specific Gravity, and comparative Wear of Gold . 193 
expansion, dependent on a certain relative proportion of the dif- 
ferent metals. 
The experiments on the comparative wear of gold, which are 
described in the last section, were attended with considerable 
difficulties ; for this reason, the conclusions have been founded 
only upon such facts as were uniformly the same under every 
circumstance. These general conclusions have been already 
fully stated ; but we may again observe, that gold of moderate 
ductility is (all things being considered) the best adapted to the 
purpose of coin, and that the real wear of such coin is, in all 
probability, very slowly effected ; so that a long period of time 
must elapse, before any considerable diminution in weight can 
be perceived. 
The experiments contained in these three sections were limited 
to standard gold ; and, allowing that some curious and instruc- 
tive facts have been discovered, still more might have been 
expected from an extension of such experiments to gold variously 
alloyed in every possible proportion. But an immense addition 
to metallurgical science would, in all probability, be derived from 
a comparative investigation of the whole of the known metallic 
substances, formed into binary, ternary, and such like combi- 
nations; proceeding from the most simple to the most compli- 
cated, and accompanied by accurate observations on the lustre, 
colour, ductility, hardness, specific gravity, and fusibility of the 
compounds. 
Our actual knowledge of the properties of metallic mixtures 
is certainly very imperfect, and has by no means kept pace with 
the rapid progress of modern chemistry. Few additions have 
been made to the compound metals employed by the ancients. 
MDCCCIIL C C 
