ON THE PHOTOGRAPHIC IMAGE. 107 



these several tests with the dark body formed by the photochemical decom- 

 position of the chloride of silver, or of this body mixed with excess of nitrate ; 

 for we find that these reactions are in the several cases identical. The light- 

 darkened chloride indeed presents a deeper and bluer colour than that formed 

 artificially ; but when it is considered that the light- formed body is a coating 

 of uniformly and completely transformed substance — superficial it is true, but 

 continuous in its surface — while the laboratory product is an intimate mix- 

 ture of discontinuous particles, the bluer tint of the one and the redder tint 

 of the other will hardly carry much weight in deciding against the identity 

 of the colorific silver-compound in each case. Nor will it perhaps be con- 

 sidered to support the view of the photochemical reduction consisting in the 

 complete severance of the metallic silver, that the product of that reduction 

 can be formed by the light in the presence of nitric acid. The production 

 .of an allotropic form of silver in the nascent state, in the presence of nitric 

 acid, seems certainly to make a larger demand on the credulity of the che- 

 mist than the assertion that the reduction stops at an intermediate stage, at 

 which a subchloride is the result of it — a subchloride, whose properties we 

 have seen to be identical with those of a substance formed in the laboratory, 

 and to which it is difficult to assign any other composition than that of a 

 subchloride of silver. 



In the photographic processes in which the chloride of silver is employed, it 

 is to be borne in mind that the chloride of silver is not used by itself — nay, by 

 itself is quite inadequate to the production of the deep colour requisite for 

 photographic effects. It is used in fact always in conjunction with nitrate of 

 silver, and also, it must be added, with organic substances, among which the 

 cellulose of the paper and the glue-like size are prominent. The action of 

 the nitrate of silver needs little explanation ; it supplies continually a fresh 

 surface of chloride of silver, formed by part of the chlorine given off from the 

 surface of the original chloride, which unites at once with the silver of the 

 nitrates, and simultaneously becomes blackened by the action of the light. 

 It is singular, however, that it has escaped the observation of the chemists 

 who have experimented on this point, that an oxide of chlorine is also formed 

 at the same time, as may be shown by the renewed deposit of chloride of 

 silver which is produced in the supernatant nitrate by the addition to it of 

 sulphurous acid. That the darker compound produced by the presence of ni- 

 trate of silver is in no respect different, save that it is a more abundant deposit, 

 from that formed from the chloride alone, is evidenced by the identity of its 

 reactions with those of the latter. For here, again, dilute nitric acid of suf- 

 ficient strength to dissolve silver at 112°, is inert in its action on this bluish- 

 black compound. Chlorhydric acid, if not sufficiently dilute, renders it 

 somewhat paler, and gives a brownish hue to its slaty violet, but otherwise 

 does not alter it. Hyposulphite of soda dissolves nearly the whole if suffi- 

 ciently strong, leaving but a trace of metallic silver ; and ammonia acts in a 

 similar manner, while cyanide of potassium appears entirely to dissolve it. 



In order to be satisfied that the bluish slate-coloured substance formed in 

 the presence of nitrate of silver by the action of light on the chloride was 

 not an oxychloride, an attempt was made to form such an oxychloride by 

 operating on the chocolate-coloured substance so often alluded to. Boiled 

 with caustic potash, this became dark brown ; but nitric acid restored to it 

 its chocolate tint. The substance operated on in this experiment was formed 

 from the citrate by the action of hydrogen (in this case in the presence of 

 nitrate of silver), and treatment of the products as before, by chlorhydric 

 and nitric acids in succession. 



We consider that we are justified in drawing the following conclusions :— ? 



