420 PRINCIPLES OF CHEMISTRY 



Chemically-pure silver has an exceeding pure white colour, and a 

 'specific gravity of 10'5. Solid silver is lighter than the molten metal, 

 and therefore a piece of silver floats on the latter. The fusing - 

 point of silver is about 950 C., and at the high temperature attained 

 by the combustion of detonating gas it volatilises. 16 By employing 

 silver reduced from silver chloride by milk sugar and caustic potash, 

 and distilling it, Stas obtained silver purer than that obtained by any 

 other means ; in fact, this was perfectly pure silver. The vapour of 

 silver has a very, beautiful green colour, which is seen when a silver 

 wire is placed in an oxy hydrogen flame. 17 



It has long been known (Wohler) that when nitrate of silver, 

 AgN0 3 , reacts as an oxidising agent, upon citrates and tartrates, it is 

 able under certain conditions to give either a salt of suboxide of silver 

 (see Note 19) or a red solution, or to give a precipitate of metallic 

 silver reduced at the expense of the organic substances. In 1889 Carey 

 Lea, in his researches on this class of reactions, showed that soluble 



The resultant precipitate of silver oxalate is dried, mixed with an equal weight of dry 

 sodium carbonate, and thrown into a gently-heated crucible. The separation of the 

 silver then proceeds without an explosion, whilst the silver oxalate if heated alone 

 decomposes with explosion. 



According to Stas, the best method for obtaining silver from its solutions is by the 

 reduction of silver chloride dissolved in ammonia by means of an ammoniacal solution of 

 cuprous thiosulphate ; the silver is then precipitated in a crystalline form. A solution of 

 ammonium sulphite may be used instead of the cuprous salt. 



16 Silver is very malleable and ductile; it may be beaten into leaves 0'002 mm. in 

 thickness. Silver wire may be made so fine that 1 gram is drawn into a wire 2 kilo-, 

 metres long. In this respect silver is second only to gold. A wire of 2 mm. diameter 

 breaks under a strain of 20 kilograms. 



17 In melting, silver absorbs a considerable amount of oxygen, which is disengaged on 

 solidifying. One volume of molten silver absorbs as much as 22 volumes of oxygen. In 

 solidifying, the silver forms cavities like the craters of a volcano, and throws qff metal, 

 owing to the evolution of the gas } all these phenomena recall a volcano on a miniature 

 scale (Dumas). Silver which contains a small quantity of copper or gold, &c., does not 

 .show this property of dissolving oxygen. 



The absorption of oxygen by molten silver is, however, an oxidation, but it is at the 

 same time a phenomenon of solution. One cubic centimetre of molten silver can 

 dissolve twenty-two cubic centimetres of oxygen, which, even at 0, only weighs 0'08 

 gram, whilst 1 cubic centimetre of silver weighs at least 10 grams, and therefore it is 

 impossible to suppose that the absorption of the oxygen is attended by the formation of 

 any definite compound (rich in oxygen) of silver and oxygen (about 45 atoms of silver to 

 1 of oxygen) in any other but a dissociated form, and this is the state in which sub- 

 stances in solution must be regarded (Chapter I.) 



Le Chatelier showed that at '300 and 15 atmospheres pressure silver absorbs so 

 much oxygen that it may be regarded as having formed the compound Ag 4 O, or a 

 mixture of Ag 2 and Ag 2 O. Moreover, silver oxide, Ag 2 0, only decomposes at 800 under, 

 low pressures, whilst at pressures above 10 atmospheres there is no decomposition at 

 800 but only at 400. 



Stas showed that silver is oxidised by air in the presence of acids. V. d. Pfordten 

 .confirmed this, and showed that an acidified solution of potassium permanganate rapidly 

 dissolves silver in the presence of air. 



