264 Mr Bevan , Some Phenomena connected with the Combination 
On some Phenomena connected with the Combination of Hydrogen 
and Chlorine under the influence of Light. By P. V. Bevan, B.A., 
Trinity College. 
\Read 28 October 1901.] 
The present note is to give an account of the results of 
experiments on the initial expansion observed on illuminating 
a mixture of Hydrogen and Chlorine when the gases are damp. 
This expansion was discovered by Draper 1 and studied more 
carefully by Pringsheim 2 . Pringsheim concluded that the ex- 
pansion took place before any combination to form HC1 occurred, 
and he attributed the expansion to dissociation of Hydrogen 
and Chlorine molecules giving rise to a larger number of systems 
in the gas mixture than before illumination. 
The apparatus used for the experiments from which the results 
in this note were obtained was essentially that of Bunsen and 
Roscoe 3 . The gas mixture to be illuminated is confined in a glass 
bulb about one-eighth filled with water saturated with chlorine. 
The bulb communicates with a capillary tube in which a water 
index defines the volume of the gas mixture in the bulb. 
The bulbs in these experiments were considerably larger than 
those used by Bunsen and Roscoe, admitting of more accurate 
observation of small percentage changes of volume. 
In the insolation bulb is placed a fine platinum wire (‘001 inch) 
with its ends sealed through the bulb. By observing the resistance 
of this wire the change in temperature of the gas mixture can be 
determined when combination takes place, or when the initial 
expansion takes place. This apparatus was well suited for ob- 
serving the expansion of chlorine alone under the influence of light. 
This expansion was found to be associated with a rise in tempera- 
ture equal, within the limits of accuracy of the experiment, to that 
required to produce the expansion under constant pressure. The 
change of resistance of the platinum wire could be observed as 
accurately as desired with a sensitive galvanometer. It was found 
that the chlorine had no sensible action on the platinum even after 
1 Phil. Mag. xxiii. 1843, p. 415. 
a Wied. Ann. 1887, xxxn. p. 384. 
3 Phil. Trans. 1857, p. 359. 
