88 
MR. P. Y. BEVAN ON THE COMBINATION OF HYDROGEN 
Expansion. 
HCl formed. 
Ratio. 
Expansion. 
HCl formed. 
Ratio. 
1 
* 2 
5 
3-5 
1-7 
2 
1 
•3 
3 
4 
2 
2 
1-2 
• 2 
6 
4 
1 -5 
2-6 
1-3 
' 2 
6-5 
•5 
3 
1-7 
1-5 
•3 
5 
' 5-2 
1-6 
3 
1-5 
•6 
2-5 
6-5 
2 
3-2 
1-5 
• 2 
7-5 
8 
3 
2-7 
1-5 
•4 
4 
8 
4 
2 
3-5 
1 
3-.5 
12 
4 
3 
As the index even with an instantaneous illumination takes a finite time to move, 
all the heat liberated by the combination of the hydrogen and chlorine will not 
contribute to expanding the gas, but some will be dissipated in the walls of the 
insolation vessel and the water contained in it, so that the larger the amount of 
hydrochloric acid formed with instantaneous illuminations of different intensities, the 
smaller will be the ratio of expansion to contraction. This we find to be the case in 
the above ex^^eriments. The ratio tabulated diminishes as the contraction repre¬ 
senting the hydrochloric acid formed increases. Thus, however small the initial 
expansion, we can conclude that there is always some hydrochloric acid formed. 
This conclusion was arrived at also by Mellor (‘ Chemical Society’s Transactions,’ 
1902). 
§ 3. The Temperature Change Associated with the Initicd Expansion. 
The next series of experiments were made to determine directly the change of 
temperature in the mixture of gases, when light falls on it, during the period of 
expansion. The method employed was to find the change in the resistance of a 
platinum wire in the gas mixture during illumination. A large insolation bulb (about 
8 centims. in diameter) was used for this purpose. A piece of platinum wire, '001 inch 
in diameter and about 9 centims. long, Avas welded at each end to a stout wire of the 
same metal, and these stout Avires Avere sealed into the bulb at opposite sides, so that 
the fine AAure hung loosely in the insolation bulb. The Avire Avas then connected Avith 
a Post Office resistance box in the ordinary Wheatstone bridge arrangement. A 
differential galvanometer Avas used, AA^iich could be easily made as sensitiAm as aa'us 
required. The temperature coefficient of the Avire Avas found in situ, and agreed A^ery 
nearly AAuth tliat of another specimen of the same Avire. The gaharnometer Avas so 
adjusted that the change of resistance of the Avire in the insolation bulb AA-as 
determined by reading the displacement of the spcit of light on the galAarnometer 
scale. The resistance of the AAore at 0° C. AAms 20'650 ohms, and a change of 
resistance of 1 ohm corresponded to a change of temperature of 23° C. 
It Avas found that on illumination of the ffas mixture the resistance of the AA’ire 
O 
increased, indicating a rise i)i temperature of the mixture. 
