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above 300° the latter chloride also loses its chlorine and leaves metallic gold. Auric chloride is the usual form in which gold occurs in solutions, and in which its salts are used in the arts and for chemical purposes. It is soluble in water, alcohol, and ether. Light has a reducing action on these solutions, and after a time metallic gold is deposited upon the sides of vessels containing the solution. Hydrogen when nascent, and even in a gaseous form, reduces gold from this solution to a metallic state. The reduction is more conveniently and usually effected by ferrous sulphate, and in general by the action of ferrous salts. 33

If a solution of potassium hydroxide be added to a solution of auric chloride, a precipitate is first formed, which re-dissolves in an excess of the alkali. On being evaporated under the receiver of an air-pump, this solution yields yellow crystals, which present the same composition as the double salts AuMCI,, with the substitution of the chlorine by oxygen--that is to say, potassium aurate, AuKO.,, is formed in crystals containing 31,0. The solution has a distinctly alkaline reaction. Auric oxide, Au,03, separates when this alkaline solution is boiled with an excess of sulphuric acid. But it then still retains some alkali ; however, it may be obtained in a pure state as a brown powder by dissolving in nitric acid and diluting with water. The brown powder decomposes below 250° into gold and oxygen. It is insoluble in water and in many acids, but it dissolves in alkalis, which shows the acid character of this oxide. An hydroxide, Au(OH)., may be obtained as a brown powder by adding magnesium oxide to a solution of auric chloride and treating the resultant precipitate of magnesium aurate with nitric acid. This hydroxide loses water at 100°, and gives auric oxide.34

33 Stannous chloride as a reducing agent also acts on auric chloride, and gives a red precipitate known as purple of Cassius. This substance, which probably contains a mixture or compound of aurous oxide and tin oxide, is used as a red pigment for china and glass. Oxalic acid, on heating, reduces metallic gold from its salts, and this property may be taken advantage of for sepurating it from its solutions. The oxidation which then takes place in the presence of water may be expressed by the following equation : 2AuCl3 + 3C,H,O, = 2Au + 6HCl + 6CO.,. Nearly all organic substances have a reducing action on gold, and solutions of gold leave a violet stain on the skin.

Auric chloride, like platinic chloride, is distinguished for its clearly-developed property of forming double salts. These double salts, as a rule, belong to the type AuMCI,. The compound of auric chloride with hydrochloric acid mentioned above evidently belongs to the same type. The compounds 2KAUC1,5H.0, ViAuC1,,2H,O, AuNH,CI,,H,O, MgAuC1,1.,2H,0, and the like are easily crystallised in well-formed crystals. Wells, Wheeler, and Penfield (1892) obtained RbAuCl, (rechalish yellow) and CsAuCl, (golden yellow), and corresponding bromides (dark coloured). AuBrz is extremely like the chloride. Auric cyanide is obtained easily in the form of a double salt of potassium, KAU(CN), by mixing saturated and hot solutions of potassium cyanide with auric chloride and then cooling.

51 If ammonia be added to a solution of auric chloride, it forms a yellow precipitate VOL. II.

GG

The starting-point of the compounds of the type AuX 35 is gold monochloride or aurous chloride, AuCl, which is formed, as mentioned above, by heating auric chloride at 185°, Aurous chloride forms a yellowish-white powder ; this, when heated with water, is decomposed into metallic gold and auric chloride, which passes into solution : 3AuCl = AuCl2 + 2 Au. This decomposition is accelerated by the action of light. Hence it is obvious that the compounds corresponding with aurous oxide are comparatively unstable. But this only refers to the simple compounds AuX; some of the complex compounds, on the contrary, form the most stable compounds of gold. Such, for example, is the cyanide of gold and potassium, AuK(CN), It is formed, for instance, when finely-divided gold dissolves in the presence of air in a solution of potassium cyanide : 4KCN +2 Au+H,O+O = 2K Au(CY)2 + 2KHO (this reaction also proceeds with solid pieces of gold, although very slowly). The same compound is formed in solution when many compounds of gold are mixed with potassium cyanide, because if a higher compound of gold be taken, it is reduced

of the so-called fulminating gold, which contains gold, chlorine, hydrogen, nitrogen, and oxygen, but its formula is not known with certainty. It is probably a sort of ammonio-metallic compound, Au.,0,,+NH,, or amidle (like the mercury compound). This precipitate explodes at 140°, but when left in the presence of solutions containing ammonia it loses all its chlorine and becomes non-explosive. In this form the composition Au.,0,3,2NH,,HO is ascribed to it, but this is uncertain. Juric sulphide, Au S., is obtained by the action of hydrogen sulphide on a solution of auric chloride, and also directly by fusing sulphur with gold. It has an acid character, and therefore dissolves in sodium and ammonium sulphides.

