MANURES. 43 



or where it acts directly upon the plant itself. All that we 

 truly know is, that certain earthy and alkaline bodies, or their 

 saline combinations, applied to the soil, promote the growth of 

 plants, and so, in the language of farmers, are manures." 



Of all the mineral substances known to us, LIME is that 

 which performs the most important part in improving the soil, 

 and promoting the growth of vegetables. It is found in nearly 

 all soils that are capable of sustaining vegetation; and, in com- 

 bination with different acids, in a vast variety of vegetable sub- 

 stances. 



Limestone, from whatever series of rocks derived, when 

 submitted to the action of heat loses the carbonic acid* with 

 which it was united, becomes a substance of an acrid nature, 

 absorbs water with an evolution of heat, and by this union 

 forms what is termed a hydrate. In absorbing water it crum- 

 bles down by degrees, while at the same time it begins to im- 

 bibe carbonic acid from the atmosphere. 



In absorbing carbonic acid, the water of the hydrate is ex- 

 pelled, the carbonic acid taking its place. In this way the lime 

 recovers the principles which it had lost by the action of heat. 

 It becomes again a carbonate, without resuming its former 

 hardness and external characters. In proportion as its recom- 

 position takes place, it loses the properties which it had ac- 

 quired by calcination, [fire,] ceases to be acrid and caustic, and 

 its solubility in water is diminished. 



Lime is applied to the soil either in a state of hydrate, that 

 is, immediately after being slacked, and when it still retains its 

 caustic qualities, or in the state of carbonate, that is, after it 

 has again absorbed carbonic acid from the surrounding atmo- 

 sphere and become mild. 



Caustic lime performs two functions apparently opposed to 

 each other. While it dissolves vegetable fibre and renders it 

 soluble, it possesses also the property of forming compounds 

 of a soapy nature, with the soluble portion of vegetable and 

 animal substances, which compounds are not dissolved till after 

 a very considerable time. Lime forms these insoluble com- 

 pounds with almost all the soft animal or vegetable substances 

 with which it can combine. 



It has been ascertained from a series of experiments care- 

 fully made in England, by Bishop WATSON, and it is believed 

 they will apply equally well in this country, that upon an 

 average of every ton of 2240 pounds, good limestone produced 

 1292 pounds of quick-lime, weighed before it was cold; and 



* Carbonic acid is a combination of carbon and oxygen. It was formerly 

 called fixed air, on account of its being so intimately combined in chalk, lime- 

 stone, magnesia, &c. 



