134 
this purpose,) will take as high a polish as glass will, un- 
less it be more brittle than glass. And indeed this pro- 
perty is common to all substances which we know, that 
are capable of such polish : they must be very hard, and, 
as such, brittle; for the polishing powder employed 
would stick and bed itself in any soft metal, instead of 
cutting and polishing it. 
From the result of my trials, I contented myself with 
the composition mentioned hereafter,, as being in every 
respect sufficient for the purpose, and inferior to none in 
whiteness, lustre, and exemption from tarnish : for,, as to 
the addition of silver, I found that, when used even in a very 
small quantity, it had an extraordinary property of ren- 
dering the metal so soft, that I was deterred from employ- 
ing it : and unless it shall be found that,, without this ef- 
fect, it makes the metal less porous than otherwise it might 
be, or less frail and brittle, I am certain that it may,, in 
every other respect, be dispensed with. I had no oppor- 
tunity to try it, in the precise quantity Mr. Edwards re- 
commends, (though I did so before, in very nearly that 
proportion, since I first saw his memoir on that subject 
Sir Isaac Newton made trial of a very small portion of it, 
and found the same effects from it as I experienced : hut 
It is possible, that, if it were added in the just proportion 
discovered by Mr. Edwards, it would be an improve- 
ment, and useful ingredient, in the composition.* 
* Having read somewhere, that sinG and gold made the best 
speculum -metal, I tried it ; and found, that the zinc was sublimed 
from the gold in fusion, and arose to the top in the crucible form- 
ing a white, hard, spongy mass. The metal, called tutanag, is. fit 
for specula, when melted with tin ; but I am certain, that what I 
procured, under the name of tutanag, was a mixture of brass and 
copper, &c. ; for the zinc, in the brass, rose from, it, during the fu- 
sion, in white flowers. 
