Horticultural Chemistry. 271 



parts coarse matter to every 100 of finely pulverised consti- 

 tuents. 



In affording warmth to plants, the earth is of considerable 

 importance, and the power of accumulating and retaining it 

 varies as much in soils as the proportions of their constituents. 

 Sir Humphry Davy found that a rich black mould, containing 

 one fourth 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 circum- 

 stances ; 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 tenacious soils are, 

 in general, so much more backward in spring, but are retained 

 longer in verdure during autumn, than those on black light 

 soils ; the latter attain a genial warmth the more readily, but 

 part with it with equal speed. An experiment which I have 

 often repeated upon light as well as tenacious soils with like 

 success, demonstrates howgreatly the colour of a soil influences 

 the accumulation of heat. Coal ashes were sprinkled over 

 half the surfaces of beds sown with peas, beans, &c, and on 

 these the plants invariably appeared above ground two or 

 three days earlier, obviously on account of the increased 

 warmth ; it being a well known fact that dark-coloured bodies 

 absorb caloric more readily, and in larger proportions, than 

 those of a lighter hue. 



Different plants affect different soils. Every gardener must 

 have observed that there is scarcely a kitchen-garden but 

 has some particular crop which it sustains in luxuriance far 

 superior to any other garden in its neighbourhood, or to 

 any other crop that can be grown in it. My own garden, 

 without the preparation of an artificial soil, will not produce 

 the common garden cress (Zepidium sativum), whilst the rasp- 

 berry is remarkably luxuriant. That the composition of a soil 

 has a main influence in these peculiarities is certain. The nettle 

 haunts, as it were, the footsteps of man, and clings, as poetry 

 might urge, in very sociality round his dwelling. This plant 

 will not flourish but in a soil containing nitrate of potassa 

 (saltpetre), a salt always abounding in the neighbourhood of 

 walls and places where there is calcareous matter. The rabbit 

 warrens near Mildenhall, in Suffolk, I have noticed frequently 

 as abounding in nettles, yet it is a houseless waste of many 

 miles' extent ; but, still, nitrate of potassa is furnished to the 

 soil by the urine of the rabbits, which contains potassa and 

 lime, in very considerable proportion. These topics, however, 

 belong more properly to a future communication upon 



