parts of sulphur to 62.77 of iron. If, therefore, two hundred grains of the ore are used, the iron should weigh from 60 to 100 grains. Filings ought never to be used, as they are always covered, to a smaller or greater amount, with oxide. Besides, it is very difficult to get them as free from impurities as wire. Weighed pieces of the latter should be kept in some vial or box for this purpose.

XVI.

Assay of Lead Ores by a We: Process.

THIS method will be found much the more accurate of the two, although it may not often happen that a practical assayer has sufficient time for this proceeding. (See Woehler's Anal. Chem.)

The ore (galena) should be powdered much finer even than in the assay by heat, after which it is moistened with fuming nitric acid, and digested in the sand-bath, by which process it is entirely changed from the sulphuret to the sulphate of lead.

If the mass be diluted with water and filtered, the merest traces only of lead can be found in the solution,-quantities too small to deserve any farther attention. If the ore contain copper, iron, or silver, they will be contained in this filtered solution;

the first are then discovered by ammonia, the latter by muriatic acid.

Should fuming nitric acid not be at hand, and if therefore a weaker kind is used, a mixture of sulphate of lead and sulphur is produced, together with a solution of nitrate of lead. From this latter the metal should be precipitated with sulphuric acid. By heating the dried residue,—after filtering and washing, by pouring water over it while yet on the filter,-sulphur is evaporated and sulphate of lead is retained.

In both cases, the lead produced is a sulphate, and in this shape it should be weighed, and from the result the amount of the former alone may be easily calculated, as will be seen from the sequel.

Sulphate of lead consists, in 100 parts by weight, of

Oxide of lead. 73.56

Sulphuric acid. 26.44

100.00

Hence we say, if 100 parts of oxide of lead contain 92.83 of pure lead, then 73.56 of pure lead contain 68.285748 parts; or, in other words, sulphate of lead consists of

For this assay about twenty grains only need be taken. A smaller amount might make us incapable of finding the real quantity of lead, while a larger one would only give us unnecessary trouble.

ONE part of the ore, about a hundred grains, thoroughly pulverized, is mixed with from thirty to a hundred grains of calcined borax. The quantity of the latter varies according to the purity of the mineral, and increases when it contains many foreign admixtures.

If the ore contains sulphur, it ought first to be roasted, as was the case with the assays of other metals already described.

A Hessian crucible is then prepared with coal, as already directed in the description of those vessels under the head of Utensils and Implements. Into the hollow in the centre, the mixture of ore and borax is poured, and on that some charcoal powder; after which the cavity is entirely covered