194 PROCEEDINGS OF THE [Feb., 1888. 



the partially hydrated and intermediate compounds standing between the bi- 

 and tri-calcic phosphates. According to its chemical combination the tetra- 

 calcic phosphate should consist of : 



Phosphoric Acid, . . . 38.4 per cent. 

 Lime, 61.6 



and, according to the representations of Dr. Paul "Wagner and others, it should 

 be readily taken up by an aqueous solution of citrate of ammonia — the usual 

 solvent employed for the estimation of that phosphoric acid which is insoluble 

 in water, but is regarded as "available" for the nourishment of plants. 



The specimen exhibited by Dr. Shepard, yielded on its examination in his 

 laboratory, 33.45 per cent, of phosphoric acid, of which only slightly more than 

 one-third was soluble in neutral citrate of ammonia when subjected to the 

 method prescribed by the Association of Official Agiicultural Chemists. 



As an illustration of the discrepancy of opinion in regard to the value of 

 the ground ' ' Thomas " slag ( very probably the result of essentially different 

 material operated upon), the following quotations may be made from the re- 

 ports of two of the leading agricultural experimental stations of the New Eng- 

 land States, viz : 



Phosphoric acid soluble in 



citrate of ammonia, 19. 57 per cent. 0.19 per cent. 



According to Dr. Shepard's experience, the finely ground "Thomas" slag 

 yields to the neutral solution of citrate of ammonia 5 to 7 per cent, of phos- 

 phoric acid, which is not much more than is afforded by the best grades of 

 South Carolina phosphate dust (actual "floats"). 



MARCH 22d, 1888. 

 The President in the Chair. 



Books Received. 

 Royal Society, London : Proceedings, No. 262. 



