AGRICULTURE. 221 



quires its hardness and durability by re-absorbing 

 carbonic acid, when it again becomes limestone. 



Magnesian limestones effervesce but little when 

 plunged into acids. 



Marie. 



Marie is a natural mixture of limestone and clay 

 in variable quantities. 



Manures. 



The decay and death of animal substances tend to 

 resolve organized forms into their chemical consti- 

 tuents, and the disagreeable effluvia disengaged seem 

 to point out the propriety of burying them in the 

 soil, where they are fitted to become the food of 

 animals. The fermentation and putrefaction of 

 organized substances in the open atmosphere are 

 noxious processes ; beneath the surface of the ground 

 they are salutary operations. In this case the food 

 of plants is prepared where it can be of use, and that 

 which would offend the senses and injure the health 

 if exposed, in the earth is gradually converted into 

 forms of beauty and utility. The foetid gas is ren- 

 dered a constituent portion of the aroma of the 

 flower, and what might be poisonous becomes the 

 nourishment of man and animals. 



Putrefaction. 



The circumstances necessary for the putrefaction 

 of animal substances are similar to those required 

 for the fermentation of vegetable substances; viz. 

 a temperature above the freezing point, and the 

 presence of water and oxygen, at least in the first 

 stage of the process. No putrefaction or fermenta- 

 tion can go on without the generation of elastic fluid, 

 and water is as necessary as air to the process of 

 fermentation. 



