specific Gravity, and comparative Wear of Gold. 165 
The loss sustained by copper, when rubbed against copper, is 
infinitely more than that of the former metals ; and, when these 
are exposed to the action of copper, they, as well as the copper, 
suffer a very considerable loss. This appears from the general 
results of these experiments, which prove, that pieces of metal 
which are the most subject to wear, are those which produce 
the greatest loss upon other pieces of metal, when rubbed against 
them ; and it is remarkable, that in such a case, the loss does not 
always fall on one in preference to the other ; so that the wear 
can only be considered in the aggregate, although one of the 
pieces may be regarded as the principal cause. 
In order, however, to illustrate the results of the preceding 
experiments, as far as they concern the softer and harder kinds 
of standard gold, and to ascertain more fully the comparative 
wear of flat and smooth surfaces with that of such as, were 
partly protuberant, the following experiment was made. 
Experiment v. 
In this experiment, two kinds of standard gold were employed, 
viz. 
1st. Gold made standard by fine Swedish copper, which was 
very ductile ; and, 
sdly. Gold made standard by a mixture of fine Swedish 
copper and dollar copper. This was as brittle as was compatible 
with rolling and stamping; and was prepared by melting 
gold made standard by fine Swedish copper, with an equal 
quantity of gold rendered brittle by the standard proportion of 
Swedish dollar copper, which was mentioned in the first section 
of this Paper. 
It may here be observed, that a distinction must be made 
