July io, 1890] 



NA rURE 



249 



absorption and decomposition. It is well known also 

 that the more refrangible rays are the most active in 

 promoting the decomposition in the case of the silver 

 haloids. This was first proved for the chloride by 

 Scheele, and is now known to be true for the other haloids. 

 It would be presumption on my part in the presence of 

 Captain Abney to enlarge upon the effects of the different 

 spectral colours on these haloids, as this is a subject upon 

 which he can speak with the authority of an investigator. 

 It only remains to add that the old idea of a special 

 "actinic" force at the more refrangible end of the spec- 

 trum has long been abandoned. It is only because the 

 silver haloids absorb these particular rays that the blue 

 end of the spectrum is most active in promoting their 

 decomposition. Many other instances of photo-chemical 

 decomposition are known in which the less refrangible 

 rays are the most active, and it is possible to modify the 

 silver haloids themselves so as to make them sensitive 

 for the red end of the spectrum. 



The chemical nature of the coloured products of photo- 

 chemical decomposition is still enshrouded in mystery. 

 Beyond the fact that they contain less halogen than the 

 normal salt, we are not much in advance of the knowledge 

 bequeathed to us by Scheele in the last century. The 

 problem has been attacked by chemists again and again, 

 but its solution presents extraordinary difficulties. These 

 products are never formed — even under the most favour- 

 able conditions of division and with prolonged periods of 

 exposure— in quantities beyond what the chemist would 

 call " a mere trace." Their existence appears to be deter- 

 mined by the great excess of unaltered haloid with which 

 they are combined. Were I to give free rein to the 

 imagination, I might set up the hypothesis that the 

 element silver is really a compound body invariably 

 containing a minute percentage of some other element, 

 which resembles the compound which we now call silver in 

 all its chemical reactions, but alone is sensitive to light. I 

 offer this suggestion for the consideration of the specula- 

 tive chemist.' For the coloured product as a whole, 

 i.e. the product of photo-decomposition with its combined 

 unchanged haloid, Carey Lea has proposed the convenient 

 term " photosalt." It will avoid circumlocution if we 

 adopt this name. The photosalts have been thought at 

 various times to contain metallic silver, allotropic silver, 

 a sub-haloid, such as argentous chloride, &c., or an 

 oxyhaloid. The free metal theory is disposed of by the 

 fact that silver chloride darkens under nitric acid of 

 sufficient strength to dissolve the metal freely. The acid 

 certainly retards the formation of the photosalt, but does 

 not prevent it altogether. When once formed the photo- 

 chloride is but slowly attacked by boiling dilute nitric 

 acid, and from the dry photosalt mercury extracts no 

 silver. The assumption of the existence of an allotropic 

 form of silver insoluble in nitric acid cannot be seriously 

 maintained. The sub-haloid theory of the product may 

 be true, but it has not yet been established with that 

 precision which the chemist has a right to demand. We 

 must have analyses giving not only the percentage of 

 halogen, but also the percentage of silver, in order that it 

 may be ascertained whether the photosalt contains any- 

 thing besides metal and halogen. The same may be 

 said of the oxyhaloid theory : it may be true, but it has 

 not been demonstrated. 



The oxyhaloid theory was first suggested by Robert 

 Hunt ^ for the chloride; it was taken up by Sahler, and 

 has recently been revived by Dr. W. R. Hodgkinson. It 



' I have gone so far as to test this idea experimentally in a preliminary 

 way, the result being, as might have been anticipated, negative. Silver 

 chloride, well darkened by long exposure, was extracied with a hot saturated 

 solution of potassium chloride, and the dissolved portion, after precipitation 

 by water, compared with the ordinary chloride by exposure to light. Not 

 the slightest difference was observable either in the rate of coloration or in 

 the colours of the products. Perhaps it may be thought worth while to 

 repeat the experiment, using a method analogous to the " method of frac- 

 tionation " of Crookes. 



» " Researches on Light," 2nd ed., 1854, p. 80. 



