PHOSPHATES. 45 



phosphate (bi-phosphate) of lime, containing more 

 than seventy per cent of the acid, and a very valu- 

 able form of plant food. There are other compounds 

 of phosphoric acid, as for instance, phosphate of 

 magnesia, which also enters into plant and animal 

 tissue; phosphate of soda, of potash, etc., but phos- 

 phate of lime is really the only compound of this 

 acid of real importance to us as a source of plant 

 food, and in bones, guano, phosphate rock, and 

 Thomas' or basic slag (phosphate meal, odorless 

 phosphate) we have the only stores worth mention- 

 ing from which we can draw our supply of phos- 

 phoric acid. 



The compound ^'phosphate of lime," wherever 

 found as a natural product, is firmly fixed, and does 

 not readily yield up its phosphoric acid to the use 

 of plants. The lime holds the acid in firm embrace. 

 In sulphuric acid, however, we have a means of 

 breaking the combination. This powerful acid, 

 when brought in contact with phosphate of lime, 

 forces some of the lime to part with its phosphoric 

 acid, and enters with this lime into a new union — 

 sulphate of lime or gypsum. The phosphoric acid 

 thus freed or driven off, attaches itself lightly to 

 the remaining lime, forming with it a double or bi- 

 phosphate. By the addition of more sulphuric acid 

 this process may be repeated until we have a treble 

 phosphate, generally called a superphosphate, 

 which is a little lime and a great deal of phosphoric 

 acid. The excess of the latter, however, is always 

 ready to leave the companionship of the lime in the 

 regular phosphate of lime combination on short 

 notice, either to sacrifice itself for use by plants, or 

 to enter new and more congenial combinations with 

 free lime, soda, etc., in the soil. When the latter 



