1218. The mechanical effect of aeration in winter is to obtain a minute division of the particles of earth by the freezing of the water in the soil; for as water in a solid state occupies more space than it did when fluid, the particles of earthy matters and decomposing stones are thus rent asunder and crumbled down to a fine mould. In summer the same effect is produced by heat, as all substances expand by heat; and in both cases the particles of earth are rent asunder and a greater sur ace is exposed to the action of the atmosphere. A secondary advantage which is derived from exposing soils to the heat of summer, and the cold of winter is, the destroying the seeds of weeds, and the eggs of insects; but this advantage is too uncertain to be counted upon.
