 
 of view occupies only the sixtieth of a degree (or one minute) 
 in a circle of twelve inches diameter, the eye being supposed 
 in the centre of the circle. This space is about the one-hun- 
 dredth of any inch measure held six inches from the eye. Now, 
 a body smaller than this, at six inches, or any thing, however 
 large, placed so far from the eye as to occupy in the field of 
 view less space than this, is invisible to ordinary sight. At 
 four miles off, a man is thus invisible. A pin-head near will 
 hide a house on a distant hill nay, will hide even the planet 
 Jupiter, although one hundred thousand times bigger than this 
 earth. 
 
 In accordance with the principle now explained, a marine 
 telescope has been constructed, having the field of view di- 
 vided by fine cross wires, or otherwise, so that the person 
 using it can say at once how much of its field any object oc- 
 cupies. Now, when ships are in chase, it is common by this 
 instrument, or some other that will detect a change of apparent 
 size, or of visual angle, to view the fleeing or pursuing ship; 
 and if the apparent size be observed to increase, it is known 
 that the ships are nearing each other; if on the contrary it di- 
 minish, the chased ship is escaping. 
 
 By applying this rule, whenever the exact size of a distant 
 ohject is known the distance is ascertainable, and, vice versa, 
 where the distance is exactly known, the size is determinate: 
 for it is evident that if a body, as a ship, known to be 100 feet 
 tall, occupy or subtend in the field of vision the 360th part of 
 a whole circle, or one degree, the whole circle must be 360 
 times 100 feet, or 36,000; and knowing the diameter of such 
 a circle to be nearly one-third as much, we learn the distance 
 of the ship, viz. half the diameter. Again, if we know the 
 distance of a ship or other object to be a mile, and if we then 
 find its visual angle to be the 1,000th part of a circle, we know 
 its true size to be the 1,000th part of a circle, of which the 
 half-diameter and radius is one mile. It is by applying this 
 rule in a manner to be afterwards explained, that we determine 
 the size of the heavenly bodies. 
 
1 74 LIGHT. 
 
 We now perceive that if the rays of light coming to the 
 eye through a plate of glass, from objects seen beyond it, could 
 leave marks in the glass at the points where they passed, and 
 marks capable of giving out the same kind of light as caused 
 them, there would be formed upon the glass a representation 
 or picture of the objects formerly viewed through it, and that 
 picture would be so perfect, that when held before the eye, it 
 would form on the retina an image or images the same in al- 
 most all respects as the objects themselves had done; for from 
 the different points of the glass, light would dart to the eye 
 in the very same directions pursued by that originally darted 
 from the objects. Now, the art of painting seeks so to dis- 
 pose lights, shades, and colours on any plane surface, as to pro- 
 duce the sort of representation of objects here contemplated, 
 while the picture frame has to recall the window frame, or 
 edge of the plate of glass through which the true scene is sup- 
 posed to be viewed. It is remarkable, indeed, how perfectly 
 this art now accomplishes its ends; and although there are still 
 trifling differences between the effect upon the eye, of a com- 
 mon picture, and of the realities, which peculiarities we shall 
 consider presently, and how they may be combated so as to 
 render the illusion quite perfect, it is not one of them, as 
 might be supposed from the small extent of the canvas, that 
 the picture appears to the retina smaller than the objects them- 
 selves. Few people, before studying this subject, are aware 
 that in a good picture the size of the figures is always made ex- 
 actly such, that at the distance from the eye at which they are 
 meant to be beheld, they produce on the retina the very same 
 size of image as would be produced by the realities seen under 
 the aspect represented in the picture. To become sensible of 
 this, let a person look through a window pane, with the eye 
 at the distance of eight inches from it, and let him draw with 
 a sharp point upon the glass previously coated with gum, the 
 outline of the scene Jbeyond perhaps a street or square, he 
 will find, that the outline of a man, seen there at the distance 
 of twenty paces, appearing perfectly to coincide with the boun- 
 daries of the person, and such as, if opaque, would just hide 
 the person, will be scarcely half an inch tall, while the figure 
 
THE EYE APPARENT SIZE OP OBJECTS. 175 
 
 of a man a few hundred paces off will appear as a little point 
 too small for the minuter features to be distinguished, even if 
 they could be drawn. 
 
 Now, as a person who reads the description of an elephant, 
 does not deem the animal larger or smaller because of the size 
 of letter used in the printing, or of the size of the accompa- 
 nying engraved representation, whether it be diminutive in 
 the page of a child's nursery book, or wide spread over the 
 page of a quarto and as a man, viewing in a picture gallery 
 miniatures and larger portraits, does not conceive of the origi- 
 nals according to the size of the representations and as a man 
 who views a picture of a temple, so perfect that it might al- 
 most be mistaken for the reality, never dreams, unless his at- 
 tention be particularly directed to the fact, that the distant pil- 
 lars of the rows are vastly smaller upon the canvas than the 
 near ones; but in all such cases the mind merely uses the signs 
 to help it to conceive of the things according to other princi- 
 ples of judging; so in any common case of seeing, the mind 
 takes so little account of the apparent size of objects passing 
 instantly from the types to the realities, generally known, that 
 it soon ceases to be aware that the apparent size of the same 
 object ever changes. Most persons would be surprised to be 
 told, br instance, that a man with whom one is shaking hands, 
 appears to the mere eye ten times taller than when he has 
 walked ten paces away, or that a chair at one end of a room 
 appears to a person sitting at the other, only half as large as a 
 chair in the middle of the room; but such are the facts; and 
 they may be immediately proved by holding a common eye- 
 glass or ring at a certain distance from the eye, and then look- 
 ing through it at any similar objects placed at different dis- 
 tances; then, while of a chair standing near, only a small part 
 will be visible through the ring of a distant chair, the whole 
 may be seen; and so of any other case. At five miles' dis- 
 tance, Nelson's fleet on the great day of Trafalgar might have 
 been seen through a marriage-ring as the picture frame. There 
 are occasions, however, where the usual collateral helps to the 
 immediate recognition of objects being wanting, the attention 
 is strongly aroused to the fact of their diminutive appearance 
 
176 LIGHT. 
 
 produced by distance for instance, when a man first from the 
 high sea approaches a land, of which the features are in a de- 
 gree new to him: an Englishman arriving in India has consi- 
 derable difficulty in believing the little specks which he sees 
 scattered along the shore to be commodious dwellings, and 
 what seem to him only luxuriant herbs or bushes, to be mag- 
 nificent palm trees. 
 
 For the same reason that a distant body to the mere eye ap- 
 pears diminutive, viz. the smallness of the visual angle be- 
 tween the extreme points observed, so does a distant motion to 
 the mere eye appear slow. A carriage dashing past a pedes- 
 trian in the street, may surprise him by its speed; but if viewed 
 at the same time by a spectator from the top of St. Paul's, it 
 seems to be but crawling along the pavement. A ship driven 
 before a tempest, scarcely allows the sailor on board to distin- 
 guish the individual masses of the white foam through which 
 she flies; but if then seen on the distant horizon by a specta- 
 tor on shore, she is scarcely perceived to change her place. A 
 balloon high in the air, and borne along on the wings of the 
 wind at the rate of seventy or eighty miles an hour, may still 
 for a time leave a spectator on earth doubtful as to the direc- 
 tion in which it is moving. The moon in her orbit wheels 
 round the earth at the astonishing rate of about 2,000 miles an 
 hour, yet, owing to her distance from it, her motion is not 
 there visible to the naked eye, except by comparing her place 
 at considerable intervals. In respect to bodies still more dis- 
 tant than the moon, the truth at present under consideration is 
 still more striking. 
 
 Having now explained how the apparent transverse measure of 
 bodies and of space, in other words the visual angle subtend- 
 ed by them, is affected by their distance from the eye, we 
 proceed to show how it is affected also by their position. 
 
 A globe at a certain distance from the eye, however turned, 
 has the same appearance or bulk, in the field of view, and its 
 outline traced upon glass held between it and the eye, is always 
 a circle; but an egg, although when held in one position it 
 produces a circular outline or image, when held in another, 
 
THE EYE FORE-SHORTENING. 177 
 
 produces an image nearly oval. A wheel when viewed side- 
 ways appears a perfect circle, when viewed edgeways it ap- 
 pears a broad straight band or line, and when in any interme- 
 diate position it also appears oval. The apparent form then is 
 only a hint to the mind from which, by former experience or 
 other means, it guesses at the true form. If a man had never 
 seen an egg but endways, he never could have known that it 
 was not a perfect sphere. 
 
 If any long straight object, as a beam, be placed with one 
 of its ends directly to the eye, that end only can be seen, and 
 according to the case must appear a square or circle of the di- 
 ameter of the beam; if it then be placed with its side directly 
 to the eye, its whole length will be seen; and if placed in any 
 intermediate position, it will appear more or less shortened; 
 in all cases, its outline on the retina being similar to that of its 
 shadow on a wall behind the person. A man has advanced on 
 a spear pointed directly to his eye without seeing it, or on a 
 bar of iron carried on the shoulder of a porter met in the street. 
 A common telescope held with its end to the eye appears a 
 perfect circle; if then inclined a little, it seems to jut out on one 
 side, and as the inclination is increased, it juts out more and 
 more, until it displays its whole length. A great ship of war 
 whose stern is towards a spectator, appears a rounded building, 
 with its rows of windows like those of a peaceful habitation; 
 but as it turns, it gradually reveals the bristling cannon in their 
 whole length of fearful batteries. A straight row of a thou- 
 sand similar objects, as of soldiers in rank, pillars, trees, &e., 
 may appear to a person at the extremity as only one object of 
 the kind, the nearest individual completely hiding all the others; 
 but if viewed from the side and at a certain distance, the indi- 
 viduals may be counted. 
 
 The appearance now treated of is called fore-shortening, 
 and is to be noted wherever any surfaces or lines are not placed 
 so as completely to face the spectator. 
 
 Perhaps the most important case of fore-shortening which 
 has to be observed is that of an extensive plane surface, along 
 which the eye looks for instance, the general surface of the 
 earth or sea, by estimating aright the fore-shortening of which, 
 
 23 
 
178 LIGHT. 
 
 we partly judge of the distance or situation of the objects 
 placed upon it. It is evident, that in all such cases, the more 
 distant portions of the surface are progressively more fore-short- 
 ened than the nearer; for a man standing on a plain as a b, and 
 looking down immediately before him with his eye at c, sees a 
 portion of the surface almost directly, or without any fore- 
 shortening, and an extent of 5 feet, as a d (if 5 feet be the 
 
 height of the eye,) 
 will subtend in the 
 eye an angle of 45, 
 viz. the angle a c d, 
 or will appear 45 
 long in his field of 
 
 view therefore half of what is subtended by the whole space 
 from his feet to the horizon ; the next five feet will subtend an 
 angle of only 18, viz. d e /, the next of S, viz. f c g, and 
 so on; and as he carries his view more and more forward, the 
 surface becomes to it more and more oblique, until at last the 
 light coming from the surface seems rather to skim along the 
 level than to rise. This explains why a person having a side 
 view of a row of separate objects, as of men in line, trees, 
 pillars, &c., may see through between the nearest of them, but 
 towards the extremes of the view sees them as if standing in 
 closest possible array, or as if forming a continued surface. 
 The same remark explains why masses of cloud scattered uni- 
 formly over the sky, may allow a spectator to see wide inter- 
 vals of the blue heaven over head, while all around there is a 
 dense cloudy wall appearing to rest on the horizon. 
 
 If a man standing on a hill look down upon a field or plain 
 which is well known to him, and if he see some objects near 
 its side, and some near its middle, and some near its distant 
 border, he knows at once how far they are from him and from 
 each other. Similarly, if viewing the ocean from a lofty cliff, 
 and seeing ships scattered over its face, he may judge correctly 
 of their distance, for he can see only a certain extent of ocean 
 which becomes to him as a known field. The man stationed 
 at the flag-staff on the High Knowl peak of St. Helena, looks 
 down upon a circular field of the Atlantic a hundred miles 
 
THE EYE PORE-SHORTENING. 179 
 
 broad, and he tells the distance of any sail in sight to within a 
 mile or two. Now although the ground plan of a landscape 
 may not be so level as the field or ocean-face now spoken of, 
 there is still an approximation to the true plain, which very 
 considerably assists a spectator's judgment of distances.' 
 
 Painters are not only careful to fore-shorten correctly, ac- 
 cording to the proportion explained above, all the objects 
 which they portray, but they often avail themselves of the 
 principle to produce most striking effects. For instance, Mar- 
 tin, in many of his beautiful designs, by judicious fore-short- 
 ening, has exhibited miles in extent of gorgeous architecture 
 and of armed men, on a space of canvas that would seem 
 scarcely more than sufficient to receive a few figures: he has 
 made a single magnificent pillar or accoutred warrior placed in 
 the foreground, become the type which first fills the mind with 
 admiration, and then sends it along the retiring lines of beau- 
 tiful perspective, where every tip or edge renews the first im- 
 pression. A man lying on a table or a bed nearly as high as 
 the eye, with his feet towards the spectator, is fore-shortened 
 into a roundish heap, of which the soles of the feet hide tho 
 greater part. This is the description of the painting which has 
 been called the miraculous entombment of Christ, and it is be- 
 cause an unreflecting spectator moving sideways with the ex- 
 pectation of seeing more of the body, still sees only the soles 
 of the feet, and may suppose the body turned round so as to 
 front him, that the painting has received its appellation. For 
 nearly the same reason the eyes of a common portrait may 
 seem to follow a spectator to whatever part of the room he goes. 
 A rifleman represented as taking aim directly in front of the 
 picture, will seem to have in his power every spectator stand- 
 ing in the room; for, as in the case of the miraculous entomb- 
 ment, every spectator present will feel as if he alone could see 
 the picture as all see it. To terrify young ladies, a little arch 
 Cupid has ingeniously been represented with his arrow pointed 
 directly at them, and just ready to let it slip from his bended 
 bow: and, oh, how they are terrified ! 
 
 As the painter, availing himself of a knowledge of the prin- 
 ciples now explained, by which the eye usually judges of size 
 
180 LIGHT. 
 
 and distance, may produce on his canvas the most charming il- 
 lusions, so may the tasteful landlord in his ornamental gardens 
 and pleasure grounds, by working his levels into artificial un- 
 dulation of hill and dale, with magnitude of tree and of edifice 
 to correspond make the eye of a spectator luxuriate in the 
 contemplation of supposed extensive plains, lofty mountains, 
 distant pagodas, and wide-spread lakes all within the narrow 
 space of an acre or two; thus, in truth, by other means, pro- 
 ducing on the retina the same impressions as Claude, Poussin, 
 or Wilson, by their finest pictures. 
 
