COPPEE, SILVER, AND GOLD 411 



oxides of lead! or mercury. Hence the hydroxide of copper dissolves in. 

 solutions of neutral cupric salts. The cupric salts are generally blue or 

 green, because cupric hydroxide itself is coloured. But some of the 

 salts in the anhydrous state are colourless. 10 



characterised by a constant pressure at a constant temperature even under a change in 

 the amount of one of the component parts (for instance, of a salt in a saturated solution), 

 while an imperfect equilibrium is such a one for which such a change corresponds with 

 a change of pressure (for instance, an unsaturated solution). The law of phases consists 

 in the fact that : n "bodies only give a perfect equilibrium when n + 1 phases participate* 

 in that equilibrium for example, in the equilibrium of a salt in its saturated solution 

 in water there are two bodies (the salt and water) and three phases, namely, the salt, 

 solution, and Tapour,- which can be mechanically separated from each other, and to this 

 equilibrium there corresponds a definite tension. At the same time, n bodies may 

 occur in n + Z phases, but only at one definite temperature and one pressure ; a change 

 of one of these may bring about another state (perfect or not equilibrium stable or 

 unstable). Thus water when liquid at the ordinary temperature offers two phases 

 {liquid and vapour) and is in perfect equilibrium (as also is ice below 0) 1L but water, ice, 

 and vapour (three phases and only one body) can only be in equilibrium at 0, and at the 

 ordinary pressure ; with a change of t there will remain either only ice and vapour or 

 only liquid water and vapour ; whilst with a rise of pressure not only will the vapour 

 pass into the liquid (there again only remain two phases) but also the temperature of 

 the formation of ice will fall (by about 7 per 1000 atmospheres). The same laws of 

 phases are applicable to the consideration of the formation of simple or double salts 

 from saturated solutions and to a number of other purely chemical relations. Thus, for 

 example, in the above-mentioned instance, when the bodies are KC1, GuCl 2 , and H 2 0, 

 perfect equilibrium (which here has reference to the solubility) consisting of four phas_es, 

 corresponds to the following seven cases, considering only the phases (above 0) 

 A = CuCL,2KCl,2H 2 O; B = CuCl 2 KCl ; C = CuCl 2 ,2H 2 0,KCl, solution and vapour: 

 (1) A + B + solution + j vapour; (2) A + C + solution t vapour ; (3) A + KC1 + solution, 

 f vapour; (4) A + B + C + vapour (it follows that B + KC1 + solution gives A); {5} 

 A + C + KC1 + vapour ; (6) B + C + solution + vapour ; and (7) B + KC1 + solution + vapour. 

 Thus above 92 A gives B + KC1. The law of phases by bringing complex instances of 

 chemical reaction under simple physical schemes, facilitates their study in detail and 

 gives the means' of seeking the simplest chemical relations dealing with solutions, dis- 

 sociation, double decompositions and similar cases, and therefore deserves consideration, 

 but a detailed exposition of this subject must be looked for" in works on physical 

 chemistry. 



10 The normal cupric nitrate, CuNoO 6 ,3H 2 0, is obtained as a deliquescent salt of a blue 

 colour (soluble in water and in alcohol) by dissolving copper or cupric oxide iix nitric acid. 

 It is so easily decomposed by the action of heat that it is impossible to drive off the water 

 of crystallisation from it before it begins to decompose. During the ignition of the normal 

 salt the cupric oxide formed enters into combination with the remaining undecomposed 

 normal salt, and gives a basic salt, CuN 2 6 6 ,2CuH 2 O 2 . The same basic salt is obtained 

 if a certain quantity of alkali or cupric hydroxide or carbonate be added to -the solution 

 of the normal salt, which is even decomposed when boiled with metallic copper, and forms, 

 the basic salt as a green powder, which easily decomposes under the action of heat and 

 leaves a residue of cupric oxide. The basic salt, having the composition CuN 2 6 ,3CuH 2 O 2 , 

 is nearly insoluble in water. 



The normal carbonate of copper, CuCO 3 , occurs in nature, although extremely rarely. 

 If solutions of cupric salts be mixed with solutions of alkali carbonates, then, as in the 

 case of magnesium, carbonic anhydride is evolved arid basic salts are formed, which vary 

 in composition according to the temperature and conditions of the reaction. By mixing- 

 cold solutions, a voluminous blue precipitate is. formed, containing an equivalent pro- 

 portion of cupric hydroxide and carbonate (after standing or heating, its composition 



