PHOTO CHEMICAL REACTIONS OF THE HALOGENS 
 
 BY 
 
 JACOB NEVYAS 
 
 A. B Swarthmore College, 1919 
 
 THESIS 
 
 SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS 
 FOR THE DEGREE OF MASTER OF ARTS IN CHEMISTRY 
 IN THE GRADUATE SCHOOL OF THE 
 UNIVERSITY OF ILLINOIS, 
 
 1922 
 
 URBANA, ILLINOIS 
 
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 UNIVERSITY OF ILLINOIS 
 
 THE GRADUATE SCHOOL 
 
 
 _ Lj 192 JL 
 
 : I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY 
 
 SUPERVISION BY JACOB NEVYAS 
 
 
 ENTITT.ED PHOTOCHEMICAL REACTIONS OF 
 
 THE HALOGENS ^ 
 
 
 i BE ACCEPTED AS FULFILLING THIS PART OF THE 
 
 REQUIREMENTS FOR 
 
 THE DEGREE OE MSTER OF ARTS . 
 
 
 
 
 Mn Charge of Thesis ; 
 
 of. 
 
 
 ■” C 
 
 Head of Department 1 
 
 Recommendation concurred in* 
 
 
 
 Committee 
 
 
 on 
 
 
 Final Examination* 
 
 •Required for doctor’s degree but not for master’s 
 
 
 <;-2SS98 
 
CONTENTS. 
 
 I. INTRODUCTION 1 
 
 II. HISTORICAL 11 
 
 III. EXPERDIENTAL 
 
 Part 1. 19 
 
 Part 3. 31 
 
 Part 3, 31 
 
 Part 4 35 
 
 IV. CONCLUSIONS 37 
 
 V. ACKNOl^/LEDGMENTS 39 
 
 VI. BIBLIOGRAPHY • 4q 
 
 PLATE I. 34 
 
 PLATE II. 30 
 
 PLATE III. 33 
 
Digitized by the Internet Archive 
 in 2015 
 
 https://archive.org/details/photochemicalreaOOnevy 
 
1 
 
 I. INTRODUCTION. 
 
 Chemical reactions which are initiated, accelerated, or 
 retarded iinder the influence of radiant energy of tie visible 
 spectrum or of the ultra-violet are given the name. Photo- 
 chemical Reactions. The most common instances of photochemical 
 activity are the action of light on the silver salts used in 
 photography and the role of sunli^t in the metabolism of plant 
 life, whereby the chlorophyl in green plants is enabled to build 
 up an almost endless variety of complex sugars and starches 
 from the simple substances: water, oxygen, and carbon dioxide. 
 
 Other well-known cases are the fading of dyed fabrics and 
 colored wall papers through photochemical oxidation of the 
 coloring materials and the darkening of li^t paints on surfaces 
 exposed to strong sunlight, Turhere the reaction is probably a 
 polymerisation of the oil used as the paint vehicle. Photo- 
 chemical darkening or decomposition of organic substances, 
 such as aniline, are quite common while halogenation of organic 
 substances as in the chlorination of acetic acid and of benzenel>2 
 in sunlight are widely used in Organic Chemistry. Less known is 
 the phot obrominat ion of toluene and xylene with the aid of ultra- 
 violet llght^. Other photochemical reactions that have been 
 extensively studied are the conversion of maleic acid into its 
 Geometric isomer, fuma^ric acid^, 
 
 the polymerisation of 
 
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 anthracene to dianthracene , the decomposition of gaseous 
 ammonia^, and the conversion of the trans -compound of o(-methyl 
 cinnamic acid into the c is -compound^ under the influence of 
 ultra-violet light. 
 
 In many of these isomeric changes and polymerisations 
 the reverse reaction sets in as soon as the activating influence 
 is removed so that in tim.e the original compound is reproduced. 
 Two examples of interest in Inorganic Chemistry are reported. 
 
 An alcoholic solution of phosphotungstic acid on exposure to 
 sunlight turns an intense blue*^. This is believed to be due 
 to an oxidation of the alcohol to aldehyde with corresponding 
 reduction of the acid to a blue compound of unknown composition. 
 In the dark this blue color gradually disappears and the solution 
 becomes thoroughly white again, Lenher® reports that a solution 
 of molybdenum trioxide in that remarkable solvent, selenium 
 oxychloride, turns to a deep indigo-blue when exposed to strong 
 sunlight or to the electric arc for a few minutes. When this 
 blue solution is removed ftpom the li^t it gradually fades to 
 the pale yellov; of the original solution. Strange to say, 
 heating this blue solution gently also discharges the color. 
 
 On the other hand, many combinations or decompositions effected 
 by photochemical means remain permanently in the reacted state 
 after the source of radiant energy has been removed. Examples 
 of these are the formation of phosgene^ by the combination of 
 carbon monoxide with chlorine and the formation of hydrogen 
 chloride from the elements, as well as the organic halogenations 
 
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 mentioned above. 
 
 However, although the number of known photochemical 
 reactions is quite large and is continually increasing, definite 
 and consistent information bearing on even the simpler photo- 
 chemical phenomena is notably lacking through-out the literature* 
 The subject of Photochemistry, itself, is too new to have as 
 yet developed an'jjtheories and explanations of photochemical 
 reactivity applicable to more than a few isolated instances. 
 
 The attempts that have been made to correlate our small store 
 of observed data with the more general laws of Physical Science 
 may be profitably considered here. 
 
 The earliest attempt to apply a generalization to 
 reactions influenced by light was made by Theodor von Grotthuss^O 
 who stated "that in a photochemical reaction only those waves 
 which are absorbed by the reacting substances can be chemically 
 active"* This law was later carefully investigated and apparently 
 verified by Draper^^ who studied the sensitivity of daguerreo- 
 type plates to various colors of visible li^t. Bancroft^^'^® 
 in his articles on the Electrochemistry of Light has extended 
 Draper’s work to the study of light sensitivity of gelatinized 
 photographic films which have been coated with organic dyes* 
 
 The dyes act as optical sensitizers and by absorbing light of 
 various wave lengths make the films reactive to rays which would 
 otherwise be without effect. However, the Grotthuss -Draper Law 
 has but a limited application, many reactions going in direct 
 contradiction to it* Bancroft^^, himself, admits that "there is 
 

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 never an exact coincidence betv/een the absorption bands of the 
 dyed films and that portion of the spectriim to which color 
 sensitiveness is increased.” Also in the photochemical oxidation 
 of quinine with chromic acid, Luther and Forbes^^ have shown 
 that the maximum reaction does not necessarily follow the region 
 of maximum absorption. Grotthuss^® considered that the action 
 of a ray of light is analogous to that of a voltaic cell and 
 that in a photochemical process positive and negative charges 
 are set up which give rise to the resultant chemical action. 
 Bancroft feels that his work substantiates this view and says 
 that " the theory of Grotthuss accounts for all action of light 
 on salts” He ajso develops a theory of the formation of 
 
 positive and negative ions by the action of light to explain 
 the halogens t ion of organic hydrocarbons by the use of "carriers”. 
 That the amount of reaction is proportional to the absorbed 
 radiation was suggested by Draper^^»^^ who studied the photo- 
 chemical combination of hydrogen and chlorine quantitatively 
 and found that the velocity of the reaction is inversely 
 proportional to the square of the distance from the source of 
 the ill\iminatlon and therefore directly proportional to the 
 intensity of the inciting radiation. 
 