 When any object or mass of objects is fore-shortened, by 
 one part being farther from the eye than another, that part ap- 
 pears also in a proportion smaller than the other. For exam- 
 ple, in a straight row of similar houses, trees, &c., those near- 
 est to the eye will, on a glass held before the eye to receive 
 their images, form the largest images, and there will be a gra- 
 dual diminution, from the largest to the least, so that lines 
 drawn upon the glass along the tops and bottoms of the images 
 would tend to a point, called, for a reason explained below, the 
 vanishing point. Thus a person looking from a window upon 
 a long straight street, must, to see to the chimneys of the near- 
 est house, look through the top of the window, and to see the 
 street-door must look through the bottom; but the most dis- 
 tant house, both top and bottom, may be concealed from view 
 by a little spot upon the glass at the height of the eye. This 
 remarkable tapering of fore-shortened objects may of course 
 be strikingly observed on looking at any correctly made draw- 
 ing or engraving meant to represent a retiring row of similar 
 objects; such drawing being in truth an attempt to realize by 
 art the appearance of the objects as seen through the window. 
 
 The art which attempts to trace objects on a plane surface, as 
 they would appear on looking at them through that surface if it 
 were transparent, with their various degrees of apparent dimi- 
 nution on account of distance, and of fore-shortening on account 
 of obliquity of position, is called from the Latin word signify- 
 ing to look through, the art of perspective. It consists en- 
 tirely of the two parts now mentioned; and notwithstanding 
 the terror with which the study of it is clothed in the imagina- 
 
THE EYE PERSPECTIVE. 
 
 181 
 
 tions of many young painters, by reason of the mathematical 
 difficulties with which it has usually been mixed up, it is in it- 
 self exceedingly simple. We hope that a person capable of or- 
 dinary attention will, from what we have already said, and from 
 the few additional remarks which we have still to make on the 
 appearances of nature, be able completely to understand the 
 great laws of perspective. Although, without a knowledge of 
 these laws, a quick eye soon enables its possessor to sketch from 
 nature with much truth; and although the two instruments, the 
 camera obscura and camera lucida, give almost mathematical 
 accuracy to drawings, without requiring other skill in the drafts- 
 man than to trace with ink or pencil the lines which he sees as 
 if on the paper, still the subject is so interesting to all who look 
 either at nature or the works of art, that no intelligent person 
 should neglect it. 
 
 Supposing a straight row of similar objects, as of the stone 
 blocks or pillars represented here from a to S, to be viewed by 
 a person standing near C, then, because, as already explained, 
 
 objects to the eye ap- 
 pear smaller in exact 
 proportion to their in- 
 creased distance from 
 it, the second block, if 
 twice as far off as the 
 first, would appear only 
 half as large; the third, 
 if three times as far, 
 would be only one- 
 third as large, and so 
 on to any extent, and for any other proportions; and if the 
 1,000th or any other nearer or more distant pillar subtended to 
 the eye an angle less than the sixtieth of a degree of the field 
 of view, it would be altogether invisible, even if nothing inter- 
 vened between it and the eye. Then, where the row ceased 
 to be visible from the minuteness of the parts, or from the fact 
 of the nearer objects concealing the more remote, it might be 
 said to have reached its vanishing point. 
 
 Now it is very remarkable that in any such case of a straight 
 
183 LIGHT. 
 
 line or row vanishing from sight, in whatever direction it points, 
 east for instance, although the eye to see the near end of it 
 would have to look about north-east, still the point in the hea- 
 vens, or in a picture, or transparent plane before the eye, where 
 the line would vanish, would be exactly east from the eye, and 
 not in the slightest degree either to the north or to the south of 
 the east point, because the pillars happened to be north or south 
 of the individual; and therefore, if there were two or more 
 rows of pillars parallel to the first, but considerably apart from 
 each other, as the lines here, a S, b S, d S, &c., still all would 
 vanish or seem to terminate in the very same point of the field 
 of view. The reason of this is easily understood. Let us 
 suppose a line drawn directly east from the eye or to the point 
 tf, viz. a line directly over the line C S, and that the line of 
 pillars a S, also pointing east, is 20 feet north of the spectator, 
 and the line of pillars b S, running in the same direction, is 20 
 feet south of him, then, evidently, for the same reason as the 
 space between the top and bottom of the pillars, that is to say 
 their height, becomes apparently less as their distance from the 
 eye increases, so will the space between each pillar and the point 
 corresponding to its place in the visual ray, or the line along 
 which the eye looks, become less, and the lines of pillars really 
 20 feet apart from the visual ray, will, at a certain distance from 
 the eye, viz. where 20 feet is apparently reduced to a point, 
 appear to join it, and the three lines will appear to meet in that 
 point, beyond which they cannot be visible, and which is there- 
 fore called the vanishing point. The conception of this truth 
 may be facilitated by our supposing a star or planet to be rising 
 in the eastern point of the heavens at the moment of observa- 
 tion; then, if the three parallel lines were continued on to the 
 planet, and were visible as far, they would arrive there with 
 the 20 feet of interval between them just as they left the earth; 
 but as any planet, although many thousand miles in diameter, 
 owing to its distance from the earth, appears only a point, much 
 more would two lines only 20 feet apart be there undistin- 
 guishable in place by human sight. And what is true of a 
 space of 20 feet between parallel lines, is equally true, as re- 
 gards human vision, of a space of hundreds or of thousands of 
 
 
THE EYE PERSPECTIVE. 
 
 183 
 
 miles: as a general rule, therefore, it holds, that all lines in reali- 
 ty parallel to each other in perspective tend to and finish in the 
 same vanishing point, viz. the situation of the line in which the 
 eye looks when directed parallel to any one of those real lines. 
 And this is true not only of lines in the same level or horizon- 
 tal plane, viz. such as might be along the surface of the sea, 
 but also of lines that are vertical or one above another, as those 
 running along the tops and bottoms of the pillars here, or along 
 the roofs and windows of the houses, and indeed of all lines in 
 whatever situation, provided they are parallel to the visual ray. 
 When it is ascertained, therefore, that a line in any natural or 
 artificial object points 10 or 20 or any number of degrees north 
 or south, or above or below, &c. the centre of a scene or pic- 
 ture, that is to say, the point of sight or principal visual ray, 
 then also is it known that all the parallels to that line have their 
 vanishing point in that spot of the field of view, and a line sup- 
 posed drawn from the eye to the heavens, or really drawn to 
 the picture in that direction, marks the true vanishing point. 
 
 It is explained now, why in a long arched tunnel, or a cathe- 
 dral with many longitudinal lines on its floor, walls, roof, &c., 
 all such lines seem by an eye looking along from one end, ap- 
 pear to converge to a point at the other, like the radii of a spi- 
 der's web; and why in the representation of a common room, 
 viewed from one end, all the lines of the corners, tops and 
 
 bottoms of windows, 
 floor, stripes on a car- 
 pet, corners of tables, 
 &c. being parallel to 
 each other, tend to the 
 same vanishing point, 
 as V, and are cut off 
 according to the rule 
 of fore-shortening for- 
 merly alluded to. The 
 same considerations 
 will explain the appearance often to be observed, of two little 
 clouds near each other and almost motionless for a long time 
 in the distant sky directly to windward, but which on approach- 
 
184 LIGHT. 
 
 ing the spectator, appear to be gradually, and at last suddenly 
 enlarged, while one of them sweeps past considerably to the 
 right hand, and the other considerably to the left, but both again 
 meet, when at the former distance beyond the spectator, ap- 
 pearing there as small as at first. Clouds being so mutable and 
 uncertain in their forms, persons have been led to deem all 
 apparent changes in them, of form, size, and place, to be real 
 changes, and not, as they generally are, mere optical or per- 
 spective illusion. 
 
 By far the most important vanishing point in common scenes, 
 is the middle of the horizon or level line, and in a picture, 
 properly placed, it is at the exact height of the eye. It is 
 marked S in the figure before the last, and V in the last figure. 
 Because in houses, the roofs, foundations, floors, windows, &c. 
 are all horizontal, the vanishing points of their lines must be 
 somewhere in the horizon, and if the spectator be in the mid- 
 dle of a street or of a building, and be looking in the direction 
 of its walls, their vanishing point will be in the centre of the 
 scene or picture; if he be elsewhere, it will be at one side. In 
 holding up a picture frame, through which to view a scene suit- 
 able for a picture, it would be found most befitting to raise it 
 until the line of the horizon appeared to cross at about one- 
 third from the bottom: this fact becomes the reason of the 
 rule in painting, so to place the horizontal line. In beginning 
 a picture, this line is usually the first line drawn on the canvas, 
 as marking the place of the vanishing points of all level lines 
 and surfaces. And the eye of the spectator is supposed to be 
 placed before the middle of it, and generally about as far from 
 the picture as the picture is itself long, such being the extent 
 of view which the eye at one time most conveniently com- 
 mands. 
 
 Understanding now that the apparent or perspective direc- 
 tion of all lines in a scene is towards their vanishing points as 
 above discovered, or parallel to the picture where the originals 
 are so, and therefore cannot have vanishing points according 
 to the rule given, we proceed to show how much of a line 
 drawn to any vanishing point belongs to the known magnitude 
 of any object through which it passes; in other words, how 
 
THE EYE PERSPECTIVE. 
 
 185 
 
 much an object is in perspective fore-shortened in consequence 
 of its obliquity of position. 
 
 If we suppose A S P to represent a plate of glass seen edge- 
 ways, and that towards the point, S, in it, an eye is looking 
 
 from the point D 9 
 evidently then, a 
 line from P, con- 
 tinued in the di- 
 rection P R until 
 , it vanished from 
 sight, could have 
 as its perspective image or representation on the glass only a 
 line reaching from P to S, the point of sight here, and the 
 pictorial vanishing point of the line. Now, to divide the repre- 
 sentative line P S so as to correspond with any given portions 
 of the original line P R, &c., it would only be necessary to 
 draw other lines from the place of the eye D to cut or touch 
 the original line in the situations desired, and these lines would 
 cut the perspective line, S P, as required: for instance, the por- 
 tion of the true line, a b, would be represented by the por- 
 tion of the image line, S P, included between the two lines 
 a D and b D, and so of any other portions. There are figures 
 drawn on many mathematical scales by which such problems 
 as this can be solved at once; and the proportions are also de- 
 tailed in common tables: but the most generally convenient 
 mode in practice is, to set off on the intended drawing, (as 
 that of which c d here marks the boundary,) from the point 
 of sight, S, a distance on the horizontal line, at D, equal to 
 
 the distance of the eye (rom the picture, and then by oblique 
 lines drawn upon the base line, P R, to cut the perpendicular 
 
LIGHT. 
 
 line, P S, in the situations desired as is seen in the last 
 sketch, which differs from the present only in having the point 
 of distance marked before its point of sight, instead of late- 
 rally, as here. And the line P S being always cut by the ob- 
 lique line from D, in proportion to the length of base line be- 
 tween P and the extremity of the oblique line, a horizontal line, 
 drawn through any point in it, cuts in corresponding propor- 
 tions all the other lines which have their vanishing points in 
 the horizontal line S D, for instance, a S, b S, &c. Thus, to 
 draw in perspective, on the surface above represented and pre- 
 pared, a chess board, or board of squares, it is necessary to 
 set off the breadth of the board on the base line to the right 
 and left of P, viz. to b and a, and then to draw to the point 
 of sight as a vanishing point, the lines a S and b S, part of 
 which lines will therefore represent the sides of the board, 
 and then to draw the diagonal, b D, which, for the reasons 
 above stated, will cut the lines P S and a S in proportion to 
 the length of base line to the right of their extremities; a efb, 
 therefore, is a square seen in perspective, and any number of 
 smaller included squares are made by drawing lines from the 
 vanishing point to equal divisions on the base, and making 
 cross lines where the diagonal cuts these. 
 
 Much of the delight which the art of painting is calculated 
 to afford is lost to the world, because persons in general know 
 not how to look at a picture. Unless a spectator place himself 
 where he can see the objects in true perspective, so that he 
 may fancy himself looking at them through a window or open- 
 ing, every thing must appear to him falsely and distorted. 
 The eye should be opposite the point of sight of the picture, 
 and therefore on a level with the line of the horizon, and it 
 should be at the required distance, which is generally, at least, 
 as great as the length of the picture. But blame not. unfre- 
 quently rests also with the artist, from his having neglected 
 the study of perspective. It is very common, for instance, to 
 see miniature resemblances of architectural structures so fore- 
 shortened and tapered, that the eye, to see them in true per- 
 spective, would require to be within an inch of the paper; 
 whence, at the usual distance of ten or twelve inches, they are 
 
THE BYE JUDGING OP SIZE. 187 
 
 seen as hideous distortions. The specimens in the few pre- 
 ceding pages necessarily exemplify, in a degree, this error, be- 
 cause the point of distance had to be marked where there 
 was but a small page. These figures, therefore, by any per- 
 son studying the subject particularly, should be drawn on such 
 a scale as that the eye may really view them at the distance 
 supposed. 
 
 A means of judging of the dimensions of bodies by the visual 
 angle, but which depends neither on the absolute size of the 
 image, nor on the fore-shortening of the ground plane on 
 which the body stands, is to use known objects in view as 
 measures for others near them which are unknown. 
 
 If any person of our acquaintance be standing at some dis- 
 tance from us near another person who is a stranger, we know 
 how tall the stranger is by taking the acquaintance as a mea- 
 sure. 
 
 In pictorial representations of objects little familiar, as to 
 many people are the Egyptian pyramids, the bodies of the 
 whale, the elephant, the camel, &c., human beings may be re- 
 presented around them to serve as measures for the less known 
 object. The Colossus of Rhodes, seen from afar, might, to a 
 stranger, have appeared but an ordinary statue of a man; but 
 the exact magnitude would have been known as soon as a ship 
 of known dimensions were seen sailing into port between his 
 gigantic limbs. 
 
 When an unpractised eye is first directed to a great ship of 
 war, it will on many accounts dwell upon it with wonder and 
 admiration; but it may not judge truly of the enormous mag- 
 nitude until near enough to perceive the sailors climbing on the 
 rigging, and appearing there, by comparison, as flies or little 
 birds appear among the branches of a majestic tree. 
 
 By having a measure of this kind presented to us, the mag- 
 nitude and elevation of some fine edifices are rendered more 
 obvious. The magnificent pile of St. Paul's in London be- 
 comes more striking still, when we discover visiters looking 
 from the balconies near the summit cross. They appear so 
 minute among the surrounding huge masses that a person is at 
 
I8< LIGHT. 
 
 iirst for a while disposed to doubt whether they be men; but 
 the fact once ascertained, the grandeur of the temple is render- 
 ed extremely impressive. 
 
 Many persons cansfot distinguish between the little pilot bal- 
 loon (sometimes despatched before a great one to show the di- 
 rection of the wind) and the great balloon itself, until with the 
 last they perceive the aeronauts as little black points suspended 
 under the globular cloud. 
 
 Strangers, visiting Switzerland, on first entering the valleys 
 there, are often much deceived as to their extent. Because 
 familiar generally with more lowly hills and shorter valleys at 
 home, but which from being near to the eyes form bulky 
 images, and having no measure at first, they almost universal- 
 ly underrate the Alpine dimensions: they will wonder, for 
 instance, in the valley of Chamouny, that they should be tra- 
 Telling swiftly for hours without reaching the end, where on 
 entering they did not believe the length to be as many miles. 
 
