fl44 LECTURE LI. 



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 externul 

 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, some- 

 times 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 iu 

 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 destroying them, so that the concussion produced by 

 breaking the drop seems to be concerned in the destruction of the equili» 

 brium. 



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 wliich accompanies it would be inconvenient 

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

 suffering them to cool more gradually, they are rendered softer and more flexible, 

 and the more as the heat which is thus applied is the more considerable. 



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