THE FOOD OF PLANTS. 275 



Carbonate of lime, in any form, cannot be regarded 

 as a fertilizing substance having a commercial 

 value. Very strenuous attempts have been made 

 to induce farmers to purchase ground clam and 

 oyster shells, the venders alleging that they were 

 equal to ground bones in fertilizing value ; but 

 this is a fraud of a serious nature. Clam shells are 

 composed of carbonate of lime, while bones are 

 made up of the phosphate, of lime, quite a dif- 

 ferent substance, chemically and agriculturally con- 

 sidered. The shells are composed of carbonic acid 

 and lime, the bones of phosphoric acid and lime, 

 the former acid having no money value, the latter 

 having a high value. 



Calcic carbonates should not be confounded with 

 sulphate of lime, which is plaster or gypsum. In 

 this substance sulphuric acid, or oil of vitriol, is in 

 combination with the lime, in place of carbonic acid, 

 and a very different chemical and fertilizing agent 

 is supplied. It has high value as an application to 

 some fields, although its action is not well under- 

 stood. The experiments which the writer has 

 made with plaster go to prove that its good effects 

 are due rather to the acid than the lime. It has 

 the power of fixing the ammonia of the atmosphere, 

 and forming sulphate of ammonia, which is a salt 

 of much value. In applying gypsum to soils, it 

 must be remembered that but a small quantity can 



