of Hydrogen and Chlorine under the influence of Light. 265 



several weeks of constant use of the apparatus, so that the method 

 of measuring the change in temperature was all that could be 

 desired. 



For experiments on hydrogen and chlorine the mixture of gases 

 was prepared by electrolysis of pure aqueous hydrochloric acid 

 saturated in the cold with HC1 gas. This method gave a mixture 

 which yielded very constant results affording thus a sufficient 

 guarantee of the purity of the mixture. 



The results to which I desire here to call attention are in 

 relation to the connection between the initial expansion, the rise 

 in temperature of the mixture associated with the expansion, and 

 the amount of hydrochloric acid formed when the pure mixture is 

 submitted to the action of light. 



Pringsheim states that the initial expansion for a given quantity 

 of light is independent of the state of the induction, that is, inde- 

 pendent of the previous history of the gas mixture in respect of 

 exposure to light and consequent combination. My experiments 

 however have shewn that the expansion depends in amount on the 

 extent to which the gas mixture has been insolated previously to 

 the illumination producing the expansion. Bunsen and Roscoe 

 found that after the gases had been combining under the influence 

 of light, if the mixture were darkened for a short time and again 

 illuminated the rate of combination attained its maximum sooner 

 than if the gas had remained in the dark for a long time before 

 illumination. Some effect then of the previous illumination 

 remained after darkening the mixture. It was not till about 

 half-an-hour of darkening that all of this after-effect passed off. 

 A similar effect was found from my experiments on the initial 

 expansion. The expansion was found to be greater if there had 

 been previous illumination than if the mixture had remained in 

 the dark for a long time. 



With regard to the relations between the initial expansion, 

 hydrochloric acid formed, and the rise in temperature of the 

 mixture, three series of experiments were made investigating the 

 expansion and quantity of hydrochloric acid formed, the expansion 

 and the rise in temperature, and the amount of hydrochloric acid 

 formed. It was found that the initial expansion was always 

 followed by a contraction to a volume less than the original volume, 

 shewing the formation, and subsequent absorption by the water 

 in the instrument, of hydrochloric acid. This was found to be the 

 case with illuminations of different intensities and durations. 

 With constant intensity of illumination the ratio of the expansion 

 to the hydrochloric acid formed measured by the final contraction 

 from the original volume increased with decreasing duration of 

 illumination ; but the value of this ratio was never greater than 5, 

 the illumination for this case being by means of a single electric 



VOL. XI. PT. iv. 20 



