156 ' THE ART OF CULTURE. 



tain certain quantities of phosphates. In the seeds 

 of different kinds of corn particularly, there is abun- 

 dance of phosphate of magnesia. 



Plants obtain their phosphoric acid from the soil. 

 It is a constituent of all land capable of cultivation, 

 and even the heath at Liineburg contains it in ap- 

 preciable quantity. Phosphoric acid has been de- 

 tected also in all mineral waters in which its pres- 

 ence has been tested; and in those in which it has 

 not been found it has not been sought for. The 

 most superficial strata of the deposits of sulphuret 

 of lead {^galena) contain crystallized phosphate of 

 lead {^greemlead ore) ; clay-slate, which forms ex- 

 tensive strata, is covered in many places with crys- 

 tals of phosphate of alumina ( Wavellite) ; all its 

 fractured surfaces are overlaid with it. Phosphate 

 of lime (^Apatite) is found even in the volcanic 

 boulders on the Laacher See in the Eifel, near 

 Andernach.* 



The soil in which plants grow furnishes them with 

 phosphoric acid, and they in turn yield it to animals, 

 to be used in the formation of their bones, and of 

 those constituents of the brain which contain phos- 

 phorus. Much more phosphorus is thus afforded to 

 the body than it requires, when flesh, bread, fruit, 

 and husks of grain are used for food, and this ex- 

 cess is eliminated in the urine and the solid excre- 

 ments. We may form an idea of the quantity of 

 phosphate of magnesia contained in grain, when we 

 consider that the concretions in the csecum of horses 



phate of lime, with small quantities of carbonate of lime and phosphate 

 of magnesia. This powder had been sent to Sir David Brewster from 

 India, with the assurance that it was the same substance which usually 

 is found in the hollows of teak timber. It has long been known that 

 silica, in the form of tabasheer, is secreted by the bamboo ; but I am 

 not aware that phosphates have been found in the same condition. 

 Without more precise information, we must therefore suppose that they 

 are left in the hollows by the decay of the wood. Decay is a slow 

 process of combustion, and the incombustible ashes must remain after 

 the organic matter has been consumed. But if this explanation be cor- 

 rect, the wood of the teak-tree must contain an enormous quantity of 

 earthy phosphates. — Ed. 



* See the analyses of soils in the Appendix. 



