36 THE FOOD OF PLANTS CHAP. 



weeds, etc. etc. ; and phosphoric acid in many 

 common manures. 



For instance, bones in all the various forms used 

 in agriculture contain large quantities of phosphoric 

 acid in combination with lime. Take a piece of 

 bone, heat it red hot on the platinum foil, and then 

 M r arm it with nitric acid and test it as before, and 

 phosphoric acid will be found in it. 



Another common source of artificial manure are 

 earth phosphates, such as apatite and coprolites ; the 

 latter are the fossil remains of fish, and on warming 

 with nitric acid and testing as before, will show 

 the presence of phosphoric acid. 



Phosphate slag (formed in certain methods of 

 making steel), will also show the presence of phos- 

 phoric acid in the same way. 



Sometimes the farmer prefers to have his phos- 

 phate soluble in water, in which case the bones or 

 coprolites are treated with sulphuric acid. The 

 experiment may be carried out as follows : 



Take some powdered ignited bone and warm it 

 with a few drops of dilute sulphuric acid and heat 

 till dry. The dry powder left is now soluble in 

 water. Boil with water, filter, and test as before. 

 Phosphoric acid will be found in the solution, while 

 on boiling the bone dust with water, before it has 

 been treated with acid, nothing will be dissolved. 



We have now learnt that the plant con- 

 tains water, charcoal, and a mineral ash, and 

 that this ash contains among other things 



