100 
also, an indirect influence on vegetation, by its power of 
attracting and retaining water and ammonia ; it is itself very 
rarely found in the ashes of plants, but silica is always pre- 
sent, having in most places entered the plants by means of 
alkalies. Potash is present in all clays ; according to Fuchs 
it is contained in marl ; it has been found in all the argillace- 
ous earths in which it has been sought clay slate contains 
three per cent, of potash, and one Hessian acre of 20 inches 
in depth, will contain 200,000 lbs." 
" The land in the vicinity of Vesuvius may be considered 
as a type of a fertile soil, and its fertility is greater or less 
according to the proportion of clay or sand which it con- 
tains. The soil, from disintegration of lava, cannot possibly, 
on account of its origin, contain the smallest trace of veget- 
able matter ; yet, when the volcanic ashes have been some- 
time exposed to the air and moisture, a soil is gradually 
formed in which all kinds of plants grow with the greatest 
luxuriance. The fertility is owing to the alkalies which are 
contained in the lava, and which, by exposure, are rendered 
capable of being absorbed by plants. The first colonists of 
Virginia grew Wheat and tobacco for a century upon the 
same fields without the aid of manure, but now, whole dis- 
tricts are converted into unfruitful pasture land, which with- 
out manure produces neither wheat nor tobacco. From 
every acre of this land there were removed, in the space of 
100 years, 1,200 lbs. of alkalies in leaves, grain, and straw; 
it became unfruitful, therefore, because it was deprived of 
every particle of alkali, which had been reduced to a soluble 
state, and because that which was rendered soluble again in 
the space of one year, was not sufficient to satisfy the 
demands of the plants. Almost all the cultivated land in 
Europe is in this condition ; fallow is the term applied to 
land left at rest for further disintegration. It is the greatest 
possible mistake to suppose that the temporary diminution of 
