AND CHLORINE UNDER THE INFl.UENCE OF LIGHT. 
75 
In 1897 Gautiee, and Helier"^ investigated the action of light on dry chlorine and 
hydrogen. They noticed that the gas they prepared, by electrolysis of aqueous 
hydrochloric acid, did not consist of a mixture of the pure gases, but that there was 
present in addition oxygen, free, and combined with chlorine. This has also been 
shown by several observers; but the proof of the presence of oxygen when aqueous 
acid saturated with hydrochloric acid gas is used is not complete. MellorI con¬ 
cluded that the lower chlorine oxides formed in the process of electrolysis are all 
removed by washing, but that some free oxygen remains in the gas mixture. It 
remains, however, somewhat doubtful whether the oxygen found by Mellor was not 
produced by decomposition of water vapour by chlorine, under the influence of light. 
In his method hydrochloric acid was electrolysed and the gases continually acted on 
by light in the upper part of the vessel used for the electrolysis, so that recombi¬ 
nation took place as rapidly as the gases were formed. In this way a residue of 
oxygen was found in the electrolysis. But it seems more reasonable to attribute 
the formation of this oxygen to the decomposition of water vapour by the chlorine 
under the intense light used. 
Because of this supposed impurity, Gautier and Helier used hydrogen and 
chlorine prepared separately. But the difficulties pointed out by Bunsen and 
Roscoe, in obtaining molecular quantities of the gases, introduce very much more 
uncertainty in the results, when this method is used, than occurs with the exceedingly 
small quantity of oxygen, if any, evolved in the electrolysis. 
The method of Gautier and Helier was different from that of Bunsen and 
Roscoe and Pringsheim, in that in their experiments the hydrochloric acid formed 
remained in the gas mixture, and so modified the action. The period of induction 
was not studied by them, and in their hands the problem was simply one of mass 
action. 
With the dried gases they find that the rate of action is not proportional to the 
density of the free hydrogen and chlorine. If M is the quantity of hydrochloric acid 
at time t, D the density of hydrogen and chlorine, then, if the rate of combination 
were proportional to the density of hydrogen, we should have 
— = cD : c beinp; a constant. 
t ^ 
From their experiments, Gautier and Helier deduce the following values for C :— 
For the 1st period, 2’5 hours . . . C = ’0217, 
„ 2nd „ 3 „ . . . C = -0310, 
„ 3rd „ 41 „ . . . C = -0130, 
„ 4th „ 191 „ . . . C = -0070. 
* Gautier and Helier, ‘Comptes Rendus,’ vol. 124, p. 1128. 
t Mellor, ‘ Journal of the Chem. Soc.,’ Feb., 1901, 
L 2 
