284 PEODUCTION OF CRYSTALS, OF PHOSPHATE 



present. Hence the minerals produced differ in quantity, 

 in kind, and in relative proportions, according to the 

 quantity and nature of the impurities which the lime- 

 stone contains. 



Again, all limestones, almost without exception, con- 

 tain the remains of animals or vegetables, or both. 

 In the former they are often very rich, and in these 

 phosphate of lime always exists in sensible quantity. 

 When the limestone is changed by heat, the animal and 

 vegetable substances are at the same time entirely 

 decomposed. All that is volatile escapes ; while the 

 earthy phosphate, being fixed, remains intermixed with 

 the limestone. But while the particles of carbonate of 

 lime attract each other, and form crystalline marble, 

 those of phosphate of lime also attract each other, and, 

 although sparingly mixed up with the limestone, gradu- 

 ally approach each other, and finally cohere into crys- 

 talline grains, and regularly crystallised forms. In this 

 way the phosphate, which was intermixed with the 

 rocky mass, perhaps in almost inappreciable proportion, 

 becomes collected together into sensible masses in par- 

 ticular places. And if the rock be one which, like some 

 of our still unchanged limestones, is unusually rich in 

 animal remains, or in mineral phosphates as a mass, the 

 quantity of the separated crystals will be great in pro- 

 portion. Hence their comparative abundance in some 

 of these white limestones, and hence also the reason 

 why the occurrence of them may be looked for, in some 

 localities, in sufficient quantity to admit of their being 

 economically extracted for use in agriculture or the arts. 

 Lastly, while the volatile parts of the animal and 

 vegetable matters contained in the limestone — those 

 which often give to limestones a bituminous character — 

 escape under the influence of heat, a portion of the more 

 fixed charcoal (carbon) remains, in the crystallised form 

 of graphite or plumbago. Hence, the deposits of this 



