COPPER, SILVER, AND GOLD 427 



pure silver nitrate obtained (its solution is colourless, while the presence 

 of Cu renders it blue), which may be ultimately purified by crystallisa- 

 tion. It crystallises in colourless transparent prismatic plates, which 

 are not acted on by air. They are anhydrous. Its sp. gr. is 4'34 ; it 

 dissolves in half its weight of water at the ordinary temperature. 22bis 

 The pure salt is nob acted on by light, but it easily acts in an oxidising 

 manner on the majority of organic substances, which it generally 

 blackens. This is due to the fact that the organic substance is oxidised 

 by the silver nitrate, which is reduced to metallic silver ; the latter is 

 thus obtained in a finely-divided state, which causes the black stain. 

 This peculiarity is taken advantage of for marking linen. Silver nitrate 

 is for the same reason used for cauterising wounds and various growths 

 on the body. Here again it acts by virtue of its oxidising capacity in 

 destroying the organic matter, which it oxidises, as is seen from the 

 separation of a coating of black metallic powdery silver from the part 

 ^cauterised. 22 tri From the description of the preparation of silver nitrate 

 it will have been seen that this salt, which fuses at 218, does not 



give more stable compounds than those of copper oxide. This is verified by the figures 

 and data of their reactions. It is impossible to calculate for cupric nitrate, because this 

 salt has not yet been obtained in. an anhydrous state ; but the sulphates of both oxides 

 are known. The specific gravity of copper sulphate in an anhydrous state is 3'53, and of 

 silver sulphate 5'36; the molecular volume of the former is 45, and of the latter 58. 

 The group S0 3 in the" copper occupies, as it were, a volume 45-18 = 82, and in the silver 

 palt a volume 58 82 = 26 ; hence a smaller contraction has taken place in the formation 

 of the copper salt from the oxide than in the formation of the silver salt, and conse- 

 quently the latter should be more stable than the former. Hence silver oxide is 

 able to decompose the salt of copper oxide, whilst with respect to the metals both salts 

 have been formed with an almost identical contraction, since 58 volumes of the silver 

 salt contain 21 volumes of metal (difference = 87), and 45 volumes of the copper salt 

 contain 7 volumes of copper (difference = 38). Besides which, it must be observed that 

 copper o'xide displaces iron oxide'/ just .as silver, oxide displaces copper oxide. Silver, 

 .copper, and iron, in the form of oxides, displace each other in the above order, but in the 

 'form of metals in a reverse order (iron, copper, silver). The cause of this order of the 

 displacement of the oxides lies, amongst other things, in their composition. They have 

 the composition Ag 2 0, Cu 2 O 2 , Fe 2 O 3 ; the oxide containing a less proportion of oxygen 

 ; displaces that containing a larger proportion, because the basic character diminishes 

 with the increase of contained oxygen. 



Copper also displaces mercury from its salts. It may here be remarked that Spring 

 (1888), on leaving a mixture of dry mercurous chloride and copper for two hours, 

 observed a distinct reduction, which belongs to the category of those phenomena which 

 demonstrate the existence of a mobility of parts (i.e. atoms and molecules) in solid- sub- 

 stances. 



22 bis The reaction of 1 part' by weight of AgN0 3 requires (according to Kremers) the 

 following amounts of water : at 0, 0'82 part, at 19'5, 0'41 part, at 54, 0'20 part, 

 at 110, 0-09 part, and, according to Tilden, at 125, 0-0617 part, and at 188, 0'0515 

 .part. 



2trl It may be remarked that the black stain produced by the reduction of metallic 

 silver disappears under the action of a solution of. mercuric chloride or of potassium 

 Cyanide, because these salts act on finely-divided silver, 



