48 REPORT — 1855. 



On the Allotropic Modifications of Chlorine and Bromine analogous to the 

 Ozone from Oxygen. By Thomas Andrews, M.D., F.E.S., M.R.l.A. 



The author explained that ozone could be produced, first, by an electric spark ; 

 secondly, by the decomposition of acids and solutions, when coming into contact witli 

 the galvanic wire ; and lastly, by oxidation. 



On Photographic Researches. By Mr. Barnett. 



Photochemical Researches, loith reference to the Laws of the Chemical Action 

 of Light. By Professor Bunsen of Heidelberg and Dr. Henry E. 

 RoscoE of London. 



The following abstract gives the results of an investigation extending over a period 

 of nearly two years, which has been carried on at Heidelberg. 



Owing to the great experimental difficulties which are met with in researches on 

 the chemical action of light, our knowledge of the laws which govern this action is at 

 present very limited. The object of the following investigation was to endeavour to 

 obtain more precise information regarding these laws, and if possible to arrive at a 

 quantitative measurement of the cliemical rays. The first substances examined in 

 their photochemical relations were aqueous solutions of chlorine, bromine, and iodine, 

 either alone in solution or mixed with hydrogenous organic substances, and the altera- 

 tion which these solutions underwent by exposure to sun-light was made the subject of 

 accurate measurement. The amount of free chlorine, bromine, or iodine present 

 both before and after insolation, was estimated most exactly by the iodometrlc 

 method, and the experiments were so conducted that all errors arising from gaseous 

 absorption or diffusion were fully eliminated. From many experiments made accord- 

 ing to this method, it was observed that no simple relation existed between the 

 amount of free chlorine which disappeared and the time of exposure or the intensity 

 of the light. 



This anomalous action may be explained by theoretical considerations. Chemical 

 affinity must be regarded as the resultant of all the forces which come into play 

 during the decomposition, and therefore the total action is dependent not only upon 

 the interchanging molecules, but also upon the atoms which more or less surround 

 these. Alteration in the mass of these surrounding particles must therefore alter the 

 resulting chemical action. The correctness of this view was remarkably established 

 by further experiment. In order to ascertain wh:it effect the hydrochloric acid, 

 formed during the decomposition, exerted upon the affinit}' of chlorine for hydrogen 

 in presence of sun-light, pure chlorine-water and chlorine-water containing 10 per 

 cent, of hydrochloric acid were insolated during the same period ; the solution of 

 pure chlorine lost 99'6 per cent. ; whilst that containing 10 per cent, of hydrochloric 

 acid lost only 1'3 per cent, of its contained free chlorine. The result of this and 

 many other series of experiments*, justifies the conclusions, — 



1. That the presence of hydrochloric acid retards in a remarkable degree the 

 aflSnity of chlorine for hydrogen. 



2. That owing to this retarding action, which is governed by entirely unknown laws, 

 the examination of the photochemical decomposition of chlorine-water cannot lead 

 to the discovery of any simple relations. 



From these circumstances it appears probable that some simple law would be 

 arrived at if the following conditions were complied with : — 



1. That two elements which have no action upon each other in the dark, simply 

 combine under the action of the light, so that the relative amounts of the uncombined 

 bodies remain the same. 



2. That the substance produced by the combination be either entirely removed 

 from the sphere of chemical action, or be reduced to a small constant amount. 



These two conditions are only found in the gas evolved by the electrolytic decom- 

 position of hydrochloric acid. This gas consists, under certain conditions, of exactly 

 equal volumes of chlorine and hydrogen, and is singularly well-fitted for a photo- 

 metric substance. It is perfectly unalterable in the dark ; it is not affected by lamp- 



* See Quart. Journ. of Chemical Society, Oct. 1855. 



