C *55 ] 
black fcurf, and had loft about It was much 
duller coloured, harder to the hammer, and cracked 
fooner about the edges, than mixtures of fine gold 
with a larger quantity of platina. By repeated fu- 
fion, and frequent nealing, it became a little fofter 
and tougher, fo as to be drawn into pretty fine wire ; 
but the colour was ftill exceeding dull, more re- 
fembling that of bad copper than of gold. 
The fpecific gravity of this compound was 17.9 15; 
a little lefs than the medium of the three ingredients 
unmixed, and a little greater than the mean gravity 
refulting from the platina by itfelf, and the copper 
and gold mixed ; for copper, in the ftandard pro- 
portion, appears to diminifh the gravity of gold 
more than it ought to do according to calculation. 
From the foregoing experiments it appears, that 
platina is mifcible with gold, in certain proportions, 
without injuring either its colour or ductility, or oc- 
cafioning any confiderable alteration in the gravity : 
experiments related in former papers have fhewn, that 
it ftands aqua- fortis, and the other trials by which 
the purity of gold is eftimated. It is to be hoped, 
that the abufes manifeftly practicable by this mineral 
have hitherto been but rarely, made ufe of. To guard 
againft them is the object of this paper $ to detect 
them, of the next* 
X 2 
XX, Ex- 
