127 
specific Gravity , a?id comparative Wear of Gold. 
present experiment, after the pieces had been subjected to long 
continued friction, there cannot be any doubt but that this is a 
general effect, which probably arises from the pieces having 
suffered expansion, in consequence of heat generated during the 
friction; and, (similar to what has been observed in pyro me- 
trical experiments, ) that these pieces of metal did not, upon the 
cessation of friction, return precisely to their original size and 
specific gravity. 
Among the other less powerful causes which produce some 
alteration in the specific gravity of gold, the processes of rolling, 
and of annealing, may also be enumerated ; for, in the course 
of these experiments, I have always found, that the specific 
gravity of the bars, &c. was in a small degree increased by 
rolling, and that the contrary effect was produced by annealing. 
The specific gravity of gold, 23 car. 3^ grs, fine, when rolled 
and stamped without being annealed, I found to be 19,277; 
but, when the same was annealed, the specific gravity was 
19,231. 
1 am, however, inclined to believe, that annealing had reduced 
the specific gravity to much less than is here stated ; and that 
the subsequent operation of stamping had, in some measure, 
compensated the effects of annealing. For, it may be recol- 
lected, that in the experiments lately mentioned, it was proved, 
that the specific gravity of the pieces which had. not been an- 
nealed, was reduced, by long continued friction, from 19,277 to 
19,1 71 ; an effect surpassing that which resulted from annealing 
by ,060 (19,231 — 19,171 = ,060) ; and, if heat was the cause, 
the reverse might have been expected, inasmuch as the anneal- 
ing heat exceeded that which was produced by friction ; but, 