 The author once sailed through the Canary Islands, and 
 passed in view of the far-famed Peak of Teneriffe. It had 
 been in sight in the afternoon of the preceding day, at a dis- 
 tance of more than 100 miles, appearing then only as an ordi- 
 nary distant hill rising out of the ocean; but, next morning, 
 when the ship had arrived within about twenty miles of it, 
 and while another ship of the fleet, holding her course six miles 
 nearer to the land, served as a measure, it stood displayed as 
 perhaps the most stupendous single object which on earth, and 
 at one view, human vision can command. That noble ship, 
 whose side showing its tiers of cannon, equalled in extent the 
 fronts of ten large houses in a street, and whose masts shot up 
 like lofty steeples, appeared as a speck scarcely rising from the 
 sea, compared with the huge prominence beyond it towering 
 sublimely to heaven, and around which the masses of cloud, 
 although as lofty as those which sail over the fields of Britain, 
 were still hanging low on its sides. Teneriffe, alone, of very 
 high mountains, rises out of the bosom of the ocean with in- 
 accessible steepness on one side, to an elevation of 13,000 feet; 
 and as an object of contemplation, therefore, is more impres- 
 sive than even the still loftier summits of Chimborazo or the 
 
THE EYE JTTDGING OP SIZE. 189 
 
 Himalayas, which rise from elevated plains, and in the midst 
 of surrounding hills. 
 
 It is because objects which are nearly on a level with us, as 
 contrasted with such as are either much above or much be- 
 low, are in general more numerously surrounded by other 
 objects which serve as measures of comparison, that we judge 
 BO much more correctly of the size and distance of the for- 
 mer than of the others. 
 
 A man walking like ourselves on the sea-shore or other le- 
 vel, is at once recognised; and probably it may not occur to 
 us, that he appears smaller on account of the distance; but if 
 the same man be seen afterwards at an equal distance above us, 
 collecting the sea fowl's eggs on the face of a cliff, or below 
 us, gathering shells on the beach, when we ourselves have 
 reached the height, he appears no bigger than a crow: yet in 
 all the cases he is where the same bulk forms the same magni- 
 tude of image on the retina. 
 
 Even on a horizontal plain, if the general surface be bare and 
 uniform, single distant objects appear very diminutive. This is 
 true, for instance, of a man seen apart from his caravan, while 
 journeying across a sandy desert; while a man viewed at an 
 equal distance, in the midst of a cultivated landscape, appears of 
 his natural size; the same is true of a boat or ship seen out on 
 the high sea, as contrasted with similar objects viewed in a har- 
 bour, where other known objects are near them. 
 
 We may now understand why the sun and moon, when rising 
 or setting, appear to us much larger than when they have at- 
 tained meridian height although, if we examine them by any 
 measure of the visual angle, as simply by looking at them through 
 the same ring or tube, we find that there is no difference. The 
 Bun and moon in appearance from this earth are nearly of the 
 same size, viz. always occupying in the field of view about the 
 half of a degree, or as much as is occupied by a circle of a foot 
 in diameter when held about 250 feet from the eye which cir- 
 cle, therefore, at that distance, and at any time, would just hide 
 either of them. Now, when a man sees the rising moon appa- 
 rently filling up the end of a street, which he knows to be 100 
 
190 LIGHT. 
 
 feet wide, he very naturally believes that she then subtends a 
 greater angle than usual, until the reflection occur to him,- 
 which it rarely will of itself, that he is using as a measure of her 
 size, a street known indeed to be 100 feet wide, but of which the 
 part concerned, owing to its distance, appears to his eye exceed- 
 ingly small. The width of the street near him may occupy 60 
 of his field of view, and he might see from between the houses 
 many broad constellations instead of the moon only; but the 
 width of the street far off may not occupy, in the same field of 
 view, the twentieth part of a degree, and the moon, which al- 
 ways occupies half a degree, will there appear comparatively 
 large. The kind of illusion now spoken of is yet more remark- 
 able when the moon is seen rising near still larger known ob- 
 jects, for instance, beyond a town or a hill which then appears 
 within her luminous circle. Any person who from the river- 
 side terraces of Greenwich has observed the sun setting beyond 
 London, with St. Paul's cathedral included in the glorious pic- 
 ture, will recollect a most interesting example of our present 
 subject. That our ocular judgment of the size of the sun or 
 moon is thus influenced by ihe presence or absence of objects of 
 comparison, and not by the place of the bodies in the sky, is 
 proved by the fact that a person viewing these bodies from the 
 bottom of some of the Swiss valleys, where he might almost 
 suppose himself placed at the centre of the earth, and looking 
 abroad along an endless extent of precipices if he can closely 
 compare them with certain known magnitudes of ridge or forest 
 bounding his view, sees them, although at a great elevation, 
 as large as they appear from other situations when rising direct- 
 ly out of the sea. Another proof is afforded by the case of a 
 balloon at a great elevation seen crossing the disk of the sun or 
 moon, and appearing, however large, as an absolute speck with- 
 in the vast luminous area In a future paragraph it will be ex- 
 plained, that the sun and moon when low appear larger than 
 when high, also, because of their apparent dimness when low. 
 
 It may be remarked here, that the visual estimate formed of 
 the great size of the sun and moon when seen on the horizon, is 
 not an illusion, as is popularly supposed, but an approximation 
 to truth, still vastly short of the reality. When we see a tree, 
 
THE EYE INTENSITY OF LIGHT. 191 
 
 or a house, or a bill, apparently within the circumference of one 
 of these orbs, it is really true that the orb is larger than the tree, 
 or house, or hill, and so much larger, that even if the whole 
 British Isles could be lifted away from the earth, and suspended 
 as a map in the sky, when brought near the moon they would 
 hide from a spectator on earth but a small part of her. 
 
 Having now shown that in relation to any object, the visual an- 
 gle or apparent size can be a measure of the true size only 
 when the distance also is known, and that the visual angle it- 
 self serves to determine the distance only in certain cases, we 
 proceed to examine other means which the eye commands for 
 guessing at distances. 
 
 2d. Intensity of light, shade, and colour. (See the Analysis, 
 
 page 122.) 
 
 It has already been explained that light, like every other in- 
 fluence radiating from a centre, becomes rapidly weaker as the 
 distance from the centre increases, being, for instance, only one 
 fourth part as intense at double distance, and in a corresponding 
 proportion for other distances; while it is still farther weakened 
 by the obstacle of any transparent medium through which it 
 passes. Now, the eye soon becomes sufficiently familiar with 
 these truths to judge from them, with considerable accuracy, of 
 the comparative distances of objects. 
 
 The fine Gothic pile of Westminster Abbey may break upon 
 the view in some situation where nearer edifices, and perhaps 
 some minor imitations of its beauties, already fill and dazzle the 
 eye with their brightness; but the mistf or less d.stinct outline 
 of the former warns the approaching stranger of its true magni- 
 tude, and prepares him for the enjoyment which a nearer in- 
 spection of its grandeur and perfection is to afford. 
 
 A small yacht or pleasure boat may be buili from the same mo- 
 del or of the same comparative dimensions as a first-rate vessel of 
 war, and may be in view from the shore at the same time, only 
 so much nearer than the ship, that both shall form images of the 
 same magnitude on the retina of a spectator. In such a case, to 
 an unpractised eye, it might be difficult to detect the difference; 
 
192 LIGHT, 
 
 but to another, the bright lights of the little vessel, contrasted 
 with the softer or more misty appearance of the larger, would 
 leave no room for doubt. A haziness occurring in the atmos- 
 phere between the little vessel and the eye, might considerably 
 disturb the judgment. 
 
 In a fleet of ships, if the sun's direct rays fall upon one here and 
 there through openings among the clouds, while the others re- 
 main in shade, the former immediately start in appearance towards 
 the spectator. Similarly, the mountains of an unknown coast, 
 if the sunshine fall upon them, appear comparatively near; but 
 if clouds again intervene, they recede and mock the awakened 
 hope of the approaching mariner. 
 
 A conflagration at night, however distant, appears to specta- 
 tors generally, as if very near, and inexperienced persons often 
 run towards it with hope of arriving immediately, but find after 
 miles travelled that they have made but a little part of their way. 
 
 A person ignorant of astronomy deems the heavenly bodies 
 vastly nearer to the earth than they are, merely because of their 
 being so bright or luminous. The evening star, for instance, 
 Been in a clear sky over some distant hill-top, appears as if a 
 dweller on the hill might almost reach it for the most intense 
 artificial light that could be placed on the height would be dim 
 in comparison with the beauteous star; yet to a dweller on the 
 hill it appears just as distant as to one on the plain; nay, at 
 thousands of miles nearer, the appearance would still be nearly 
 the same. 
 
 The concave of the starry heavens appears flattened above, or 
 nearer to the earth in its zenith than towards its horizon, because 
 the light from above having to pass through only the depth or 
 thickness of the atmosphere, is little obstructed, while of that 
 which darts towards any place horizontally through hundreds of 
 miles of dense vapour-loaded air, only a small part arrives. 
 
 The sun and moon appear larger at rising and setting than, 
 when midway in heaven, partly, as already explained, because 
 while below they can easily be compared with other objects, of 
 which the size is known, but partly, also, because of their less 
 light in the former situation, while their diameters are always 
 the same. 
 
THE EYE LIGHT AND SHADE. 193 
 
 A fog or mist is said to magnify objects seen through it. The 
 truth is, that by reason of the diminished intensity of light, it 
 makes them appear farther distant without lessening the visual 
 angles subtended by them; and because an object at two miles, 
 subtending the same angle as an object at one mile, must be 
 twice as large, the conclusion is drawn that the dim object is 
 large. Thus a person in a fog may believe that he is approach- 
 ing a great tree, fifty yards distant, when the next step throws 
 him into the bush which had deceived him. Two friends meet- 
 ing in a fog have often mutually mistaken each other for persons 
 of much greater stature. A row of foxglove flowers on a neigh- 
 bouring bank has been mistaken for a company of scarlet-clad 
 soldiers on the more distant face of the hill. There are, for si- 
 milar reasons, frequent misjudgings in late twilight and early 
 dawn. The purpose and effect of a thin gauze screen interposed 
 between the spectators in a theatre and some person or object 
 meant to appear distant, are intelligible on the same principle: a 
 boy near, so screened, will appear to be a man at a distance. 
 The art of the painter uses sombre colours when his object is to 
 produce in his picture the effect of distance. On the alarming 
 occasion of a very dense fog coming on at sea, where the ships 
 of a fleet are near to each other, without wind, and where there 
 is considerable swell or rolling of the sea, much damage is of- 
 ten done; but it is to be remarked in such a case, that the size of 
 ships approaching to the shock is always in idea exaggerated. 
 
 The celebrated Spectre of the Brocken, among the Hartz 
 Mountains, is a good illustration of our present subject. On a 
 certain ridge, just at sunrise, a gigantic figure of a man had of- 
 ten been observed walking, and extraordinary stories were re- 
 lated of it. About the year 1800 a French philosopher went 
 with a friend to watch the phenomenon; but for many morn- 
 ings they had paraded on an opposite ridge in vain. At last, 
 however, they discovered the monster, but he was not alone; 
 he had a companion, and singularly he and his companion aped 
 all the motions and attitudes of the observer and his companion: 
 in fact, the spectres were merely shadows of the observers, 
 formed by the horizontal rays of the rising sun falling on the 
 morning fog which hovered over the valley beyond; but be- 
 
 25 
 
194 LIGHT. 
 
 cause the shadows were very faint, they were deemed distant, 
 and therefore seemed men walking on the opposite ridge, and 
 because a comparatively small figure seen near, but supposed 
 distant, appears of gigantic dimensions, these shadows were ac- 
 counted giants. 
 
 While under common circumstances the comparative inten- 
 sities of light furnish an indication of difference of distance, 
 there are other cases of comparative intensity, in which bodies 
 are to be considered not as wholes, but as made up of parts un- 
 equally exposed to the source of light, and therefore reflecting 
 it to the eye, or being illuminated in different degrees. In ob- 
 serving, for instance, a white house exposed to the sun, it is 
 seen that the side directly receiving the rays is highly illumi- 
 nated or bright, while the other sides are less so, and are said 
 to be in the shade: and they are luminous in proportion to other 
 sources of reflected light near them. The different faces or 
 walls of such a house are as strongly distinguished from each 
 other, by the mere difference of shade, as if they were of dif- 
 ferent colours. If the object were a ball instead of a square 
 house, there would still be as great differences of shade in the 
 half not receiving direct rays, but the parts, instead of forming 
 abrupt contrasts like ther walls of a house, would melt into each 
 other and mark the beautiful round contour of the object. The 
 consideration of all such cases forms the subject of chiaro-oscu- 
 ro, so interesting to the painter. 
 
 Had there not been in nature the provision of light and shade 
 now described, the sense of sight would have been of compa- 
 ratively little use; for it is that provision chiefly which enables 
 us to distinguish the profile or outlines of different bodies placed 
 near to each other, and the protuberant or other form of the 
 surfaces next the observer. But for this, it would have been 
 impossible to distinguish, for instance, between a flat circle, a 
 sphere, and a cone, all directly exposed to the eye; but, in re- 
 ality, the uniformly bright surface of the circle, the soft rounded 
 shadowing of the sphere, and the shade coming to a point on 
 the cone, at once declare the true forms. But for the shadows, 
 the fag.ade of a white palace of varied architecture would have 
 been an unmeaning sheet of light; the lights, however, and 
 
THE EYE LIGHT AND SHADE. 195 
 
 shadows produced by the juttings and recesses, mark the varie- 
 ty of surface most completely: and the round pillar is distin- 
 guished from the square, and every pediment, and capital, and 
 architectural ornament, stands out pleasingly conspicuous. But 
 for light and shade, again, the human face divine would have 
 been an unmeaning patch of flesh, for there are few other lines 
 in it than those made by different exposures to the light, and 
 yet every prominence and depression are so truly indicated to 
 the eye that it becomes full of meaning or expression. How 
 clearly mere light and shade serve to convey all that the eye 
 can learn of a scene or object, may be perceived by examining 
 any of the admirable engravings which now abound scarcely 
 inferior in expression to the most finished paintings. 
 
 The student of painting soon learns that the lines called out- 
 lines, by which he first sketches subjects, do not exist at all in 
 nature, and have to be again effaced in his finished work: for 
 they only mark the place where lights and shades happen to 
 meet. Much may be conveyed to the mind, however, by a 
 mere outline, and particularly if lines of different breadth or 
 thickness are used to mark the situation of the lighter and deeper 
 shadows. 
 
 The subject of chiaro-oscuro is not so simple as, from the fact 
 of the sun being the great source of light, might at first be 
 supposed; for although this be true, still every body which re- 
 flects the sun's light becomes a new source to those about it, 
 and the shading of a picture must have reference to- all such 
 sources, and to the colours of the body itself, and of the neigh- 
 bouring bodies. 
 