 It is generally conceded that photochemical reactions 
 are more sensitive to the shorter wave lengths. In their work 
 on the oxidation of ketoses and aldoses Bertholet and Gaudechon^*^ 
 state that the ease of reaction is favored by an increase in the 
 

 
 
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 postulate the hypothesis that "an increase in vibrational 
 frequency of exciting light should have the same effect on a 
 photochemical reaction as an increase in temperature on a 
 thermal reaction", 
 
 Byk^® regards the mechanism of photochemical action as 
 essentially that of a rapidly alternating electrolytic process 
 in which the light energy is propagated in the form of a rapidly 
 alternating electric current. This is in harmony with our 
 conceptions of the nature of light as an electromagnetic 
 phenomenon. He further assumes that in a photochemical reaction 
 the work performed against the chemical forces is proportional 
 to the energy falling on the reacting system. With this as a 
 basis he calculates from thermodynamic reasoning the work done 
 in transforming one mole of a substance through the influence 
 of radiant energy^^. Byk applied his equations to the 
 polymerisation of anthracene to dianthracene and obtained a 
 fairly close agreement of theory with experiment for high 
 concentrations of anthracene. For low concentrations of 
 anthracene the amount of light energy required to transform one 
 mole was considerably higher. To account for this discrepancy 
 Byk^O get forth his Electromagnetic Theory in v/hich he assumed 
 that light has a loosening effect on the electronic constituents 
 of the molecules which results in the temporary formation of 
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 one. This suggests the earlier assumptions of Grotthuss . Byk 
 then assumes that these ions are in vibration and that in a 
 photochemical reaction some of the incident radiant energy is 
 used up by the ion in its oscillations before it can collide and 
 react with an oppositely charged ion. In a concentrated system 
 the mean time between collisions of ionized molecules is small 
 so liiat the amount of light energy converted to heat throu^ 
 translatory motion may be relatively negligible. In a dilute 
 system, however, the mean distance travelled by an ionized 
 molecule will be many times larger and the amoimt of light energy 
 used up to traverse space may increase to an appreciable 
 proportion of the incident energy. Byk was able to get but a 
 qualitative idea of the amoiint of light energy thus dissipated 
 but he has shown thermodynamically that for very low temperatures 
 even in dilute systems this value would be negligible and the 
 electronic assumption unnecessary^®. Weigert^^'®^ has developed 
 a set of thermodynamic equations substantially equivalent to 
 Byk*s equations. He, however, rejects the Electromagnetic Theory 
 altogether and takes exception to Byk's calculated values for 
 dilute systems in the anthracene -dianthracene reaction on the 
 grounds that the constants used by Byk were not obtained from 
 truly representative experiments®®, Luther and Weigert ^4,25 
 had studied the same reaction earlier and proposed a theory of 
 the formation of intermediate compounds to account for the fact 
 that only about fifty percent of the incident light energy was 
 

 
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7 . 
 
 chemically effective. They made a distinction between chemically 
 absorbed light and thermally absorbed light and assumed that by 
 the absorption of light one or more intermediate compounds were 
 formed which in turn absorbed heat to a marked degree and broke 
 down to give the final reaction products. Weigert^^*^*^ supports 
 this view again in opposition to Byk’s Electromagnetic Theory 
 in citing his observations on the photochemical oxidation of 
 dye solutions, 
 
 Einstein^®'^^ has applied the principles of the (^uantTom 
 Theory to explain the mechanism of photochemical reactions with 
 a fair degree of success. He assumes that a reacting system 
 absorbs energy in integral units corresponding to Planck* s 
 conception of quanta and postulates the Einstein Law of the 
 
 Photochemical Equivalent as follows: For the decomposition of 
 
 g ram 
 
 ^equivalent of a reacting substance through photochemical 
 means an amount of radiant energy equal to Whv must be absorbed. 
 Here : 
 
 N - Avagadro*s Number 
 h = The Planck Constant 
 
 y' - The characteristic vibrational frequency of 
 the absorbed radiation. 
 
 According to Stark ^ the primary reaction in a photochemical 
 
 process is always the liberation of an electron from a molecule. 
 31 
 
 Millikan finds that in the case of a metal the work necessary 
 
 to displace an electron from its molecule is equal to hf^, where 
 
 l^in this case is equal to the frequency of vibration of the 
 light which^acting . on the metal is just able to dislodge the 
 
4 
 
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8 
 
 electron, itself. It is possible that this expression may also 
 apply to non-metals and compounds and that the difference between 
 Einstein’s /lyand Millikan's /rv^may be the energy with which the 
 electron leaves the molecule when the latter is activated by 
 light. Warburg has tested the Einstein Law and foimd that it 
 applies within reasonable limits to the formation of ozone^^*^^ 
 and the decomposition of gaseous ammonia^^»25 i,y ultra-violet 
 light. However, the large majority of photochemical reactions 
 give experimental values for the absorbed energy per mole of 
 reacted product which fail signally to agree with the Law of the 
 Photochemical Equivalent, Such reactiona are the combination 
 of chlorine with hydrogen^^, the hydrolysis of chloroplatinic 
 acid^*^, the decomposition of water vapor^®, and the hydrolysis 
 of acetone^^. In this last reaction, Henri and Wurmser report 
 that the energy used was onl^ about one percent of that required 
 by the Einstein Law. Bodenstein^® discusses at some length the 
 amounts of radiant energy absorbed in a large number of photo- 
 chemical reactions in relation to the amount of products realized. 
 It should be pointed out here that Einstein's Law states merely 
 that for a given amoimt of reacted products a definite quantity 
 of radiation is absorbed. Further than that it says nothing 
 concerning the conditions under which a photochemical reaction 
 is carried out; neither does it take into account the possibility 
 of the reaction products undergoing further change and entering 
 into additional reactions after the influence of the exciting 
 light has made itself felt. Light reactions are generally 
 complex and many of them probably consist of a number of distinct 
 

 
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9 
 
 processes so closely associated with the initial photochemical 
 effect as to he impossible of separate study and Identification, 
 
 A proper judgment of the value of the Law of the Photochemical 
 Equivalent is, therefore, made exceedingly difficult. It seems 
 quite possible that with closer study of photochemical phenomena 
 leading to a clearer conception of the relation of t he effects 
 due to light as opposed to those due purely to thermal and 
 other conditions that the Einstein Law might prove of much 
 greater applicability. 
 
