. - 
Binilbaphy). mn 
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 Ibs. of ammonia, or 65 lbs. of nitrogen. is 
is much more nitrogen than is contained in the form of vegetable al- 
bumen in 2650 Ibs. of wood, or 2800 lbs. of hay, which are the an- 
nual products of sucha 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 (cxteris 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 
osed for some time to the influence of air and moisture, a soil is 
ally formed in which all kinds of plants grow with the greatest 
laxuriance. 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. Itis 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 eovered 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 
