Sept. 12, 1878] 



NATURE 



529 



which has been explored by others in relation to a dif- 

 ferent subject ; but this may be stated as a matter of 

 observation that some compounds of silver which are 

 sensitive to light are capable of forming two molecular 

 groups, one of which absorbs the blue rays, and the other 

 the red rays. 



The iodide and bromide are the salts of silver which, 

 either separately or together, are chiefly employed for 

 securing a developable photographic image; and it is 

 these with which we shall principally deal, though the 

 chloride and one or two other combinations will come 

 under review when considering certain new phases in 

 photography. The point to which attention must now 

 be directed are the radiations to Avhich these compounds 

 are sensitive ; and these are evidently dependent upon 

 the absorption that takes place in them individually. If we 

 take precipitated silver bromide and fuse it into a crystal- 

 line mass, and examine it spectroscopically, we find that 

 it energetically absorbs all rays from the extreme violet 

 to the green, and also less markedly in the yellow of the 

 spectrum ; whilst if Ave place a slab of it before the slit of the 

 spectroscope, and photograph the spectrum of white light 

 passing through it, we find that it completely cuts off the 

 altra-violet rays; so that we may say that the red and 



perhaps the ultra-red rays are the parts of the spectrum 

 in which but slight absorption takes place. Now, since 

 absorption means work done in the absorbing body, it is 

 evident that we may expect some action to take place in 

 the silver bromide when exposed to these rays ; and the 

 action may be a chemical change, or a rise in tempera- 

 ture, it being remembered that the latter may co-exist 

 with the former, since it may be produced by the result 

 of chemical action as well as by the absorption of the 

 radiation. The question then arises, On Avhat does the 

 possibility of a chemical change in a compound depend ? 

 This is a question which is very easily asked but not so 

 easily answered. It must evidently depend amongst other 

 things on the capability of the molecules of the compound 

 to throw off some atom or atoms ; or on their capability 

 of acquiring some vibrating atom or atoms of a body with 

 which they may be brought in contact ; in other words, 

 that the molecule shall be in a state verging on indifferent 

 equilibrium, and seeking rearrangement of the atoms 

 when the impulses of the waves forming the radiations 

 impinge against it. Taking for granted that the chemical 

 theory of the formation of the photographic image holds 

 good, we know that the atom of the halogen is thrown off 

 from the molecule of the silver compound, leaving 



SOLAR SPrCTRUM, 



Fig, 



behind two "atoms of the metal combined with one atom 

 of the halogen. Thus silver bromide (Ag^Brg) is split up 

 into argentous bromide (AgjBr) and bromine. If a film 

 of the yellow sensitive salt be exposed to the spectrum 

 an image is developed on all the parts which it absorbs, in 

 other words, absorption in this case means chemical action. 

 Similarly with the iodide where the absorption of radiation 

 is most energetic there we have a chemical change. With 

 the chloride we have a different phenomenon ; crystalline 

 chloride is nearly colourless, and the absorption of every 

 part (though not necessarily of the invisible regions) of 

 the spectrum must be nearly equal, but we find that this 

 salt is sensitive only to about the same radiations, under 

 ordinary conditions, as the iodide. An explanation of 

 this will be offered at a more advanced stage of these 

 articles, but it may be as well to note the fact now. In 

 the accompanying diagram the ordinary results of the 

 exposure of the three chief silver compounds to the spec- 

 trum are shown (Fig. 2). 



When we obtain a molecular grouping in which the 

 red is absorbed, we may expect that the same che- 

 mical change may take place as when in the former 

 state. This is not a necessity, since, instead of a reduc- 

 tion to a less complex form of molecule, the new com- 

 pound, formed by the aid of light, might be rendered 



more complex by annexing other foreign atoms. An 

 example of this we have in Hunt's experiment, already 

 quoted ; where it is evident that the violet subchloride 

 would be sensitive to light if it were proved that light alone 

 caused the absorption of oxygen. Again, in the combina- 

 tion of chlorine and hydrogen, when exposed to light, we 

 probably have a greater complexity of molecular struc- 

 ture introduced. 



In the ordinary state in which silver bromide is formed 

 in a collodion film (we will take this compound as it is 

 this one which has been principally examined), we have 

 it in the state in which the blue is absorbed and the red 

 transmitted ; but there are means by which it may be 

 made to absorb the red. We may obtain the bromide in 

 either state by Avhat is known as the " emulsion process." 

 Briefly, this process may be described as one in which 

 an emulsion of silver bromide is formed in a viscous 

 liquid, such as collodion or a solution of gelatine, by first 

 dissolving a. soluble bromide in it and then adding to 

 it a solution of silver nitrate. The particles of solid 

 bromide thus formed remain suspended in the fluid. 

 In the emulsion with collodion Ave can get the silver 

 bromide in various states according to the rapidity 

 Avith Avhich Ave add the silver nitrate. Thus if we 

 add it rapidly, Ave get a mass of silver bromide which 



