38 FIELD, FOREST AND FARM 



acid for the growth of a hundred kilograms of wheat. 

 Despite their great value as a fertilizer bones will 

 never be thus used except to a limited extent, because 

 they are not abundant enough and also because they 

 are much in demand in various arts and manufac- 

 tures. Fortunately in some localities phosphate of 

 lime is found in certain coarse pebbles called nodules 

 or coprolites. These precious stones are carefully 

 collected and ground to powder in a mill. Then, in 

 order to make the substance more soluble in damp 

 soil, and thus better fitted for the nutrition of plants, 

 it is sprayed with an extremely corrosive liquid 

 called sulphuric acid or, more commonly, oil of vit- 

 riol. In this way is obtained the superphosphate of 

 lime which manufacture gives to agriculture as one 

 of the most powerful of fertilizers, especially for the 

 raising of grain. 



"We were wondering a little while ago what sub- 

 stances could be contained in the ashes of a burnt 

 plant, and we have now found potash to be one of 

 them. Moreover, since all vegetation must have 

 phosphate in order to thrive, this also ought to be 

 found in the ashes, phosphate being indestructible 

 by heat. And, in fact, after the incineration of any 

 vegetable matter whatever, as a bundle of hay or a 

 handful of grain, the delicate processes of science 

 can always recover this compound of phosphorus; 

 and they further find lime, iron in the form of rust, 

 the silicious component of pebbles, and divers other 

 substances of less interest. 



