Note 25 — Chapter XXIV · COPPER, SILVER, AND GOLD

In photography these are called ‘developers.’ The most common developers are: solutions of ferrous sulphate, pyrogallol, ferrous oxalate, hydroxylamine, potassium sulphite, hydroquinone (the last acts particularly well and is very convenient to use), &c. The chemical processes of photography are of great practical and theoretical interest; but it would be impossible in this work to enter into this special branch of chemistry, which has as yet been very little worked out from a theoretical point of view. Nevertheless, we will pause to consider certain aspects of this subject which are of a purely chemical interest, and especially the facts concerning subchloride of silver, Ag2Cl (see Note 19), and the photo-salts (Note 23). There is no doubt that under the action of light, AgCl becomes darker in colour, decreases in weight, and probably forms a mixture of AgCl, Ag2Cl, and Ag. But the isolation of the subchloride has only been recently accomplished by Güntz by means of the Ag2F, discovered by him (see Note 24). Many chemists (and among them Hodgkinson) assumed that an oxychloride of silver was formed by the decomposition of AgCl under the action of light. Carey Lea's (1889) and A. Richardson's (1891) experiments showed that the product formed does not, however, contain any oxygen at all, and the change in colour produced by the action of light upon AgCl is most probably due to the formation of Ag2Cl. This substance was isolated by Güntz (1891) by passing HCl over crystals of Ag2F. He also obtained Ag2I in a similar manner by passing HI, and Ag2S by passing H2S over Ag2F. Ag2Cl is best prepared by the action of phosphorus trichloride upon Ag2F. At the temperature of its formation Ag2Cl has an easily changeable tint, with shades of violet red to violet black. Under the action of light a similar (isomeric) substance is obtained, which splits up into AgCl + Ag when heated. With potassium cyanide Ag2Cl gives Ag + AgCN + KCl, whence it is possible to calculate the heat of formation of Ag2Cl; it = 29·7, whilst the heat of formation of AgCl = 29·2—i.e. the reaction 2AgCl = Ag2Cl + Cl corresponds to an absorption of 28·7 major calories. If we admit the formation of such a compound by the action of light, it is evident that the energy of the light is consumed in the above reaction. Carey Lea (1892) subjected AgCl, AgBr, and AgI to a pressure (of course in the dark) of 3,000 atmospheres, and to trituration with water in a mortar, and observed a change of colour indicating incipient decomposition, which is facilitated under the action of light by the molecular currents set up (Lermontoff, Egoroff). The change of colour of the halogen salts of silver under the action of light, and their faculty of subsequently giving a visible photographic image under the action of ‘developers,’ must now be regarded as connected with the decomposition of AgX, leading to the formation of Ag2X, and the different tinted photo-salts must be considered as systems containing such Ag2X's. Carey Lea obtained photo-salts of this kind not only by the action of light but also in many other ways, which we will enumerate to prove that they contain the products of an incomplete combination of Ag with the halogens, (for the salts Ag2X must be regarded as such). The photo-salts have been obtained (1) by the imperfect chlorination of silver; (2) by the incomplete decomposition of Ag2O or Ag2CO3 by alternately heating and treating with a halogen acid; (3) by the action of nitric acid or Na2S2O3 upon Ag2Cl; (4) by mixing a solution of AgNO3 with the hydrates of FeO, MnO and CrO, and precipitating by HCl; (5) by the action of HCl upon the product obtained by the reduction of citrate of silver in hydrogen (Note 19), and (6) by the action of milk sugar upon AgNO3 together with soda and afterwards acidulating with HCl. All these reactions should lead to the formation of products of imperfect combination with the halogens and give photo-salts of a similar diversity of colour to those produced by the action of developers upon the halogen salts of silver after exposure to light.