 That photochemical phenomena are of more general 
 occurence than is commonly supposed is the contention of several 
 workers in the field of chemical activity. Trautz^^ has 
 advanced the idea that in order to take part in a chemical 
 reaction a molecule must be in an activated state. Based on 
 the theory of the existence of activated and unactivated 
 molecules, Arrhenius^ has developed an equation for chemical 
 reaction similar to the van’t Hoff equation: 
 
 d In K _ E 
 dT R? 
 
 in which K represents the equilibrium constant for the reaction 
 between activated and unactivated molecules of the same species 
 and E represents the quantity "heat of activation" instead of 
 the usual term "heat of reaction". Perrin^^ puts forth the 
 hypothesis that the action of light plays an essential part in 
 all chemical processes while Kruger^"^ even assumes that 
 phenomena such as solution, solution pressure of metals, and 
 
^ t: 
 
 
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10 
 
 electrolytic dissociation may be explained on the basis of the 
 absorption of energy in the form of radiation. Following the 
 work of these men and several others W. C, Me, Lewis"^^ suggests 
 the theory that catalysis is essentially a radiation phenomenon 
 consisting in the activation of the reacting substances by the 
 absorption of one quantum (hV) of energy per molecule. He 
 assumes that ordinary thermal reactions are in reality photo- 
 chemical processes activated by the long wave lengths of the 
 infra-red region and that the infra-red radiation, which appears 
 to us as heat, is emitted in quanta by the catalyst and absorbed 
 by lthe reacting substancesf^ Daniels and Johnston^*^ support 
 this view in discussing their work on the photochemical 
 decomposition of nitrogen pentoxide by stating that " there is 
 no fundamental difference between the mechanism of photochemical 
 and thermal action", Langmuir^® hov/ever feels that the 
 radiation hypothesis is not justified. He bases his objections 
 in part on the fact that in the dissociation of phosphine at 948° 
 Centigrade the amount of energy in the form of infra-red 
 radiation that could be possibly supplied by even a perfect black 
 body at this temperature is millions of times too small to 
 account for the observed heat of dissociation. As an alternative, 
 he offers the hypothesis that the energy of activation is obtained 
 at the expense of the internal energy of the molecules. 
 
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11 . 
 
 II. HISTORICAL. 
 
 The purpose of this research was to study the action of 
 the halogens, bromine and iodine, on hydrogen under varying 
 conditions of temperature and illumination and to determine, if 
 possible, whether the resultant formation of the hydrogen 
 halides is to be regarded (1) as a purely thermal reaction, (2) 
 as a true photochemical reaction in which temperature conditions 
 play but a negligible part, or (3) as a composite reaction in 
 which both thermal and photochemical factors enter simultaneously 
 in such a manner that the presence of one factor influences 
 materially the effect produced by the other. There seems to 
 be less consistency in the literature regarding the action of 
 these two halogens than is the case with their two more negative 
 allies, fluorine and chlorine. It is well known that fluorine 
 combines explosively on contact with cold hydrogen in the dark^^ 
 so that this element offers no ready field for photochemical or 
 thermal investigation. The action of chlorine with hydrogen 
 at ordinary temperatures is noticeably influenced by the intensity 
 and frequency of the light that is allowed to fall on the 
 mixture. In the dark there is practically no combination even 
 over a long period of time; in diffuse sunlight the action is 
 moderate; in bright sunlight the action becomes very rapid and 
 can be made explosive; while in ultra-violet light the mixture 
 combines immediately with explosive violence. That temperature 
 

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12 
 
 may have some effect on the sensitivity of chlorine to light is 
 suggested by an early article of Amato'^^ who claims that there 
 is no combination of hydrogen with chlorine in the sunlight at 
 a temperature of -12° Centigrade, There is no later con- 
 firmation of this observation. It has been shown that the 
 photochemical combination of hydrogen and chlorine does not 
 take place in accordance with the Einstein Law of the Photochem- 
 
 £>! RO 
 
 ical Equivalent , Bodenstein has calculated that as many 
 as 10 molecules of chlorine are brought into combination by 
 one quantum of light energy. There have been several theories 
 put forth to explain this high activity of chlorine, Nernst^® 
 suggests that the chlorine molecule is split up into uncharged 
 chlorine atoms by the action of light and that these atoms being 
 hi^ly reactive are able to disrupt the hydrogen molecule with 
 the formation of one molecule of hydrogen chloride and the 
 liberation of a considerable quantity of heat energy. The 
 remaining hydrogen atom is then able to disrupt a passive chlorine 
 molecule with the accompanying formation of more hydrogen chloride 
 and the liberation of an additional quantity of heat. This 
 latter reaction in t\irn gives rise to an additional activated 
 chlorine atom so that the process gives the impression of being 
 an unending chain of reactions which once initiated continues 
 without further addition of energy from the out-side till one 
 of the reacting constituents is either entirely removed or 
 rendered so low in concentration as to reduce the velocity of 
 combination practically to zero, Nernst formulates this 
 
 reaction as follows; 
 
l,U fk 
 
 , ' ■'’ijA' Vf '■’ '^i^'TyT«ft 
 
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 tkYltCf r. 
 
 y »#<■.■ P; 2 i 
 
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 iitffA 
 
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 •to 4>fsc« '5':. 
 
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 •' f V *"i^. . ' .Wi 
 
 
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 ^ ; «p? y ? ' tfi. ' ^^atyjyjigig ^ 
 
( 1 ) 
 
 ( 2 ) 
 
 (3) 
 
 13 
 
 Gig = Cl + Cl 
 
 Cl + Hg =HC1 -f II + 25,000 cal* 
 
 H i Clg=HCl 4 Cl f 19,000 cal. 
 
 Bodenstein^^ assumes that the action of one quantum of 
 radiant energy on a mixture of hydrogen and chlorine is to first 
 dislodge an electron from the chlorine molecule leaving a 
 positive residue which immediately reacts with hydrogen to form 
 hydrogen chloride. He calls this the "primary" light reaction 
 but does not go into any explanation as to how the positive 
 charge on the chlorine residue is neutralized. The "secondary" 
 reaction, which he regards as the more important part of the 
 process, results from the attachment of the free electrons to 
 passive chlorine molecules. The chlorine molecules thereby _ 
 become activated and react with the hydrogen molecules present 
 with the formation of hydrogen chloride and the re-liberation 
 of the electrons. The electrons in turn go through another 
 cycle in an endless sequence till the reaction is either 
 completed or has reached a point of stable equilibrium. 
 
