52 AGRICULTURE. [en. 



96. The action of lime as a manure is entirely 

 regulated by the form and manner in which it is 

 employed. It is very desirable for these variations 

 in practice to be distinctly understood. The form 

 in which lime is most largely employed is as burnt 

 lime. In its preparation limestone or chalk rock 

 is placed in a kiln, with some fuel, and after an expos- 

 ure to the fire thereby produced, we find the "lime" 

 has changed its character and its composition. The 

 carbonic acid which was present has been driven off, 

 it is therefore no longer a carbonate of lime, but lime. 

 It is sometimes called quick-lime, sometimes burnt 

 or calcined lime, or caustic lime, but you must 

 remember that it is no longer a carbonate of lime, 

 because the carbonic acid has been driven off. 



97. The burnt lime is very different from the lime- 

 stone or chalk rock, as you will readily see if you 

 take a lump of each, and put some water upon them. 

 The limestone and chalk are not changed by the 

 water, but with the burnt lime it is very different. 

 A violent action takes place, which produces much 

 heat, and breaks the burnt lime into a fine powder. 

 This is commonly known as " slaking the lime," 

 but viewed from a chemical point of view, the change 

 which has taken place is a combination of the water 

 with the lime. It is not simply a mixture of the 

 water with the burnt lime, which would only have the 

 effect of wetting the lime, but a definite union has 

 taken place between the water and the lime, and a 

 new product is obtained, viz., hydrate of lime, or, 

 as it is more commonly known, slaked lime. 



98. If this slaked lime is allowed to remain exposed 

 to the air, the carbonic acid of the atmosphere readily 

 enters into combination with it, and we have carbonate 

 of lime again produced. Carbonic acid, which we 

 drove away by burning in the kiln, again associates 

 itself with the lime, and the practical change which 

 has taken place is the reduction of the carbonate of 



