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the purposes of agriculture ; but I shall enlarge most 

 upon the subject of lime ; 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 en- 

 quiry ; 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 combination 

 with carbonic acid or fixed air. If a piece of lime- 

 stone, 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 car- 

 bonic acid gas is expelled, and then nothing remains 

 but the pure alkaline earth ; in this case there is a loss 

 of weight ; and of 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 burn- 

 ing, 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 at- 

 mosphere 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 lirne 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 com- 



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