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sider the operation of the pure calcareous element 
as a manure, and as a cement. 
Quicklime in its pure state, whether in powder 
or dissolved in water, is injurious to plants. — I 
have in several instances killed grass by watering 
it with lime-water. — But lime, in its state of com- 
bination with carbonic acid, as is evident from the 
analyses given in the Fourth Lecture, is a useful 
ingredient in soils. Calcareous earth is found in 
the ashes of the greater number of plants ; and 
exposed to the air lime cannot long continue 
caustic, for the reasons that were just now assigned ; 
but soon becomes united to carbonic acid. 
When newly burnt lime is exposed to air, it soon 
falls into powder ; in this case it is called slacked- 
lime ; and the same effect is immediately produced 
by throwing water upon it, when it heats violently, 
and the water disappears. 
Slacked-lime is merely a combination of lime with 
about one-third of its weight of water ; i. e. 55 
parts of lime absorb 17 parts of water ; and in this 
case it is composed of a definite proportion of lime 
to a definite proportion of water, and is called by 
chemists hydrate of lime ; and when hydrate of 
lime becomes carbonate of lime by long exposure 
to air, the water is expelled, and the carbonic acid 
gas takes its place. 
When lime, whether freshly burnt or slacked, is 
mixed with any moist fibrous vegetable matter, 
there is a strong action between the lime and the 
vegetable matter, and they form a kind of compost 
together, of which a part is usually soluble in 
water. 
