28 5 
and if I should enter into some details which may 
be tedious and minute, I trust, my excuse will be 
found in the importance of the enquiry; and it is 
one which has been greatly elucidated by late 
discoveries. 
The most common form in which lime is found 
on the surface of the earth, is in a state of combin- 
ation with carbonic acid or fixed air. If a piece of 
limestone, or chalk, be thrown into a fluid acid, 
there will be an effervescence. This is owing to 
the escape of the carbonic acid gas. The lime 
becomes dissolved in the liquor. 
When limestone is strongly heated, the carbonic 
acid gas is expelled, and then nothing remains but 
the pure alkaline earth ; in this case there is a loss 
of weight ; and if the fire has been very high, it 
approaches to one half the weight of the stone ; 
but in common cases limestones, if well dried 
before burning, do not lose much more than from 
35 to 40 per cent., or from seven to eight parts out 
of 20. 
I mentioned, in discussing the agencies of the 
atmosphere upon vegetables, in the beginning of 
the Fifth Lecture, that air always contains carbonic 
acid gas, and that lime is precipitated from water 
by this substance. When burnt lime is exposed to 
the atmosphere, in a certain time it becomes mild, 
and is the same substance as that precipitated from 
lime-water ; it is combined with carbonic acid gas. 
Quicklime, when first made, is caustic and burning 
to the tongue, renders vegetable blues green, and 
is soluble in water ; but when combined with car- 
bonic acid it loses all these properties, its solubility 
