10 PHOSPHORUS AND THE PHOSPHATES. 



of a phosphate. Whether the phosphate is formed in the soil, or in the plant, is a point 

 difficult to determine. It is an element of food, and not of mechanical support. It is 

 required as a constituent part of the grain ; and the grain itself, in consequence of its 

 composition, becomes an important nourishment for animals. It is necessary, however, 

 to say that magnesia enters into the composition of the straw of the cereals, but in small 

 proportion to the quantity in the grain. So it is found in the ash Of fruit and forest trees, 

 but the bark is almost destitute of it. In animals, it enters into the composition of the 

 harder parts, as the internal and external skeletons, but in less proportion than lime. It 

 forms salts with the same acids as lime. We have no facts which go to show that lime 

 may replace the salts of magnesia, or the contrary. 



The state in which magnesia is required by the cereals, is that of a phosphate. No 

 grains contain the carbonate or other salts of magnesia, except in combination with phos- 

 phoric acid. I have so arranged the analyses of the cereals, that this fact may assume 

 its proper importance. It will be seen, on consulting these analyses, that both magnesia 

 and lime exist in the straw and chaff, while in the kernels they appear only in traces as 

 carbonates. 



PHOSPHORUS, PHOSPHORIC ACID, PHOSPHATE OF LIME, .fee. 



Phosphorus, which is one of the most remarkable of the simple substances or elements, 

 exists in all organized bodies. It is never found simple in nature, and never enters as such 

 into organized bodies. We have therefore nothing to say of it as such ; but on its compound 

 forms, or combinations with oxygen, lime and magnesia, and some other bodies, it is ne- 

 cessary to dwell for a moment. In plants, it is found combined in the four principal alkalies 

 and alkaline earths, viz. lime, magnesia, potash and soda. It is also in combination 

 with iron. 



If the general presence of a substance is an evidence of its importance, the phosphoric 

 compounds rank high in this respect. It is, however, in the fact that grains are its main 

 receptacles, that its real importance is indicated. In respect to the quantity present in any 

 particular part or organ, its distribution follows the same law as that which governs the 

 distribution of other bodies ; thus it is present in the wood, bark, leaves and fruit, but not 

 in equal quantities. In the bark of trees, straw and chaff of grain, it is comparatively 

 trifling in amount. In the leaves of many plants it is abundant. In all edible parts, 

 however, it forms the largest proportion of the whole inorganic matter. These facts indi- 

 cate very clearly the utility of its presence in soils ; and not only this, but its amount ; for 

 as it is comparatively only in small quantities that it is always found, it is probable that it 

 is one of the first which becomes exhausted by culture. In most soils, the phosphates are 

 only appreciable, or just susceptible of being weighed in one hundred grains. A total 

 absence of phosphates in a soil will render it barren and unproductive. It is not impro- 

 bable that phosphate of lime may replace and become a substitute for that of magnesia, 

 when the latter is very deficient in quantity. 



