Page:Proceedings of the Royal Society of London Vol 1.djvu/126

Rh turned gold much paler, yet did not materially affect its ductility. With respect to platina, one-twelfth of this metal, alloyed with gold, turned the latter metal to a colour similar to that of tarnished silver, but did not essentially diminish its ductility. Hence it is inferred, that a mixture of platina with gold, with a View to the adulteration of coin, need not be so much apprehended as was once the case, since the remarkable change of colour is a sufﬁcient criterion to de- tcct the fraud. The ultimate results of the experiments on coppa‘ and silver are, that tbese, either jointly or separately, are the only metals ﬁt for alloys to reduce ﬁne gold to the standard; care only must be taken that they, especially the copper, be of the purest sort; for which purpose, the ﬁne granulated Swedish copper is recom- mended as the most proper. A mixture of thc two metals ought to have the preference, as the colour of the gold is least aﬁected by it.

2. In examining, in the second section, the speciﬁc gravity of gold made standard by different metals, single or mixed, it was found that several variations take place from causes independent of any defects in the hydrostatical operations. These are imputed to occasional imperfections in thc interior texture of the mass duringr the processes of melting and casting; to a diﬁ'erence of density in parts of even the same mass; to the nature and position of the mould in which the metal is cast,—a long mould in a vertical position always pr0< ducing a bar of metal more dense at the hottom than towards the top; to peculiar eﬁ'ects which certain metals produce when employed as alloys, and which are often very different from the results of cal- culation; and, lastly, to the effect of friction, which, as it is well known to generate heat, cannot, by the expansion it occasions, but affect the speciﬁc gravity of the metal. It hence follows, that as the speciﬁc gravity of metals is liable to be inﬂuenced by such a nume- rous variety of causes, it is almost in vain to expect absolute precision in the results of such experiments, and that a near approximation is all that can be demanded.

From the experiments made upon separate and entire ingots of gold, reduced to standard by silver and copper, separately and con- jointly, it was proved that their speciﬁc‘gravities were as follows :— gold made standard by silver, 17'927; gold made standard by equal parts of silver and copper, 17-344; and gold made standard by cop- per, 17'157. Hence it appears that the speciﬁc gravity of our gold coin, which is generally alloyed by a mixture of the two metals, must be found somewhere between the two extremes just now men- tioned; or, making allowances for small variations, arising from accidental'causes, between 18 and 17.

3. In the third section, which treats of the comparative wear of gold when variously alloyed, we ﬁnd, in the ﬁrst place, an account of three modes or contrivances for ascertaining the quantity of abrasion by friction, according to the different circumstances of alloy and ﬁgure in the coins. In the ﬁrst, two sets of coins were fastened, each in a frame, one of which was made to move backwards and forwards over the other with certain determined degrees of velocity