Arable Lands in Dry Climates. 
345 
Peaty soils arc an exception ; but the more sandy a soil is, the 
more dense it is, and the heavier it weighs, whilst the more 
clayey so much lighter it is. Almost pure sand weighs nearly 
twice as much per cubical foot as water. Porcelain or pipe-clay 
is only about one-and-a-half times heavier than water. These 
proportions vary, of course, according to whether sand or clay pre- 
dominates, also as to the state of moisture and porosity influenced 
by cultivation and manure. On digging out a cubic foot of 
a clay soil at the surface, and another from the subsoil, the 
upper foot, being more porous, was lighter by some pounds ; and 
if water were added to the upper soil, the space occupied by a 
foot moderately dry, would be increased ; and, as water is lighter 
than soil, a foot of dry clay would be more than a foot of wet 
clay, and would thus, space for space, be lighter. Sand is so 
dense and fits so closely together that there is very little space 
left for the retention of moisture ; it therefore neither sensibly 
expands nor contracts by drought or moisture. Clay, on the 
other hand, if saturated to its fullest power of retaining water, 
will contract quite a tenth part by the ordinary heat of the sun. 
Any one may observe this on clay soils in a dry season, when a 
bare field will be cracked about in all directions ; the spaces 
between the cracks, not a foot perhaps, having such large 
openings at times that a walking-stick may be thrust down 
several feet. This at once exemplifies the porosity of clays, 
although they appear when wet to be dense and heavy. No 
one ever saw cracks in a field of sand. 
Many interesting experiments have been made for testing the 
percentage of water which dry soils would absorb and retain 
without dripping. Strong clay, when thoroughly dry, has retained 
50 per cent, of water. Peat, which is the skeleton of land, 
absorbs much more than half its own weight of water. Pure 
sand will not retain even 5 per cent. It may well be acknow- 
ledged, therefore, that such and such kinds of soil are dry and 
others retentive. Sandy soils, after being wetted, part with their 
moisture very rapidly, whilst clay, on the other hand, not only 
retains more moisture in the first place, but keeps it with great 
pertinacity afterwards. Some of the early chemists thought it 
a good test of the fertility of soils to ascertain the percentage 
of water they could absorb, and the length of time they took to 
dry again. Such tests, however, are now abandoned, but the 
idea was pretty good upon the whole. If our stiffest clays could 
be completely dried, they would indeed have an excellent chance 
of being fertile when moderately moistened again for a crop ; but 
there is no such chance in general for a field being dried so that 
the soil would absorb 50 per cent, of moisture ; hence the com- 
parative fallacy of artificially drying a portion of soil only, and 
2 A 2 
