106 



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 he so much apprehended as was once the case, 

 since the remarkable change of colour is a sufficient criterion to de- 

 tect the fraud. The ultimate results of the experiments on copper 

 and silver are, that these, 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 the two metals ought to 

 have the preference, as the colour of the gold is least affected 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 the interior texture of the mass during the processes 

 of melting and casting ; to a difference 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 pro- 

 ducing a bar of metal more dense at the bottom than towards the 

 top ; to peculiar effects 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 abra- 

 sion 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 



