is only 0.002 gram, but this equals 2 per cent. of the silver present.
In the same way the student should see that the two results, 91.5 per
cent. and 92.0 per cent., are really more concordant than the results
9.1 per cent. and 9.2 per cent.

A device often adopted in practice where a large number of assays of one
kind are made, and the report is given as so many ounces or pounds to
the ton, is that known as the _assay ton_. The assay ton may be any
arbitrary and convenient weight, but its subdivisions must bear to it
the same relations as pounds and ounces bear to the actual ton. On the
other hand, in a laboratory where many kinds of work are performed,
different sets of weights of this kind would only tend to confusion,
even if they were not unnecessary. With a set of gram weights and its
subdivisions anything may be done. If it is desired to report as pounds
to the ton, then, since there are 2240 lbs. to the ton, a weight of
2.240 grams may be taken as the assay ton, and each 0.001 gram yielded
will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01
gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton;
if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1
oz. to the ton. In some cases it may be convenient to have, in addition
to the usual gram weights, one or other of the "assay tons" mentioned
above, but generally it is better to work on a purely decimal system,
and convert when required into ounces per ton, &c., either by actual
calculation or by reference to a set of tables.


PRACTICAL EXERCISES.

The student should practise such calculations as the following:--