892 
Jarities in the specific gravity of alloys 
of gold take place, from the chemical 
action of their ingredients, Mr. Hatchiett 
proceeds to notice another cause of ano- 
maly, arising from the unequal mixture 
of the hetercgeneous particles; im conse- 
quence of which the lower part of a bar, 
when cast in‘a vertical direction, will be 
of greater specific gravity than the upper 
one, from the particles of gold having 
principally settled to the bottom; hence 
there will be a considerable difference in 
the results of an assay, as the specimen 
is taken from one or the other end of a 
bar. The circumstance, too, of casting 
in iron or sand will generally make a 
difference; the bar which is formed in 
an iron mould being ceteris paribus of 
greater specific gravity than if it had 
been cast in sand. Mr. Hatchett also 
discovered that even where the parts of 
an alloy were so perfectly mixed as not 
_ to separate at all on being cast, yet the 
bar is by no means of the same specific 
ravity throughout; for the lower part 
eing pressed upon, while congealing, by 
the superincumbent metal, is of the 
greatest specific gravity; and the oppo- 
site end having to sustain only the atmo- 
spheric pressure is, of course, of the small- 
est specific gravity. Thus, a bar which 
proved to be throughout ten grains better 
than standard, was found to have the 
specific gravity of thetop = 17.203, and 
that of the bottom = 17.387. Fricticn 
also was found to have a very sensible 
-effect_in lowering the specific gravity of 
metallic alloys. 
The third section treats of the effects 
of friction on the comparative wear of 
o'd and its alloys, which was ascertained 
in three circumstances: Ist, the friction 
between gold coin of the same or dif- 
ferent qualities ; 2dly, the friction of gold 
coin against silver and copper coin; and, 
3dly, the friction of gold against gritty 
powders, metallic filings, &c. these being 
the different circumstances under which . 
gold,‘tn the course of circulation, is sub- 
ject to wear. Instruments for friction 
were invented by Mr. Cavendish; and, 
from a vast multitude of experiments, it 
wis concluded, 
. ’ 
‘¢tst. That when equal friction, assisted by 
a moderate pressure, takes- place between 
pieces of coin which are in-each series of a 
similar quality, then, abrasion is most com- 
mouly produced in an inverse ratio to the de- 
gree of ductility. ; 
‘« adly. That the contrary effect happens, 
when pteces of different quality rub against 
, 
GENERAL SCIENCE. 
each other ; for then, the more ductile metaf 
is worn by that which is harder. . 
«<3dly. That earthy powders and metallic 
filings produce similar effects, and tend to 
wear the different kinds of gold in proportion 
to their respective degrees of ductility., Fine 
gold, being extremely soft and duetile, sus- 
tains a considerable loss, uader many of the 
general circumstances of friction; and as at all 
times it appears certain, that the impressions 
which have been stamped upon it are most 
easily obliterated, even. when actual abrasion 
does not take place, there is much reason to 
conclude, that gold of such extreme ductility 
is not that which is most proper to beformed 
into coin. 
«* But gold of the opposite quality, or at 
least so hard as to be just capable of being 
rolled and stamped, seems to be ms im- 
proper for the purpose of coin. For, even 
supposing that hard gold suffered, in every 
case, less by friction than that which is mo 
derately ductile, (which is not however the 
fact) and allowing that standard gold may, 
by a fixed alloy, be rendered as hard as gold 
reduced by copper to 18 carats; without 
changing the standard proportion of gold, 
yet it would be very difficult always to make 
such standard gold of an uniform degree of 
hardness. Mcreever, by some experiments 
which I purposely made at the mint, upon the 
rolling and stamping of gold of different quali- 
ties, it evidently appeared, that gold equal in 
hardness to that of 18 caratscould not'beern- 
ployed with advantage ; for, the additional la- 
bour which was required for the rolling and 
stamping of this hard gold, the frequent fui- 
lure in making the impression, and the bat. 
tering and breaking of the dies, fully proved, 
that the expence and difficulty attending the 
working of such gold, would by no means be 
compensated by any small degree of durability 
which it might possess over any other. 
«* Phe extremes of ductility and of hard- 
ness being therefore equally objectionable, it 
follows of conrse, that gold of moderate duc+ 
tility must be that which is the best adapted 
for coin; and as nothing but silver or copper 
can be employed to alloy gold which is in- 
tended to be coined, it may be here observed, 
that whatever might have been the original 
motive for introduciag the present standard 
of twenty-two carats, yet it appears, from the 
experiments lately described, that this pro- 
portion of 1-12th of the above-mentioned me- 
tals is (every circumstance being considered) 
the best, or, at least, as good as any which 
could have been chosen, 
* * * + 
‘there is every reason to believe, that 
our gold coin suffers but little by friction 
against itself; and the chief cause of natural 
and fair wear probably arises from exttaneons 
and eritty particles, to the action of which 
the pieces may occasionally be exposed in the 
course of circulation. But still at must be 
repeated, that the united effects of every spe* 
