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 