 Bodenstein represents his theory by means of the equations; 
 
 (1) 
 
 Cl2 
 
 + 
 
 Light 
 
 
 (2) 
 
 01 
 1 — 1 
 o 
 
 + 
 
 e 
 
 - Clg- 
 
 (3) 
 
 Clg 
 
 + 
 
 H2 
 
 = 2 HCl + © 
 
 The representation of the photochemical activity of 
 chlorine as taking place by the formation of various positively 
 
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 j^%fl *>t *--i c ^i■^VIli'0' 
 
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14 . 
 
 and negatively charged particles through the liberation or 
 
 attachment of electrons appeals to us because of our present-day 
 
 development of the electronic nature of matter as well as because 
 
 effect 
 
 of its analogy to the photo-electric^^in metals. However, there 
 is a serious objection to this sort of an explanation in the 
 minds of some workers in view of the fact that we have at present 
 no evidence of ionization taking place in a strongly illuminated 
 system of pure chlorine or of any mixture of chlorine and 
 hydrogen^^, Gohring^^ has studied the above reaction and 
 subjected both Nernst's and Bodenstein's views to careful 
 analysis. He lists eighteen possible reactions which might 
 occur in the photochemical interaction of hydrogen and chlorine 
 on the assumption that the chlorine is activated by light to 
 the forms; Cl, Clg, CI5. These active radicals may then react 
 with hydrogen molecules (Hg) to form the appropriate amounts of 
 hydrogen chloride and hydrogen atoms (H), or of hydrogen chloride 
 and chlorine atoms (Cl); or the various species of chlorine 
 radicals may react among themselves thus neutralizing some of 
 their activation and generally changing the relative amounts 
 of each species present; or, finally, each species may react 
 with the hydrogen atom previously formed yielding hydrogen 
 chloride and a lower species of chlorine radical. Gohring 
 inclines to a choice of Nernst*s ideas and to the theory of the 
 formation of activated Cl^ molecules to best explain the 
 mechanism of the whole process. However, he admits that the 
 solution of the problem is still far from completion and that 
 much additional evidence from a study of other types of reactions 
 

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15 
 
 is needed to permit of any suitable decision amid such a maze 
 of possibilities. 
 
 Bromine and iodine are elem.ents less negative than 
 
 chlorine and consequently would be expected to have less 
 
 an 
 
 affinity for combinations with ( i i . " . element such as 
 hydrogen. Iodine combines partially with hydrogen at bright 
 red heat to form an equilibrium mixture in the reversible 
 reaction: 
 
 Hg + I 2 2 HI 
 
 the amount of combination or decomposition being readily 
 Influenced by a change in temperature. Prom this it is readily 
 seen that higher temperatures accelerate the reaction of iodine 
 with hydrogen in both directions. On the other hand, the 
 influence of light is to accelerate the reaction in but one 
 
 direction that which favors the decomposition of hydrogen 
 
 iodide into its elements. Lemoine^*^ reports that a gradual 
 decomposition of hydrogen iodide is readily brought about by 
 exposure to sunlight at ordinary temperatures, the blue and 
 the violet rays being most effective. The same author claims 
 that no combination of hydrogen with iodine was observed in 
 sunlight at ordinary temperatures; neither was there any evidence 
 of decomposition when a sample of pure, dry hydrogen iodide was 
 kept in the dark for a long period of time. Bodenstein^® has 
 made the Interesting observation that the photochemical 
 decomposition of hydrogen iodide proceeds as a monomolecular 
 reaction while the thermal decomposition is obviously a reaction 
 
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16. 
 
 of the second order. Bromine though resembling iodine in mg^ny 
 
 particulars shows some decided differences in its behavior with 
 
 hydrogen. For one thing, the direct combination of bromine 
 
 with hydrogen at red heat is not a reversible reaction. Newth®^ 
 
 shows how to prepare pure hydrogen bromide by passing pure 
 
 hydrogen saturated with bromine vapor over a coil of glowing 
 
 platinum. According to Bodenstein and Lind^^ the combination of 
 
 hydrogen with bromine goes to completion at 224.7° to 301.3° 
 
 fil 
 
 Centigrade. Kastle and Beatty report that a slight union of 
 these elements occurs at 100° C. in the light and that practically 
 complete combination is effected at 196° C. in sunlight. They 
 also make the statement that no combination of hydrogen and 
 bromine was observed on heating the mixture at 196° C. in the dark. 
 In contrast to hydrogen iodide, dry pure hydrogen bromide shows 
 no evidence of decomposition on exposure to sunlight at ordinary 
 temperatures^^>®^. However a qualitative decomposition, though 
 not a combination, of both hydrogen iodide and hydrogen bromide 
 by means of ultra-violet light is reported for the case of 
 ordinary temperatures.®^. From analogy with chlorine we would 
 expect any photochemical reaction involving bromine or iodine 
 to depart from the Einstein Law by a wide margin. In this 
 connection it is interesting to note the work of Fraulein Pusch®® 
 who working under Nernst claims that the photochemical bromination 
 of several hydrocarbons in sunlight takes place in such a way 
 that the amount of reacted products corresponds to but l/l,000 
 to l/l0,000 of that which should be realized from the amount of 
 radiant energy absorbed by the system. Some years earlier than 
 

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17. 
 
 this Nernst^^had shown by calculations from his heat theorem 
 that the combination of hydrogen and bromine by the action of 
 light would necessarily be of very limited extent. Assuming 
 that the mechanism of the reaction in this case is similar to 
 the case of chlorine and hydrogen, he shows that the second 
 step in the process would be endothermic according to the 
 equation; 
 
 Br + Hg — ^ HBr + H - 15,000 cal. 
 and that unless the system received heat energy from the outside 
 the reaction would not occur. A comment on the possible photo- 
 chemical reactivity of bromine is made by Lind®*^ who says, "It 
 is well known that a mixture of hydrogen and bromine is not 
 light sensitive at ordinary temperatures". 
 