 In looking at an extended landscape, it is seen that the near 
 objects considered as wholes, are comparatively bright, that 
 their shadows are strongly marked, and that their peculiar co- 
 lours are every where easily distinguishable as of flowers, 
 fruit, foliage, &c., but farther off the colours become dim, the 
 lights and shadows melt into each other or are confused, and 
 the illumination altogether becomes so faint that the eye at last 
 sees only an extent of distant blue mountain or plain appear- 
 ing bluish because the transparent air through which the light 
 must pass has a blue tinge, and because the quantity of light 
 
LIGHT. 
 
 arriving through the great extent of air is insufficient to exhibit 
 the detail. The ridge called Blue Mountains in Australia, and 
 another of the same name in America, and many others else- 
 where, are not really blue, for they possess all the diversity of 
 scenery which the finest climates can give; but to the discover- 
 er's eye, bent on them from a distance, they all at first appeared 
 blue, and they have ever since retained the name. 
 
 In a picture by an artist, who on his canvas stretched on a 
 frame disposes the lights, shades, and colours in the very situ- 
 ations and with the intensities which on coming from the land- 
 scape to the eyes, through a plate of glass filling up the frame, 
 they would have had, all that we have now been saying is strict- 
 ly exemplified. In the fore-ground the objects are large and 
 bright, but as the scene is supposed to be gradually more re- 
 mote, the size and brightness of the objects correspondingly di- 
 minish, until at last there is only a dim mixture of bluish or 
 grayish masses forming the horizon and sky. 
 
 A child, during what may be called the education of the 
 sense of sight, has a strong perception of the vast differences 
 of appearance which things assume according to their acciden- 
 tal distance from the eye, their position, their exposure to 
 light, &c.; for these differences, being often calculated to de- 
 ceive the young judgment, many of them have been noted 
 with surprise. Thus, a boy when he first discovers that a ship, 
 which at the quay, with sails spread, concealed from him half 
 the heavens, is in an hour or two afterwards seen by him on 
 the distant horizon as a speck hardly big enough to hide one 
 star; or, again, when he discovers that the faint blue un- 
 ehanging mass which he had always observed bounding in one 
 direction the view from the home of his infancy, is a distant 
 mountain-side thickly inhabited, and covered with fields and 
 gardens, where in succession all the bright colours of the dif- 
 ferent seasons predominate, his attention is strongly awakened, 
 and he feels surprise. But as soon as experience has enabled 
 him to interpret readily and correctly the visual signs under 
 every variety of circumstance, his attention passes so instant- 
 ly from them to the realities which alone are interesting to 
 him- just as it might pass from the paper and printing of ? 
 
THE EYE PERSPECTIVE. 197 
 
 newspaper to the important intelligence communicated by 
 them, that he very soon ceases to be aware that the sign, which 
 in every case similarly suggests the object, is not also in every 
 case similar to itself, and the very same true and complete re- 
 presentation of the reality. The prejudice that the sign is of 
 this nature, becomes quickly so strong, that even a difficult ef- 
 fort has to be made by a grown person again to attend to the 
 mere appearances, in any scene of which the realities are 
 known. 
 
 This attempt to analyze the appearances, and which in one 
 sense is a trying to unlearn something, or to retrograde, is 
 called the study of perspective and when it regards the appa- 
 rent reduction of size, and the fore-shortening of bodies under 
 various circumstances, it is called linear perspective; when it 
 regards the fading of light and the modifying of colour, it is 
 called aerial perspective. As the art of painting depends en- 
 tirely upon the understanding of these two departments, the 
 gradual progress which it has made in different countries is a 
 measure of the degree in which the common prejudice that 
 things appear as they are has in them been overcome. Where 
 this prejudice exists, any untaught person conceives a good 
 painting to be merely a miniature representation drawn accord- 
 ing to a certain reduced scale, as of an inch to a yard, and 
 in which all the dimensions of things are to be measured as 
 simply as in the reality while the colours, as to vividness, &c., 
 should perfectly agree in both. This statement is remarkably 
 illustrated by the fact, that children in their rude attempts to 
 paint, always aim at realizing the notion of the art given above, 
 and that such has been the first stage of painting in every coun- 
 try. In Europe now, owing to the labours of men of genius, 
 art in painting may be said almost to rival nature, producing 
 scenes as lovely as the finest of nature's scenes, and scarcely 
 distinguishable from them, but in other countries, as in China 
 and India, among the native artists, the first stage of the art is 
 still in existence. In a Chinese picture, owing to the absence 
 of perspective proportions, an extensive subject is only a col- 
 lection of portraits of men and things drawn on the same 
 scale, and placed near one another, and where all the colour? 
 
19S LIGHT. 
 
 are as vividly shown as if the objects were only a few feet 
 from the eye: there the figures at the bottom or fore-ground 
 are supposed to represent the objects nearest to the spectator, 
 while the figures higher up are supposed to be of more remote 
 objects, all appearing as they might be seen in succession by a 
 person who had the power of flying over the country. This 
 kind of picture or representation, although not natural if all 
 viewed at once, may communicate more information than a 
 single common painting, for it is equivalent to many such. In 
 Europe, and lately, the principle has been usefully acted upon 
 for certain purposes, as for representing on one long sheet, or 
 on a succession of sheets connected in a suitable manner, the 
 banks of a river or a line of road. The banks of the Rhine, 
 particularly, have thus been admirably portrayed, so that the 
 spectator, directing his eye along the paper, feels almost as if 
 carried in a balloon to view, in great detail, the whole of the 
 real and enchanting scenery. The principle might perhaps 
 with advantage be acted upon still more extensively for in- 
 stance, to produce, instead of common maps or charts of coun- 
 tries, true bird's-eye views, over which the eye, moving from 
 place to place, at every new point of sight, would see a cer- 
 tain portion of the country just as a bird or aeronaut would, 
 the sketch being supposed to be taken from that certain eleva- 
 tion deemed most suitable for the ends in view. 
 
 3d. Divergence of the rays of light. 
 
 This is the next circumstance to be mentioned, by which the 
 eye judges of distance. Supposing the line E F to mark the 
 place or breadth of the pupil of the eye, the light entering 
 
 from an object at , which 
 * s near, is very divergent, 
 or j s spreading with a 
 
 J? large angle; from b the 
 
 extreme rays are less divergent, or they open at a smaller an- 
 gle; from c they are less divergent still, and so on. Now, the 
 eye, to form an image on its retina, requires to exert a bend- 
 ing power exactly proportioned to the divergence jf the rays; 
 and it appears to have a sense of the effort made, which be- 
 
THE EYE DIVERGENCE Ol' KAYS. 199 
 
 comes to the person a kind of measure of the distance of the 
 object. This great divergence of the rays entering the eye, 
 is the chief circumstance in which the most perfect painting 
 must still differ in its effect upon the eye from a natural scene; 
 for while in nature every ohject according to its distance is 
 sending rays which reach the eye with different divergence, 
 and which, therefore, can produce distinct images on the retina 
 at any one time only of the objects which are at due distance 
 from it, the rays from a picture which is a single plane surface, 
 come from every part with the same divergence, and the eye 
 must feel a disappointment in not having to accommodate its 
 power of bending to the different distances attempted to be 
 portrayed on the canvas. It might be expected that this kind 
 of disappointment would be more felt on looking at a common 
 picture placed a few feet from the eye, than at the sort of pic- 
 ture called panorama, which is on a larger scale and proportion- 
 ately more distant, but such is not the case; and the reason 
 seems to be, that in the former the illusion is not intended to 
 be complete, the fact of its being but a picture not being at all 
 concealed, and the eye is therefore at once told to expect a dif- 
 ference of feeling; but in the panorama, the whole circum- 
 stances are arranged to deceive the eye entirely, if possible, 
 and to make it believe that the images on the retina are formed 
 by light from the objects themselves; then, to the eye really 
 deceived in all other particulars, the non-accordance with na- 
 ture in this one is strongly, and by some persons even painful- 
 ly felt, so as on their first entering the place to cause them 
 headach or giddiness. The illusion, and consequently the plea- 
 sure from viewing any picture, may be made more complete by 
 the spectator using lenses or spectacles, such that the focal dis- 
 tance shall be equal to the distance of the painting from the 
 eye; because such lenses, as was formerly explained, would 
 render all the rays entering the eye nearly parallel, and there- 
 fore very nearly such as would arrive from objects at any con- 
 siderable distance. 
 
200 LIGHT. 
 
 4th. Convergence of the axes of the eye. 
 
 This is the last circumstance to be mentioned, by which a 
 person, through the eye, judges of the distance of objects. In 
 consequence of there being in the two eyes corresponding 
 parts, which must be similarly affected by any object, that the 
 person may have single vision of it as was explained in a 
 former page, the axes of both eyes must point to the object, 
 and if it happen to be very near, they will meet and cross each 
 other so near the face as to produce the appearance of squint- 
 ing, seen when a person tries to look at the point of his 
 nose, but if the object be more distant, the obliquity will be 
 less, until at last the eyes, directed to a thing at a very great 
 distance, will have their axes almost parallel. The last figure 
 may serve also to explain this subject. Supposing E and F to 
 mark the places of the two eyes, if the object looked at be 
 near them, as at a, they must be very much turned inwards, 
 that their axes may meet; if it be at b, they will be less turned; 
 if at c, less still, and so forth. 
 
 When the eyes are not directed to any thing in particular, 
 the axes generally become parallel, or as if they were pointed 
 to a very distant object: and because this happens generally 
 when persons are reflecting on things which are absent and 
 seen only by the mind's eye, it is an expression of countenance 
 held to mark contemp- ition or thoughtful ness. 
 
 The direction of the visual axes is a particular like the diver- 
 gence of light, as to which a mere picture can never produce 
 upon the eye precisely the effect of the objects themselves. To 
 see a picture, the axes must meet at a few feet from the eye; 
 while to see the objects of nature, they often do not meet nearer 
 than at miles. By a glass, however, as will be explained a lit- 
 tle farther on, it is possible to correct also this defect, and to 
 render an optical illusion, as reg-rds still objects, absolutely 
 complete. 
 
 When a picture has to represent objects supposed far from the 
 eye, the farther the picture itself is placed from the eye, sup- 
 posing the figures to be made proportionately large, the more 
 nearly perfect will the illusion become, because the divergence 
 
 
THE EYE CONVERGENCE OF VISUAL AXES. . 201 
 
 of rays and convergence of the axes (the two circumstances in 
 which the effect on the eye of a mere picture must always dif- 
 fer frgm that of a real scene) will be in proportion more near- 
 ly natural. This explains in part why the picture called pano- 
 rama (from Greek words, signifying a view of all) is an exhi- 
 bition so charming; for usually the painting is far removed from 
 the eye, and is drawn on a proportionately large scale, and the 
 eyes feel that the light comes from a considerable distance, and 
 that their axes do not need to converge very much; and when, 
 in such a case, the first impression of the want of perfect con- 
 formity has passed away, the illusion becomes nearly complete. 
 But a not less important peculiarity in the panorama is, that in- 
 stead of being a painting on a plane surface like common pic- 
 tures, and embracing only a small part of the field of view, it 
 is on a surface which entirely surrounds the spectator, and on 
 which all the objects, visible in every direction from the sup- 
 posed place, are seen in the very situations which in nature they 
 hold; and the spectator is enabled to conceive much more dis- 
 tinctly of each particular by seeing it in relation to others 
 around. Few persons can forget the impression made on them 
 by the first panorama which they may have seen; and with in- 
 creased maturity of judgment, still more and stronger reasons 
 are discovered for admiring this miraculous mode of instantly 
 transporting persons to any distance, beyond seas and other dan- 
 gers, to contemplate at their ease the most interesting scenes of 
 nature, represented under the most favourable circumstances of 
 light, season, weather, &c. Hence, few persons of good taste 
 neglect the opportunity now, in most great towns so frequently 
 offered, of obtaining at so little cost so high a gratification. 
 
 To correct the slight remaining optical defects of a common 
 panorama, a large lens may be used, of which the focal distance 
 is the distance of the picture from the eye. This has the effect 
 of converting the divergence of the rays of light into the paral- 
 lelism which belongs to the supposed remoteness of the objects, 
 and it also bends the light so that the axes of the eyes become 
 parallel. The author has found a convenient mode of using the 
 lens for such a purpose to be, to cut out two round pieces from 
 opposite sides of it, and to form them into a pair of spectacles: 
 
202 . LIGHT. 
 
 from one lens three pairs may be formed. Panorama exhibiters 
 should always keep such lenses or spectacles for the use of vi- 
 siters. 
 
 The effect of the magnitude and distance of the ordinary large 
 panoramic views might be had with the assistance of proper 
 glasses, from even the smallest picture or engraving embracing 
 the same field; and it is remarkable that some enterprising per- 
 son has not undertaken to publish a series of interesting views 
 fitted to be used in that way. A common panorama occupying 
 a circular wall of 150 feet circumference and twenty feet high, 
 might be reduced, still retaining the same truth of proportions, 
 to appear on a sheet of paper five feet long and eight inches 
 high or broad-'-or on a common sheet of paper, which might 
 afterwards be cut into three stripes to be joined endways; and 
 if this paper were set up in a suitable frame, like a wall round the 
 head of a spectator, while its edges were concealed by drapery 
 or otherwise, and the eye could only view it through fit glasses 
 placed in its centre and made to turn round so as to command 
 the whole, it could not by an ordinary spectator be distinguished 
 from the large panorama. With the art of lithography, now so 
 well adapted to producing soft representations of scenery, the 
 expense of such views might be very moderate, allowing them 
 to form a common part of library furniture. When we reflect 
 upon the expansion of mind obtained by travelling, and that not 
 a few of the advantages would follow a familiarity with a good 
 selection of panoramic views, it is not perhaps too much to sup- 
 pose that courses of instruction on geography, history, &c. may 
 before long be illustrated by this most interesting mode of aid- 
 ing the conception and memory. 
 
 Common paintings and prints may be considered as parts of 
 a panoramic representation, showing as much of that general 
 field of view which always surrounds a spectator, as can be seen 
 by the eye turned in one direction, and looking through a win- 
 dow or other opening. The pleasure from contemplating these 
 is much increased by using a lens or such spectacles as above de- 
 scribed. There is such a lens fitted tip in the shops, with the 
 title of optical pillar machine, or diagonal mirror, and the 
 print to be viewed is laid upon a table beyond the stand of the 
 
THE EYE COSMORAMA. 203 
 
 lens, and its reflection in a mirror supported 
 diagonally over it, is viewed through the lens. 
 The illusion is rendered more complete in 
 such a case by having a box, as a, b, to receive 
 the painting on its bottom, and where the 
 lens and mirror, fixed in a smaller box above 
 at a, are made to slide up and down in their 
 place to allow of readily adjusting the focal distance. This box 
 used in a reverse way becomes a perfect camera obscura. The 
 common show*stalls seen in the streets are boxes made some- 
 what on this principle, but without the mirror; and although the 
 drawings or prints in them are generally very coarse, they are 
 not uninteresting. To children whose eyes are not yet very 
 critical, some of these show-boxes afford an exceeding great 
 treat. 
 
