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 colour, causes the more rapid absorption of heat from the sun's rays. It will be thus understood, that though it does not directly supply food to the plant, it ministers indirectly in a most important manner to its well-being, and that to so great an extent that it must be considered an indispensable constituent of a fertile soil. But it is important to observe that it must not be present in too large a quantity, for an excess does away with all the good effects of a smaller supply, and produces soils notorious for their infertility.