15. As a metal chloride is a compound of a metal with chlorine, so a metal oxide is a compound of a metal with oxygen, and a metal sulphide, or sulphuret, is a metal combined with sulphur, while a metal sulphate is a metal combined with both oxygen and sulphur - that is, a metal oxide with a sulphur oxide; the latter being the same which, when combined with a certain proportion of water, is called sulphuric acid, or oil of vitriol. The metal oxides, sulphides, and sulphates are distinguished in the same way as the chlorides.

16. The purpose and effect of roasting ore for lixi-viation is, as to gold, to burn all base metals, sulphur, and other substances, such as arsenic, antimony, and tellurium, and either expel them by volatilization, or leave them in such condition as to be harmless in the chlorination of the gold, and, as to silver, to change its condition in the ore, from various insoluble compounds, into soluble silver chloride.

17. In order that roasting may be effective, the ore must first be crushed to powder. The most suitable degree of comminution must be found by trial for each particular ore. The more coarsely it is crushed, consistently with good roasting, the more easily is it leached. A powder which will pass through wire gauze of 40 meshes to the running inch is fine enough, and, in some cases, a sieve of 20 meshes to the running inch may be used with advantage. For crushing there is, as yet, nothing better in the market than the stamp battery.

Oxidizing Roast

18. Oxidizing Roast. The crushed ore is exposed to heat, with abundant access of air. The metal sulphides take fire and burn, both metal and sulphur being oxidized by combining with oxygen from the air. A part of the oxidized sulphur flies off with the well known sulphurous smell. Another part combines with a portion of the oxidized metal, forming metal sulphate. The rest of the metal remains as oxide, except silver, which, if not converted into sulphate, becomes metallic. In this way iron, copper, zinc, and lead sulphides are changed, partly into the respective sulphates, and partly into oxides. Nearly the whole of the silver is converted into sulphate, or reduced to the metallic state; gold remains unchanged. Antimony and arsenic are oxidized, and partly fly off, while a part remains, to combine with metal oxides, forming antimonates and arsenates, much in the way in which sulphur makes sulphates. An oxidizing roast is a roast so conducted that the gold is metallic; the silver is either metallic or in the form of sulphate, and the base metals are converted into sulphates or oxides.

Dead Roast

19. Dead Roast. Under an increase of heat, some of the metal sulphates which were formed during the oxidation are decomposed; sulphur oxide flies off, and metal oxide remains, although some of the metal oxides also volatilize to some extent. The order in which some of the principal sulphates are decomposed is, iron, copper, silver; the last requiring a very high heat. Lead sulphate is not decomposed, nor is it usual to push the heat so far as to decompose silver sulphate. A dead roast is an oxidizing roast, carried forward to decomposition of iron and copper sulphates.