NO. 1080, VOL. 42] 



has been thought that this theory is disposed of by the 

 fact that the chloride darkens under liquids, such as 

 hydrocarbons, which are free from oxygen. I have been 

 repeating some of these experiments with various liquids, 

 using every possible precaution to exclude oxygen and 

 moisture ; dry silver chloride heated to incipient fusion 

 has been sealed up in tubes in dry benzene, petroleum, 

 and carbon tetrachloride and exposed since March. [Tubes 

 shown.] In all cases the chloride has darkened. The 

 salt darkens, morever, in a Crookesian vacuum.^ By 

 these experiments the oxychloride theory may be scotched, 

 but it is not yet killed ; the question now presents itself, 

 whether the composition of the photosalt may not vary 

 according to the medium in which it is generated. 

 Analogy sanctions the supposition that when the haloid 

 darkens under water or other oxygen-containing liquid, 

 or even in contact with moist or dry air, that an oxychloride 

 may be formed, and enter into the composition of the 

 photosalt. The analogy is supplied by the corresponding 

 salt of copper, viz. cuprous chloride, which darkens rapidly 

 on exposure. [Design printed on flat cell filled with cuprous 

 chloride by exposure to electric light.] Wohler conjec- 

 tured that the darkened product was an oxychloride, and 

 this view receives a certain amount of indirect support 

 from these tubes [shown], in which dry cuprous chloride 

 has been sealed up in benzene and carbon tetrachloride 

 since March ; and although exposed in a southern window 

 during the whole of that time, the salt is as white as when 

 first prepared. Some cuprous chloride sealed up in water, 

 and exposed for the same time, is now almost black. 

 [Shown.] 



When silver is precipitated by reduction in a finely 

 divided state in the presence of the haloid, and the pro- 

 duct treated with acids, the excess of silver is removed 

 and coloured products are left which are somewhat 

 analogous to the photosalts proper. These coloured 

 haloids are also termed by Carey Lea photosalts because 

 they present many analogies with the coloured products 

 of photo-chemical change. Whether they are identical 

 in composition it is not yet possible to decide, as we have 

 no complete analyses. The first observations in this 

 direction were published more than thirty years ago in a 

 report by a British Association Committee,^ in which the 

 red and chocolate-coloured chlorides are distinctly de- 

 scribed. Carey Lea has since contributed largely to our 

 knowledge of these coloured haloids, and has at least 

 made it appear highly probable that they are related to 

 the products formed by the action of light. [Red photo- 

 chloride and purple photobromide and iodide shown.] 



The photographic image is impressed on a modern film 

 in an inappreciable fraction of a second, whereas the 

 photosalt requires an appreciable time for its production. 

 The image is invisible simply because of the extremely 

 minute quantity of haloid decomposed. In the present 

 state of knowledge it cannot be asserted that the material 

 composing this image is identical in composition with the 

 photosalt, for we know the composition of neither the one 

 nor the other. But they are analogous in so far as they 



1 Some dry silver chloride which Mr. Crookes has been good enough to 

 seal up for me in a high vacuum darkens on exposure quite as rapidly as the 

 dry salt in air. It soon regains its original colour when kept in the dark. It 

 behaves, in fact, just as the chloride is known to behave when sealed up in 

 chlorine, although its colour is of course much more intense after exposure 

 than is the case with the chloride in chlorine. 



" These results were arrived at in three ways. In one case hydrogen was 

 passed through silver citrate suspended m hot water, and the product e.xtracted 

 with citric acid. " The result of treating the residue with chlorhydric acid, 

 and then dissolving the silver by dilute nitric acid, was a rose-tinted chloride 

 of silver." In another experiment the dry citrate was heated in a stream of 

 hydrogen at 212° F.. and the product, which was partly soluble in water, gave 

 a brown residue, which furnished "a very pale red body on being transformed 

 by chlorhydric and nitric acids." In another experiment silver arsenite was 

 formed, this being treated with caustic soda, and the black precipitate then 

 treated successively with chlorhydric and nitric acids : " Silver is dissolved, 

 and there is left a substance .... [of] a rich chocolate or maroon, &c." 

 This on analysis was found to contain 24 per cent, of chlorine, the normal 

 chloride requiring 24*74 and the sub-chloride i4"o8 per cent. The Committee 

 which conducted these experiments consisted of Messrs. Maskelyne, Hadow 

 Hardwick, and Llewelyn. B.A. Rep., 1859, P- »°3' 



