1816.) | On the Physical Analysis of Soils. 211 
state of perfect dryness, as well as when drenched in moisture ; 
because the weight of the earths varies considerably according to 
the degree of their humidity, You will find accordingly in the 
second and third columns of the preceding table, the weight of a 
cubic foot and inch of each earth examined, given in the medi- 
cinal weight of Nuremberg, (the pound containing 12 ounces, 
‘poids de marc, and the ounce 480 grains.) 
Vegetable earth has the smallest specific-gravity, and sandy soil 
has the greatest, whether they be dry or moist. Clay soils, whether 
dry or moist, are always lighter than sandy soils. 1 call a soil per- 
fectly dry when, exposed to the temperature of from 100° to 122°, 
it loses no more weight. I do not venture to expose the soils toa 
higher temperature, because then the humus would be decomposed 
or volatilized. 1 call a soil completely moist when it ceases to 
drop if put upon a filtering paper. Compound soils are always 
the lighter the more humus they contain. These researches on 
the specific gravity of soils lead naturally to the observation that 
the agricultural terms, heavy and light lands, are founded on phy- 
sical qualities quite different from specific gravity. 
The fourth colunm points out the different degrees of force with 
which the different arable soils retain moisture. I mean by this 
capacity, the faculty which each soil has of retaining a certain 
quantity of water, without letting it escape in the form of drops. 
1 usually employ 400 grains of soil, and indicate the quantity of 
moisture retained at so much per cent. 
Of all the substances usually found in arable soil the humus 
absorbs and retains the greatest quantity of water; almost double 
its own weight. In this respect magnesia alone surpasses it in a 
remarkable manner, retaining 41 times its own weight of water. 
This remarkable property of pure magnesia must render it preju- 
dicial to vegetation, while it must increase the fertility of a dry and 
sandy soil. It would appear at first sight that we might calculate 
the force with which different soils retain water, by comparing a 
cubic inch of the soil perfectly dry with the same bulk perfectly 
moist; but as the earths are condensed very unequally when moist- 
ened, this comparison would not give an accurate result. 
The fifth and sixth columns give the results of experiments on 
the consistence of the earths, both their solidity when dry and their 
tenacity when moist. I determined the first of these qualities by 
measuring their cohesion, For this purpose I formed on a model 
paralellopipeds of each of the same length, six lines broad and as 
much in thickness; these I placed upon two supports distant 15 
lines from each other. Upon these, when dry, weights were 
laid successively till they broke. Thesum of the weights employed 
gave me the cobesion. The weight necessary to break the soils 
containing a great deal of clay, astonished me. No less than 
178,300 grains were necessary to break pure clay, I adopted 
by way of comparison this degree of the cohesion of clay = 1000; 
the cohesion of sand was = 0, 
