No. 144.] 495 



The principal use of tiie proce?ses of tillage seems to be to pro- 

 duce tliat spongy cliaracler of the soil that enables it to absorb 

 and retain the falling rains. A turnpili:e road dries up and be- 

 comes dusty sooner than the adjoining plowed land, and if a load 

 of loose earth be thrown down on the dry, hard bed of such a 

 road, on removing it after a day or two the ground will be found 

 moist underneath, although there shall have been no rain. The 

 earth is always moist a few inches beneath the surface, and deep 

 culture seems to facilitate the passage of this moisture upwards 

 in the same waylhat potted plants are watered from the saucer, 

 or that a towel becomes damp when it hangs with the lower part 

 of it in water. The land to which I at first alluded was sadly 

 affected by drought until it was trench plowed a measured foot 

 deep, after which there was no trouble of that sort, and running 

 the plow through the rows of the growing crops in a dry time re- 

 vived them like a shower. 



As to the precipitation of moisture from the air into the inter- 

 stices of tilled soil, it does undoubtedly amount to something. 

 Yet we must remember that the temperature of the soil is not as 

 low as that of fresh spring or well water, and that it is only on 

 certain days in summer that water vessels sweat. This effect de- 

 pends on the hygrometrical condition of the air, or the amount of 

 water that happens to be dissolved in it. 



II. Carhonic acid, which forms naturally about one part in a 

 thousand of the atmosphere, though it is a variable constituent. 

 Though this amount may appear insignificant, yet when we take 

 the whole height of the air into account, and the fact that it ex- 

 tends over the sea as well as the landj we shall find that there is 

 a greater amount of carbon in this reservoir than would suffice 

 to construct another organized system of nature. It is to this 

 substance that plants are, after all, most intimately connected, as 

 it is their principal food. Many varieties thrive when growing 

 in pounded charcoal, without effecting any change in the nature 

 of that substance ; and a plant of the " Ficus Australis " flourished 

 and bore fruit in the hot house of the Botanical Garden at Edin- 

 burgh, after every root had been successively removed from the 



