~Examination of Tin Slags.~--In tin smelting works the term "slag" is applied to the unfused portion of the charge. It is made up of unburnt anthracite and small lumps of slag proper together with some buttons of metallic tin. This is rarely, if ever, assayed. The slag proper (or, as it is generally called, "glass") is a silicate of iron, alumina, and lime, containing from 3 to 7 per cent. of tin. It is thus examined:--The sample after bruising on an iron plate, is reduced to a very fine powder by grinding in an agate mortar. In this state it is in most cases readily decomposed by hydrochloric acid. ~Determination of Tin.~--Where the percentage of tin only is required, take 2 grams of the powdered slag and well mix with it 20 c.c. of hydrochloric acid, and heat to boiling. Add 1 c.c. of nitric acid, allow to stand for fifteen minutes, dilute with water, and filter. Pass a rapid current of sulphuretted hydrogen for some time. Allow to settle, and filter. The precipitate, after washing with hot water, is dried, and gently calcined until the greater part of the sulphur is burnt off. It is then strongly ignited in the muffle (or over the blowpipe) with the addition of a small lump of ammonic carbonate. The residue is weighed as stannic oxide (SnO_{2}); and is calculated to metallic tin by multiplying by 0.787. The percentage on the slag is calculated in the usual way. The tin is always best determined in the examination of slags by a separate assay carried out in this way. The determination of the other constituents is thus made:-- ~Silica.~--Take 2 grams of the powdered slag and cover them, in a small evaporating dish, with 20 c.c. of hydrochloric acid; mix well by stirring with a glass rod; and evaporate to dryness. If (as is generally the case) tungsten is present the solution will be blue. Take up with 20 c.c. of hydrochloric acid. Add 1 c.c. of nitric acid; and reduce by boiling to about half the bulk. Add about 20 c.c. of water, boil, and filter. Wash the residue with hot dilute hydrochloric acid. It consists of silica with the tungstic acid. Wash it back into the dish; and digest with 5 or 10 c.c. of a cold solution of ammonic carbonate. Filter; and collect the filtrate and washings in a weighed porcelain dish. Dry the residue, ignite strongly, and weigh as silica, SiO_{2}. In certain exceptional cases this may contain some unaltered cassiterite, which is easily recognised by its appearance. ~Tungsten.~--The ammonic carbonate filtrate from the silica is evaporated to dryness, ignited strongly over the blowpipe, and weighed. The residue is tungstic acid, WO_{3}. The tungsten may be conveniently reported in this form, although it is probably present as a lower oxide.