178 
different proportions, and tliat in general those are absorbed in 
greatest quantity which are most injurious to vegetation. Saus- 
sure explains this apparent aiiomally by supposing that a por- 
tion of the roots were soon destroyed by the injurious liquids, 
and that then they absorbed the solution indiscriminately. 
When various salts were dissolved at once in the same solutions, 
and plants made to vegetate in them, it was found that differ- 
ent proportions of the salts were absorbed. Thus it appears 
that plants do not absorb saline bodies indiscriminately. Saus- 
sure supposes that the difference depends rather upon the 
degree of liquidity which the solution possesses, than upon any 
discriminating power in the root. But if this were the case it 
would be difficult to explain how so much greater a proportion 
of water should be absorbed than of the salt which it holds in 
solution. 
181 Animal and Vegetable Manure. Water, carbonic acid, 
and oxygen, and perhaps also earths and salts, constitute a part 
of the food of plants; but is very clear that the whole food is 
not furnished by these substances. It is well known that if 
vegetables be successively raised on the same ground, they at 
last exhaust it, or render it sterile ; and to prevent this, farmers 
are obliged to supply their grounds annually with a quantity of 
manure. Without this manure or some equivalent, plants can- 
not be made to thrive, or to perfect their seeds. Neither water, 
air, nor earths, nor salts, will prevent them from perishing. 
Giobert mixed together the four earths, silica, alumina, lime, 
and magnesia, in the proper proportions to constitute a fertile 
soil ; and after moistening them with water, planted several 
vegetables in them; but none of them grew well till he moist- 
ened his soil with water from a dunghill. Lampadius planted 
different vegetables in compartments of his garden, filled each 
with one of the pure earths, and watered them with the liquor 
which exuded from a dunghill. They all grew, notwithstand- 
ing the diversity of the soil ; and each contained the usual 
earthy constituents of plants, notwithstanding the absence of 
these constituents from the soil. 
It is not the earths which constitute a fertile soil, but the 
remains of animal and vegetable substances, and the propor- 
tion of these capable of being held in solution by water. It 
appears from the experiments of Mr.Hassenfratz, that substances 
