DIMORPHISM. 153 



of the silicates from each other and their crystallization ; and 

 the less mixed glasses are known to be most liable to this 

 change. It is probable that all bodies differ when in the 

 vitreous and in the crystalline forms in the proportion of com- 

 bined heat which they possess, as has been observed of melted 

 sugar (page 43) in these two conditions. 



Arsenious acid when fused or newly sublimed appears as a 

 transparent glass of a light yellow tint. But left to itself, it 

 slowly becomes opaque and milk white ; the change commencing 

 at the surface and advancing to the centre, and often requiring 

 years to complete it in a considerable mass. The arsenious acid 

 is no longer vitreous, being changed into a multitude of little 

 crystals, whence results its opacity ; and it has altered slightly 

 at the same time in density and in solubility. But the passage 

 from the vitreous to the crystalline state may take place in- 

 stantaneously, and give rise to an interesting phenomenon lately 

 observed by H. Rose. The vitreous arsenious acid seems to 

 dissolve in dilute and boiling hydrochloric acid without change, 

 but the solution on cooling deposits crystals which are of the 

 opaque acid, and a flash of light, which may be perceived in the 

 dark, is emitted in the formation of each crystal. This pheno- 

 menon depends upon and indicates the transition, for it does 

 not occur when arsenious acid already opaque is substituted for 

 vitreous acid, and dissolved and allowed to crystallize in the 

 same manner. 



A still greater change than those described, is induced upon 

 certain bodies by exposure to a high temperature, without any 

 corresponding change in their composition. Several metallic 

 peroxides, such as alumina, oxide of chromium and peroxide of 

 tin cease to be soluble in acids after being heated to redness. 

 The same is true of a variety of salts, such as many phos- 

 phates, antimoniates and silicates. All these bodies contain 

 water in combination, when most readily dissolved by acids, 

 which constituent is dissipated at a high temperature, but in 

 general before the loss of solubility occurs, so that the con- 

 tained water alone is not the cause of the solubility. Berzelius 

 remarked an appearance often observable when such bodies are 

 under the influence of heat, and in the act of passing from the 

 soluble to the insoluble state. They suddenly glow or become 

 luminous, rising in temperature above the containing vessel, 

 from a discharge of heat. The rare mineral gadolinite affords a 



