Bibliography. 181 



of rain water contains only \ grain of ammonia, then a field of 40,000 

 square feet (one Hessian acre, or 26,917 English square feet) receives 

 annually upwards of 80 lbs. of ammonia, or 65 lbs. of nitrogen. This 

 is much more nitrogen than is contained in the form of vegetable al- 

 bumen in 2650 lbs. of wood, or 2800 lbs. of hay, which are the an- 

 nual products of such a field ; but it is less than the straw, roots and 

 grain of corn, which might grow on the same surface, would contain. 

 As nitrogen is always present in considerable quantity, in some part 

 or other of plants, the importance of food containing it can scarcely 

 be overrated, especially as, according to the view of Prof. Liebig, the 

 assimilation of substances generated in the leaves will (cseteris pari- 

 bus) depend on the quantity of nitrogen contained in the food. The 

 great efficacy of animal manures is shown to depend mainly on the 

 nitrogen and carbonic acid which they furnish. 



But we must hasten to close this very imperfect notice, passing al- 

 most in silence the author's remarks on the mineral constitution of 

 soil, and on culture and rotation of crops, which are as important and 

 original as the foregoing parts. Speaking of the composition of soils, 

 he cites the neighborhood of Vesuvius as the type of a fertile soil, and 

 as it is formed entirely from the disintegration of lava, it cannot 

 possibly, on account of its origin, contain the smallest trace of vegeta- 

 ble matter ; yet it is well known that when volcanic ashes have been 

 exposed for some time to the influence of air and moisture, a soil is 

 gradually formed in which all kinds of plants grow with the greatest 

 luxuriance. This fertility is owing to the alkalies contained in the lava, 

 and which, by exposure to the weather, are rendered capable of being 

 absorbed by plants. It is the greatest possible mistake to suppose that 

 the temporary diminution of fertility in a soil is owing to the loss of hu- 

 mus ; it is the mere consequence of the exhaustion of the alkalies. The 

 fallow time is that period of culture during which land is exposed to a 

 progressive disintegration, by means of the influence of the atmos- 

 phere, for the purpose of rendering a certain quantity of alkalies ca- 

 pable of being appropriated by plants. Now it is evident that the 

 careful tilling of fallow land must increase and accelerate this disinte- 

 gration. For the purpose of agriculture it is quite indifferent whether 

 the land is covered with weeds, or with a plant which does not abstract 

 the potash enclosed in it. Hence the secret of the success of that 

 greatest of all improvements in modern agriculture, the rotation of 

 crops ; especially if we consider, in connexion with it, the fact that 

 many plants excrete from their roots those matters not fit for assimi- 

 lation to form their organs, and the accumulation of which soon ren- 

 ders the soil unfit to support a succession of the same plants, although 

 the matter thus rejected may be salutary, or at least innoxious to plants 