 Prom the above discussion it is seen that the observations 
 made by various authoritive investigators on the action of 
 bromine to light are in themselves quite contradictory and that 
 an acceptable explanation for the photochemical reactions of 
 the halogens is still lacking. Of particular interest to this 
 
 go 
 
 problem is the work of Coehn and Stuckardt ° on the action of 
 light on the formation and decomposition of the hydrogen halides. 
 These men put the pure hydrogen halides of chlorine, bromine, 
 and iodine into flasks of quartz, uviol glass, and Jena glass 
 and subjected the flasks and contents to the rays of ultra- 
 violet light for periods of time sufficient to produce equil- 
 ibrium. Their values obtained for the decomposition effected 
 in each case can be best expressed in tabular form; 
 

 
 
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18 
 
 Percent Decomposition. 
 
 
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 Jena 
 
 
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 • • 92.29 
 
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 The differences 
 
 in the effects 
 
 on the same gas 
 
 were ascribed 
 
 as due to the selective action of the range of wave lengths w 
 which were able to pass through the walls of the flasks. 
 
 Similar experiments on the formation of the halides from 
 equivalent mixtures of hydrogen and halogen were tried. The 
 amount of corribination effected in each flask checked the decompo- 
 sition percentage exactly to give the same equilibrium mixture. 
 The authors make no direct mention of the temperature conditions 
 affecting the flasks during the reaction period^ though a causal 
 reading creates the impression that all reactions took place at 
 room temperature. This is the assumption made by the abstractor 
 in a later journal. However a description of the ultra-violet 
 light apparatus used here is given in an earlier article by 
 Coehn and Sieper®^ in which it appears that the reacting vessel 
 is entirely surrounded by the mercury arc in such a way as to 
 receive the full heating effect of the lamp. In this article 
 mention is made that the temperature within the field of 
 illumination was usually at 240° Centigrade. From this it 
 seems quite possible that Coehn and Stutgard have really measured 
 the thermal equilibrium of their reactions after any possible 
 photochemical effects have taken place. 
 

 
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19 
 
 III. EXPERIMENTAL . 
 
 Part 1. 
 
 As already stated on p.ll, the purpose of this research 
 was to study the influence of heat and light on the action of 
 bromine and iodine on hydrogen and to differentiate if possible 
 the effects due to each of these agents. The first part of 
 this Investigation was a qualitative experiment to obtain some 
 idea of the action of ultra-violet light on bromine, A clear 
 quartz flask of about 200 c.c. capacity was filled with a mixture 
 of hydrogen and bromine by means of the same apparatus described 
 later in Part 2 (Plate I.) which was used for filling the Pyrex 
 glass tubes. The hydrogen ?/as in considerable excess, there 
 being just about enough bromine present to give a perceptible 
 color to the flask. This color could be best observed and 
 roughly estimated by sighting through the flask to a sheet of 
 white paper held up against its side. The ratio of bromine to 
 hydrogen was about one volume of bromine to six or more volumes 
 of hydrogen. This flask was first exposed at room temperature 
 to the ultra-violet rays emitted by a mercury vapor lamp. The 
 lamp and flask were both enclosed in a large light-proof box, 
 the flask being set below the lamp at a distance of about twelve 
 inches in such a way that it could get a good intensity of the 
 light and still be reasonably unaffected by the temperature of 
 the lamp. In addition, a slight current of air was maintained 
 

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20 
 
 upwards by means of suction to carry off the ozone formed in the 
 box. This procedure incident ly created a draught of air from 
 the flask to the lamp thereby minimizing greatly the chances of 
 heat transfer from the lamp to the flask. After an exposure 
 of twenty hours no noticeable dimunition in the color of the 
 flask could be observed. It is felt that if any action had 
 oc cured it would have been sufficient to have almost if not 
 entirely removed the color. This view is strengthened when one 
 considers that the total amount of bromine ?/as exceedingly small 
 and that any possible equilibrium would be materially displaced 
 by the presence of so large an excess of hydrogen. The flask 
 
 was then heated in the free flame of a Bunsen burner for about 
 three minutes when the color was observed to have entirely 
 disappeared. That complete combination had taken place was 
 decided when a sheet of writing paper appeared perfectly white 
 on being viewed through the flask. The flask, now containing 
 a mixture of hydrogen bromide and hydrogen, was first allowed 
 to cool and was then again exposed to the action of the ultra- 
 violet rays for another twenty hours. At the end of this 
 period, the flask on careful examination showed no evidence of 
 possessing color. From these two experiments it appears that 
 the action of ultra-violet light at room temperature will neither 
 cause the combination of hydrogen and bromine even in the small- 
 est amount nor will it cause the decom.position of hydrogen 
 bromide into its elements once the compound is formed. 
 

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21 . 
 
 Part 2 
 
 It was next thought advisable to determine the influence 
 of temper atiire on the action of bromine with hydrogen both in 
 complete darkness and in the presence of a strong light. Here 
 obviously any difference in behavior could be correctly ascribed 
 to photochemical effects. For this pxirpose a number of Pyrex 
 glass tubes 1" in diameter and 8'* long were prepared and filled 
 with a mixture of bromine and hydrogen by means of the apparatus 
 shown in Plate I. and described below; 
 
 Descrip t ion of Filling Apparatus . 
 
 (A) is a U-shaped electrolytic hydrogen generator filled 
 with a strong solution of potassium hydroxide and provided with 
 pure nickel electrodes. The generator was made by bending a 
 large piece of glass tubing 2" in diaraeter and 30” long. The 
 current performing the electrolysis is controlled by means of 
 a lamp bank (not shown) in series with the generator. The 
 U-bend of the generator is filled to a height of 3" with glass 
 beads to decrease the tendency of dissolved oxygen to diffuse 
 to the hydrogen side during electrolysis. Next to the generator 
 is a tower (B) filled with glass wool to collect any spray that 
 may be carried over from the electrolysis by the escaping 
 hydrogen. Connected to (B) is a wash bottle (C) fitted with a 
 tightly ground stopper and containing sulphuric acid to a height 
 of 2" to serve as a drying agent for the hydrogen. Then, 
 following (C) is a tower (D) containing soda-lime as an 
 
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22 
 
 additional drying agent and protection against any spray from 
 the sulphuric acid being carried over by the hydrogen. This 
 constitutes the system for generating and drying pure hydrogen 
 for the experiment. The hydrogen was generated continuously 
 but was drawn over the liquid bromine and into the reaction 
 tubes intermittently as will be described later. The remainder 
 of the apparatus is made of Pyrex glass tubing one-half inch in 
 diameter. The hydrogen generating system is made of soft glass 
 and is connected to the second part of the apparatus by a piece 
 of soft, tight-fitting, black rubber tubing (a). The bromine 
 is contained in the short U-tube (E) which immediately follows 
 the rubber connection (a). (P) is a large U-bend 12” high 
 
 which serves as a trap for any bromine carried over while the 
 apparatus is being washed out with hydrogen and later serves 
 as an additional mixing chamber for bromine and hydrogen when 
 the tubes (d) are being filled. The Pyrex stop-cock (b) serves 
 to isolate the tubes (d) from the rest of the apparatus when 
 they are being evacuated and also controls the admission of the 
 bromine -hydrogen mixture to the tubes. The second Pyrex stop- 
 cock (c) leads to the vacuum line and to a mercury manometer 
 (not shown). All joints were of glass with the exception of 
 the rubber connection (a); a soft, tight-fitting rubber stopper 
 set in the delivery end of the hydrogen generator; and the 
 rubber tubing leading from the stop-cock (c) to the mercury 
 manometer and the vacuum line. As mentioned above, the wash 
 bottle (C) was fitted with a well-ground glass joint to permit 
 changing of the sulphuric acid before each run. Every 
 

 
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23 
 
 precaution was taken to exclude air from the apparatus during 
 the filling. 
 
