calculation it is well to take such a quantity of ore for each assay as
by a simple multiplication will yield the percentage. In these
calculations decimals are freely employed, and students should make
themselves familiar with the methods of using them.

Other methods of statement are in use, and have advantages in certain
special cases. With bullion the parts in a thousand are given, and in
those cases in which the percentage is very small, as in water analysis,
it is convenient to report on parts in 100,000, or even on parts per
1,000,000. These are easily got from the corresponding percentages by
shifting the decimal point one, three, or four places to the right. Thus
92.5 per cent. is 925 per thousand; and 0.0036 per cent. is 3.6 per
100,000, or 36 per million.

With ores of tin, silver, and gold, the result is stated as so many
cwts., lbs., or ozs., in the ton. With dressed tin ores as they are
sent to the smelter, the produce is given in cwts. and quarters to the
ton. The corresponding percentage may be obtained by multiplying by
five; or, inversely, if the percentage is given, the produce may be got
by dividing by five. A produce of 13-1/2 equals a percentage of 13.5◊5
= 67.5; and a percentage of 70.0 equals a produce of 70/5 = 14. With
tin ores as raised (in which the percentage is small) the reduction must
be carried to pounds per ton. One per cent. equals 22.4 lbs. to the ton;
consequently, if we multiply the percentage by 22.4, the produce will be
given. Thus, if an ore contains 6.7 per cent. of oxide of tin, the
produce is 6.7◊22.4 = 150 lbs. (or 1 cwt., 1 quarter, and 10 lbs.) to
the ton. With gold and silver ores, the proportion of precious metal is
small, and it is necessary to carry the reduction to ozs. and dwts. to
the ton; and since gold and silver are sold by troy weight, whilst the
ton is avoirdupois, it is of importance to remember that the ounces in