THE FERTILITY OF SOILS. 157 



consist of phosphate of magnesia and ammonia, 

 which must have been obtained from the hay and oats 

 consumed as food. Twenty-nine of these stones 

 were taken after death from the rectum of a horse 

 belonging to a miller, in Eberstadt, the total weight 

 of which amounted to 3*3 lbs. ; and Dr. F. Simon has 

 lately described a similar concretion found in the 

 horse of a carrier, which weighed 1'6 lb. 



It is evident that the seeds of corn could not be 

 formed without phosphate of magnesia, which is one 

 of their invariable constituents ; the plant could not 

 under such circumstances reach maturity. 



Some plants, however, extract other matters from 

 the soil besides silica, potash, and phosphoric acid, 

 which are essential constituents of the plants ordi- 

 narily cultivated.* These other matters, we must 

 suppose, supply, in part at least, the place and per- 

 form the functions of the substances just named. 

 We may thus regard common salt, sulphate of pot- 

 ash, nitre, chloride of potassium, and other matters, 

 as necessary constituents of several plants. 



Clay-slate contains generally small quantities of 

 oxide of copper ; and soils formed from micaceous 

 schist contain some metallic fluorides. Now, small 

 quantities of these substances also are absorbed into 

 plants, although we cannot affirm that they are 

 necessary to them. 



It appears that in certain cases fluoride of calci- 

 um f may take the place of the phosphate of lime in the 

 bones and teeth; J at least it is impossible otherwise 

 to explain its constant presence in the bones of 



* For more minute information regarding soils see the supplemen- 

 tary chapter at the end of Part I. 



t Fluorine is the base of the acid contained in Fluor or Derbyshire 

 spar ; with hydrogen it forms the hydrofluoric acid. The acid is separ- 

 ated by heating fluor spar with sulphuric acid, and is distinguished by 

 its power of corroding glass, and of uniting with its silica. Compounds 

 of Fluorine are called Fluorides, of the acid Hydrofluates. Calcium is 

 the metallic base of lime. 



t The earthy parts of bones are composed principally of the phos- 

 phate and carbonate of lime in various proportions, variable in different 

 animals, and mixed with small quantities, equally variable, of phos- 



14 



