24 LECTURES TO SCIENCE TE AGREES. 



it a final wash with silver nitrate, and allow it to darken 

 in white light, and then expose it to the spectrum, or 

 "beneath red, green, yellow, and blue glasses. In this 

 case we shall find the silver reddens in the red light, becomes 

 greenish in the green, and blue in the blue. I have here a 

 plate that has been so treated and exposed to the different 

 coloured lights, and you can note the colours though they 

 are somewhat spoilt by subsequent experiments. Mark the 

 difference between the two plates : one had a chloride and 

 sub-chloride of silver alone, the other had chloride, sub- 

 chloride, and a chlorine absorbent present. 



We may class the result obtained on the silver chloride on 

 paper arid that on the last coloured plate as identical, both 

 being exposed under the same conditions. Leaving out, for 

 the present, the theory of the production of the colour, let us 

 examine the fact that a change has been effected by the rays 

 of low refrangibility. We have silver chloride and sub-, 

 chloride together in close contact on the surface, and I think 

 we may take it that we have the case of a loaded molecule, 

 whose swing is in discord with the longer waves ; that such a 

 discord causes an atom of chlorine to be thrown off (as before 

 explained), and that the atom so separated is from the portion 

 of the compound molecule which by itself would be the sub- 

 chloride. As a result of the impact of the light we should 

 have remaining metallic silver (from the sub-chloride) and 

 unaltered silver chloride. The amount of sub-chloride formed 

 by the preliminary exposure would be small, hence the total 

 amount of reduced silver would be little in comparison with 

 that which would be due to the reduction of the chloride to 

 the sub-chloride by the more refrangible portion of the 

 spectrum. The well-known reversal of the lines of the red 

 end when photographed on iodide or bromide of silver, to 

 which a slight preliminary exposure has been given, can 

 be accounted for in the same way, on the supposition that 

 the silver reduced from the sub-iodide or bromide is not so 

 actively attractive as the sub-iodide or sub-bromide itself. 

 The fact that such reversed photographs are always more or 

 less veiled is rather confirmatory of this view. 



With the plate treated with copper chloride alone, and no sub- 

 sequent addition of silver nitrate and preliminary exposure, 

 the same line of argument still holds good. Part of the sub- 

 chloride is reduced to silver, and chlorine is evolved, the latter 



