CONDITIONS OF CHEMICAL CHANGE 71 



which was slowly decomposed at the ordinary temperature in 

 the dark and gave rise to the formation of nitrogen chloride. 

 In order to test this explanation an actinometer was con- 

 structed which could be charged and heated at a little over ioo°. 

 When the heating had been continued during about twelve 

 hours the actinometer was allowed to cool and exposed to 

 light. Interaction at once set in and even after keeping the 

 actinometer in the dark during several weeks the photo- 

 chemical change was not preceded by a preliminary inert 

 period. The heating had destroyed the nitrogen chloride and 

 other substances from which nitrogen chloride could be de- 

 veloped by the action of chlorine. It was subsequently found 

 that inhibitors are formed slowly when chlorine acts on water 

 containing albumen. 



The so-called induction period is therefore caused by the 

 presence in the gas of a powerful inhibitive impurity — nitrogen 

 chloride — which must be almost completely removed from the 

 gases before the chlorine and hydrogen can interact. 



The facts detailed above were discovered and published early 

 in 1905 in the Proceedings of the Royal Society and in the 

 Manchester Memoirs. 



A year later it was suggested by Luther and Goldberg 1 — in a 

 paper in which our work was mentioned but curiously enough 

 not contested — that induction is essentially due to the contamina- 

 tion of the mixture of hydrogen and chlorine with oxygen. Any 

 one who peruses the papers of Bunsen and Roscoe, Bevan, 

 Mellor or those of the author and his collaborators will per- 

 ceive that such a suggestion cannot possibly be entertained. 

 Oxygen is not removed from a mixture of chlorine and hydro- 

 gen on exposure of the latter to light ; if it be, the rate of 

 removal is so slow that the effect of its disappearance cannot 

 be detected by measurements of the velocity with which chlorine 

 and hydrogen interact. 2 



The most remarkable feature of the inhibitory influence 

 of nitrogen chloride is the enormous effect produced by an 



1 Zeitschr. Phys. Chem. 1906, 56, 43. 



2 I take this opportunity of proclaiming the untenability of Luther and Gold- 

 berg's suggestion, since the views of these authors on this question have been 

 accorded a prominent place in the new edition of Nernst's Theoretical Chemistry 

 and may through that source find their way into other text-books, dealing with 

 the subject of photochemistry. 



