CONDITIONS OF CHEMICAL CHANGE 



II. PHOTOCHEMICAL CHANGE IN GASES 



PART I. HISTORICAL 



By D. L. CHAPMAN, M.A. 



In comparison with other branches of chemistry, photochemistry 

 in its scientific aspect in the past has received little attention and 

 is a science relatively in its infancy. The classical researches of 

 Draper and of Bunsen and Roscoe and a few other interesting 

 but isolated investigations which will be alluded to in this and 

 a subsequent article almost complete the list of researches on 

 a subject which promised from the outset to aid in an excep- 

 tional degree in the elucidation of the mechanism of chemical 

 change. When it is remembered that the vast stores in plants 

 of energy derived from the radiation of the sun are the result 

 of the operation of photochemical changes, it can scarcely be 

 doubted that a precise knowledge of the laws that control 

 photochemical transformations would be of immense benefit 

 to mankind. The comparative neglect of the subject in the 

 past is therefore a matter of surprise. 



The photochemical change which has been most extensively 

 investigated is the interaction of chlorine and hydrogen. Change 

 sets in when a mixture of these two gases is exposed to light 

 and the hydrogen chloride that is produced dissolves with such 

 ease in water that when the gases are confined in a glass vessel 

 over water the decrease in volume is almost an exact measure 

 of the amount of chemical change. At the ordinary temperature, 

 in the dark, these two elements do not interact except in the 

 presence of a catalyst such as platinum. That change takes 

 place under the influence of light was first observed by William 

 Cruickshank (Nicholson's Journal, 1801 (1), 5, 202). In sun- 

 light the action is so energetic that the mixture explodes (Gay 

 Lussac and Thenard, Memoires d'Arceuil, 1809, 2, 340). Cruick- 

 shank and later Dalton {A New System of Chemical Philosophy^ 



657 



