380 REPORT OF NATIONAL MUSEUM, 1899. 



Muntz and Marcano' have called attention to the extensive deposits 

 of guano, sometimes amounting to millions of tons, in caves in Vene- 

 zuela and other parts of South America. 



According to them the deposits consists not merely of the excreta 

 of the birds and l)ats which frequent the caves, but also of the dead 

 bodies of these and other animals. The excreta were found to consist 

 almost wholly of the remains of insects. Through the agency of bac- 

 teria, nitrification takes place, whereby the organic nitrogen is con- 

 verted in nitric acid which combines with the lime from the bones or 

 the carbonate of lime in the soils to form nitrates, as descriljed on 

 page 391. 



^^g5._The phosphates of the classes thus far described are used 

 wholly for fertilizer purposes. In their natural condition they exist 

 in the form known to chemists as tribasic phosphates — that is a com- 

 pound in which three atoms of a base mineral, usually calcium, are 

 combined with one of phosphoric anhydride (PgOg). Thus the com- 

 mon tribasic phosphate of lime has the formula (CaO)3 PgOj (= 45.81 

 parts by weight T,,0, and 51.19 CaO). Other bases, as alumina, iron, 

 or magnesia, may partially replace the lime, but the phosphate is 

 always deteriorated thereby. This is particularly the case when alu- 

 minum and iron are the replacing constituents. Although when finely 

 ground the tricalcic phosphates are of value for fertilizers, it is cus- 

 tomary to first submit them to chemical treatment in order to render 

 them more readily soluble. 



This treatment consists, as a rule, in converting them into a super- 

 phosphate by treatment with sulphuric acid whereby a portion of the 

 base becomes converted into sulphates and the anhydrous and insoluble 

 tribasic phosphate into a hydrous and soluble monobasic form of the 

 formula CaO. (HaO)^. P^Og. There are other reactions than those 

 above given, but the process is one too complicated for discussion here, 

 and the reader is referred to especial treatise on the subject. 



BIBLIOGRAPHY. 



R. A. F. Penrose, Jr. Nature and Origin of Deposits of Phosphate of Lime. Bul- 

 letin No. 46, U. S. Geological Survey, 1888, pp. 143. Gives a bibliography, up 

 to date, of publication. The following have appeared since: 

 W. H. Adams. List of Commercial Phosphates. 



Transactions of the American Institute of Mining Engineers, XVIII, 1889, 

 p. 649. 

 John D. Frossard. About some Apatite Deposits of Ontario. 



Engineering and Mining Journal, VIII, 1889, p. 194. 

 Paul Levy. Des phosphates de chaux. De leurs principaux gisements en France et 

 a I'etranger des gisements r^cemment decouvertes. Utilisation en agriculture; 

 assimilation par les plants. 



Annales des Sciences Geologique, XX, 1889, p. 78. 



^Comptes Rendus de 1' Academic des Sciences, Paris, 1885, p. 65. 



