EVOLUTION AND MUTABILITY OF MATTER.. 275 



zero is not entirely established ; but we may suppose, 

 with the author of the Traitc de Tlicrnioinctrie, that in 

 glass, as in alloys, are to be found compounds which 

 vary according to the temperature. At each tem- 

 perature glass tends to assume a determinate com- 

 position and a corresponding state of equilibrium ; 

 but the previous temperature to which it has been 

 subjected clearly has an influence on the rapidity 

 with which it attains its state of repose. The effect 

 of variation is more marked when we observe glass ot 

 more complicated composition. We can understand 

 that those which contain comparable quantities of the 

 two alkalies, soda and potash, may be more subject 

 to these modifications than those having a more 

 simple composition based on a single alkali. 



Effects of Annealing. — A piece of brass wire that 

 has been drawn and then heated is the scene of 

 certain very remarkable internal changes, and these 

 have been only recently recognized. The violent 

 treatment of the metallic thread in forcing it through 

 the hole in the die has crushed the crystalline 

 particles ; the interior state of the wire is that of 

 broken crystals embedded in a granular mass. 

 Heating changes all that. The crystals separate, 

 repair themselves, and are built up again; they are 

 then hard, geometrical bodies, in an amorphous, 

 relatively soft and plastic mass; their number keeps 

 on increasing; equilibrium is not established until the 

 entire mass is crystallized. We may imagine how 

 many displacements, enormous when compared with 

 their dimensions, the molecules have to undergo when 

 passing through the resisting mass, and arranging 

 themselves in definite places in the crystalline 

 structures. 



