Revieiv of Essays on Calcareous Manures. 149 



must pardon us, if we devote a few lines to matters in which they 

 need no instruction. 



Pure lime is so rarely found in nature, that it may be stated as a 

 general rule, that it can be obtained only artificially, in the process 

 of calcination, as practiced in preparing limestones for making mor- 

 tar. Thus obtained, lime retains the original figure of the stone 

 from which it is manufactured, is acrid and caustic, soluble in small 

 proportion in water, and possesses alkaline properties, that is to say, 

 it is capable of neutralizing acids, and forming with them substances 

 of the class to which chemists give the name of salts. Of these 

 salts, those most important to the agriculturist are, the carbonate, 

 which forms the principal part of common limestones, chalk, and the 

 shells of testaceous fish; sulphate of lime, which in combination with 

 water constitutes gypsum or plaister of Paris ; and phosphate of lime, 

 which is the basis of the bones of animals, and has been found in the 



9 



ashes of plants. * 



When exposed to air, lime attracts carbonic acid, and passes back 

 to the state whence it was reduced by fire, but loses its figure and 

 falls slowly to powder. When water is thrown on lime, it under- 

 goes the process called slaking, and falls rapidly to powder, produ- 

 cing a hydrate, which is a combination with water. This powder, 

 when exposed to the air, also rapidly passes into carbonate of lime. 

 Both lime and its combination with water promote the decomposi- 

 tion of animal and vegetable substances, and absorb the gases which 



■ 



are generated by their putrefaction. The latter property is also pos- 

 sessed by the carbonate of lime, when in a state of fine powder, or 

 even when it merely exposes a large surface to their action, and al- 

 though rarely mentioned by chemists, and in itself purely mechani- 

 cal, is of the most familiar character, being habitually called into use 

 in our domestic economy, to correct offensive effluvia, and absorb the 

 miasmata which produce disease. 



The two other earths, as usually found in soils, (silica the base of 

 flint and sand ; and alumina, the base of clay,) appear to possess this 

 mechanical property, either not at all or in a very inferior degree. 



Lime, when mixed with these earths, gives them this property, 

 and at the same time modifies their characters in a most remarkable 

 manner. Silica, which has no attraction for moisture, is rendered 

 retentive of water by lime ; and clay which forms with abundance of 

 water, a plastic paste, and hardens, on partial drying, into a tough 

 clod, loses its plasticity and is rendered friable. 



