On the Art of Hardening Copper. 275 
was made, of 20 parts of tin and 80 of copper, and a knife - 
-blade was made of it, as before. This was much whiter and 
hatder, but alfo brittler in the fame proportion, and there- 
fore broke by careleffnefs in the polifhing. The edge, how- 
ever, was fo fharp that it could be ufed for making pens ; 
but it did not ftand long, as notches were formed in it by 
each cut. 
When the tin makes twenty-five hundred parts in the 
mixture, it becomes rather white than red, but eXceedingly 
brittle. If the addition_of tin be increafed to thirty in the 
hundred, or more, both thefe properties are increafed in the 
fame proportion, and the compofition becomes fit for {fpecula. 
In gun-metal tin makes nine parts, or more, in the hundred : 
in bronze, 84 parts of copper are mixed with about 16 parts 
of tin, but a confiderable portion of zine or brafs is fometiines 
fubftituted for the latter. Bell-metal contains in general 76 
parts of copper, 19 of tin, and 5 of brafs, or thereabouts. 
What has been here faid may perhaps be fufiicient to con- 
firm the opinion refpecting the art employed by the ancients 
to harden copper, and may furnifh fome hints for the em- 
ployment of fuch compounds in common life. Befides, we 
are hereby enabled to appreciate the different opinions enter- 
tained on this fubject.. M. Monnet imagined that the cop- 
per in ancient times was mixed with arfenic, which rendered 
it hard. No real objection can be made to this being pof- 
fible; but as long as no ancient implements made of this 
mixture are found, the above affertion may at any rate be 
doubted, without mentioning other circumftances which 
feem dire&tly to oppofe it. 
M. Dizé * mentions the addition of iron to copper, as the 
means of rendering the latter harder; and endeavours to 
prove that Geoffroy the younger, who firft drew this con- 
clufion, (from an experiment he made, where copper, mixed 
with fixteen parts of iron in the hundred, was found to 
be equally hard, to have the fame grain on the fracture, and 
to be as fit for making cutting inftruments as the hardened 
copper of the ancicnts,) was too precipitate in forming his 
* Yourmal de Phyfique 1790, April. 
Nn2 opinion, 