 Method of Eilling the Tubes . 
 
 The tubes (d) were sealed to the Manifold (M) by means 
 of short sections of capillary tubing (e). The Large U-tube 
 (P) was enclosed in a Dewar flask filled with ice end brine 
 solution while a beaker of ice -water was brou^t up to the small 
 U-tube (E) so as to enclose it entirely. With the rubber 
 connection (a) removed, about 5 c.c. of liquid bromine was run 
 into (E) from a pipette. The rubber suet ion- tubing connecting 
 stop-cock (c) to the vacuum line was disconnected; the rubber 
 tubing (a) was fitted in place; and the stop-cocks (b) and (c) 
 were then turned so as to give access to the air. A current 
 of 3 to 4 amperes was sent through the electrolysis cell (A) 
 and hydrogen gas was allowed to flow through the apparatus for 
 about two hours to displace the air. This procedure was 
 expected to remove practically all traces of oxygen. The 
 U-tubes, (E) and (F), were kept cold in order to reduce the 
 vapor pressure of the bromine and prevent loss, as well as to 
 avoid the presence of objectionable vapors in the laboratory. 
 Some of the bromine was carried over from the bend (E) but was 
 readily solidified in the cold tube (F) and caused no incon- 
 venience . 
 
 After the apparatus had been thoroughlt washed with 
 hyc3rogen the cold baths were removed from around (E) and (F) 
 

 
 
 
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24 
 
 acuum PLATE 
 
9 
 
25 
 
 and replaced by baths of warm water of varying temperatures. 
 
 This raised the vapor pressure of the bromine causing the stream 
 of hydrogen to become more highly saturated. It was fairly 
 easy to regulate within rough limits the ratio of bromine to 
 hydrogen in the flowing mixture by setting the temperature of 
 the baths to afford the desired vapor pressure. After a suit- 
 able wait to permit temperature equilibrium to be established 
 throughout the system, the stop-cock (b) was closed and the 
 vacuum line and manometer were immediately connected to the 
 stop-cock (c). This procedure caused two effects to take place: 
 first, a head of hydrogen immediately began to build up in the 
 apparatus on the side of the generator causing a difference of 
 levels in the potassium hydroxide solution used as electrolyte; 
 second, the reaction tubes (d) became rapidly evacuated as 
 could be observed by noting the mercury manometer. About 150- 
 200 c.c. of hydrogen was allowed to form above the solution in 
 the generator. The stop-cock (c) was then closed and the 
 stop-cock (b) opened cautiously to permit the bromine -hydrogen 
 mixture to be drawn into the tubes. This was the most important 
 and delicate part of the procedure. Since the capacity of the 
 tubes was greater than the amount of hydrogen that could be 
 safely accomodated over the potassium hydroxide level, it was 
 necessary to be particularly careful not to create a partial 
 vacuum in the hydrogen-generating system. This error would 
 cause air to diffuse into the apparatus through the rubber 
 tubing or through the rubber stopper and might even cause the 
 solution of potassium hydroxide to be drawn up through the exit 
 
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26 
 
 tube and into the glass-wool tower (B). To guard against error 
 
 from this direction a heavy, easily-visible line was drav/n on 
 
 the oxygen side of the electrolysis vessel (A) in such a position 
 
 that when the level of the solution on this side coincided with 
 
 the line, the level of the solution on the hydrogen side was a 
 
 full inch below it. Then, when the stop-cock (b) was opened 
 
 the amount of hydrogen admitted to the tubes was just enough to 
 
 permit the level of the solution on the oxygen side of the 
 
 generator to fall to the mark indicated. Since the oxygen 
 
 electrode was open to the atmosphere, this procedure ensured a 
 
 pressure of hydrogen within the apparatus of about 2 ram. of 
 
 mercury which was considered sufficient to prevent entrance of 
 
 air by diffusion. Two or three "charges" of hydrogen gas were 
 
 necessary to fill the tubes. The removal of the last vestige 
 
 of air as well as any chlorine which might have come over in 
 
 the first fraction of bromine was assured by closing (b) and 
 
 ly 
 
 re-evacuating the tubes. This process of alternate filling 
 and evacuating was repeated three or four times. Both stop- 
 cocks were then closed; the hydrogen-generating system was 
 disconnected ; and the tubes were sealed off at the capillaries 
 (e) by means of an oxygen-gas flame. The tubes were then used 
 in the following experiments: 
 
 It should be noted here that the quartz flask used in 
 part one was filled with the above apparatus. The flask was 
 provided with a thin quartz tube which was sealed to one of 
 the capillary tubes (e) by means of de£ot insky cement. The 
 
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27 
 
 remaining capillaries were sealed shut and the flask was given 
 the same treatment accorded to the Pyrex tubes except that the 
 ratio of bromine to hydrogen was made considerably less by omitt- 
 ing the warm baths around the U- tubes (E) and (F). 
 