 A still more perfect contrivance of the same kind has been 
 exhibited for some time in London and Paris under the title of 
 Cosmorama (from Greek words signify in gw'e&j,? of the world) 
 because of the great variety of views.) Pictures of moderate 
 size are placed beyond what have the appearance of common win- 
 dows, but of which the panes are really large convex lenses fit- 
 ted to correct the errors of appearance which the nearness of 
 the pictures would else produce. Then by farther using various 
 subordinate contrivances, calculated to aid and heighten the ef- 
 fects, even shrewd judges have been led to suppose the small 
 pictures behind the glasses to be very large pictures, while all 
 others have let their eyes dwell upon them with admiration, as 
 magical realizations of the natural scenes and objects. Because 
 this contrivance is cheap and simple, many persons affect to de- 
 spise it; but they do not thereby show their wisdom: for to have 
 made so perfect a representation of objects, is one of the most 
 sublime triumphs of art, whether we regard the pictures drawn 
 in such true perspective and colouring, or the lenses which as- 
 sist the eye in examining them. 
 
 It has already been stated, that the effect of such glasses irt 
 looking at near pictures, is obtainable in a considerable degree 
 without a glass, by making the pictures very large and placing 
 them at a corresponding distance. The rule of proportion in 
 
204 LIGHT. 
 
 such a case is, that a picture of one foot square at one foot dis- 
 tance from the eye, appears as large as a picture ,of 60 feet 
 square at 60 feet distance. The exhibition called th*e Diorama 
 is merely a large painting prepared in accordance with the prin- 
 ciple now explained. In principle it has no advantage over the 
 cosmorama or the show box, to compensate for the great ex- 
 pense incurred, but that many persons may stand before it at a 
 time, all very near the true point of sight, and deriving the 
 pleasure of sympathy in their admiration of it, while no slight 
 motion of a spectator can make the eye lose its point of view. 
 
 A round building of prodigious magnitude has lately been 
 erected in the Regent's Park in London, on the walls of which 
 is painted a representation of London and the country around, 
 as seen from the cross on the top of St. Paul's Cathedral. The 
 scene taken altogether is unquestionably one of the most extraor- 
 dinary which the whole world affords, and this representation 
 combines the advantages of the circular view of the panorama, 
 the size and distance of the great diorama, and of the details being 
 so minutely painted, that distant objects may be examined by 
 a telescope or opera glass. 
 
 From what has now been said, it may be understood, that for 
 the purpose of representing still nature, or mere momentary 
 states of objects in motion, a picture truly drawn, truly co- 
 loured, and which is either very large to correct the divergence 
 of light and convergence of visual axes, or, if small, is viewed 
 through a glass, would affect the retina exactly as the realities. 
 But the desideratum still remained of being able to paint mo- 
 tion. Now, this too has been recently accomplished, and in 
 many cases with singular felicity, by making the picture trans- 
 parent, and throwing lights and shadows upon it from behind. 
 In the exhibitions of the diorama and cosmorama there have 
 been represented with admirable truth and beauty such pheno- 
 mena as the sun-beams, occasionally interrupted by passing 
 clouds, and occasionally darting through the windows of a ca- 
 thedral and illuminating the objects in its venerable interior 
 the rising and disappearing of mist over a beautiful landscape, 
 running water, as, for instance, the cascades among the sublime 
 precipices of Mount St. Gotharcl, in Switzerland ; and, most 
 
THE EYE PAINTING REPRESENTING MOTION. 205 
 
 surprising of all, a fire or conflagration. In the cosmorama of 
 Regent Street, the great fire of Edinburgh was admirably re- 
 presented: first, that fine city was seen sleeping in darkness, 
 while the fire began, then the conflagration grew and lighted up 
 the sky, and soon, at short intervals, as the wind increased, or 
 as roofs fell in, there were bursts of flame towering to heaven, 
 and vividly reflected from every wall or spire which caught the 
 direct light then clouds of smoke were seen rising in rapid 
 succession and sailing northward upon the wind, until they dis- 
 appeared in the womb of distant darkness. No one can have 
 viewed that appalling scene with indifference, and the impres- 
 sion left by the representation, on those who knew the city, can 
 scarcely have been weaker than that left on those who saw the 
 reality. The mechanism for producing such effects is very sim- 
 ple; but spectators, that they may fully enjoy them, need not 
 particularly inquire about it. 
 
 It is remarkable, when the imagination is once excited by 
 some beautiful or striking view, how readily any visual hint 
 produces clear and strong impressions. One day in the cosmo- 
 rama, a school boy visiter exclaimed with fearful delight, that 
 he saw a monstrous tiger coming from its den among the rocks; 
 it was a kitten belonging to the attendant, which by accident 
 had strayed upon the paintings. And another young spectator 
 was heard calling that he saw a horse galloping up the moun- 
 tain-side; it was a minute fly crawling slowly along the canvas. 
 There is in this department a very fine field yet open to the ex- 
 ercise of ingenuity, for the contemplation of pictures represent- 
 ing the motion or progressive events, may be made the occasion 
 of mental excitement the most varied and intense. For in- 
 stance, there are few scenes on earth calculated to awaken more 
 interesting reflections on the condition of human nature than 
 that beheld by a person who sails along the river Thames from 
 London to the sea, a distance of about sixty miles, through the 
 wonders which on every side there crowd on the sight the fo- 
 rest of ships from all parts of the world the glorious monu- 
 ments of industry, of philanthropy, of science the marks of 
 the riches, the high civilization, and the happiness of the peo- 
 ple. Now, this scene was last year in one of our theatres stri- 
 
206 LIGHT. 
 
 kingly portrayed by what was called a moving panorama of 
 the southern bank of the Thames. It was a very long painting, 
 of which a part only was seen at a time gliding slowly across 
 the stage, and the impression made on the spectators was, that 
 of their viewing the realities while sailing down the river in a 
 steam boat. In the same manner the whole coast of Britain 
 might be most interestingly represented or any other coast, or 
 any line of road, or even a line of balloon flight. There was 
 another moving panorama exhibited about the same time at 
 Spring Gardens, aiming at an object of still greater difficulty, 
 viz. to depict a course of human life; and the history chosen, 
 was that of the latter part of Buonaparte's career. Scenes re- 
 presenting the principal events were, in succession, and appa- 
 rently on the same canvas, made to glide across the field of 
 view, so designed that the real motion of the picture gave to 
 the spectator the feeling of the events, being only then in pro- 
 gress, and with the accompaniments of clear narration and suit- 
 able music, they produced on those who viewed them the most 
 complete illusion. The story began with the blow struck at 
 Buonaparte's ambition in the battle of Trafalgar, and to mark 
 how completely, by representations of various moments and si- 
 tuations of the battle, the spectators were in imagination made 
 present to it, the author of this work may mention, that on the 
 occasion of his visiting the exhibition, a young man seeing a 
 party of British preparing to board an enemy's ship, started 
 from his seat with a hurra, and seemed quite surprised when 
 he found that he was not really in the battle. To the first views 
 there succeeded many others, similarly introduced and ex- 
 plained, in each of which the hero himself appeared: there 
 were, his defeat at Waterloo his subsequent flight his delive- 
 ry of himself to the British Admiral his appearing at the 
 gangway of the Bellerophon to thousands of spectators, wait- 
 ing in boats around, while he was in Plymouth Harbour, pre- 
 vious to his departure for ever from the shores of Europe his 
 house and habits during his exile, with various views of St. 
 Helena: and, last of all, that solemn procession, in which the 
 bier with his lifeless corpse appeared moving slowly on its way 
 to the grave under the willow tree. The exhibition now spoken 
 
THE EYE HISTORICAL PAINTING. 207 
 
 of might have been made much better in all respects, yet in its 
 mediocrity it served to prove how admirably adapted such 
 unions of painting, music, and narration, or poetry, are to af- 
 fect the mind, and therefore to become the means of conveying 
 most impressive lessons of historical fact and moral principle. 
 
 Painting, whether employed to portray scenes of entirely 
 still nature, or scenes involving some kind of motion as above 
 described, has still as its great aim or end merely to represent 
 interesting subjects, and to give to the spectator as much as pos- 
 sible the clear conception of them, which is obtained by ocu- 
 lar examination of realities. Painting thus, as a system of vi- 
 sual signs of thought, becomes like language, which is a system, 
 of audible signs, a means of expanding the boundaries of indi- 
 vidual human existence into wider space and time, and thus of 
 ennobling human nature. While it portrays only strict matters 
 of fact, whether of past or present time, as particular human in- 
 dividuals, objects of natural history, the beautiful and magnifi- 
 cent scenes of nature, interesting events which the artist had 
 the means of faithfully representing, &c., it may be called truly 
 historical painting, embodying the materials of true history, 
 both natural and civil, and then is of singular value. But 
 even when applied to other purposes, it may still be fraught with 
 delight; and just as language, of which the grand object or use 
 is to express strict truths, has still been admirably employed in 
 giving a permanent existence to a variety of fictions, from the 
 wildest fables and rhapsodies to the historical plays and novels 
 of modern times, as those of Shakspeare and of Scott which 
 plays and novels, although not furnishing true portraits of indi- 
 vidual human nature, are yet most correct portraits of general 
 human nature so may painting be employed in embodying fic- 
 tions adapted to its peculiar powers, and so as to prove the ar- 
 tist endowed with the highest degree of human genius. It 
 should always be recollected, however, that what is usually dig- 
 nified with the name of historical painting, really bears to his- 
 torical truth only the kind of relation which novels and plays 
 bear to it, and often approaches much less nearly to the truth: 
 for it pretends to relate a thousand minute circumstances which 
 no history has preserved, and which, therefore, only the imagi- 
 
208 LIGHT. 
 
 nation of the artist can supply. Thus, when a painter, know- 
 ing that Lucretia stabbed herself in the presence of her father 
 and others, after the crime of Tarquin, exhibits a woman dying, 
 and a certain number of persons around her in horror and as- 
 tonishment, he no more represents the real Lucretia and her 
 friends than he represents any other particular young woman and 
 her friends; for he is quite assured that not one of the figures 
 in such a picture is a portrait of the individual whose name 
 it bears: his picture, therefore, in so far is an untruth or fiction, 
 while it very probably shares the additional errors and even ab- 
 surdities so common among historical painters errors, for in- 
 stance, in respect of national usage in costume, religion, man- 
 ners, &c., and in respect to general appearance, as when a Reu- 
 bens, wishing to represent Sabine or other ladies, gave them the 
 degree of corpulency deemed comely in his own country, al- 
 though it notably contrasted with the true forms of Italian or 
 Grecian nymphs. From all this it appears that historical pic- 
 tures may often be regarded as portraitures, not of the realities, 
 but of comedians acting scenes in historical plays intended to 
 represent the realities. 
 
 In dealing with the events of ordinary history, there is no 
 strong reason why artists may not please themselves and their 
 spectators as we have now been describing; but it may admit of 
 doubt whether similar liberties should be allowed with respect 
 to religion. Yet any painting of the. last supper, for instance, 
 or of the ascension, is not likely to be more true than a thea- 
 trical representation. To judge of the nature of such a picture, 
 we have only to suppose any of the events recorded in the New 
 Testament to be represented by a painter in China with the 
 countenances such as are seen on Chinese tea-boxes; such a re- 
 presentation would appear in Europe revoltingly absurd; but 
 the common practice here is only a degree better; as twenty 
 painters undertaking to treat the same subject, will put different 
 persons into all the situations. Then it can produce no pleasing 
 impression on a Christian's mind to be told, that an admired 
 painting of the crucifixion was made chiefly from the body of 
 an executed murderer; or that for a praised representation of 
 the triumphal entry into Jerusalem, the painter had deemed his 
 
THE EYE ART OF PAINTING. 209 
 
 own physiognomy the most befitting for the principal figure, 
 while he copied the portrait of a noted modern sceptic as a spe- 
 cimen of the had men, of an equally noted believer as a speci- 
 men of the good, while wife, cousins, acquaintances, and old 
 clothes-men, served to make up the remaining groups. With 
 the knowledge that such things have often been, it need not sur- 
 prise, that many persons of correct feeling turn with horror from, 
 all these mimicries and falsehoods, to seek their idea of God 
 and his providence in the sublime descriptions of his attributes, 
 which written language conveys, and which all creation, in a 
 mute language, scarcely less impressive, so strongly confirms. 
 When men generally could not read, and as a mass were ex- 
 tremely ignorant, various means of fixing their attention upon 
 religious subjects might be useful, and therefore proper, as sa- 
 cred plays, certain processions, pictures, &c., which have now 
 in many countries ceased to be either; but a person of good 
 sense will continue to regard with a certain respect whatever at 
 any time may have contributed to reclaim portions of mankind 
 from barbarism and wickedness, to the just appreciation of the 
 divine charities of a pure religion. 
 
 There are in painting other classes of fictions, which pretend 
 to nothing beyond fiction, and which are yet truly admirable: 
 such are personifications of the virtues and vices, serving to re- 
 commend the practice of the former, and to deter from that of 
 the latter; almost all Hogarth's works are of this character, 
 and evince the highest mental acumen and genius: then may 
 be mentioned the personifications of what have been called the 
 elements and powers of nature, including many of the person- 
 ages of the Heathen mythology then other generalizations of 
 the characteristics of human or other nature, as scenes of do- 
 mestic affection, of the play of the passions, &c. &c. ; and be- 
 cause many subjects when so sketched, are intelligible to the 
 eye with the suddenness of lightning, where longest verbal de- 
 scription would convey the idea but imperfectly, the art of 
 painting in regard to them possesses a truly magical and inesti- 
 mable power. 
 
 As painting, whether engaged in representing matters of fact 
 or of fiction, can accomplish its ends only through the arts of 
 
 27 
 
210 LIGHT. 
 
 drawing or linear perspective, and of shading and colouring, 
 or aerial perspective, these subjects require to be studied by 
 every artist with great attention; but it is important for all to 
 be aware that the greatest mastery over these, which are mere- 
 ly the mechanical parts of the art, will go a very short way to- 
 wards producing good performances, unless there be present 
 also the genius to select or to compose subjects worthy of being 
 represented. The latter remark seems the more necessary, be- 
 cause there is in human nature a disposition to value so much 
 the means by which important ends are attained, that often the 
 end itself is forgotten in the contemplation of the means, 
 while among painters, as among persons of other occupations, 
 the talent for the inferior or more mechanical departments of 
 the art, is more common than for the higher. It is hence, that 
 the subordinate accomplishments of the painter are, by not a 
 few, both artists and pretended connoisseurs, supposed to be the 
 principal. But this is evidently to value the dress or clothing, 
 instead of the person, as might be observed of the criticism of 
 a mere tailor or dress-maker on a courtly assembly of indivi- 
 duals distinguished by rank and talent. The same very com- 
 mon error, arising from the weakness or short-sightedness of 
 ordinary human nature, of confounding means with ends, is 
 strikingly exemplified in the case of the person, who, per- 
 ceiving that money will procure all desirable things, at last be- 
 comes the insane miser, and dies from want of the common ne- 
 cessaries of life rather than touch his hoarded treasures: it is 
 seen also in the case of the mere pedagogue, who values the ab- 
 stractions of language or grammar, and of mathematics more 
 than the useful knowledge which they may serve to convey; 
 or, lastly, still more suitably for our purpose, in the case of the 
 bibliomaniac, who regards the type and binding of his books 
 more than the subject. To prove how unessential what is 
 called high-finishing in painting, is to the complete obtainment 
 of the purposes of the art, we have the cartoons of the immor- 
 tal Raphael, which to the mere mechanic appear almost to be 
 daubs: and many of the mere sketches of genius are to a true 
 taste more precious than some of the most finished pieces in 
 our galleries. Again, of what consequence is it to a man whe- 
 
THE TELESCOPE. 211 
 
 ther he see approaching the friend of his heart by day-light or 
 candle light, or with the source of light above or below, &c., 
 provided there is light enough for him to distinguish clearly the 
 friend of his heart. A painter will discover the difficulties 
 which a brother artist had to surmount in representing an ob- 
 ject in some particular predicament, as regards the light, &c., 
 and may estimate the talent accordingly; but the great mass, 
 even of the most accomplished ordinary spectators, will gene- 
 rally be looking beyond the sign to the thing signified; and 
 perhaps few will at all heed the difficulties. In consequence of 
 the prejudice in favour of " a sweet or adorable bit of colour- 
 ing," as it will sometimes be called, and which in truth may 
 have the merit of most natural colouring, there are preserved 
 in many galleries, pictures disgusting in almost all other re- 
 spects, as of drunken Dutch boors, with fiery noses and phy- 
 siognomies degrading to human nature, &c. &c.; on seeing which 
 the man of taste deplores that the art of representing should be 
 prostituted to the vile purpose of representing things of worse 
 than no interest, and that its nobler objects should be so often 
 forgotten. 
 
 ee Whan the image formed, as above described, beyond a lens 
 is viewed in the air by an eye placed still farther beyond 
 in the same direction, the arrangement, according to mi- 
 nor circumstances, constitutes either the common TELES- 
 COPE or MICROSCOPE." (Read the whole second paragraph 
 of the Analysis, page 122.) 
 
