specific Gravity , and comparative IVear of Gold. 151 
for it not only much injures the colour of the precious metal, 
but also renders it extremely susceptible of the effects of friction. 
The presence of tin, or iron, appears also to render standard gold 
more liable to wear, than when the alloy consists only of cop- 
per, or of silver. So rapid was- the loss of the pieces composed 
of equal parts of gold and copper, and of the others in which 
iron was present, that it was found necessary to remove the 
former, as well as those pieces which contained iron, after 105480 
revolutions had been performed. The pieces containing tin 
were worn so thin, after 189000 revolutions, that they also were 
obliged to be taken out. As, therefore, the whole of the others 
sustained 200300 revolutions, it may be concluded, that the 
comparative loss of the pieces which were taken out, although 
very considerable, would have been much greater, had it been 
possible to have kept them in the apparatus during the whole 
period of the experiment. 
The preceding experiment was made upon smooth, flat, un- 
stamped pieces ; it was therefore thought necessary to repeat it, 
in some measure, upon those which had been stamped by 
the die already described. In the following experiment, there 
was also a small variation, in respect to the quality of the series 
which were examined ; for, the pieces composed of gold and 
copper in equal proportions were omitted, and some pieces of 
standard silver, and some of fine copper, were added. 
Experiment 11. 
In this experiment, as in the former, pieces of similar quality 
were opposed to each other ; and, in general, every circumstance 
was the same, excepting that the pieces were stamped, that the 
number of revolutions amounted only to 20680, and that all 
