104 PHOSPHATIC MANURES [chap. 



considerable proportion of this nearly insoluble salt is 

 contained in most fertile soils. . . . These alkaline 

 phosphates (potash and soda) will be found to promote 

 vegetation in a very great degree, the substance of 

 which they are composed, viz., alkaline salts and 

 phosphoric acid, are found in the ashes of most 

 vegetables." Again, Kirwan writes in 1796 about the 

 constituents of plants : " Phosphorated calx is found in 

 greatest quantity in wheat, where it contributes to the 

 formation of animal gluten. . . . Hence the excellence 

 of bone ashes as a manure for wheat. . . ." Finally, 

 de Saussure, in his Recherches Chimiques sur la Vegeta- 

 tion, published in 1804, writes : " Le phosphate de chaux 

 contenu dans un animal, ne fait peut-etre pas la cinq 

 centieme partie de son poids : personne ne doute 

 cependent qu ce sel ne soit essential a la constitution 

 de ses os. J'ai trouve ce meme sel dans les cendres de 

 tous les v^getaux ou je I'ai recherche, et nous n'avons 

 aucune raison pour affirmer qu'ils puissent exister sans 

 lui." This opinion was repeated by Davy, and adopted 

 and disseminated by Liebig, by which time various 

 other experimenters had reached the conclusion that 

 the mineral and not the organic matter contained in 

 bones was their chief fertilising constituent. 



Since all the phosphatic manures which possess any 

 practical importance are phosphates of calcium, it is 

 necessary to discuss these compounds a little before 

 proceeding further. The phosphatic material which is 

 most widely disseminated, occurring in all the primitive 

 crystalline rocks and occasionally found massive, is the 

 true crystalline mineral apatite, Ca5(POj3F, in which 

 the flourine atom may be wholly or partially replaced 

 by chlorine. This is a definite crystalline compound, 

 the undoubted source of all the other compounds of 

 phosphoric acid, but being very hard and difficult of 



