specific Gravity, and comparative IVear of Gold. 139 
sdly. Friction of gold coin against coin of other metals, such 
as silver and copper. 
3dly. The friction which gold coins of various qualities suffer, 
when exposed to the action of certain substances, such as the 
particles or filings of metals, gritty powders, &c. 
The consideration of these different modes of wear, points out 
the best method to be pursued in an experimental investigation. 
The whole of the experiments which compose this section 
may therefore be divided into three subordinate series ; the two 
first of which have been directed to the consideration of that 
part of the diminution of the coin which arises from the rubbing 
of one piece of metal against another ; while, 
The third of these subordinate series was intended to show 
the comparative power of gold, differently alloyed, to resist abra- 
sion from sand or other gritty powders. 
In the first set of experiments, 28 pieces of coin were fixed to 
a frame, and over each of them was placed another piece of 
coin, which was pressed against it by a weight. These upper 
pieces were all attached to a second frame, so that, by means of 
the motion communicated thereto by cranks, each upper piece 
was made to move about of an inch backwards and forwards 
on the lower one. This mode of experiment afforded an oppor- 
tunity of trying the comparative diminution of gold differently 
alloyed, both when rubbed against pieces of the same and of a 
different alloy ; and also of examining the difference of wear 
between pieces with plain and with stamped faces. 
In the second series, 200 pieces of gold, differently alloyed, 
were inclosed in a wooden box, of a cubic figure, which was 
kept constantly turning round, till, by the repeated rubbing and 
striking of the pieces against each other, and against the sides of 
T 2 