 Results of Heating Tubes 
 With and Without Illumination , 
 
 For heating the tubes a resistance furnace was constructed 
 by winding eighty feet of #16 Nichrome wire around a section of 
 stove pipe 18" long and two inches in diameter. The pipe was 
 first insulated with a coating of bundin' s cement which was 
 allowed to dry thoroughly. The Nichrome wire was then wound 
 carefully over thia layer of cement and secured in place by 
 twisting the end coils together, A second layer of the cement 
 was then added over the wire to protect it from the action of 
 the insulating material which was next put on. An insulating 
 wall of "85^ Magnesia" was built around the furnace to a depth 
 of four inches and allowed to dry thoroughly. This material 
 has a high efficiency as an insulator though it contains as a 
 binder some corrosive material, such as sodium silicate, which 
 attacks metals at high temperatures. The furnace proved 
 highly satisfactory for the purpose to which it was put. 
 Temperatures in the furnace were measured by means of a copper- 
 constar^^Sierrao-couple attached to a milli-voltmeter graduated 
 to 17 milli-volts. The thermo-couple was made by fusing the 
 ends of the two wires together in a free flame. The hot- 
 

 
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28 
 
 Junction was enclosed in a glass tube while one of the terminals 
 of milli-voltmeter was used to serve as the cold- Junction at 
 room temperature. This obviated the use of a second fused 
 Junction as well as the need of a cold-bath of melting ice to 
 serve as the zero of reference. The thermo-couple and milli- 
 voltmeter were then calibrated as used. The hot-Junction 
 was placed in turn in the vapors of boiling water at 100° C., 
 of boiling napthalene at 218° C., and in the vapors of boil- 
 ing mercury at 357° C. and the readings of the milli-voltmeter 
 observed when the thermo-couple had come to equilibrium. These 
 values were then plotted on co-ordinate paper so that the read- 
 ing of the milli-voltmeter could be directly translated to 
 degrees from the straight line obtained by connecting the three 
 points. The current through this furnace could be controled 
 within very narrow limits by means of a lamp bank and a plate 
 i^ostat connected in series. About one hour was required by 
 the furnace to heat up for a given current and to acquire a 
 constant temperature. After the amount of current was adjusted 
 for the changed restance of the hot coils, the furnace would 
 remain at a constant temperature almost indefinitely. It is 
 assumed that a fluctuation of 5^ was possible during the 
 adjustment period and that an additional error or variation of 
 5 more degrees was present in the thermo-couple due to a time 
 "lag” necessary to warm up the air enclosed in the glass tube 
 with the hot-Junction and to any errors of calibration. Although 
 it was read??y^4o^read temperatures on the milli-voltmeter to 
 within two degrees, the temperatures observed are recorded here 
 
p 
 
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29 . 
 
 as having taken place over a range of ten degrees. Also since 
 a qualitative comparison of the effects of light on the velocity 
 of combination at various temperatures was the information really 
 desired, the time of heating the tubes was fixed at one hour* 
 
 In the first set of experiments the ends of the furnace 
 were closed tightly by means of plugs made by rolling strips of 
 asbestos paper into cylinders of the required diameter. This 
 reduced the losses of heat by convection and served to make the 
 furnace light-proof. In the second set of experiments one of 
 the asbestos plugs was removed and a 100 watt nitrogen-filled 
 tungsten lamp was brought up close to the opening so as to 
 thoroughly illuminate the interior of the furnace. The two 
 
 sets of results are tabulated for comparison as follows: 
 
 Pyrex Tubes Filled VJith Mixture of Hg and Brg . 
 
 (About three moles Hg to one mole Brg.) 
 
 
 4 
 
 
 
 Heated 
 
 in the Dark. 
 
 Heated in Light of Ng-filled 
 
 
 
 Tungsten Lamp. 
 
 Temp. °C. 
 
 Observations . 
 
 Temp, oq. 
 
 Observations . 
 
 Above 265 
 
 Colorless 
 
 Above 270 
 
 Colorless 
 
 255-265 
 
 (Appreciable 
 
 260-270 
 
 (Appreciable 
 
 
 Ide color izat ion 
 
 
 (decolorization 
 
 245-255 
 
 /Slight decolor 
 
 - 245-255 
 
 /Slight decolor- 
 
 
 iization 
 
 
 IjLzation 
 
 230-240 
 
 Paint color 
 
 235-245 
 
 Faint color 
 
 Belov/ 230 
 
 /no apparent 
 
 Below 230 
 
 |no apparent 
 
 
 {change 
 
 
 (change 
 

 
 V 
 
 tj 'Of:. 
 
 K 1 ; 
 
 - f 
 
 
 
 I ' : 
 
 r r 
 
 
 ' ' = -‘■■‘•^0; t^/v.i "fj' 
 . ,'CfQ. 
 
 .^o/oo In » 
 
 
31 
 
 The arrangement for simultaneously illuminating and 
 heating the tubes is shown in Platell., page 30, An examination 
 of the data given above, which was taken from a larger number 
 of concordant observations, shows that the amount of reaction 
 occur ing in one hour for a given range of temperature was 
 practically the same in both cases. Thus, reaction was initiated 
 in the dark at 230°- 240° and in the light at 235°-245°; while 
 complete combination was effected at about 265° in the first 
 case and at about 270° in the second. The small difference of 
 5° between these ranges of temperature is negligible in view 
 of the limits of error earlier accepted. Also, it would natur- 
 ally be expected that the illuminated reaction would occur at 
 the lower temperature so the acceptance of these temperature 
 ranges as practically identical is justified. In view of this 
 experiment the conclusion is made that the presence of visible 
 light has no easily detectable influence on the rate of comb- 
 ination of bromine with hydrogen. 
 
 Fart 3 
 
 The influence of Ultra-violet light on the action of 
 hydrogen and bromine contained in a quartz flask kept at 
 various ranges of temperatures similar in method to that 
 employed in the case of the nitrogen-filled lamp was next studied. 
 The filling apparatus described in Part 2 and shown in Plate I 
 was modified by replacing the manifold with a long glass tube 
 

 
 
 
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32 
 
 
 
 
 •d 
 
 PLATE III 
 
33 
 
 to the end of which was connected the quartz flask (H) hy 
 means of a joint of de Kotinsky cement. The tubing was bent 
 so as to bring the quartz flask into the electrically heated 
 oven (G). The light-proof box containing the mercury vapor 
 lamp was brought up to the oven so that the ultra-violet rays 
 could shine through a hole cut in the door and thereby 
 illuminate the interior of the oven. The arrangement of the 
 apparatus ia shown In Plate III. Page 32. The quartz flask 
 was filled in the regular maimer and then isolated from the 
 rest of the apparatus by closing the stop-cocks (b) and (c). 
 Two series of experiments were then carried out ?n^?fee flask 
 and contents were heated first, while under the influence of 
 the ultra-violet light and second, while exposed to ordinary 
 diffuse day-light. The time of exposure was still one hour 
 though the temperatures required to obtain any given effect 
 would be expected to differ from that required in the case of 
 the Pyrex tubes because of the difference in capacity between 
 flask and tubes. The ratio of hydrogen to bromine was made 
 three to one in this case, also. Results are tabulated: 
 
 Heating Mixture of Hp and Bro in Quartz Flask . 
 
 Exposed to U-V . Light . 
 Temp. °C. Observations. 
 
 Exposed to Day -light . 
 Temp, oc. Observations. 
 Above 310 Colorless 
 
 Above 310 Colorless 
 
 295-305 Appreciable 
 
 Decolor izat ion 
 280-29© Slight Decol. 
 
