266 Mr Sevan, Some Phenomena connected, etc. 
spark. With constant duration the ratio increased, with increase 
of intensity of illumination. 
The expansion was also alwaj^s 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. 