 The name of telescope (a compound Greek term, signifying 
 to see far, as microscope signifies to see what is small,) applies 
 to that wondrous instrument of modern invention by which the 
 intelligent soul on the beams of light as its path, is enabled as 
 it were, to dart widely into space for the purpose of contem- 
 plating the distant glories of creation; or, again, by which dis- 
 tant objects are instantly brought near to the eye for the purpose 
 of convenient inspection. In ancient times, a man, while look- 
 ing with admiration on the bright face of the moon, might have 
 exclaimed, " Would that I had the power to fly upwards to that 
 celestial orb, the better to understand its nature and beauties;" 
 
212 LIGHT. 
 
 but he could little have dreamed that the day was coming when 
 human ingenuity would in effect achieve the wish: now by the 
 telescope it is achieved; for, an instrument which merely dou- 
 bles apparent magnitudes, shows the moon exactly as she would 
 appear to a person who had ascended toward her from the earth 
 a distance of 120,000 miles, while one of greater power pro- 
 duces effects correspondingly great. But to examine the hea- 
 venly bodies is by no means the only use of the telescope, man 
 being often more concerned to discover what is passing on the 
 surface of the earth around him. Thus, by a telescope the military 
 chief descries approaching friends or foes, while yet concealed 
 from others and from the naked eye, in the blue mist of distant 
 mountain or plain or similarly, the sailor, whose attention has 
 been attracted to a little speck on the sea horizon, discovers 
 there a ship of class and nation at once evident to him, and with 
 the crew of which, by the additional use of signal flags, he is 
 enabled readily to converse; at midnight a telescope directed 
 to a distant cathedral will so effectually call it into the presence 
 of the observer, that on the clock turret may be read from the 
 slow-moving hands the unceasing lapse of time. A man, in the 
 midst of a wide plain, or on a lofty hill-top, or far on the face 
 of a lake, nay, even in his own garden, or in his house near 
 some open window, who might suppose himself quite alone and 
 unseen, would yet by a telescope be instantly placed under 
 the observation of whoever chose to watch him. Some remark- 
 able cases of actions, imagined by the parties to have been done 
 in perfect secrecy, have thus been brought to light. 
 
 Now, the telescope with its extraordinary powers exhibits but 
 another modification of the simple case described at page 144, 
 and exemplified in the camera obscura, &c., of an image formed 
 for visual inspection beyond a lens. And we shall here ex- 
 plain that its powers depend altogether on the two circum- 
 stances; first, of its large lens collecting for the formation of the 
 image (subsequently transferred to the observer's retina) a thou- 
 sand or more times the quantity of light which the naked pu- 
 pil could receive; and, second, of its forming by this light an 
 image, which to the eye brought near it appears vastly larger 
 than the object itself appears. 
 
THE TELESCOPE. 213 
 
 To understand this well, we must recal, that the nature of 
 the bending of light in passing through a lens is such, that all 
 the rays reaching the lens from any point of a visible object in 
 
 front, and form- 
 ing what is called 
 a pencil of light, 
 as that spread- 
 ing from the point A of the cross here represented to the lens 
 L are collected in a corresponding point, as r/, at the focal 
 distance beyond the lens, and so as always to meet the central 
 ray of the pencil: and therefore when the light comes from 
 above the centre of the lens, the focal meeting is below, as 
 shown here; and when it comes from below, the meeting is 
 above: then the same happening as regards every visible point 
 of the object (the rays from only the two extreme points A 
 and B are here represented) at corresponding points beyond the 
 lens in the space between a and b, the collected light, if re- 
 ceived on a white screen placed there, as in the camera obscu- 
 ra, will make apparent to an eye in any direction a beautiful in- 
 verted image of the object. Now, in the place where the rays 
 meet to form this image, if no screen be interposed, the rays are 
 not lost or destroyed, but merely cross each other in the air 
 nearly as they previously crossed, without interference, in the 
 lens, and spread again beyond the focal points, or towards c } as 
 here shown, as they originally spread from the several points of 
 the object itself: an eye therefore placed any where beyond 
 c, must receive portions of the pencils from every point of 
 the image, and may see it in the air as an object situated 
 in the focus of the lens. This may be observed at once by 
 holding a spectacle glass or any lens at proper distance be- 
 tween an object and the eye. 
 
 Now, a telescope is merely a long tube, blackened within to 
 exclude and destroy useless light, and having a large lens called 
 the object glass, filling one end of it as a window, to gather 
 the light from the objects in front, and to form with it images 
 near the other end of the tube, where they may be convenient- 
 ly inspected. These images, for a purpose to be immediately 
 explained, are examined through another lens called the eye 
 
LIGHT. 
 
 glass, which is fixed in a smaller tube made to slide backwards 
 and forwards in the larger, so as to admit of the focal distances 
 being adjusted to the power of different eyes, &c. The accom- 
 panying sketch 
 
 p j, _____ s ^ ows th e P ro ' 
 
 gress of the light 
 from the object 
 A, through the 
 object glass L, to form an image at b a, and afterwards to be 
 bent by the eye glass D, so as to enter the pupil of the eye at 
 E, to form the last image on the retina. 
 
 In the simple telescope, with only two lenses, as above re- 
 presented, called the astronomical telescope, the image is in- 
 verted: but this is a circumstance of no importance in viewing 
 the heavenly bodies; to fit the instrument, however, for view- 
 ing terrestrial objects, it is necessary to place in the tube ano- 
 ther simple or compound lens, which shall form a second image 
 from the first, and by inverting a second time, shall produce an 
 image really upright. 
 
 To determine how much larger an object will appear when 
 viewed through a certain telescope, for instance, one with an 
 object glass of three feet focus, than when viewed by the naked 
 eye, we must recollect that the image is formed in the focus of 
 the object glass, or at b a, and subtends from the centre of that 
 lens, as at c, the same visual angle as the object itself, (a fact ex- 
 plained at page 152,) and to an eye placed there, would appear 
 of the same size as the object; but if the eye can be brought 
 fifty times nearer to the image, this will appear fifty times taller 
 and broader, and therefore with 2,500 times the surface, and 
 thus, as compared with the object, may be called much magni- 
 fied. Now, as the naked eye cannot see distinctly an object 
 nearer to it than at about six inches, because of the great diver- 
 gence of light from a nearer radiant point, the telescope in 
 question, without an eye glass, would allow the eye to come 
 only six times nearer to the image than when at the centre of 
 the object glass, and would only magnify the diameter six times; 
 but if then an eye glass, as D, of half an inch focus, were 
 placed half an inch from the image, so as to render the rays of 
 
THE TELESCOPE. 215 
 
 ever)'- pencil parallel, and therefore fitted to the powers of the 
 eye, while the different parcels would cross each other a little 
 way beyond the glass, as shown above, an eye placed to receive 
 in its pupil the crossing parcels, would see the image as large as 
 if at half an inch from it, and therefore 72 times nearer than if 
 viewed from the object glass, and therefore again as of 72 times 
 greater diameter. Now, as in all cases, the image in a telescope 
 is in the focus both of the object glass and eye glass, and is 
 therefore nearer to the latter than to the former in proportion 
 as their focal distances differ, the magnifying power is mea- 
 sured by that difference in the case at present supposed, the 
 difference is as 72 to 1, and 72 is the magnifying power of the 
 telescope. The rule is generally thus expressed: "divide the 
 focal distance of the object glass by that of the eye glass, and 
 the quotient is the magnifying power." It is always to be re- 
 membered, that if the diameter of an object be magnified ten 
 times, the surface is magnified 100 times, and so in proportion 
 for other numbers. 
 
 With such means of aiding the sight, then, is it that we dis- 
 cover the mountains of our moon, and can even measure their 
 altitudes; that we can see the four beautiful moons of the planet 
 Jupiter; that we can perceive marks and irregularities on the 
 surfaces of the other planets, enabling us to say at what rate 
 they severally whirl round their axes, experiencing the pheno- 
 mena of day and night; and that we can determine many other 
 interesting particulars. 
 
 The discovery of the telescope is said to have been first made 
 accidentally by the children of a Dutch spectacle maker, while 
 they were playing with their father's work; but it was turned 
 to no use until Galileo, led by science, fell upon it again, and 
 with the knowledge of its worth, obtained from it the most sub- 
 lime results. If ever human heart throbbed with delight, it 
 must have been when Galileo first directed his optic tube to the 
 heavens, and through it contemplated so many glorious objects 
 which no human eye before had seen! as Venus, our beautiful 
 morning and evening star, appearing not a circle, but a crescent, 
 like our moon in her quarters as the satellites of Jupiter the 
 rings of Saturn myriads of stars, until then invisible to man; 
 
216 LIGHT. 
 
 and, in a word, when he beheld the undoubted proofs of the 
 true system of the universe, as his genius had before conceived 
 it, uniting the greatest simplicity with its unspeakable gran- 
 deur. 
 
 The Galilean telescope was simply a large object glass to col- 
 lect much light, with a small concave eye glass placed so as to 
 intercept the converging rays before they reached their focus, 
 and to change their convergency into the parallelism which the 
 eye could command. This telescope, although magnifying less 
 than that made of two convex glasses, as above described, still 
 from occasioning no loss of light by the crossing of rays in 
 forming an image, was of considerable power. The same prin- 
 ciple is now adopted for the common opera glass. 
 
 It was explained at page 143, that a ray of light, in being 
 bent or refracted by transparent media, as by a lens, is also di- 
 vided into rays of the different colours seen in the rainbow. 
 Hence, an image formed behind a simple lens has coloured 
 edges or fringes. This fact rendered the images of small ob- 
 jects, formed in the first telescopes, rery indistinct; and, but 
 for the important discovery originally made by Dollond the 
 optician, that different kinds of glass have the dispersive and 
 refractive powers with different relations, so that a concave 
 lens of a certain curve applied to a convex lens might com- 
 pletely counteract the dispersion of colour by the latter, while 
 it left enough of the convergence of the rays for the formation 
 of an image refracting telescopes would have always been 
 very imperfect. Dollond called his telescopes achromatic, or 
 without colouring power. It is very remarkable, that he had 
 the fortune to obtain some glass for his purposes more suitable 
 than any which has been procured since, or which could be 
 made by known rules, until the late improvements in the ma- 
 nufacture suggested by the ingenuity of Mr. Farraday. The 
 author carried abroad with him a small Galilean telescope of 
 Dollond 's, which often gave more correct information respect- 
 ing minute coloured objects at a distance, as signal flags at sea, 
 than much larger glasses of modern make. 
 
 The MICROSCOPE, of greatest power, and with the form called 
 compound, in its structure approaches very closely to the tele- 
 
THE MICROSCOPE. ^17 
 
 scope, the chief difference being, that while in the telescope a 
 large distant object forms in the focus of the object glass, an 
 image exactly as much smaller than itself as the distance of 
 the image from the glass is Jess, in the microscope, conversely, 
 a small object placed near the focus of the object glass pro- 
 duces a more distant image, as much larger than- itself as the 
 image is more distant, -and in the one case as in the other, 
 the image is viewed through an appropriate eye glass. The 
 object glass in the telescope is large, in the microscope it is 
 generally very small. If, .in the latter, an object glass be used 
 of one-eighth of an inch focal distance, and the object be so 
 placed that its image is formed at six inches, the image will be 
 of diameter 48 times as great as the object, or will have near- 
 ly 2,500 times as much surface; and if that image be viewed' 
 through an eye glass of half an inch focus, the image will ap- 
 pear still twelve times larger, or 30,000 times larger than the 
 object. 
 
 One convex lens is called a single microscope, and it mag- 
 nifies, as already explained, chiefly by allowing the eye to be 
 brought so much nearer to the object than would be possible 
 to see it without the glass: but even if the distance of the eye 
 and object is not changed, a lens interposed will still magnify 
 
 by bendiiii; the light, as at d, 
 
 -j <; J _ > 7 
 
 and making that which comes 
 to the eye at c, from the top 
 .of such an object, as the little 
 cross, a, appear to come from 
 
 b, and that from the bottom to come from c, thus magnifying 
 the cross here represented by the black lines, to appear of the 
 size represented by the dotted lines. A concave lens minifies 
 for the contrary reason. 
 
 Perhaps there is not a greater treat for a person who has 
 feeling for the beauties of nature,' than to explore with the mi- 
 croscope. While the telescope lifts the mind to the contem- 
 plation of boundless space, occupied by myriads of suns, and 
 exhibits this globe of ours as less, compared with the universe 
 around it, than a leaf is compared with, a forest, or a grain of 
 sand compared with alf which lies on ocean's shore; the mi- 
 
2 IS 
 
 LIGHT. 
 
 croscope, again, excites new astonishment by showing on a leaf, 
 or in a single drop of water in which the leaf has-been infused, 
 thousands of living creatures, and of creatures not imperfect 
 because thus small, but endowed with organs and parts as com- 
 plex and curious as those of an elephant. And he who ad- 
 mires the curious structure of a honey comb, may bend his 
 eye through the microscope upon the cut surface of a willow- 
 branch, or of other wood, there to see a similar structure more 
 wonderful still: or he may compare the lace of a fly's wing with 
 the most perfect which human art can weave; or the beautiful 
 proportions and perfection of the limbs and weapons of an in- 
 sect, invisible perhaps to the naked eye, with any larger ob- 
 jects of the kind already known to him. 
 
