152 DIMORPHISM. 



its metallic lustre, and acquires a good deal of the appearance* 

 of the precipitated sulphuret. The nitrites are sometimes white 

 and sometimes yellow ; and crystals of sulphate of manganese 

 are often deposited from the same solution, some of which are 

 pink and others colourless, although identical in composi- 

 tion. 



Such differences of colour are permanent, and not to be con- 

 founded with changes which are peculiar to certain tempera- 

 tures : thus oxide of zinc is of a lemon yellow colour, when 

 strongly heated, but milk-white at a low temperature; the 

 oxide of mercury is much redder, at a high than at a low tem- 

 perature, and bichromate of potash, which is naturally red^ 

 becomes almost black when fused by heat. Even bodies in the 

 gaseous state are liable to transient changes of this kind, the 

 brown nitrous fumes being nearly colourless below zero, and 

 on the other hand deepening greatly in colour at a high tempe- 

 rature. 



The condition of glass is a remarkable modification of the 

 solid form assumed by many bodies. Matter in this state is 

 not crystallized, and on breaking presents curved and not plain 

 surfaces, or its fracture in mineralogical language is conchoidal, 

 and not sparry. The indisposition to crystallize, which causes 

 solidification in the form of glass, is more remarkable in some 

 bodies, such as phosphoric and boracic acids, and their com- 

 pounds, than in others. The biphosphate and binarseniate of 

 soda have the closest resemblance in properties, yet when both 

 are fused by a lamp, the first solidifies on cooling into a trans- 

 parent colourless glass, and the second into a white opaque 

 mass composed of interlaced crystalline fibres. The phosphate 

 at the same time discharges sensibly less heat, than the arse- 

 niate in solidifying, retaining probably a portion of its heat of 

 fluidity or latent heat in a state of combination, while a glass. 

 None of the compounds of silicic acid and a single base, or 

 simple silicates, becomes a glass on cooling from a state of 

 fusion, with the exception of the silicate of lead containing a 

 great excess of oxide : they all crystallize. But a mixture of 

 the same silicates when fused, exhibits a peculiar viscosity or 

 tenacity, appears to have lost the faculty of crystallizing, and 

 constantly forms a glass. The varieties of glass in common use 

 are all such mixtures of silicates. Glass is sometimes devitrified 

 when kept soft by heat for a long time, owing to the separation 



