196 PHOSPHATE OF LIME. 



washing fhem in water, before they are exposed to a heat sufficient to 

 burn away tlie organic matter they contain. 



The details already entered into in the preceding lecture (pp. 159 to 

 163) regarding the general action of nitric acid, in promoting the natural 

 vegetation of the globe, render it unnecessary forme to dwell here on the 

 special action of its compound with lime — njore particularly as the entire 

 subject of the action of lime upon the land will hereafter demand from 

 us a separate consideration. 



The nitrate of lime cannot, as yet, be formed by art, at a sufficiently 

 cheap rate to allow of its being manufactured for the use of the agricul- 

 turist. 



Phosphates of Lime. — Lime combines with phosphoric acid in sev- 

 eral proportions, forming as many different compounds. Of these b^' 

 far the most important and abundant in nature, certainly the most use- 

 ful to the agriculturist, is the earth of hones. It will be necessary, how 

 ever, to advert shortly to two others, with the existence of which it is 

 important for us to be acquainted. 



A. Earth of Bones is the name given to the white earthy skeleton that 

 remains when the bones of animals are burned in an open fire until 

 every thing combustible has disappeared. This earthy matter consists 

 chiefly of a peculiar phosphate of lime, composed of 51i per cent, of 

 lime, and 48i of phosphoric acid. • This compound exists ready formed 

 in the bones of all animals, and is the substance selected in the economy 

 of nature to impart to thern their strength and solidity. It is found in 

 smaller quantit}' in those of young animals, while they are soft, and 

 cartilaginous, — and the softening of the bones, which in after-life occurs 

 as the result of disease, is caused by the unnatural abstraction of a greater 

 portion of this earthy matter than is replaced by the food. 



This earthy phosphate constitutes about .57 per cent, of the dried bones 

 of the ox, is present in lesser quantity in the horns, hoofs and nails, and 

 is never absent even from the flesh and blood of healthy animals. It 

 exists in the seed of many plants, in all the varieties of grain which are 

 extensively cultivated for food, and in the ashes of most common plants. 

 The ashes of leguminous, cruciferous, and composite plants, are es- 

 pecially rich in this compound. 



If we consider that when animals die, their bones are chiefly buried in 

 the earth, and that over the entire globe, animal life, in one or other of 

 its forms, prevails, we shall not be surprised that, in almost every soil, 

 the earth of bones should be found to exist in greater or less abundance. 

 Nor can we have any difficulty in conceiving, if such be the case, 

 whence plants draw their constant and necessary supplies of this 

 substance. 



At the same time, it is true of this compound, as of all the others we 

 have yet spoken q^, as occurring in, and as necessary to the growth of, 

 vegetables, — that some soils contain it in greater abundance than others, 

 and that from some soils, therefore, certain plants will not readily obtain 

 as much of this substance as they require. This is the natural principle 

 on which the use of bone-dust as a manure chiefly depends. 



Hence of two marls both containing carbonate of lime, that will be 

 miDst useful to the land which contains also, as many do, a notable por- 

 tion of jihosohate of lime ; and of two limestones, that will be preferred 



