356 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHE^nCAU EESEAECHES. 
of a greater or less amount according as the volumes of the solvent and the free and 
absorbed gases increase or diminish. In the method employed by Deapee for the pre- 
paration of his sensitive gas, the conditions necessary for obtaining this relation, are 
not even approximately attained, so that from this cause alone, an exact agreement 
between the indications of the Tithonometer cannot be possible. 
Another source of more considerable error is to be found in the alterations of pressure 
to which the liquid and the gaseous mixture are subjected during the whole course of 
the observations. This variation of pressure causes a certain quantity of gas to escape 
from solution in the liquid, and as the atmosphere in the liquid has a different com- 
position from that above it, the relation between the chlorine and hydrogen vrill be 
altered, and all regularity in the indications of the instrument impaired. 
As we were unable, from the foregoing reasons, to employ the mode of observation 
used by Deapee, we were compelled to seek for some new method, fr-ee from the eiTors 
above alluded to. 
With this object we have been obliged to examine all the cfrcumstances which could 
possibly affect the combining power of chlorine and hydrogen, in a series of experiments 
which have extended over a long space of time, and in which difficulties of no ordinary 
kind have had to be overcome. For this purpose we required a mixture composed of 
equal volumes of chemically pure chlorine and hydrogen gases. Amongst the various 
methods which presented themselves for the preparation of such a mixture, that by 
electrolysis of aqueous hydrochloric acid appeared at first sight the least hkely to give the 
wished-for result, as Deapee especially mentions in his research that the electrolysis of 
this acid, however long continued, never yields equal volumes of chlorine and hydrogen. 
A series of careful experiments has however proved, most distinctly, that Deapee’s state- 
ment is erroneous ; and we have found that the electrolytic chlorine and hydrogen mix- 
ture not only consists of exactly equal volumes of chlorine and hydrogen, but that it is 
perfectly free from oxygen and all oxides of chlorine which might have been formed by 
secondary electrolytic action. 
As the possibility of executing accurate photo-chemical determinations entfrely depends 
upon this purity of the gas, we deem it necessary here to detail the experiments made 
upon this subject. 
When a current from three or four common-sized carbon-zinc elements is passed, by 
means of carbon-poles, through hydrochloric acid of about specific gravity 1T48, all the 
chlorine set free at the anode is first absorbed by the liquid. After a short time the 
evolution of chlorine becomes visible, and gradually increases, whilst the colour of the 
liquid becomes deeper, until a point is attained, more or less quicldy according to 
the intensity of the current and the quantity of hydrochloric acid employed, at which 
a statical equilibrium between the free gases and those absorbed in the hquid ensues. 
From this point forwards the composition of the free and absorbed gases remams con- 
stant, provided that the temperature and pressure do not vary, and the amount of hydro- 
