34 THE CHEMISTRY OP THE FARM. 



similar in value to farmyard manure. It becomes more 

 valuable as it loses water. 



Guano. This manure consists chiefly of the dried ex- 

 crements of sea fowl. When guano has been deposited 

 in the absence of rain it contains a large amount both of 

 nitrogenous matter and phosphates. If exposed to rain 

 the original nitrogenous matter is decomposed, and the 

 nitrogen volatilised in the form of carbonate of ammonium ; 

 the guano remaining is then almost purely phosphatic. 

 Ichaboe guano, for example, is a recent deposit, containing 

 about 12 per cent, of nitrogen, and 10 per cent, of 

 phosphoric acid ; while Arbrohlos guano is a phosphatic 

 guano, containing 1 per cent, of nitrogen, and 33 per cent, 

 of phosphoric acid. The largest deposits of guano are on 

 the Peruvian coast and the adjacent islands. The present 

 imports contain 4 8 per cent, of nitrogen, 14 23 per 

 cent, of phosphoric acid, and 2 4 per cent, of potash. 

 From its great variation in composition guano should 

 always be purchased on analysis. 



In a nitrogenous guano the nitrogen is chiefly present 

 as uric acid, and as ammonium salts. The strong smell 

 of a damp guano is due to carbonate of ammonium. 

 The phosphoric acid exists principally in the form of 

 phosphate of calcium, but in nitrogenous guanos a small 

 part exists as phosphate of ammonium, a salt readily 

 soluble in water. 



Damp Peruvian guano is sometimes treated with a 

 small proportion of sulphuric acid, it is then called " Dis- 

 solved guano. 9 ' Such guano contains no volatile carbon- 

 ate of ammonium, and nearly the whole of the phosphates 

 has become soluble in water. 