 Telescopes and microscopes might, with propriety? be both 
 called microscopes, for often the telescopic object appears to 
 the naked eye even smaller than that which the microscope ex- 
 amines. The minutest visible insect at hand may hide from 
 the eye a planet at a distance. The image in the telescope is 
 smaller than in the microscope, because the rays from a dis- 
 tance being nearly parallel, must form the image nearly in the 
 focus of the object glass; while for the microscope, the rays 
 from the near object being very divergent, form the image far 
 beyond 'the focus, and praport 
 
 " Light falling on very smooth or polished and flat sur- 
 faces is reflected so nearly in the order in which it falls, 
 
 as to appear to the eye as if coming direc fly from the ob- 
 jects originally emitting it, and such surfaces are called 
 
 MIRRORS." (Read the Analysis, page 122.) 
 
 If a marble slab, or other flat surface. 
 were in the situation M R, with its 
 edges towards the spectator, a ball pro- 
 jected from A perpendicularly towards 
 it at D, would rebound directly back to 
 A, but if projected from it obliquely, 
 as from B to D, it would not return to 
 the first situation B, but to b, a situa- 
 tion as distant on the opposite side of 
 the perpendicular; thus making the angle of the return or re- 
 
MIBKOKS 
 
 flection equal to the angle of approach or incidents : the same 
 would be true of a ball approaching obliquely from any other 
 point, as C, and returning to c. Now, light is reflected from 
 polished surfaces according to the same law, so that an eye at 
 A would see itself as if placed at d, an eye at b would see an 
 object really at B as if it were at e, and so forth. Where the 
 existence of a mirror is not suspected, the objects reflected 
 from it are held to be realities placed beyond where it is. A 
 wild animal will attack its image in a glass; and a dog crossing 
 a brook, will quit the piece of meat in its mouth to catch the 
 tempting image which the eye sees in the water below. The 
 reason that an object seen in a plane mirror appears to be just 
 as far beyond the mirror as its true distance on the side of the 
 spectator, is, that the diverging rays of a pencil of light have 
 the same divergence after as before reflection. 
 
 Any plane very smooth surface reflects light as now de- 
 scribed, and is a mirror; but different substances send back 
 very different proportions of the light which falls on them. 
 A highly polishp.d mptaHin. surfapp is thp hest mirror, often re- 
 turning three-fourths of the whole light. Hence, in reflecting 
 telescopes, the mirrors are made of polished metal. 
 
 Our common looking-glasses are really metallic mirrors, for 
 it is the smooth clear surface of the quicksilvered tin foil be- 
 hind the glass which reflects the light, the glass itself merely 
 serving the purpose of preserving the metallic surface perfect- 
 ly clear and flat. There is always an imperfection in such 
 glass mirrors, when used for viewing oblique objects, because 
 the glass bends the light a little, and because the external sur- 
 face of the glass, acting also as a mirror, although so much 
 more feebly than the metal behind, forms an image which 
 mixes with and confuses the other. 
 
 The mirror power of glass, unaided, is seen from the panes 
 of a plate-glass window, which make objects in front very vi 
 sible, although by no means with clearness comparable to that 
 from a metallic surface. All common panes of glass in win- 
 dows, or covering print-frames, &c., reflect as much light as 
 plate glass, but the reflection being irregular because the sur- 
 face is irregular, it does not attract notice. 
 
 The smooth surface of a fluid is a mirror, and which more- 
 
220 LIGHT. 
 
 over is horizontal; and when that surface is metallic, as of mer- 
 cury, the mirror is most perfect. In water, spirits, oil or any 
 other liquid, it is also perfect, but feebler. 
 
 The mirror of liquid quicksilver is sometimes used by as- 
 tronomers in observing the apparent altitudes of the heavenly 
 bodies; for the image in the mirror appearing exactly as much 
 below the horizon as the object is really above it, half the dis- 
 tance between them is the true height. 
 
 A varnished picture, or any japanned surface, is a mirror;, 
 nay, also, even a polished piece of wood, for instance, a ma- 
 hogany table, as is well known among playful children. The 
 author, while writing this, is looking on a table covered with 
 black leather, and in that covering, as a mirror, he clearly sees 
 all bright objects beyond the table. Polished stones, as mar- 
 ble slabs, &c., reflect as much as glass. But even a surface of 
 air may be a mirror, as where a cold and dense stratum hap- 
 pens to be in contact with a warmer and rarer stratum; hence, 
 the trees, islands, &c., reflected from below, and seen in the 
 sky where particular causes have unequally heated different 
 levels of the atmosphere. On the burning sands of Africa a 
 stratum of air near the surface being heated, that above some- 
 times becomes a mirror, thus reflecting more remote objects: 
 certain kinds of mist and thin clouds will elsewhere produce 
 a similar effect, so that in them a ship may be seen as if sus- 
 pended aloft, with keel uppermost. 
 
 An object seen by the light reflected from a mirror appears 
 always reversed, as, for instance, when the right hand of a per- 
 son standing before a glass becomes the type for the left hand 
 of the image and it may be a stump: or when a tree or rock, 
 or mountain, seen in the mirror of a lake, has its top down- 
 wards. 
 
 It is on this account, that a man painting his own portrait 
 from a mirror, is apt to reverse all accidental characteristics of 
 the countenance or person, unless they are the same on both 
 sides; and then if, as is generally true, one eye be higher than 
 the other, or the nose be a little to one side, a very incorrect 
 resemblance will be produced.' Hence, a person with a coun- 
 tenance at all thus peculiar, never sees himself in a mirror as 
 he appears to others: and a belle or beau, who has decided that 
 
CAMERA LUCIDA. 2QI 
 
 a curl is mote graceful on the left temple, may unconsciously 
 leave it on the right. 
 
 By reflecting any image, however, from a first mirror to a 
 second, and from that to the eye, persons may see the object, 
 or themselves, if they choose, as others see them. What a 
 pity that there are not some moral mirrors to answer an ana- 
 logous purpose, and occasionally to tell persons how hideous 
 they are to virtue's eye, although they think of themselves 
 with such complacency. 
 
 A candle placed between two parallel mirrors, makes visible 
 in either glass to a spectator on one side an endless straight line 
 of lights. If the glasses be incline-! to each other, the lights 
 will appear as if in the circumference of a circle, having its cen- 
 tre where the prolonged mirrors would meet: this fact' is well 
 illustrated in the beautiful toy called the kaleidoscope. By 
 placing several mirrors in particular situations around an apart- 
 ment, a man entering it may see himself multiplied into a* crowd; 
 and a few ornamental pillars may produce the effect of thousands 
 formed into long colonnades or retiring lines. 
 
 The sun or moon reflected in a still lake, appear as they do 
 in the sky; but if the surface of the water become at all ruffled 
 by the breeze, instead of one distinct image, there will be a long 
 line of bright tremulous reflection. The reason of this appear- 
 ance is, that every little wave, in an extent perhaps of miles, 
 has some part of its rounded surface with the direction or ob- 
 liquity which, according to the required relation of the an- 
 gles of incidence and reflection, fits it to reflect the light to the 
 eye, and hence every wave ,in that extent sends its momen- 
 tary gleam, which is succeeded by others. 
 
 Although the external surface of glass does not reflect but 
 a small part of the light which falls upon it, being therefore a 
 feeble mirror, very curiously, if light, which has entered- a 
 piece of glass, fall upon a back or internal surface, very oblique- 
 ly, instead of passing out there, it is more perfectly reflected 
 than it would be by the best metallic mirror. Thus light from 
 A entering at B, is entirely reflected at 
 C, and escapes at D towards E. The 
 back of a wedge of glass, or common 
 prism, thus becomes a perfect mirror. 
 It is this fact which enabled Dr. Wol- 
 
222 LIGHT. 
 
 laston to construct that beautiful little instrument called by bim 
 the Camera Lucida. The two surfaces 
 at the back of the small prism of glass A 
 become mirrors, the first reflecting to the 
 I C second, and the second to the eye at E, 
 the objects in the landscape before it while 
 the eye also sees through the glass to the 
 paper below at B, and may suppose the 
 
 imagery to be feebly portrayed on the paper: with a pen- 
 cil that appearance is made permanent, and a correctly-drawn 
 outline of the scene is at once obtained. This instrument for 
 assisting draftsman is still simpler than the camera obscura. 
 Other modifications of the instrument have since been contrived. 
 
 The same fact of the back and internal surface of a transparent 
 mass becoming a mirror, gives us the explanation of that phe- 
 nomenon so admired before it was understood, and not less ad- 
 mired since, viz. the rainbow, or arc in the sky, as in France 
 and elsewhere it is named the object which the poets of na- 
 ture have almost worshipped for its beauty, one of the de- 
 lights of our boyish days, when we saw it stretching over the 
 haunts of our young pleasures, and when we pursued it in the 
 hope of catching some of the falling rubies and emeralds, or 
 bright-coloured dew of which it might be composed. 
 
 When a partial shower of rain falls on the side of the land- 
 scape opposite to where the sun is shining, this beautiful phe- 
 nomenon immediately appears, viz. a variegated arch, red at 
 its external border- or confine, and then successively orange, 
 j^ellow, green, &c. (in the order of the colours of the prismatic 
 spectrum described at page 142,) to wards its inner border or con- 
 fine: its centre is directly opposite to the sun, as if at the end of a 
 straight line drawn from the sun through the eye of the specta- 
 tor towards the opposite horizon, and being therefore always un- 
 der the horizon, the bow is less than a semicircle. The dia- 
 ameter of the circle of which the bow is a part is of about 82 of 
 the field of view. There is a second bow of much fainter light 
 than the first, and with the colours in reverse order: it is of 
 108 diameter, and therefore external to the other. 
 
 Now, the explanation of this miracle of beauty is simply as 
 follows. While the sun shines upon the spherical drops of fall- 
 ing rain, and its light falling upon the whole central part of any 
 
HAISBOW. 
 
 drop, passes completely through, still that portion which enters 
 near the edge of the drop, as at , is refracted, and reaches the 
 back surface of the drop at y so slantingly, or at an angle so 
 great, that there occurs an entire reflection 
 of it instead o,f transmission; the ray therefore 
 is returned to b, where it again escapes from 
 the drop, and as here shown, descends to the 
 earth or eye at e. Thus every little drop of 
 rain on which the sun shines is a little mir- 
 ror suspended in the sky, and is returning at 
 a certain angle all round it, viz at an angle of 41, a portion 
 of the light which falls on it; and an eye placed in the required 
 direction receives that reflected light. If in this case there were 
 reflection only, and not also refraction with separation of co- 
 lours* the rainhow would be only a very narrow resplendent arc 
 of white light, built up of millions of little images of the sun; 
 but in truth, because the light which enters near the edge of the 
 drop traverses the surface very obliquely, it is much bent or re- 
 fracted before its reflection, as seen at a, and is divided into 
 rays of its seven colours, as it would be on passing through a 
 prism (as explained at page 143;) and this division or separation 
 continuing after the light again escapes from the drop at b, in- 
 stead of one white ray descending from each drop to a 'certain 
 
 point of the earth, seven rays de- 
 scend (here marked by dotted lines 
 from the figure I on-the left hand, 
 to 7, 6, 5, &c., on the right,) and 
 of these an eye can only catch one 
 at a time: but for the same reason 
 *l* that seven eyes placed in a line 
 from above downwards (viewing 
 
 the centre of the bow) would be required to see the seven co- 
 lours from one drop, so one eye looking in the direction of 
 seven drops situated in a corresponding line, as from 1 to 7, 
 will catch the lower or red ray of the upper, the orange or se- 
 cond ray of the next, the yellow or third ray of that which fol- 
 lows, and so on, while it will lose all the others, and thus will 
 see the several drops as if they were each of one colour only. 
 Of such elements, then, found in the same relative directions 
 all around the eye, the glorious arch is formed. No two eyes 
 
224 LIGHT. 
 
 can see the same rainbow, that is, can receive light from the 
 same drops at the same time; and the same eye does not for 
 two instants receive light from the same drops. This rainbow 
 can never appear to a person on a plain, unless when the sun is 
 within 41 of the horizon, for otherwise the centre of the rain- 
 bow would be more than 41 under the horizon, and 41 is the 
 whole semidiameter of the bow. 
 
 We have described above what is called the principal bow, 
 formed in the drops by t\vo refractions and one reflection of 
 light. To produce the fainter second or external bow, men- 
 tioned above, and of which the colours are in reverse order, 
 the light which enters at a is reflected 
 first at y, then again at b, and escapes 
 at c towards thp eye: hence there are 
 'Or Sw two. reflections as well as two refractions. 
 
 As the semidiameter of this bow is 54 ? 
 it may be visible when the internal bow is not. 
 
 An artificial rainbow may be produced in sunshine at any 
 time by scattering water drops from a brush or otherwise. The 
 cut-glass ornaments of chandeliers, &c., produce colours on the 
 same principle as rain drops; as do also mist and particles of 
 frozen water between a luminous body and the eye, exhibiting 
 the circular Aa/os often observed around the sun and moon. 
 
 e( ,ZI/m'0r,y may be plane, convex, or concave; and certain 
 curvatures will produce images by reflection* just as lenses 
 produce images by refraction; so that there are reflecting 
 telescopes, microscopes, fyc., as there are refracting instru- 
 ments of the same names." (See the Analysis, page 122.) 
 
 While a plane surface reflects light, so that what is called the 
 image in it of a known object may readily be mistaken for the 
 reality, convex or concave mirrors reflect as if every distinct 
 point of them were a separate small plane mirror, and their effects 
 on light correspond with the relative inclination of the different 
 parts. The only forms of much importance are the regularly 
 spherical or parabolic concave and convex mirrors. We shall 
 now find that these have on light similar effects with lenses, 
 only the concave mirror answers to the convex lens, and the 
 convex mirror to the concave lens. It is the concave mirror 
 which gathers the light to form images in the most perfect te- 
 
CURVED MIRRORS. 
 
 lescopes that exist, as those of Herschell and others. Admira- 
 ble as in certain respects is the refracting telescope, it falls much 
 short of the telescope acting by reflection. 
 
 In a holiow sphere, or part of a sphere with polished internal 
 surface, if rays radiate from the centre in all directions, they 
 
 reach every part perpendicularly, and 
 therefore are thrown back to the centre. 
 C Thus, if A B were a concave spherical 
 mirror, of which C were the centre, rays 
 issuing from C would again meet there. 
 
 It can be proved also, that any ray 
 parallel to the axis, falling upon such 
 a mirror, will be reflected inwards 
 
 /^ 
 
 j so as to cut the axis half way be- 
 tween the mirror and its centre, viz. 
 at D. Then, as all parallel rays meet in the same point, that 
 point becomes a focus, as already explained for lenses; about 
 that part an image of the sun, for instance, will be formed 
 when the mirror is held directly towards the sun. This point 
 is called the principal focus of the mirror. 
 
 For the same reason that parallel rays meet in the focus, so 
 will rays, issuing from the focus, become parallel, after reflec- 
 tion, as seen in the figure at page 50; and if they be then caught 
 in a second and opposite mirror, as there also represented, cor- 
 responding effects will follow. 
 
