DIMORPHISM. l.,i 



diamond and graphite, the two forms of carbon. The particular 

 form assumed by sulphur and carbonate of lime, which may be 

 made to crystallize in either of their forms at will, is found to 

 depend upon the degree of temperature at which the solid is 

 produced; carbonate of lime being precipitated, on adding 

 chloride of calcium to carbonate of ammonia, in a powder, of 

 which the grains have the form of calc-spar or of arragonite, 

 according as the temperature of the solution is 50 or 150.* 

 A large crystal of arragonite when heated by a spirit-lamp, 

 decrepitates and falls into a powder composed of grains of 

 calc-spar. The crystals of sulphur produced at the higher of 

 two temperatures, become opaque when kept for some days in 

 the air, and pass spontaneously into the other form ; while the 

 crystals produced at the lower temperature are disintegrated, 

 and changed into the other form by a moderate heat. These 

 observations are important as establishing a relation between 

 dimorphism and solidification at different temperatures. 



A considerable variation of properties is likewise often 

 observable in a solid, which is not crystalline, or of which the 

 crystalline form is indeterminate. Thus sulphuret of mercury 

 obtained by precipitating corrosive sublimate by sulphureted 

 hydrogen, is black; but N the same body when sublimed by 

 heat, or produced by agitating mercury in a solution of the 

 persulphuret of potassium, forms cinnabar, of which the powder 

 is the red pigment ver million ; while vermillion itself, if heated 

 till sulphur begins to sublime from it, and then suddenly thrown 

 into cold water, becomes black ; although if allowed to cool 

 slowly it remains red. Yet it is of the same composition ex- 

 actly in the black and red states. The iodide of mercury newly 

 sublimed is of a lively yellow colour, and may remain so for a 

 long time, but it generally begins to pass into a fine scarlet 011 

 cooling, and may be made to undergo this change of colour, 

 in an instant, by strongly pressing it. The precipitated sul- 

 phuret of antimony may be deprived of the water it contains, 

 at the melting point of tin, without losing its peculiar orange 

 colour ; but when heated a little above that temperature, it 

 shrinks and assumes the black colour and metallic lustre of the 

 native sulphuret, without any loss of weight. Again the black 

 sulphuret when heated strongly and thrown into water, loses 



* G. Rose, Phil. Mag. .'id Smi-s, vol. 12, p. Ktt. 



