PHYSICAL CHARACTERS OF THE SOII. 159 
nourishment to crops as the latter, since they present one- 
half more absolute weight in a given space. 
Peat soils are light in both senses in which this word 
is used by agriculturists. 
The Specific Gravity of Soils is the weight of a given 
bulk compared with the same bulk of water. A cubic 
foot of water weighs 624 Ibs., but comparison of this num- 
ber with the numbers stated in the last table expressing 
the weights of a cubic foot of various soils does not give 
us the true specific gravity of the latter, for the reason 
that these weights are those of the matters of the soil 
contained in a cubic foot, but not of a cubic foot of these 
matters themselves exclusive of the air, occupying their 
innumerable interspaces. When we exclude the air and 
take account only of the soil, we find that all soils, except 
those containing very much humus, have nearly the same 
density. Schéne has recently determined with care the 
specific gravity of 14 soils, and the figures range from 
2.53 to.2.71. The former density is that of a soil rich in 
humus, from Orenberg, Russia; the latter of a lime soil 
from Jena, The density of sandy and clayey soils free from 
humus is 2.65 to 2.69. (Bulletin de la Soc. Imp. des 
Naturalistes de Moscou, 1867, p. 404.) This agrees with 
the density of those mineralé which constitute the bulk 
of most soils, as seen from the following statement of their 
specific gravity, which is, for quartz, 2.65; feldspar, 2.62; 
mica, 2.'75-3.10; kaolinite, 2.60. Calcite has a sp. gr. of 
2.72; hence the greater density of calcareous soils. 
§ 2. 
STATE OF DIVISION OF THE SOIL AND ITS INFLUENCE ON 
FERTILITY. 
On the surface of a block of granite only a few lichens 
and mosses can exist; crush the block to a coarse powder 
and a more abundant vegetation can be supported on it; 
