CH. II.] THE ROOT. 85 



of lime, 23 ; decomposing animal and vegetable mat- 

 ter, 34 ; silica, 100; alumina, 28 ; oxide of iron, 13 ; 

 sulphate of lime, 2 ; soluble, vegetable, and saline 

 matter, 7 ; loss, 8, total, 400." 



I have already stated what fonns it a fertile soil ; 

 it may be added, that, to constitute it eminently 

 such, its earthy particles must be in a minute state 

 of di^dsion ; the more so the more fertile it will be. 

 In the above analysis, 185 parts only were separable 

 by sifting through a fine scarce, 215 parts were im- 

 palpable ; whereas poorer soils will often have 300 

 parts coarse matter to e\ery 100 of finely pulverized 

 constituents. 



In affording wannth to plants, we have seen that 

 the earth is of considerable importance, and the 

 power of accumulating and retaining heat vaiies as 

 much in soils as the proportions of their constituents. 

 Sir Humphrey Davy found that a rich black mould, 

 containing one-fouith of vegetable matter, had its 

 temperature increased in an hour from 65° to 88° 

 by exposure to the sunshine, whilst a chalk soil was 

 heated only to 69° under similar circumstances ; but 

 the first, when removed into the shade, cooled in 

 half an hour 15°, whereas the latter lost only 4''. 

 This explains why the crops on light coloured tena- 

 cious soils are, in general, so much more backward 

 in spring, but are retained longer in verdm'e dm'ing 



