456 



crystalline plates in the acoustic researches of Savart, and how 

 strongly distinguished that texture is from a crystalline texture or 

 structure, a nearer analogy than that of a solution ready to cry- 

 stallize, I think, will be found in the condition of water cooled below 

 the freezing-point but still remaining liquid, until by a tremor, 

 or the percussive contact of a solid body, or the mere contact of a 

 crystal of ice, its temperature rises to 32 and it becomes ice. If so, 

 glass will be a substance in which this state of arrested liquidity, or 

 potential solidity, is permanent. And this inference will harmonize 

 with known facts. Gregory Watt proved that heat is evolved when 

 mineral glasses crystallize or become (permanently and truly) solid *. 

 The preparation of sugar called barley-sugar is the vitreous condi- 

 tion of that body, already taken as a type of simple glass ; while 

 granular sugar, and more perfectly sugar-candy, exhibit its crystal- 

 line state. Prof. Graham has shown that, at a certain temperature, 

 by mechanical means the former may be converted into the latter, 

 the temperature quickly rising 70 on the transition of the sugar 

 from the glassy to the crystalline state. This and similar facts in- 

 duced him to refer the peculiar constitution and properties of glass 

 in general to the permanent retention of a certain quantity of heat in 

 a latent state, which becomes sensible on its crystallization ; and this 

 will take place on its being preserved in a soft state at certain 

 temperatures. 



There are some remarkable and instructive parallels between the 

 phenomena of the crystallization of water, and that of glass and 

 some other bodies. It follows from the experiments and inductions 

 of Gregory Watt already cited, that during the crystallization of 

 glass a higher temperature must be communicated to the interior 

 than that existing over its surface, by the evolution of heat at the 

 points where the crystalline form is assumed, which will be gradually 

 conducted throughout the mass. So that, in the express words of 

 Faraday, in relation to ice, " by virtue of the solidifying [crystal- 

 lizing] power at points of contact, the same mass may be freezing and 

 thawing at the same moment ;" and the " freezing process in the 

 inside may be a thawing process on the outside," and thus contribute 

 to the slowness of the cooling, and allow the crystallization therefore 

 to be the more perfect. We here seem to have the explanation 

 * Phil. Trans. 1804, pp. 285-290. 



