CONDITIONS OF CHEMICAL CHANGE 79 



mercury — and the fluorescent light is analysed, it has been 

 found to consist of a number of lines, designated by Wood a 

 resonance spectrum. The line spectrum becomes a band 

 spectrum when helium is present in the vapour and at the same 

 time the proportion of light in the red to that in the green is 

 increased. The helium transforms and simultaneously degrades 

 the energy. Wood also finds that after the iodine vapour has 

 been mixed with the electro-negative gases chlorine or oxygen, 

 the degradation is so rapid that the fluorescence can no longer 

 be made manifest. Now all the gases which retard or prevent 

 the interaction of chlorine and hydrogen are likewise electro- 

 negative in character. This close coincidence would be most 

 remarkable if merely fortuitous but if, as we are disposed to 

 think, it arises from a causal connexion between the two classes 

 of phenomena, it could scarcely be disputed that it does afford 

 strong presumptive evidence in favour of the view that photo- 

 chemical inhibition results from the property possessed by the 

 inhibitor of degrading the energy essential to the progress ol 

 the chemical change. The fact (for which this communication 

 contains evidence) that the gases which behave as inhibitors 

 towards the action between chlorine and carbon monoxide are 

 also inhibitors in the case of the interaction of chlorine and 

 hydrogen, lends further support to the same view." 



A direct and obvious consequence of the views that we 

 hold on the mode in which light brings about a chemical 

 transformation and on the nature of the influence of certain 

 impurities in modifying the action of the light is that the 

 impurities in question should not diminish the rate of the same 

 chemical change when the action is promoted by merely 

 elevating the temperature and the system is in thermal 

 equilibrium with all the surrounding objects from which it can 

 receive radiant energy. To put this conclusion to the test of 

 experiment, the interaction of chlorine and carbon monoxide 

 was the most suitable and nitrosyl chloride appeared to be 

 the best inhibitor, as it is capable of almost entirely preventing 

 the photochemical action but is not destroyed by light and 

 is stable at the temperature at which the thermal change 

 proceeds with a moderate velocity. An apparatus was con- 

 structed in which a mixture of equal volumes of chlorine and 

 carbon monoxide, enclosed in a glass bulb, could be kept at a 

 constant high temperature in an electric furnace and at the 

 same time exposed to light, the velocity of combination being 

 measured in the usual manner by the rate of contraction of 

 the contained gases. With the aid of this apparatus, it was 



