1903.] Combination of Hydrogen and Chlorine by LljJit. 



" The Combination of Hydrogen and Chlorine under the Influence 

 of Light." By P. V. BEVAN, Trinity College, Cambridge. 

 Communicated by Professor J. J. THOMSON, F.R.S. Received 

 April 1, Read May 14, 1903. 



(Abstract.) 



The present investigation was undertaken primarily to study the 

 initial expansion observed when light is allowed to fall on a mixture of 

 hydrogen and chlorine. This expansion was first noticed by Draper 

 and studied more carefully by Pringsheim. The latter writer attri- 

 buted the expansion to a dissociation of H 2 and C1 2 molecules giving 

 rise to a larger number of systems in the gas mixture than before 

 illumination. For this part of the investigation the apparatus in- 

 vented by Bunsen and Roscoe for determining the actinic properties of 

 light was used. Bulbs considerably larger that Bunsen and Roscoe's 

 were employed, admitting of more accurate determinations of small 

 changes of volume. The expansion was found to be due to a rise in 

 temperature, caused by the combination of hydrogen and chlorine to 

 form hydrochloric acid. This rise in temperature was measured by the 

 change of resistance of a fine platinum wire, sealed through the bulb, 

 in which the gas mixture was exposed to light. The rise in tempera- 

 ture in all cases fully accounted for the initial expansion, and the rise 

 in temperature was itself fully accounted for by the heat of formation 

 of the hydrochloric acid produced. The initial expansion is thus 

 shown to be only a side effect in the general case of the induction. 

 The investigation then considers the period of induction. The action 

 is shown to stop almost instantaneously on cutting off the light, so 

 that combination only goes on while the light is continuously acting. 

 The induction period or time in which the velocity of action has not 

 reached its maximum value can be prolonged indefinitely, but its 

 general character remains the same. The combination is made much 

 more rapid by the presence of water vapour, and it seems probable 

 that were the gases perfectly dry no action would take place. 



If chlorine be first of all exposed to light, and then mixed with its 

 own volume of hydrogen, the mixture shows a greater readiness to 

 combine than if the chlorine had not been previously illuminated. 

 This property is lost if the chlorine be bubbled through water after the 

 preliminary illumination. Previous illumination of the hydrogen 

 <done is without effect. The first step in the process of combination 

 is thus an action between chlorine and water vapour or an action on 

 chlorine alone. Some evidence as to the formation of an intermediate 

 body is afforded by the production of a nucleus-forming substance in 

 chlorine alone, and in the mixture of hydrogen and chlorine on which 



