V.] PHOSPHATES OF LIME. 35 



equivalents of water to make up the deficiency of 

 lime. 



65. You will also carefully note that in each case 

 we have three equivalents of base combined with 

 the one equivalent of phosphoric acid. In one case 

 lime is the only base, in the two others they consist of 

 lime and water, but in each case there are three 

 equivalents of base. Hence, phosphate of lime is fre- 

 quently spoken of as a tri-basic phosphate, or a 

 three-base phosphate. I have gone somewhat fully 

 into these details, because if you clearly under- 

 stand these terms, you will be the better able to 

 trace the many important uses which these three 

 phosphates of lime serve in the nutrition of our 

 crops. 



66. We are now in a position to follow out our ex- 

 planation of the changes which take place in bones 

 after they have been applied to the soil. The phos- 

 phate of lime present in bones is the tri-calcic 

 phosphate. When the bones are acted upon 

 in the soil by rain-water, which, as you know, con- 

 tains carbonic acid or when acted upon by the 

 carbonic acid produced in the soil in each case 

 we get one equivalent of the lime removed by the 

 carbonic acid, and the tri-calcic phosphate acted upon 

 then becomes bi-calcic phosphate and car- 

 bonate of lime. The bi-calcic phosphate dissolves 

 gradually in water, and is thus taken up into the cir- 

 culation of plants in a soluble form. The changes in 

 the size and condition of the bones which have been 

 mentioned as being adopted (60, 62, 63) all helped in 

 various degrees to promote their decomposition. The 

 action of the carbonic acid and water was greater when 

 the bones were broken into small pieces, because a 

 larger surface was thus exposed to their influence. 

 The fermented bones were quickly acted upon in the 

 soil, because by this fermentation they had been made 

 soft, and consequently they soon broke up in the soil 



