138 Specula . 
the calx of copper ; and, if the tin be prevalent, a blueish 
die ought to appear. Either of these colours, therefore, 
appearing unmixed, shews the redundance of that metal, 
to which each belongs. iVnd, as brass, when cast alone, 
has always a yellow tinge, so, when these three colours 
are exhibited in a cloud-like mixture, they shew an equa- 
lity and due proportion of their respective metals in the 
composition. When too large a mass of the metal is cast 
together, its intense and lasting heat calcines the surface 
so deeply, as (when exposed to the air) to obscure the co- 
lours ; so that a small quantity will best serve to exhibit 
them. 
As to the method of casting the mirrors, it has been di- 
rected, to leave the ingate, or superfluous part of the cast, 
so large, as to contain a quantity of metal, equal to that in 
the mirror itself ; which would occasion a great waste of 
it, and render it not easy to cast, at once, more than one 
mirror in each mould ; and even this might be done so 
injudiciously as not to afford security against a miscar- 
riage of the cast. But it will appear, that this great quan- 
tity of metal and incommodious manner of casting it, are 
by no means necessary. However, a judgment cannot 
be formed, of what may be the safest and most eligible 
method for casting the mirrors, unless it be considered, 
what are the circumstances attending this operation, in the 
case of malleable metals ; and how the management of 
speculum- metal, in this respect, must differ from that of 
them : since there must be peculiar difficulty in casting, 
in sand, a metal more brittle than glass. 
When any fused metal is poured into the flask, the ex- 
ternal parts of it, which are in contact with the mould, 
congeal and harden sooner than the internal parts, and 
form a solid shell, filled with the rest of the metal, in a 
fluid state. This will, consequently, remain in a state of 
greater expansion, from its heat, than the external crust ; 
