GEOLOGICAL SURVEY 
218 
highticlay..22 26s. 2ea= eee nee L617 
Sitishmol ei: Bae eee etree re 1.925 
Stroneg@¢lay.=-- == ae ene So) 
Puretclaye so =. a eae) 
Caleareous soil in fine powder_-------.---- 2.002 
Iams | Sees oo = ee = oe Ol 
Garden earth! =\22-. 522s eineeeeeee OU 
This table was continued farther, and showed that after the soils had absorbed a certain amount 
of moisture, their absorbent power decreased. It appears also that clays are hygrometric in pro- 
portion to the absence of sand ; and that humus exceeds all the rest in this respect. 
Absorption of oxygen. — We owe to the illustrious Humboldt a knowledge of the fact that argil- 
laceous soils, and certain slaty rocks, absorb oxygen. Before the year 1793 he had observed, in 
the mines of Saltzburg, that the galleries excavated in the rock absorbed that gas from the atmos- 
phere, so as to render it unfit for respiration, and incapable of supporting combustion. He also 
observed that the earth taken from the mines, only became fertile after long exposure to the atmos- 
phere. He deduced from this the fact, so often confirmed since, that oxidation is absolutely neces- 
sary to the fertility of the soil. Bousingault attributes this absorption to the iron that exists in the 
soil, in the lowest state of oxidation, and which is converted into the peroxide, by the additional 
dose of oxygen. This explains the injury often done to the arable soil by turning up the sub-soil ; 
for the protoxide, or lowest combination of oxygen with iron, is injurious to plants, and it requires 
exposure to atmospheric agencies to convert it into the peroxide, which is not hurtful. Increasing 
the depth of the soil by deep ploughing should, therefore, be practiced with caution, and that por- 
tion of the subsoil brought up should not be mixed with the arable soil, till it has been exposed for 
some time to the atmosphere. 
Schubler confirms the observations of Humboldt. Sands, he finds, absorb but little oxygen, but 
it is absorbed very decidedly by clays, loams, and largely by humus. He also corroborates the 
statements of Humboldt and M. DeSaussure, in relation to the conversion of a portion of the 
oxygen absorbed by humus, into carbonic acid. He supposes that oxygen is condensed in the 
pores of the soil, and another portion converts the protoxide of iron into the peroxide. 
Capacity for heat—This property depends inversely upon the conducting power of the soil. 
It was determined by heating, in a vessel of known capacity, a quantity of the substance under 
experiment, to 144° Fahr. and the time noted that it required to fall to 70° Fahr. the temperature 
of the surrounding air remaining the same. The following are the comparative results. 
Kind of earth. Power of retaining heat. 
G@lcareous sands 25 eA eee 100.0 
Silicious/sand=_= - o> sj2ees es eee E950) 
Gypsumta =. Sea eee see ee eee 73.2 
Sand yiclaysey Shee coe aemasi= 76.9 
Stitishighays2 heen Sooo. oo coe @a 
Stil claye see oo ce les an a oe OO 
Pure (olive wee senate voce setae se OUNt 
Caélcareous soils ..22oo-0o4--5a-~ 28 (O18 
Mumus=-542-cceececee cece SE AOI 
Garden-eatth- <<-eeceecawccan «=e O46 
