[Sidenote: The retentive power of a soil is due to its surface attractions.] The retentive power of a soil is due to the surface attraction of its particles, and when the space between them is small, or when, in other words, the pores are small, this attractive power will be all the greater. [Sidenote: When, after the limit of the retentive power has been reached, of water are poured upon the soil, a portion of the previously stored water is driven out, and its place in the pores taken up by the fresh supply.] It is further of interest to observe here, that if after the limit of the retentive power has been reached further quantities of water are poured upon the soil, the water retained in the lower layers will commence to drain away. This means that the water freshly poured upon the soil will drive out a portion of the water previously stored in the pores. It is important to bear this in mind when dealing with polluted water, as owing to this action the water penetrating into deeper layers will to some extent at least have become purified in the upper layers. 5. CAPILLARY MOVEMENTS OF WATER IN SOIL. [Sidenote: Capillary attraction causes an upward movement of the water.] Through capillary attraction an ascending movement of the water is caused in direct opposition to the laws of gravity, and the height to which water will thus ascend depends mainly on the smallness of the pores; large pores do not assist in this movement. As the same, however, extends over the whole pore-volume the quantity of water thus raised may exceed the water-retentive power of soil. [Sidenote: Capillary attraction also causes lateral and downward movements.] In addition to the upward movement brought about by capillary attraction, this power is also continually at work in a lateral and downward direction; but for the present purposes only the upward movement will be noticed. [Sidenote: Time occupied by upward movement. Height reached by upward movement.] In observing the upward movement, it is interesting to notice the time occupied by it and the total height reached. As to the time occupied, it has been established that the upward movement in gravel and coarse sand is much quicker than in fine and loamy sand, but the heights attained are reversed. For whereas the height in a material consisting of coarse or large pores amounts to from 2 inches to 4 inches; a height of about 4 feet after thirty to thirty-five days has been recorded in fine or loamy sand; in peaty soil one observer states that the upward movement of the water may reach a height of 20 feet. 6. TEMPERATURE OF SOIL. [Sidenote: Three principal sources of heat.] The earth’s crust receives its supply of heat from three principal sources, viz.: 1. From the sun through its rays; 2. From the interior of the earth through conduction; and 3. From various physical and chemical processes which take place in it and create heat. [Sidenote: Heat through sun’s rays.]