266 Mr Bevan, Some Phenomena connected, etc. 



spark. With constant duration the ratio increased, with increase 

 of intensity of illumination. 



The expansion was also always accompanied by a rise in 

 temperature as indicated by the increase in the resistance of the 

 platinum wire in the insolation bulb. This rise in temperature 

 was always sufficient to account for the expansion. And finally 

 the experiments on the rise in temperature and the hydrochloric 

 acid formed shewed that for very short illuminations the rise in 

 temperature was proportional to the hydrochloric acid formed, and 

 further that the heat of formation of the hydrochloric acid was 

 always rather more than that required to produce the observed 

 rise in temperature of the whole gas mixture, and therefore that 

 the heat of formation of the hydrochloric acid formed is more than 

 sufficient to account completely for the initial expansion. 



The final conclusion is therefore that the initial expansion 

 occurs only when combination of the hydrogen and chlorine takes 

 place, and is due to the heat which is liberated by this combination. 



Other experiments shewed that the contraction which occurs 

 after the illumination of the gases is stopped is due to the gas 

 mixture cooling to the temperature of the surrounding medium 

 after being heated by the combination, so that the action ceases at 

 once on cutting off the light. 



In conclusion I wish to thank Prof. J. J. Thomson for having 

 suggested the investigation and for his advice on the experiments 

 as far as they have as yet been carried. 



