HISTORY OF THE AMERICAN MENHADEN. 235 



occurs iu bones, and in South Caroliua and otlier fossil and mineral 

 phosphates, 'contains 3 parts of lime to 1 of phosphoric acid. This 

 is often called bone phosphate, and is insoluble in water. When the 

 bone phosphate is treated with sulphuric acid, the latter takes part of 

 the lime to itself, forming sulphate of lime, and leav^es the phosphoric 

 acid in the form of a superphosphate. This last is soluble in water, is 

 more readily diffused through the soil, and when used as a fertilizer 

 can be taken up by the plant at once, while the bone phosphate is 

 slowly available as plant food. Phosphoric acid which has been ren- 

 dered soluble often enters into other forms of combiuation, with lime, 

 aluminia, &c., which, though insoluble in water, are soluble in citrate of 

 ammonia. The terms "reverted," "reduced," and "precipitated" are 

 applied to it when in this form. The reverted phosphoric acid ranks in 

 solubility, in capability of diffusion through the soil, and consequently 

 in value, between the soluble and insoluble. The soluble and reverted 

 are sometimes classed together as available phosphoric acid. 



In some analyses the percentage of phosphoric acid is not stated sep- 

 arately, that of "bone phosphate of lime" being given in its stead. 

 Sometimes the expression "soluble bone phosphate of lime" is met 

 with, which is certainly a misnomer. One hundred parts by weight of 

 phosphoric acid unite with about 118 parts of lime to form 218 parts of 

 bone phosphate ; 100 parts or pounds of phosphoric acid ar-e said, there- 

 fore, to be equivalent to 218 parts of bone phosphate. I lay especial 

 stress on this point, because- those not familiar with chemistry are apt 

 to be deceived in comparing analyses in some of which the term phos- 

 phoric acid and in others the term bone phosphate is used. It would 

 be more accurate and clear, and in every way better, to discard the term 

 bone phosphate of lime in analyses of fertilizers, and speak only of 

 phosphoric acid. 



Potash, or potassa, is the compound of the metal potassium with 

 oxygen, which is represented by the chemical formula KaO or KO. 

 This, combined with sulphuric acid, forms sulphate of potash. Potas- 

 sium and chlorine together form chloride of potassium, or "muriate of 

 potash," as it is called by dealers. 



As the analyses and the valuations of the fertilizers to be discussed 

 can be given most concisely and clearly together in tables, explanations 

 of the latter subject may properly be given here. 



Valuations of commercial fertilisers. 



308. The agricultural value of a fertiliser^ the gain which will result 

 from its use in a given case, is subject to such varying conditions of soil, 

 climate, culture, and crop, as to preclude the possibility of exact esti- 

 mate. The commercial value, being dependent upon its composition and 

 the state of the market, admits of more nearly correct calculation. 



It is customary to make estimates of the commercial values by attrib- 

 uting to each of the important ingredients a certain value per pound j 



