139 . 
Specula . 
and its particles will, in the act of shrinking as it cools* 
recede from one another, as being more easily separable* 
and cohere, on each side, with the particles already fixed 
and grown solid : by which means a vacuum will be 
formed in the middle, and this will be gradually filled by 
the superincumbent metal, which has been later poured in, 
and remains longer in a fluid state. But, when there is 
no more metal supplied, the void, which was in this way 
latest formed, remains unfilled ; and then the shell of the 
metal, adjacent to the vacuum, as yet remaining soft, and 
unable to bear the weight of the atmosphere, resting on it* 
sinks, and is pressed down into the vacuum : by which 
means, a pit or cavity will be constantly and necessarily 
formed in the face of the cast, in that part of it which was 
last congealed ; which cavity will commonly be larger 
or smaller, in proportion to the quantity of metal in the 
cast. 
The event will, in this respect, be the same with specu- 
lum-metal, as it is, in the case of that which is tough and 
malleable : only that, as the former, in cooling, arrives 
sooner at its natural state of hardness and brittleness, its 
external solid shell will not bend, but break, and fall into 
the void part under it ; and thus form cracks, or abrupt 
chasms, in the places, where tougher metals would con- 
tract only regular depressions. And also, when the body 
of the cast is small, or the mould is so damp or cold, as 
to congeal, not only the surface, but the substance, of the 
cast too soon, and thus prevent a gradual influx of the fluid 
metal, to keep the central part as distended, as the exte- 
rior shell was, when it became fixed ; the farther contrac- 
tion of the interior parts of this brittle, refractory metal^ 
after it has become solid, will be apt to form rents in it, 
because its substance will not bear extension, without 
rupture. 
