CONDITIONS OF CHEMICAL CHANGE 663 



Bunsen and Roscoe {Phil. Trans. 1857, 147 > 88l )» on the 

 other hand, regard chemical affinity as an inherent property 

 of matter which is as unalterable as the mass of the atoms 

 themselves. To speak, therefore, of the exaltation of the 

 elective affinity of an element is, according to them, a badly 

 chosen form of speech. The circumstance that chlorine and 

 hydrogen can be intimately mixed without acting upon one 

 another is to be explained by there being in that case a 

 resistance which opposes the force of affinity. The removal 

 of this resistance can be effected by various agencies, amongst 

 which light must be included. They say : " The act by which 

 resistance to combination is diminished and the combining 

 power thus brought into greater activity, we call chemical in- 

 duction ; and we specify this as photo-chemical, thermo- 

 chemical, electro-chemical and idio-chemical, according as light, 

 heat, electricity or purely chemical action is the active agent 

 concerned in overcoming the resistance." 



When a freshly prepared mixture of chlorine and hydrogen 

 is exposed to light, energy is expended in overcoming the 

 resistance to combination ; it is only after the induction has 

 reached a definite stage that a visible formation of hydrogen 

 chloride accompanies the illumination. The change therefore 

 is only in the properties of the mixture. They were unable 

 to confirm Draper's statement that chlorine which has been 

 exposed to light and chlorine which has been made and kept 

 in the dark differ in their behaviour towards hydrogen; they 

 therefore discarded the hypothesis that the first action of the 

 light was to produce an allotropic modification of chlorine. 

 They further found that a mixture which had been illuminated 

 sufficiently to carry it through its induction period returned 

 slowly to its original condition if kept in the dark, i.e. the 

 inductive effect was gradually and spontaneously lost. 



An entire paper {Phil. Trans. 1857, 147, 601) is devoted to 

 answering a question of great theoretical significance, namely : 



" In the combination effected by the light between the 

 particles of chlorine and hydrogen, are the chemical rays ex- 

 pended in a relation proportional to the hydrochloric acid 

 formed ? " 



Bunsen and Roscoe were led by a series of indirect measure- 

 ments to conclude that a mixture of chlorine with an equal 

 volume of an inert gas such as nitrogen did not absorb so much 