 Now, for a concave mirror, as already explained for a lens, 
 when rays fall on it obliquely from one side of the axis, their 
 focus will be on the opposite side, and therefore the mirror will 
 form an inverted image of any body placed before it, just as a 
 lens does; and the image will be near or distant, and large or 
 small, according to the divergence of the approaching rays, ex- 
 actly as happens with lenses: and thus, the camera obscura, ma- 
 gic lantern, telescopes and microscopes may all be formed by 
 mirrors, as they may be by lenses. Moreover, concave mirrors 
 magnify, and convex mirrors minify, as concave lenses of the 
 opposite names do. The two subjects of images by refraction 
 and by reflection run so nearly parallel, that it would be useless 
 repetition here to enter upon the detailed consideration of the 
 htter subjectj and we shall therefore content ourselves with sffbw- 
 
Liulii 
 
 "=" 
 
 i. 
 
 ib 
 
 ing why a concave mirror magnifies, and why a convex mirror 
 minifies. 
 
 A concave mirror magni- 
 
 C ... "-... ^ A fies, because the .light from 
 
 A reaching the mirror where, 
 it can be reflected to an eye 
 placed at F. viz. at E, seems 
 to the eye to come from C, 
 
 and the light of B similarly appears to come from D, so that the 
 cross A B, by the reflection, seems to the eye to be of the greater 
 dimensions C D. 
 
 In a convex mirror, again, for cor- 
 responding reasons, the cross A B ap- 
 pears only asC D, and therefore much 
 smaller than the reality. 
 
 Concave, or magnifying mirrors, are 
 often used by persons in shaving. 
 A convex mirror is a common ornament of our apartments, 
 exhibiting a pleasing miniature of the room and its contents. 
 
 Any polished convex body is a mirror, and therefore the bail 
 of the human eye is one in whioh we may contemplate most 
 perfect miniatures of surrounding tilings. It is the image of 
 the window, or of the sun, in the convex mirror of the eye, 
 which painters usually represent by a spot of white paint there; 
 and a similar luminous spot or line must be made when they re- 
 present almost any of the pieces of furniture which have round- 
 ed polished surfaces, a % s bottles, glasses, smooth pillars, &c. 
 
 Convex lenses thus are mirrors to all the objects around them, 
 and very strikingly so, owing to the perfection of the form of 
 a lens. The polished back of a watch, often in the same way 
 attracts the attention of a child, who wonders to see there so 
 clearly ' a little baby.' 
 
 It has been a mathematical amusement to calculate what kind 
 of distortion mirrors of unusual forms will produce, and then 
 to make distorted drawings, which, reflected from such mirrors, 
 might produce in the eye the natural image of the objects. 
 
 When a concave mirror is used for a telescope, the image 
 formed in front of it, and examined usually through a powerful 
 magnifying eye glass, may be viewed, as in HerschelFs tele- 
 scope, by the spectator turning his back to the real object, and 
 
REFLECTING TELESCOPES. 227 
 
 looking in at the mouth of the telescopic tube, near to the edge 
 of which the image is thrown by a slight inclination of the mir- 
 ror at its bottom: or as in the' <\eivtonian telescope, through 
 an opening in the side of the tube, after being reflected by a 
 small plane mirror placed diagonally in the centre of the tube: 
 or as in the Gregorian telescope, through an opening cut in 
 the principal mirror or speculum, after being reflected towards 
 that opening by a smaller mirror placed in the centre of the tube: 
 this last arrangement is that preferred for smaller telescopes, be- 
 cause the spectator, while seeing the image, is also looking in 
 the direction of the object. 
 
 Reflecting telescopes have the advantage of being perfectly 
 achromatic, that is, of producing no coloured or rainbow edges 
 to the images: for compound light is reflected, although not re- 
 fracted, entire, all the colours following the same law of equal 
 angles of incidence and reflection. 
 
 HerschelPs largest telescope had a mirror of 48 inches in 
 diameter, and therefore received about 150,000 times more 
 light than an unassisted eye could, making with it, at a focal 
 distance of 40 feet, a large image admirably distinct. It was 
 with this that, in the obscurity of remote space, he discovered 
 rolling along the immense planet, which, in honour of his 
 royal patron, he called the Georgium Sidus, but which now, 
 by the decision of the scientific world, bears his own name; 
 and with this he discovered moons before unseen, of other 
 planets, and he unravelled the celestial nebulae and clustered 
 stars of the milky way, and, in a word, unveiled, vastly more 
 than had before been done, the system of the boundless uni- 
 verse. If this world were to last for millions of years, the 
 discoveries of HerschelPs telescope would mark a memorable 
 epoch of its early history. 
 
 " Light returned from, or passing through bodies of 
 rougher or irregular surface, or which have other pecu- 
 liarities, is so modified as to produce all those phenome- 
 na of colour and varied brightness seen among natural 
 bodies* and giving them their distinctive characters and 
 beauty." (See the Analysis, page 122.) 
 
 General remarks on this part of our subject were made in 
 
J-JS LIGHT. 
 
 the beginning of the section, in the explanations of how ob- 
 jects, riot self-luminous, become visible by reflecting the light 
 of other bodies, and of how the prism separates a 'ray of white 
 light into rays of the several colours of the rainbow which 
 rays, on being again mixed, reproduce white light as before: 
 and much beyond these remarks we have not the intention of 
 now proceeding. To give a full account of the matters that 
 come within the scope of this depaitment, would occupy the 
 pages of a large volume; for there would be to pass in review 
 the various opinions which have existed on the intimate na- 
 ture, of light, the facts connected with what has been called 
 the polarisation of light, the relation of light in double re- 
 fraction, to the ultimate structure of material masses, &c , all 
 which subjects are in certain respects highly interesting; but 
 as some of them are not yet completely investigated as re- 
 specting others, various opinions prevail, as they involve few 
 matters applicable to common use, as the reasonings about 
 them are far removed from ordinary trains of thinking, and 
 refer to facts altogether unknown to common observation 
 we hold them not to be fit parts of a popular treatise on light. 
 We may state, however, that persons who have the leisure and 
 the mathematical preparation necessary for pursuing the study, 
 will find their labour in it richly rewarded. 
 
 What we deem it necessary then here to aJd is, that white 
 light, in falling upon any transparent substance, as air, water, 
 glass, &c., reduced to thin plates or films, is so affected, that 
 for certain degrees of thinness, different for each substance, 
 it is decomposed, and is reflected or transmitted not as white 
 light, but as some of the colours of the rainbow, and the co- 
 lour reflected in any case is always the opposite or complement 
 of that which passes through, that is to say, such that the two 
 brought together again make white light. The facts may be 
 studied, as Newton originally studied them, in the thin plate 
 of air which occupies the space between a convex lens and a 
 plane surface of glass upon which the lens is laid, in which 
 plate, as the distance from the point of the apparent contact of 
 the glasses increases, there appear successive rings of vivid co- 
 lours. The same truth is exemplified in the colours of a soap 
 bubble, which brighten as the bubble swells and becomes of 
 
COMPARISON OF LIliHT AND SOUND. JJJ>H 
 
 thinner substance, and are different as the thickness increases 
 from above downwards; and it is exemplified in numerous 
 other common facts. Now, whatever' be the reasons of such 
 decomposition of light and the explanation is not yet com- 
 plete we cannot doubt that in natural bodies generally, the 
 colours, opacity, transparency, &.C., depend entirely upon the 
 volume and arrangement of the minute fibres or plates, with in- 
 cluded interstices, which constitute the volume or structure of 
 each mass. Accordingly, whatever changes that arrangement, 
 may change also the colour of the mass. Thus, by drawing a 
 certain number of minute lines on a certain extent of any me- 
 tallic surface, we may make it of what colour we please; and 
 mother-of-pearl owes its beauty entirely to its furrowed or stri- 
 ated surface, as is proved by our taking an impression of that 
 surface on sealing-wax, and perceiving that the wax then exhi- 
 bits similar colours. 
 
 The investigations in progress respecting the phenomena of 
 light, are furnishing new proofs of the extreme simplicity of 
 nature, amidst the boundless extent and infinite variety. When 
 men thought of the sense of touch only as it exists at the tips 
 of the fingers, or on the general surface of the body, they 
 were far from suspecting that the sense of hearing had the near 
 relation to it which subsequent discoveries have proved, and 
 still less did they think, that the sense of sight was similarly 
 related; but step by step they ascertained, 1st, of sound coming 
 to the ear through the air that air was a material fluid as much 
 as water, consisting of the same or similar particles, only more 
 distant among themselves that a motion or trembling in the 
 air, by affecting nerves exposed in the ear, produced the sensa- 
 tion of sound, as the trembling in a log of wood caused by the 
 action of a saw produces a peculiar sensation of touch in a hand 
 laid on the log, and, finally, that common sound in all its va- 
 rieties, is merely such trembling of the air, affecting a struc- 
 ture of nerve so exposed in the ear, as to be as much more rea- 
 dily excitable than the nerves in the fingers and elsewhere in 
 the skin, as the action or impulse of moving air is more deli- 
 cate than that of common solids and liquids. And now, in 
 the investigations respecting light, this kind of comparison is 
 carried a step farther; for it is become matter almost of certain- 
 
230 LIGHT. 
 
 ty that the sensation of light is produced in a suitable nervous 
 tissue in the eye, by a trembling motion in another fluid than, 
 air, which fluid pervades all space, and in rarity or subtlety of 
 nature surpasses air vastly more than air does water or solids; 
 and while in sound, different tones or notes depend on the 
 number of vibrations in a given time, so in light do different 
 colours depend on the extent of the single vibrations. Can 
 human imagination picture to itself a simplicity more magni- 
 ficent and fruitful of marvellous beauty and utility than this! 
 But, farther, as air answers in the universe so many important 
 purposes besides that of conveying sounds, although this alone 
 comprehends language, which almost means reason and civili- 
 zation so also does the material of light minister in numerous 
 ways, in the phenomena of heat, electricity, and magnetism. 
 
 The truths now positively ascertained with respect to the 
 nature of light and vision, are perhaps those in the wide field 
 of human inquiry which, acting on ordinary apprehension, 
 most forcibly place,the individual as it were in the presence of 
 Creative Intelligence, and awaken the most .elevated thoughts 
 of which the human mind is capable. Had there been no 
 light in the universe, all its other perfections had existed in 
 vain. Men placed on earth would have been as human 
 exiles with their eyes put out, abandoned on an unknown 
 shore, of climate and productions totally new to them: every 
 movement might be to destruction, for their perceptions would 
 be limited by the length of their arms, and of their fearful 
 groping steps, and the wretched beings, separating when im- 
 pelled by hunger to search for food, would probably scatter to 
 meet no more. But the material of light exists, pervading all 
 space, and certain impressions made upon it in one place ra- 
 pidly spread over the universe, the progressive impression be- 
 ing called a ray, or beam of light. The beams of light, then, 
 from all parts coming to every individual, may be regarded as 
 supplementary arms or feelers belonging to the individual, and 
 which reach to the end of the universe, so that each person, 
 instead of being as a blind point in space, becomes nearly om- 
 nipresent: then these limbs or feelers have no weight, they 
 are never in the way, they impede nothing, and they are only 
 known to exist when their use is required! But this miracle of 
 
PERFECTION Otf THE L\L. 
 
 light would have been totally useless, and the lovely paradise 
 of earth would have been to man still a dark and dreary de- 
 sert, had there not been the twin miracle of an organ of com- 
 mensurate delicacy to perceive the light, viz of the Eye; 
 in which there is the round cornea of such perfect transparence, 
 placed exactly in the anterior centre of the ball, (and elsewhere 
 it had been useless,) then exactly behind this, the beautiful cur- 
 tain the iris, with its pupil dilating and contracting to suit the 
 intensity of light and exactly behind this again, the crystal- 
 line lens, having many qua'ities which only complex structure 
 in human art can attain, and by the entering light forming on 
 the retina beautiful pictures or images of the objects in front, 
 the most sensible part of the retina being where the images 
 fall.. Of these parts and conditions, had any one been other- 
 wise than as it is, the whole eye had been useless, and light use- 
 less, and the great universe useless to man, for he could not have 
 existed in it. Then, farther, we find that the precious organ, 
 the eye, is placed, not as if by accident, somewhere near the 
 centre of the person, but aloft on a proud eminence, where it 
 becomes the glorious watch-tower of the soul; and, again, not 
 so that to alter its direction the whole person must turn, but 
 in the head, which on a pivot of admirable structure moves 
 while the body is at rest; the ball of the eye, moreover, being 
 furnished with muscles which, as the will directs, turn it with 
 the rapidity of lightning to sweep round the horizon or take 
 in the whole heavenly concave; then is the delicate orb se- 
 cured in a strong socket of bone, and there is over this the 
 arched eye-brow as a cushion to destroy the shock of blows, 
 and with its inclined hairs to turn aside the descending perspi- 
 ration which might incommode; then is there the soft and 
 pliant eye-lid with its beauteous fringes, incessantly wiping 
 the polished surface, and spreading over it the pure moisture 
 poured out by the lachrymal glands above, of which moisture 
 the superfluity by a fine mechanism is sent into the nose, there 
 to be evaporated by the current of the breath: still farther, in- 
 stead of there being only one so precious organ, there are two, 
 lest one, by accident, should be destroyed, but which two have 
 so entire a sympathy, that they act together as only one more 
 perfect; then the sense of sight continues perfect during the 
 
LIfctHT. 
 
 period of growth from birth to maturity, although the distance 
 from the lens to the retina is constantly varying; and the 
 pure liquid which fills the eye, if rendered turbid by disease 
 or accident, is by the actions of life, although its source be the 
 thick red blood, gradually restored to transparency. The 
 mind which can suppose or admit that within any limits of 
 time, even a single such organ of vision could have been pro- 
 duced by accident, or without design, and still more, that 
 the millions which now exist on earth, all equally perfect, can 
 have sprung from accident or that the millions of millions in 
 past ages were all hut accidents and that the endless millions 
 throughout the animate creation, where each requires a most 
 peculiar fitness to the nature and circumstances of the animal, 
 can be accident must surely be of extraordinary character, 
 or must have received unhappy bias in its education. 
 
 As a concluding reflection with respect to vision we may re- 
 mark, that all the provisions above considered have mere uti- 
 lity in view, for any one of them wanting would leave a ne- 
 cessary link in the chain of creation wanting: but we have 
 shown in a preceding part of the work, that if there had been, 
 white light only, susceptible of different degrees of intensity 
 and shade, the merely useful purposes of vision would have 
 been answered about as perfectly as with all the colours of the 
 rainbow which truth is instanced in the facts, that many per- 
 sons do not distinguish colours, and that it imports not whe- 
 ther a person view objects in the morning, or at mid-day, or 
 at even-tide, or through plane glass or coloured glass. While, 
 therefore, the existence of light generally, and of the eye, 
 speaks of Creative Power and Intelligence, the existence of 
 colours, or of that lovely variety of hues exhibited in flowers, 
 in the plumage of birds, in the endless aspects of the earth and 
 heavens, in a word, in the whole resplendent clothing of na- 
 ture, because appearing expressly planned, as a source of de- 
 light to animated beings, speaks of Creative Benevolence, and 
 may well excite in us, towards the Being in whom these attri- 
 butes concentrate, the feelings associated in our minds during 
 this earthly scene, with the endearing appellation of 'Father.' 
 
 END OF THE SECTION ON LIGHT- 
 
NOV 19 1917 
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