18 INTRODUCTION. I. 



animals. Bone-ash contains about 4% of calcium fluoride, 

 while many mineral phosphates contain as much as 7 or 8 %. 

 According to Lorenz*, the evolution of hydrofluoric acid by a 

 phosphate on treatment with sulphuric acid may be taken as 

 a proof that a mineral phosphate is present. This test is not 

 entirely satisfactory, since many mineral phosphates do not 

 give it, indeed they are known to contain practically no fluoride, 

 but chloride. Thus, there are two varieties of apatite cor- 

 responding to the formuhe 3CaP 2 H -CaR 2 and 3Ca 3 P a O H -CaCl 2 , 

 and many specimens have a composition intermediate between 

 those expressed by the above formulae. Of course the chloride 

 variety is preferable for the manufacture of superphosphate. 



Boron always occurs in combination as boric acid (HB0 2 or 

 H 3 BO 3 ) or borates. It is not a very abundant element except in 

 a few localities, e.g., in Tuscany and in California, but of recent 

 years its presence in a number of plants and products from 

 plants, particularly in wines t and the leaves, stalks, &c., of the 

 vinej has been detected by various observers, leading to the 

 conclusion that boric acid must be present in soils, which 

 probably derive it from igneous rocks. 



There is no evidence that it is essential to plant life. Boric 

 acid or boracic acid is possessed of considerable anti-putrefac- 

 tive properties and is often used for preserving milk and other 

 food products. 



Relative Abundance of the Elements. F. W. Clarke,^ of 

 the U.S.A. Geological Survey, has estimated very carefully 

 the relative proportions of the more common elements con- 

 stituting the earth's crust to a depth of ten miles from the 

 surface. He estimates that of this 



93 % is composed of solid rock, &c. 

 7 % ,, ,, water and air 

 the latter only amounting to about '03 %. 



The following table gives the relative abundance by weight 

 of the elements named : 



Jour. Soc. Chem. Ind. 1889, 307. t Crampton, Amer. Chein. Jour. , 11, 227. 



{ Baumert, Ber. 21, 3290. Bull. Philos. Soc. Washington, xi., 227. 



