CONDITIONS OF CHEMICAL CHANGE 



661 



is in the indigo but towards the blue. The following table 

 shows the activity assigned by him to the different rays : 



Too much importance, however, must not be attached to 

 these numbers, since it is now known that the results ob- 

 tained in this way would vary with the character of the 

 sunlight at the time when the experiment was made, the 

 mode adopted for dispersing the light and the length of 

 the mixture of gases traversed by the rays, the maximum of 

 action as the column of gas is lengthened appearing to move 

 towards the end of the spectrum at which the rays are least 

 absorbed by chlorine. Numbers of any precise scientific im- 

 port concerning the influence of the character of the light are 

 still wanting. 



Various views relating to the mode in which the light 

 brings about the change in the system have been expressed. 

 These have for the most part been elaborated with a view 

 to explaining the nature of the changes in the mixture during 

 the induction period. Draper, as we have already stated, was 

 the first to establish the reality of this phenomenon. He 

 sought further, by direct experiment, to discover its cause. 

 The facts as stated by him are these : 



" When a mixture of chlorine and hydrogen is exposed in 

 a tithonometer to the rays of an Argand lamp, with due 

 precautions to ensure a correct result, four distinct periods of 

 action may be traced : 



" 1 st. For a brief space the mixture expands. 



" 2nd. For a much longer period it then remains wholly 

 stationary, neither expanding nor contracting, though the rays 

 are constantly falling on it and it is absorbing them. 



" 3rd. Contraction arising from the production and solution 

 of muriatic acid begins, commencing at first slowly and then 

 more and more rapidly. 



" 4th. And, after that contraction has fairly set in, it goes 

 on with uniformity, equal quantities of muriatic acid being 

 produced in equal times by the action of equal quantities of 

 rays." 



The change effected in the properties of the mixture, 



