There aro a few other aub,tatices besides the above metallic ores, which have now become of so much importance as articlea of commerce that it has been thought desirable to incorporate into thia article the methoda employed to ascertain their respective commercial values. Some f these assays, such as those of fuel, sulphur, or limeatone, enter as largely into the gveryday work of the professional analyst as do thoae of copper, tin, or iron ores. The methods given are, in all oases, the aimpleat and readiest.
Assay of Arsen;c.—If it be required to determine the amount of pure arsenious acid in n sample of the crude article, the assay is best performed in the dry way. If no other volatile matters be present, the powder is sublimed iu an earthenware retort, and the sublimate collected and weighed. In the other case, the acid must be dissolved out by means of boiling water, and the residue is wei,;11cd, the difference giving the amount of pure arsenious acid in the sample.
The following method of assaying arsenical ores by the wet way is given in Crooke's' ' Practical Assaying' :—" Digest the oro in strong nitric acid, adding occasionally a crystal or two of chlorate of potash until no more is dissolved, and there is no further action on adding fresh acid. Dilute with water, and filter ; add to the filtrate nitrate of lead ; wash the precipitate formed well on a filter, and digest it in dilute nitric acid ; this dissolves the precipitated araeniate of lead, leaving the sulphate. Filter, and Batman.' the filtrate with soda, which precipitates the arseniate ; this is col lected, waahed, dried, and weighed. Every 100 parts correspond to 22.2 parts of metallic arsenic, or to 29 parts of commercial amnions acid." Araenical orea may be asaaycd in the dry way by heating them slowly le rednesa in au earthenware retort or cylinder. If they be sulphides, they must be previously mixed with potash or quicklime. A considerable portion of the sublimed arsenic is collected upon a thin iron plate, rolled up, and inserted in the neck of the retort, and the remainder in a cone, made of copper foil, and luted upon the neck ; the cone has a small opening at the top for the free escape of gas. The sublimate is afterwards carefully detached and weighed. Commercial arsenic may be assayed in the same way by mixing it with 16 to 20 per cent. of charcoal.
Assay of Coal.—The assay of coal is frequently an operation of much importance ; it consists generally in determining the moisture, the volatile and combustible matters, the fixed carbon (coke), and the sulphur contained in it.
(1) Moisture.—Powder the coal finely, and heat 1 or 2 grains of it in a covered platinum or porcelain crucible, placed in an air-bath at from 100° to 115° for fifteen minutes. Then allow the crucible to cool, and weigh it carefully, afterwards replacing in the air-bath, until its weight is constant, or begins to rise. The loss of weight indicates the amount of moisture in the quantity employed.
(2) Volatile and Combustible Matters.—Heat the same crucible and its contents to bright red ness over an ordinary Bunsen lamp for three and a half minutes, and then for the same space of time over a foot blowpipe flame. Allow it to cool and weigh it ; the loss from the previous weighing represents the volatile and combustible matters present, and includes half of the sulphur of any sulphide of iron which may be present in the coal.
(3) Fixed Carbon.—Heat the same crucible over the Bunsen lamp until its contents are perfectly white, and its weight is constant ; the loss represents the carbon present in the coal, together with half the sulphur contained in the sulphide of iron.
(4) Sulphur.—Weigh out 15 to 20 grains of the finely powdered sample, and treat it with a little nitric acid and chlorate of potash in a flask until there is no further action ; then wash the whole carefully through a filter. If the residue in the filter contain any particles of unoxidized sulphur, dry and weigh it, and then ignite and weigh it ; the difference indicates the amount of sulphur unoxidized. Now add to the filtrate a little pure hydrochloric acid ; boil, and add solutions of chloride of barium until the precipitate ceases to form. Boil well, and allow the precipitate to settle. Filter carefully, washing with hydrochloric acid, and finally with boiling water. Dry, ignite, and weigh the precipitate ; this weight, less that of the filter ash, multiplied by 16 and divided by 116.5, gives the amount of sulphur in the weight of coal taken. The weight of unoxi dized sulphur, previously determined, must; of course, be added in.