Cobalt is also gravimetrically determined, like nickel, by electrolysis, or by precipitation with sodic hydrate. In the latter case, the ignited oxide will be somewhat uncertain in composition, owing to its containing an excess of oxygen. Consequently, it is better to reduce it by igniting at a red heat in a current of hydrogen and to weigh it as metallic cobalt. PRACTICAL EXERCISES. 1. In the dry assay of an ore containing cobalt, nickel, and copper, the following results were obtained. Calculate the percentages. Ore taken, 5 grams. Speise formed, 0.99 gram. Speise taken. 0.99 gram. Arsenides of cobalt, nickel, and copper got, 0.75 gram. Arsenide of nickel and copper got, 0.54 gram. Gold added, 0.5 gram. Gold and copper got, 0.61 gram. 2. Calculate the percentage composition of the following compounds: Co_{2}As, Ni_{2}As, and Cu_{2}As. 3. A sample of mispickel contains 7 per cent. cobalt. What weight of the mixed sulphates of potash and cobalt will be obtained in a gravimetric determination on 1 gram of the ore? 4. 0.3157 gram of metal was deposited by the electrolysis of a nickel and cobalt solution. On dissolving in nitric acid and determining the cobalt 0.2563 gram of potassium and cobalt sulphates were got. Find the weights of cobalt and nickel present in the deposit. 5. What should be the percentage composition of pure cobaltite, its formula being CoAsS? ZINC. Zinc occurs in nature most commonly as sulphide (blende); it also occurs as carbonate (calamine) and silicate (smithsonite). Each of these is sufficiently abundant to be a source of the metal. The metal is known in commerce as "spelter" when in ingots, and as sheet zinc when rolled. It is chiefly used in the form of alloys with copper, which are known as brasses. It is also used in the form of a thin film, to protect iron goods from rusting--galvanised iron. Ores of zinc, more especially blende, are met with in most lead, copper, gold, and silver mines, in larger or small quantities scattered through the lodes. Those ores which generally come under the notice of the assayer are fairly rich in zinc; but alloys and metallurgical products contain it in very varying proportions. Zinc itself is readily soluble in dilute acids; any residue which is left after boiling with dilute hydrochloric or sulphuric acid consists simply of the impurities of the metal; this is generally lead.