CONDITIONS OF CHEMICAL CHANGE 81 



points to the conclusion that a certain small proportion ol 

 the effective vibrational energy is not modified by the oxygen 

 molecules. For lower concentrations of oxygen the relation 

 found to hold for mixtures of chlorine, hydrogen and variable 

 small amounts of oxygen is approximated to. It would appear 

 that, as a first approximation, the sensitiveness of a mixture of 

 chlorine and carbon monoxide (and possibly also of a mixture 

 of chlorine and hydrogen) containing oxygen at different partial 

 pressures is given by the formula S = A + B/[_0], in which 

 A and B are constants and S and [0] are the sensitiveness and 

 concentration of oxygen respectively. If [O] be small, S is 

 so large that in comparison A becomes negligible and the 



relation S = j) holds within the limits of experimental error; 



but if [O] be large, S becomes almost equal to A, Further 

 experiments on the retardation of the photochemical interaction 

 of chlorine and hydrogen by oxygen are now in progress and 

 an attempt is being made to prepare chlorine and hydrogen 

 uncontaminated with oxygen. 



It will be seen that some advance has been made in eluci- 

 dating the nature of the process which takes place when 

 chlorine and hydrogen or carbon monoxide interact under the 

 influence of light ; but what is of equal importance is the fact 

 that we are now in possession of sufficient information to 

 enable us to investigate, with reasonable hope of obtaining 

 results in which confidence can be placed, the important 

 question of the effect of the concentration of the interacting 

 substances on the rate of the chemical process. 



A number of investigations on the displacement of equilibria 

 by the agency of light have been carried out during recent 

 years. A description of these has been omitted purposely from 

 the present article, our knowledge of the quantitative laws of 

 photochemistry, in the opinion of the writer, being at present 

 too vague and inexact to admit of the results of these re- 

 searches being discussed profitably. 



In conclusion reference may be made to some of the effects 

 of ultraviolet light and the cathode rays. Ultraviolet light is 

 a much more efficient agent in promoting chemical changes 

 than visible light. Under its influence chemical transforma- 

 tions will proceed in colourless gases at an appreciable rate. 

 Light of short wave length owes its high efficiency to two 

 6 



