operation, in the case of malleable metals; and how the 

 management of speculum-metal, in this respect, must differ 

 from °hat 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 

 external 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. Tliis will, consequently, remain in a state of 

 greater expansion, from its heat, than the external crust; 

 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 wiU 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 

 yray latest formed, remains unfilled; and then the shell 

 of the metdl, 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 va- 

 cuum: by which means, a pit or cavity will be con- 

 stantly and necessarily formed in the face of the cast, 

 in that part of it which was last congealed; which ca- 

 vity 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 



R 2 speculum- 



