482 . PRINCIPLES OF CHEMISTRY 



The insolubility of the halogen compounds of silver forms tlid 

 basis of many methods used in practical chemistry. Thus by means of 

 this reaction it is possible to obtain salts of other acids from a halogen 

 salt of a given metal, for instance, RC1 2 + 2AgN0 3 =R(N0 3 ) 2 4- 2AgCl. , 

 The formati6n of the halogen compounds of silver is very frequently 

 used in thes investigation of organic substances ; for example, if any 

 product of metalepsis containing iodine or chlorine be heated with a 

 silver salt or silver oxide, the silver combines with the halogen and 

 gives a halogen salt, whilst the elements previously combined with the 

 silver replace thq halogen. For instance, ethylene dibromide, C 2 H 4 Br 2 , 

 is transformed^ into ethylene diacetate, C 2 H 4 (C 2 H3O 2 ) 2l and silver 



to consider certain^ aspects of this subject which are of a purely chemical interest, and 

 especially the facts concerning subchloride of silver, AgoCl (see -Note 19), and the photo- 

 salts (Note 28). There is no doubt that under the action of light, AgCl becomes darker 

 in .colour, decreases iu weight, and probably forms a mixture of AgCl, Ag 2 Cl, and Ag. 

 But the isolation of the subchloride has' only been recently accomplished by Giintz by 

 means of the Ag 2 F, 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 aft all, and the 

 change in colour produced by the-, action of light upon AgCl is most probably due to the 

 formation of Ag 2 Cl. This substance was isolated by Giintz (1891) by passing HC1 over 

 crystals of AgoF. He also obtahied Ag 2 I in a similar manner by passing HI, and Ag 2 S 

 by passing H 2 S over AgoF. Ag 2 Cl is best prepared by the action of phosphorus tri- 

 chloride upon Ag 2 F. At the temperature of its formation Ag 2 Cl has an easily changeable 

 tint, with shades of violet red to violet black. Under the action of light a similar 

 fisomeric) substance is obtained, which splits up into AgCl + Ag when heated. With 

 potassium cyanide Ag 2 Cl gives Ag + AgCN + KCl, whence it is possible to calculate the 

 heat of formation of Ag 2 Cl ; it = 29 : 7, whilst the heat of formation of AgCl = 29'2 i.e. the 

 reaction 2AgCl = Ag 2 Cl + 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 

 Agl 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 o 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 AgoX, and the different tinted photo-salts must be considered as 

 systems containing such AgoX^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 Ag 2 X must be regarded as. such). The photo-salts have been obtained (1) by the 

 imperfect chlorination of silver ; (2) by the incomplete decomposition of Ag 2 O or Ag 2 COs 

 by alternately heating and treating with a halogen acid ; (3) by the action of nitric acid 

 or Na 2 S 2 O 3 upon Ag->Cl; (4) by mixing a solution of AgNO 3 with the hydrates of FeO, 

 MnO and CrO, and precipitating by HC1 ; (5) by the action of HC1 upon the product 

 obtained by the reduction of citrate of silver in hydrogen (Note 19), and (6) by the action 

 of milk sugar upon AgNO 3 together with soda and afterwards acidulating with HC1. 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. 



