C 2S3 ] 



modify their properties, it will be necessary to consi- 

 der 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 combination 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 can- 

 not long continue caustic, for the reasons that were 

 just now assigned ; but soon becomes united to car- 

 bonic acid. 



When newly burnt lime is exposed to air, it soon 

 falls into powder ; in this case it it 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 j i. e. 55 

 parts of lime absorb 1 7 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 che- 

 mists hydrate of lime ; and when hydrate of lime be- 

 comes 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 



