( 491 ) 



red fusion is put on an object glass, it may be let'l for hours at the 

 temperature of the room without a trace of crystallisation being 

 noticed. The liquid is now greatly undereooled and exists in a state 

 of metastable equilibrium. For all that, it has the same chemical 

 composition as the solid phase from which it was formed. 



On prolonged exposure, small liquid globules appear locally in the 

 fairly viscous mass, probably owing to local cooling, or by a spon- 

 taneous evaporation of water at those 

 points. These liquid globules are quite 

 isotropous and are surrounded by a 

 delicate aureole having an index of 

 refraction different from that of the 

 rest of the liquid (tig. 3a). The ob- 

 servation shows that, optically, they 

 are, practically, no denser than the 

 liquid, and from the fact that they 

 afterwards become, in their entirety, a 

 Fig. 3u. spherolite of the hexahydrate, we must 



conclude that their chemical composi- 

 tion does not differ from that of the 

 fused mass. 



These globules of liquid are con- 

 verted gradually into doubly-refracting 

 masses whose section is that of a 

 regular hexangle with rounded off 

 angles ; individual crystals are not yet 

 visible in the doubly-refracting mass 

 and the luminous zone around still 

 appears to exist (fig. 3/>). Fig. 3b. 



Here and there, hexangular, sharply 

 limited, very small plate-shaped crystals 

 are also seen to form in the liquid 

 without previous formation of liquid 

 globules l ). In the end, the doubly- 

 refracting hexangular mass gets gra- 

 dually limited by more irregular sides, 

 while a greater differentiation of the 

 mass into light and dark portions 

 points to a crystallisation process coin- 

 Fig. 3c. mencing and progressing slowly. 



l ) These may, however, be formed perhaps owing to the presence of traces of 

 sal ammoniac. 



