AGKICULTURAL CHEMISTRY. 81 



proportion to each other. Its purity is tested by its 

 weight. The more water it contains, the lighter it is ; 

 and no one should buy it for good, unless it is once 

 and J as heavy as water. It has generally been re- 

 tailed for 121 cents a pound, but can now be procured 

 for agricultural purposes at 2h cents. It is a very 

 powerful acid, and may undoubtedly be used to ad- 

 vantage in composting some manures, and especially 

 for dissolving bones, to be used as fertilizers. It is 

 more commonly known as oil of vitriol 



47. Phosphoric Acid (PO*) exists largely in the 

 bones of animals, and in the phosphate of lime, a min- 

 eral called appatiiCj and is found in all soils, not en- 

 tirely exhausted by cropping. How best to restore it 

 to soils deprived of it by bad management, so as to 

 enable them to produce the cereals in abundance, will 

 be considered in another place. It may be obtained 

 in a pure state by burning phosphorus in oxygen gas. 

 In this state it gathers moisture from the air, and as- 

 sumes the appearance of a white, flaky cloud, but is 

 readily absorbed by water, rendering it intensely sour. 



48. Carbonic Acid (CO") is made up of 1 atom of 

 carbon, 6, to 2 of oxygen, 16, making its atomic 

 weight 22. That is to say, in 22 lbs. of carbonic acid 

 are 6 lbs. of carbon and 16 of oxygen. This is a gas. 

 It is 1^ times heavier than common air; and conse- 

 quently, when produced in large quantities, it falls 

 into low places, as dry wells, cellars, or cisterns, de- 

 stroying sometimes the lives of those who descend ; 

 but, in accordance with a general law of gases, it soon 



