494 LECTURE LI. 



tity of heat disappears, except some cases in which a chemical decomposi- 

 tion has accompanied the change ; thus, in the detonation of gunpowder, a 

 large quantity of gas acquires the state of elasticity, but at the same time a 

 great degree of heat is produced. 



The effects of the expansion of bodies by heat, and of their contraction 

 by cold, are observed in the frequent accidents which happen to glass and 

 to porcelain from a sudden change of temperature. Glass conducts heat so 

 slowly, that one side of a vessel may become much heated, and conse- 

 quently expanded, while the other remains much colder, and if the vessel 

 cannot readily accommodate its form to this change of proportions, it will 

 most commonly crack, the colder parts dividing, in consequence of their 

 being too much stretched by the adjoining hotter parts. Hence the thinner 

 the glass is, the less liable it is to crack from any sudden expansion ; and 

 if it be very thick, however simple its form may be, it will still crack ; 

 for no flexure, which it can assume, can be sufficient for the equilibrium 

 of the external parts without being too great for that of the parts near the 

 middle. 



When glass in fusion is very suddenly cooled, its external parts become 

 solid first, and determine the magnitude of the whole piece ; while it still 

 remains fluid within. The internal part, as it cools, is disposed to contract 

 still further, but its contraction is prevented by the resistance of the 

 external parts, which form an arch or vault round it, so that the whole is 

 left in a state of constraint ; and as soon as the equilibrium is disturbed in 

 any one part, the whole aggregate is destroyed. Hence it becomes necessary 

 to anneal all glass, by placing it in an oven, where it is left to cool slowly ; 

 for, without this precaution, a very slight cause would destroy it. The 

 Bologna jars, sometimes called proofs, are small thick vessels, made for 

 the purpose of exhibiting this effect ; they are usually destroyed by the 

 impulse of a small and sharp body, for instance a single grain of sand, 

 dropped into them ; and a small body appears to be often more effectual 

 than a larger one ; perhaps because the larger one is more liable to strike 

 the glass with an obtuse part of its surface. In the same manner the 

 glass drops, sometimes called Prince Rupert's drops, which are formed by 

 suffering a portion of green glass in fusion to fall into water, remain in 

 equilibrium while they are entire ; but when the small projecting part is 

 broken off, the whole rushes together with great force, and rebounding by 

 its elasticity, exhibits the effect of an explosion. The ends of these drops 

 may, sometimes, but not always, be gradually ground off without destroy- 

 ing them, so that the concussion produced by breaking the drop seems to 

 be concerned in the destruction of the equilibrium.* 



The tempering of metals appears to bear a considerable analogy to the 

 annealing of glass ; when they are made red hot, and suddenly cooled, they 

 acquire a great degree of hardness, which renders them proper for some 

 purposes, while the brittleness which accompanies it would be inconvenient 

 for others. By heating them again to a more moderate temperature, and 



* Hooke's Microg. Bruni, Ph. Tr. 1745, p. 272. Watson, ibid. 1745, p. 505. 

 Lecat, ibid. 1749, p. 175. Hanow, Versuche mit den Spring-Kolbchen, 4to, Danz. 

 1751. 