35 Many double salts of suboxide of golil belong to the type AuX--for instance, the cyanide corresponding to the type AuKX , like PtKX , with which we became acquainted in the last chapter. We will enumerate several of the representatives of this class of compounds. If auric chloride, AuCls, be mixed with a solution of sodium thiosulphate, the gold passes into a colourless solution, which deposits colourless crystals, containing a double thiosulphate of gold and solium, which are easily soluble in water but are precipitated by alcohol. The composition of this salt is VasauS.0;)..,2H.O. If the sodium thiosulphate be represented as Vas.,03Na, the double salt in question will be Au Nais,O,Na),2H. O, according to the type AulaX,. The solution of this colourless and easily crystallisable salt has a sweet taste, and the gold is not separated from it either by ferrous sulphate or oxalic acid. This salt, which is known as Forrios and Gelis's salt, is used in medicine and photography. In general, aurous oxide exhibits a distinct inclination to the formation of similar double salts, as we saw also with PtX:--for example, it forms similar salts with sulphuurous acid. Thus if a solution of sodium sulphite be graclually added to a solution of oxide of gold in sodium hydroxide, the precipitate at first formed re-dissolves to a colourless solution, which contains the double salt Naz Ju(SO:), -- AuNa(SO,Na). The solution of this salt, when mixed with barium chloride, first forms a precipitate of barium sulphite, and then a red barium double salt which corresponds with the above sodium salt.

The oxygen compound of the type AuX, aurons oride, Au,0, is obtained as a greenish violet powder on mixing aurons chloride with potassium chloride in the cold. With hydrochloric acid this oxide gives gold and auric chloride, and when heated it easily splits up into oxygen and metallic gold.

by the potassium cyanide into aurous oxide, which dissolves in potassium cyanide and forms KAu(CN)2. This substance is soluble in water, and gives a colourless solution, which can be kept for a long time, and is employed in electro-gilding--that is, for coating other metallic objects with a layer of gold, which is deposited if the object be connected with the negative pole of a battery and the positive pole consist of a gold plate. When an electric current is passed between them, the gold from the latter will dissolve, whilst a coating of gold from the solution will be deposited on the object.

APPENDIX I

AN ATTEMPT TO APPLY TO CHEMISTRY ONE OF THE

PRINCIPLES OF NEWTON'S NATURAL PHILOSOPHY

By PROFESSOR MENDELÉEFF

A LECTURE DELIVERED AT THE ROYAL INSTITUTION OF GREAT BRITAIN

ON FRIDAY, MAY 31, 1889

NATURE, inert to the eyes of the ancients, has been revealed to us as full of life and activity. The conviction that motion pervaded all things, which was first realised with respect to the stellar universe, has now extended to the unseen world of atoms. No sooner had the human understanding denied to the earth a fixed position and launched it along its path in space, than it was sought to fix immovably the sun and the stars. But astronomy has demonstrated that the sun moves with unswerving regularity through the star-set universe at the rate of about 50 kilometres per second. Among the so-called fixed stars are now discerned manifold changes and various orders of move. ment. Light, heat, electricity-like sound-have been proved to be modes of motion ; to the realisation of this fact modern science is indebted for powers which have been used with such brilliant success, and which have been expounded so clearly at this lecture table by Faraday and by his successors. As, in the imagination of Dante, the invisible air became peopled with spiritual beings, so before the eyes of earnest investigators, and especially before those of Clerk Maxwell, the invisible mass of gases became peopled with particles : their rapid movements, their collisions, and impacts became so manifest that it seemed almost possible to count the impacts and determine many of the peculiarities or laws of their collisions. The fact of the existence of these invisible motions may at once be made apparent by demonstrating the difference in the rate of diffusion through porous bodies of the light and rapidly moving atoms of hydrogen and the heavier and more sluggish par. ticles of air. Within the masses of liquid and of solid bodies we have been forced to acknowledge the existence of persistent though limited motion of their ultimate particles, for otherwise it would be impossible to explain, for example, the celebrated experiments of Graham on diffusion through liquid and colloidal substances. If there were, in our times, no belief in the

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