not an Oxy chloride. 359 



formation of subchloride. As the product is subchloride and 

 not oxychloride it is not necessary that the substances present 

 should contain oxygen, as has just been shown. The sub- 

 chloride thus formed instantly combines with a portion of the 

 silver chloride as yet unacted upon by light, forming a photo- 

 chloride of great stability, capable for a time of resisting the 

 action of nitric acid (I have shown that the photochloride made 

 by purely chemical means also shows this stability.) 



This combination is not by equivalents but it is of the nature 

 of a lake, and the affinity of silver chloride for the subsalt is 

 of a progressively diminishing character. Small quantities of 

 subchloride are held with great tenacity ; as the proportion of 

 subchloride increases the affinity diminishes. This is no as- 

 sumption, it is easy to form chemical photochloride containing 

 a large proportion of subchloride. Much of this latter is in- 

 stantly decomposed by cold nitric acid ; with heat, an addi- 

 tional quantity disappears and so on until the last portions may 

 require hours of boiling with strong acid, for decomposition. 



This stable combination of the chloride and subchloride con- 

 stitutes alike the material of darkened chloride, of the latent 

 image, and of the photochloride. 



An excellent mode of testing the value of a chemical theory 

 is to observe its ability to explain not only the general result 

 of a reaction, but also the secondary facts observable. In the 

 present matter, the action of light on silver chloride, there are 

 two such secondary facts of a quite remarkable nature, for 

 which, though long familiarly known, no explanation has 

 hitherto been found, but which, I think, will be found to be 

 readily and satisfactorily explained by the photochloride 

 theory. 



1. When silver chloride is exposed to light, there is a certain 

 pause, an interval during which very little action takes place. 

 After this, the darkening sets in rapidly. This fact is so con- 

 spicuous as to attract the attention of everyone who exposes 

 chloride j)aper. 



The explanation is : light, pink or violet photochloride is 

 vastly more sensitive to light than white chloride, a fact easily 

 proved by preparing light colored photochloride by any of 

 the chemical means I have elsewhere described, brushing it 

 and white chloride, each in a pasty condition, over respective 

 strips of paper and exposing side by side ; the difference in 

 the darkening is very striking. — Now the first action of light 

 on white chloride is to form this light-colored photochloride, 

 and whilst that is going on but little visible effect is produced. 

 As soon as the photochloride is once formed, the darkening 

 becomes rapid. The time required for the first formation of 

 the photochloride gives rise to the pause which is observable. 



