370 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHE^nCAL EESEAECHES. 
Experiment 1 shows that in the first 10" after the withdrawal of hght, the Tolume of 
gas diminished 1’6 volume, in the following 10" 0’2, in the next 10" 0-1 volume, and 
that then no further diminution was perceptible. In experiment 2 the diminution of 
volume amounted in the first 10" to 1'5, in the following 10" to O'S volume, and in the 
next 10" to O’l volume, after which the gas preserv’ed its constant volume. In experi- 
ment 3 the volume diminished after the first 30" by 5‘0 volume, after the second 30" by 
I'O volume, and then the water in the tube remained stationary. This subsequent action, 
after removal of light, may arise from several causes. 
In the first place, the slow combustion of the chlorine and hydrogen produces a known 
amount of heat, and the increased degree of temperatiu’e Wo, arising therefrom, becomes 
constant when the amount of heat lost by radiation and conduction is equal to the 
amount added by combustion of the gases. The gas in the insolation-vessel is therefore 
always warmer than the surrounding medium by a temperatme (W^ — W„) where W„ 
represents that of the ah. When the insolation, and with it the supply of heat, ceases, 
the gas cools down from W^ to W,^, and the contraction ensuing from this diminution of 
temperature accounts for at least a portion of the action observed after ’withdrawal of 
the light. This subsequent contraction may, in the second place, be caused by the non- 
instantaneous absorption, by the water, of the hydirochloric acid formed by the slow 
combustion of the chlorine and hydrogen. In this case, as 'with the heat, an equilibiium 
between the amount of hydrochloric acid formed and that absorbed by the water, -will 
ensue as soon as the quantity of acid absorbed is equal to that produced, and a constant 
amount of hydirochloric acid will remain in the gaseous mixture. With the exclusion 
of light the supply of hydrochloric acid ceases, and this constant amomit of acid is 
absorbed by the water, causing a corresponding diminution in the volume of the gaseous 
mixture. Lastly, it is possible that the combination of the chlorine and hydirogen does 
not instantly cease with the exclusion of light, but that by insolation the gases are 
brought into a condition of increased affinity, which continues for a certain time in the 
dark. As these three conditions might exert a great infiuence upon the action of our 
instrument, we thought it necessary to determine then relative importance. 
The three questions to be solved were, — 
1st. "SAhat is the infiuence exerted by the slow combustion of the chlorine and hydro- 
gen upon the action of the instrument 1 
2ndly. To what extent can the hydrochloric acid formed by insolation, but not absorbed 
by the water, cause an irregularity in the indications 1 
3rdly. Is the presence of light necessary for the combination of the chlorine and 
hydrogen, or can this combination occur for a short time after exclusion of light ? 
The following considerations prepare the way for the solution of these questions. 
When a sensitive mixture of chlorine and hydrogen is exposed to the action of light, 
the temperature of the gas gradually rises up to a pomt at which it remains stationary, 
o'wing to the amount of heat absorbed being equal to that evok ed from the combustion 
of the gases. As the amount of heat evolved by the combination of the chlorine can be 
calculated from experiment, we only require to determine the amount of heat absorbed 
