PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 371 
during the same space of time in order to obtain the value of (W^— W„), or the differ- 
ence of temperature between the gas and the surrounding medium. If the volume of 
gas burnt in the unit of time immediately before exclusion of light, measured in cubic 
centimetres at the temperature W,, and pressure P, be called R, the weight in grammes 
of the chlorine combined with hydrogen in the unit of time is found from the formula 
Si P R 
773 " 0-76 (1 + 0-00366 Wo) 2~^’ 
where Si is the specific gravity of chlorine. If, therefore, the amount of heat evolved by 
the combustion of 1 grm. of chlorine with hydrogen, measured by the rise of temperature 
of 1 grm. of water, be called C, the amount of heat evolved from combustion of the 
chlorine in a unit of time expressed in units of heat is AC. 
The quantity of heat absorbed can be calculated when the cooling-velocity of the 
insolation-vessel for a degree Centigrade is known. In order to determine this value, 
Avhich we may call a, we exchanged our insolation-vessel for another of the same dimen- 
sions, in the interior of which a fine platinum wire was fixed by fusion through the glass. 
AMien a weak current of electricity was passed through the wire, the sensitive gas being 
protected from the action of the light, an immediate expansion by heat occurred, which, 
as soon as the current was interrupted, gave place to a corresponding contraction. As 
the weight of the insolation-vessel was at least 1000 times greater than that of the gas 
contained in it, the amount of heat communicated from the gas to the vessel may be 
regarded as infinitely small. 
The observations for the determination of a were thus conducted. First, the con- 
stant position of the water in the tube was noted; then, after the gas had been 
expanded to the position by means of the current passing through the platinum wire, 
the electricity was cut off and the time observed, by means of a pendulum vibrating 
half seconds, during which the gas diminished in volume from to Wj. According to 
this method of observation, it was only necessary to determine the interval of time during 
which the volume of gas diminished a constant amount; hence results of sufficient 
accuracy were obtained, notwithstanding the rapidity at which the gas cooled down. 
The determination gave the following numbers : — 
Series of Experiments VIII. 
Exp. 1. 
Exp. 2. 
Exp. 3. 
Exp. 4. 
Exp. 5. 
^0 
320 
353 
427 
420 
427 

310 
340 
420 
410 
420 
W', 
300 
330 
410 
400 
410 
Time fQ— q in which 1 
1"-1 
the change to 
occurred J 
0"-9 
1"-1 
l"-2 
1"-1 
The figures in the first three horizontal divisions may be supposed to represent tempera- 
tures measured on a relative but arbitrary scale. 
3 c 2 
