OF THE FERTILITY OF SOILS. 155 



of the quantity of phosphate of magnesia contained 

 in grain, when we consider that the concretions in 

 the coscum of horses consist of phosphate of mag- 

 nesia 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 Eber- 

 stadt, the total weight of which amounted to 3lbs. ; 

 and Dr. F. Simon has lately described a similar 

 concretion found in the horse of a carrier, which 

 weighed l^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 

 ordinarily cultivated. These other matters, we 

 must suppose, supply, in part at least, the place 

 and perform the function of the substances just 

 named. We may thus regard common salt, sul- 

 phate of potash, 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 



