180 GUANO. 



must be considered the most valuable constituent 

 of bird manure ; it is found here invariably in com- 

 bination with lime, and hence, in analyzing, it is 

 usually defined and represented, for the sake of 

 greater simplicity, as phosphate of lime. When 

 guano is subjected to heat, it is left as a residue in 

 the ashes, since it does not undergo combustion and 

 is not volatile. The more phosphate of lime (ash) 

 and the less ammoniacal salts (combustible sub- 

 stances) contained in a guano, the lower must its pe- 

 cuniary worth be estimated. Good Peruvian guano 

 contains from one fourth to one third phosphate of 

 lime ; the washed-out and comparatively worthless 

 varieties (African, Patagonian, etc.), from, three 

 fourths to four fifths. The assertion is frequently 

 made, indeed, that the remarkable efficacy of guano 

 must be attributed chiefly to the phosphate of lime 

 which it contains ; but guano itself most positively 

 refutes this assumption, for, were it true, then inferior 

 guanos must be far more efficacious than those of 

 better quality, since they contain from twice to three 

 times the quantity of phosphate of lime. That they 

 are not, however, so effective, has been demonstrated 

 by hundreds of practical experiments made in Sax- 

 ony by farmers, who, in order to save 2d. or Zd. in 

 purchasing bird-manure, have made use of Patago- 

 nian or African guano. 



The remaining constituents of guano — salts of 

 potash^ salts of soda (Glauber salt, common salt), and 

 gypsum — exist in such inconsiderable quantity in 



