Assay. Three general methods are adopted for this purpose:—
1. (Mechanical.) This consists in pulverising the ore by any convenient method, and expertly washing a given weight of it (say 1000 gr.) in a wooden bowl or capsule with water, so as to remove the earthy gangues from the denser and valuable metallic matter in such a way that none of the latter may be lost. This is the common plan adopted with auriferous sands, the ores of tin after they have passed the stamping-mill, galena, grey antimony, &c., and may either be employed as an independent process or merely as preparatory to more exact investigations. When galena is thus tested, the product is a nearly pure sulphide of lead, of which every grain is equivalent to ·8666 of metallic lead, the rest being sulphur. The results with grey antimony ore are still more direct, since the product is only melted into pigs before being sent to market. In this state it contains 73% (nearly) of metallic antimony.
2. (Humid.) Assays in the ‘humid way’ are true chemical analyses, and are described under the head ‘Estim.’ attached to most of the more important minerals noticed in this work. This plan offers greater facilities and gives more accurate results than either of the other methods.
3. (Dry.) Of the methods of assay in the ‘dry way’ the following are the most accurate, generally useful, and easily applied:—
a. (Dr Abiche.) The mineral is reduced to powder, and mixed with 5 or 6 times its weight of carbonate of barium, also in powder; this mixture is fused at a white heat in a platinum crucible, and the resulting slag, after being powdered, is exhausted with hydrochloric acid. This process answers well with both stony and metallic minerals, the most refractory of which give way under this treatment.
b. (Liebig.) Into a crucible containing commercial cyanide of potassium, a weighed quantity of the ore, in the state of fine powder, is sprinkled, when the metallic oxides and sulphides
which it contains are almost immediately reduced to the metallic state, and may be separated from the scoria by edulcoration with water. With the oxides and sulphides of antimony and tin this reduction occurs at a dull red heat; with the compounds of copper it occurs with the disengagement of light and heat; but an ore of iron requires to be mixed with a little carbonate of potassium or of sodium before throwing it into the fused cyanide, and to be then submitted to a full red heat for a short time, before it is reduced to the reguline state. In this case any manganese present in the ore of iron is left under the form of protoxide. A mixture of about equal parts of dry carbonate of sodium and cyanide of potassium answers better for the crucible than the cyanide alone. See Alloys, Metallurgy, &c.; also Percy’s metallurgy.
ORGAN′IC BA′SES. These interesting bodies may be divided into two classes: the first comprising those which occur ready formed in nature (ALKALOIDS); and the second those produced by artificial processes in the laboratory (ARTIFICIAL ALKALOIDS, ARTIFICIAL ORGANIC BASES). They all contain the element NITROGEN. The natural bases have already been described under ALKALOID. Hitherto they have none of them been produced by artificial means. The bases of artificial origin are mostly volatile, and their constitution is much simpler than that of the native bases. Of the vast number which have been formed the following are, perhaps, the most interesting:—Ethylamine, METHYLAMINE, AMYLAMINE, ANILINE, NAPHTHYLAMINE, CHINOLINE, and PICOLINE. These and other bodies of the class are noticed under their respective heads.
By Berzelius the natural organic bases (owing to the invariable presence in them of hydrogen and nitrogen) were regarded as compound ammonias, or combinators of ammonia with a variety of neutral principles.
He conceived the greater part of these neutral bodies were incapable of isolation, and further more that the closest union existed between them and the ammonia. Thus it was his opinion that quinine C20H12NO2, 3HO (halving the modern formula) was a compound of the group C20H9O2 with oxide of ammonium and water of crystallisation thus (C20H9O2H4NO)2HO. He believed the organic base owed its basicity to the ammonia. Berzelius’ opinion carried weight at the time, from the circumstance that certain neutral substances when directly combined with ammonia were capable of forming a number of artificial bases very similar in qualities and also in composition to the natural ones, or those obtained from living plants. Thus, the artificial base thiosinamine having the formula C4H5NS is produced by the combination of oil of mustard and ammonia; and another base may be artificially obtained from the union of oil of bitter almonds with ammonia.