204 OF MANURE. 



glue, viz., 5*28 per cent., then 100 parts of bones 

 must be considered as equivalent to 250 parts of 

 human urine. 



Bones may be preserved unchanged for thousands 

 of years, in dry or even in moist soils, provided the 

 access of rain is prevented ; as is exemplified by 

 the bones of antediluvian animals found in loam or 

 gypsum, the interior parts being protected by the 

 exterior from the action of w^ater. But they become 

 virarm when reduced to a fine powder, and moistened 

 bones generate heat and enter into putrefaction; the 

 gelatine which they contain is decomposed, and its 

 nitrogen converted into carbonate of ammonia and 

 other ammoniacal salts, which are retained in a 

 great measure by the powder itself. (Bones burnt 

 till quite white, and recently heated to redness, 

 absorb 7-5 times their volume of pure ammoniacal 

 gas.) 



ARTIFICIAL MANURES. 



We have now examined the action of the animal 

 or natural manures upon plants ; but it is evident, 

 that if artificial manures contain the same constitu- 

 ents, they will exercise a similar action upon the 

 plants to which they are applied. We shall only 

 notice here one or two of those principally employed. 



Since it has been ascertained that animal manures 

 act (as far as the formation of orga?iic matter is 

 concerned) only by the ammonia which they contain, 

 attention has been devoted by chemists to discover 

 a more economical means of presenting this ammonia 

 to plants. The water which distils from the retorts 

 in the preparation of coal gas is strongly charged 

 with this alkali, but is at the same time mixed with 

 tar and other empyreumatic impurities. It has been 

 customary to allow the tarry matter to subside, and 

 decant off the clear, supernatant liquor. This liquor, 



