664 SCIENCE PROGRESS 



light as a mixture of chlorine and hydrogen in equal propor- 

 tions. The explanation which they offered was this : by virtue 

 of its optical properties the chlorine in both cases absorbs the 

 same amount of light, which is rapidly dissipated as heat ; but 

 when hydrogen is present, an additional amount of light is 

 absorbed in bringing about the combination of chlorine and 

 hydrogen. The additional quantity of light thus expended is the 

 photochemical induction. It is the energy which overcomes 

 the resistance to combination. 



E. Pringsheim {Weid. Ann. der Phy. 1887, 32, 384), whilst 

 accepting Bunsen and Roscoe's result that a mixture of chlorine 

 and hydrogen is more opaque than one composed of chlorine 

 and an inert gas, criticises their hypothesis at some length 

 on theoretical grounds and advances an explanation of his own. 

 According to this observer, the first effect of the light is to decom- 

 pose the molecules into atoms. The atoms, however, have only 

 a very transitory existence apart and are rapidly re-associated 

 in such a manner that they give rise to intermediate compounds. 

 In the case of chlorine and hydrogen the intermediate com- 

 pounds occupy the same volume as the original gas : in the 

 dark, they slowly change back into the substances from which 

 they were produced but when exposed to light they are trans- 

 formed into hydrogen chloride. The preliminary dissociation 

 being ignored, the successive changes are possibly embodied 

 in the following equations : 



1. H 2 + Cl 2 + H 2 = C1 2 + 2H 2 

 Vol. 111 12 



2. C1 2 + 2H 2 = 2HCI + H 2 

 Vol. 1 2 2 1 



The rate at which the change expressed by the second 

 equation takes place is of course regulated by the amount of 

 chlorine monoxide present in the system, which again must de- 

 pend on the duration of the previous exposure to light : conse- 

 quently the formation of hydrogen chloride will be inappre- 

 ciable during the first moments of illumination. Moreover, 

 after the rate of formation and decomposition of the chlorine 

 monoxide become equal, the quantity of hydrogen chloride 

 produced in unit time will be proportional to the intensity of 

 the chemical rays. To all appearance the hypothesis affords a 

 simple and satisfactory explanation of the induction period. 



