344 



On the Construction of Kite lieu 



of its substance, this heat, however intense it may be, 

 will have no tendency whatever to cause the vessel to 

 crack ; for, the expansion being equal at the two oppo- 

 site surfaces, the tension at those surfaces will be equal 

 also. But, if cold water be suddenly poured into a 

 vessel so heated, its internal surface will be suddenly 

 cooled and as suddenly contracted ; and as the ex- 

 ternal surface cannot contract, being forcibly kept in 

 a state of expansion by the heat, the inside surface 

 must necessarily crack, in consequence of its contrac- 

 tion, and this fracture will make its way immediately 

 through the whole solid substance of the vessel from 

 the inside to the outside surface. 



Sudden heat applied to one side or surface of a 

 brittle vessel causes the opposite side of it to crack ; 

 but sudden cold causes the side to crack to which the 

 cold is applied. 



By forming distinct ideas of what happens in these 

 two cases, every thing relative to the subject under 

 consideration will be rendered perfectly clear and in- 

 telligible. 



The form of a vessel has a considerable effect in 

 rendering it more or less liable to be cracked and 

 destroyed by sudden heat or cold. All flat surfaces, 

 sharp corners, and inequalities of thickness, should, as 

 much as possible, be avoided. 'The globular form is 

 the best of all, and next to it are those forms which 

 approach nearest to it ; and the thinner the utensil is 

 made, consistent with the requisite strength to resist 

 occasional blows, the better it will be in all respects. 



The best composition for earthen-ware for culinary 

 purposes is, I am told, pounded Hessian crucibles, or 

 any kind of broken earthen-ware of that kind, reduced 