 300-310 
 
 280-290 
 
 Appreciable 
 Decolonization 
 Slight Decol. 
 
 260-270 
 
 II 
 
 II 
 
 265-275 
 
 II 
 
 II 
 
 Below 255 No Change 
 
 Below 260 
 
 No Change 
 

 
 
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34 
 
 A comparison of these values justifies the assumption that 
 ultra-violet light has no more effect on the combination of 
 hydrogen with bromine than has ordinary day-light. If the 
 results of Part 2 on the comparison of visible light with total 
 darkness are taken into account, one can say with full justi- 
 fication that the combination of bromine and hydrogen is purely 
 a thermal phenomenon and the rate of this combination is 
 determined by the concentrations, amounts of reacting substances, 
 and the existing temperature conditions_,and in no way influenced 
 by existing conditions of illumination. 
 
 An experiment to determine whether hydrogen^^Suld be 
 decomposed by the agency of ultra-violet was next tried. The 
 quart used in the first part of this experiment was filled 
 in the ordinary manner and then sealed off from its quart tube. 
 The flask and contents were then heated in a Bunsen flame till 
 total combination was effected as in Part I . This flask was 
 then put into the oven (H) and exposed to the rays of ultra- 
 violet light at various ranges of temperatures as was done in 
 the case of t he flask filled with the -uncornbined mixture. 
 Temperatures in steps of 10° from 260° to the room temperature 
 were tried and the flask carefully observed to note any appear- 
 ance of color. To the best of o\ir knowledge no indication of 
 the presence of free bromine could be observed at the end of any 
 of the heatings. Therefore, from these collective experiments 
 the conclusion is drawn that the inter-action of bromine and 
 hydrogen is not an easily recognized photochemical process. 
 
* 
 
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35 
 
 Part 4, 
 
 A recent article by Baly and Barker*^^ makes reference to 
 the observance of a sudden Increase in volume in the case of 
 a sample of chlorine gas which had been subjected to the action 
 of ultra-violet light. Draper had also observed this phenomenon 
 which has been called after him, the Draper Effect. It was 
 suggested by Balyjand Barker that the volume change was due to 
 an activation of the chlorine molecules, probably a splitting 
 of the molecule as has been postulated by Nemst, and that the 
 increase or activation was due to the violet end of the spectrum 
 and was independent of any heating effects* It was decided to 
 try out this effect with both chlorine and bromine in ordinary 
 sun-light and to note the comparative action of the blue and 
 the red ends of the spectrum by shutting out part of the rays 
 with appropriate screens. The apparatus for trying out this 
 experiment was made as follows; 
 
 Two glass bulbs of about 200 c.c. capacity were blown and 
 sealed to the two opposite ends of a long T-tube made of 
 fine capillary tubing. The open end of the T-tube was connected 
 to a chlorine-generating apparatus and an evacuating system. 
 
 The bulbs were washed with chlorine gas by successive evacuation 
 and filling and the whole apparatus was sealed off by closing 
 the rem.aining end of the T-tube by the use of a air-gas flame. 
 
 The bulbs containing bromine were similarly filled except that 
 the bromine was distilled over from a flask instead of being 
 generated. Also, all joints in the case of the bromine were 
 of glass while rubber connections were thought permissable in 
 

 
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36 
 
 the case of the chlorine. The bulbs were enclosed in a light - 
 proof box with a window cut in the top for t he admission of 
 sunlight. The fine capillary of the apparatus joining the 
 bulbs was made to serve as the manometer of the apparatus by 
 the condensation of some of the enclosed halogen to form a 
 meniscus which was very sensitive to any changes in pressure 
 between the bulbs. In the case of the bromine a small piece of 
 ice served very well to give the bubble desired. In the case 
 of the chlorine use was made of solid carbon dioxide snow which 
 readily condensed some of the chlorine to give a bubble which 
 served as the manometer. One of the bulbs was kept in the 
 dark while the other was ill\iminated through the window and 
 any change in position of the small bubble observed. It was 
 found that when light was suddenly admitted to the bulb that 
 a definite expansion took place but that there was no noticeable 
 difference in effect between the use of the two colors of screens. 
 It was hoped that the blue screen would cause a far greater 
 expansion in the gas but any greater shift in position by the 
 sensitive bubble was considered explanabiejon behalf of the 
 difference in absorption of the two colors by the two gases. 
 
 We put forth the theory that the expansion of chlorine and 
 bromine, too, when subjected to light is due to a heating effect 
 caused by the conversion of light energy to heat by adsorption. 
 
4 
 
 i 
 
 
37 . 
 
 IV. CONCLUSIONS. 
 
 The combination of bromine and hydrogen at various 
 temperatures and under various conditions of illumination has 
 been studied. It has been found that this reaction is not a 
 true photochemical phenomenon and that the velocity of combin- 
 at ion is unaffected by existing conditions of illumination. 
 
 The decomposition of hydrogen bromide by the action of ultra- 
 violet light has also been attempted. It has been found that 
 hydrogen bromide will not decompose into its elements under the 
 influence of ultra-violet light. The expansion of chlorine 
 and of bromine on exposure to various regions of the solar 
 spectrum has been observed and the theory put forth that this 
 expansion can be explained on the basis of the conversion of 
 radiant energy into heat by adsorption. 
 
 Since it is well-kno^wn that chlorine combines readily 
 with hydrogen under the influence of light and that hydrogen 
 iodide decomposes even more readily under the sa^ie influence, 
 this indifferent behavior of bromine and hydrogen bromide to 
 light is worthy of note. The fact that bromine is intermediate 
 in position between chlorine and iodine in our Periodic System 
 is also of interest, though the fact may or may not be of 
 special significance in this connection. 
 
 The results obtained in this research fail signally to 
 
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38 . 
 
 agrse with the work of Coehn and Stuokardt who report a total 
 decomposition of hydrogen bromide in quartz and a total combin- 
 ation of hydrogen and bromine in Jena glass under the influence 
 of ultra-violet light. 
 
 Similar work on the combination of hydrogen v/ith iodine 
 and the decomposition of hydrogen iodide in the light is now 
 in progress. 
 
39 . 
 
 V. ACKNOWLEDGMENTS.. 
 
 In concluding this thesis the author wishes to express 
 his appreciation to Dr. W. H. Rodebush for his kind interest 
 and supervision in the pursuance of this research. Thanks 
 are also due Dr. Gerald Dietrichson and Dr. Einmett K. Carver 
 for occasional help and personal interest. 
 

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 1 .. 
 
 o 
 
 
 
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