urse of
four hours four-fifths of that it had taken up, while humus, which
imbibes nearly twice its weight, retains nine-tenths of that quantity
after four hours' exposure. Long-continued and slow evaporation of the
water absorbed by a soil is injurious in another way, for it makes the
soil "cold"--a term of practical origin, but which very correctly
expresses the peculiarity in question. It is due to the fact, that when
water evaporates it absorbs a very large quantity of heat, which
prevents the soil acquiring a sufficiently high temperature from the
sun's rays. The soils which have absorbed a large quantity of moisture
shrink more or less in the process of drying, and form cracks, which
often break the delicate fibres of the roots of the plants, and cause
considerable injury: the extent of this shrinking is given in the fourth
column.

The relation of the soils to heat divides itself into two
considerations: the amount of heat absorbed by the soil, and the degree
in which it is retained. Of these the latter only is illustrated in the
table. The former is dependent on so many special considerations, that
the results cannot be tabulated in a satisfactory manner. It is
independent of the chemical nature of the soil, but varies to a great
extent according to its colour, the angle of incidence of the sun's
rays, and its state of moisture. It is, however, an important character,
and has been found by Girardin to exercise a considerable influence on
the rapidity with which the crop ripens. He found in a particular year
that, on the 25th of August, 26 varieties of potatoes were ripe on a
very dark-coloured sandy vegetable mould, 20 on an ordinary sandy soil,
19 on a loamy soil, and only 16 on a nearly white calcareous soil.

The tenacity of the soil is very variable, and indicates the great
differences in the amount of power which must be expended in working
them. According to Schuebler, a soil whose tenacity does not exceed 10,
is easily tilled, but when it reaches 40 it becomes very difficult and
heavy to work.

On examining the table it becomes manifest, that as far as its
mechanical properties are concerned, humus is a substance of the very
highest importance, for it confers on the soil, in a high degree, the
power of absorbing and retaining water, diminishes its tenacity and
permits its being more easily worked, adds to its hygrometric power and
property of absorbing oxygen from the air, and finally, from its dark
c