136 
Brass . 
and, because the quantity of it was only the one-eighth 
part of the copper employed, which, I imagined, would 
receive too fierce a heat, if put alone into the melted cop- 
per ; I first added to the brass, in fusion, about an equal 
quantity of the tin, and put the mass cold into the melted 
copper ; supplying afterwards the remainder of the tin, 
and then the arsenic ; the whole being generally in the 
following proportion : viz. 32 parts best bar copper, pre- 
viously fluxed with the black flux, of two parts tartar, and 
one of nitre, 4 parts brass, 16 1-2 parts tin, and 1 1-4 ar- 
senic. I suppose, with others, that, if the metal be gra- 
nulated, by pouring it, when first melted, into water, and 
then fused a second time, it will be less porous than at 
first. 
In this process, whatever metals are used, and in what 
proportions soever, the chief object is, to hit on the exact 
point of saturation of the copper, he. by the tin. For, if 
the latter be added in too great quantity, the metal will 
be dull-coloured and soft ; if too little, it wall not attain 
the most perfect whiteness, and will certainly tarnish. It 
is too late to discover the imperfections of the metal, af- 
ter the mirrors are cast and polished ; and no tokens giv- 
en of them (that I know) are sufficiently free from ambi- 
guity. But I observed the following, which proved, in 
my trials, at first view T , indubitable marks of the degree of 
saturation ; and I think it fit to describe them particular- 
ly, as they have not, to my knowledge, been noticed by 
others. 
When the metal was melted, and before I poured it in- 
to the flask, I always took about the quantity of an ounce 
of it, with a small ladle, out of the crucible, and poured 
it on a cold flag; and observed the following appear- 
ances : 
First. If the metal assumed, in cooling, a lively blue, 
or purple colour, commonly intermixed with clouds, or 
