When water assumes the gaseous (or vapory) form, it takes up 1723 times as much _heat_ as it contained while a liquid. A large part of this heat is derived from surrounding substances. When water is sprinkled on the floor, it cools the room; because, as it becomes a vapor, it takes heat from the room. The reason why vapor does not feel hotter than liquid water is, that, while it contains 1723 times as much heat, it is 1723 as large. Hence, a cubic inch of vapor, into which we place the bulb of a thermometer, contains no more heat than a cubic inch of water. The principle is the same in some other cases. A sponge containing a table-spoonful of water is just as _wet_ as one twice as large and containing two spoonsful. If a wet cloth be placed on the head, and the evaporation of its water assisted by fanning, the head becomes cooler--a portion of its heat being taken to sustain the vapory condition of the water. The same principle holds true with the soil. When the evaporation of water is rapidly going on, by the assistance of the sun, wind, etc., a large quantity of heat is abstracted, and the soil becomes cold. When there is no evaporation taking place, except of water which has been deposited on the lower portions of soil, and carried to the surface by capillary attraction (as is nearly true on under-drained soils), the loss of heat is compensated by that taken from the moisture in the atmosphere by the soil, in the above-named manner. This cooling of the soil by the evaporation of water, is of very great injury to its powers of producing crops, and the fact that under-drains avoid it, is one of the best arguments in favor of their use. Some idea may, perhaps, be formed of the amount of heat taken from the soil in this way, from the fact that, in midsummer, 25 hogsheads of water may be evaporated from a single acre in twelve hours. [When rains are allowed to _enter_ the soil, how do they benefit it? How do under-drains prevent the formation of a crust on the surface of a soil?] 18. When not saturated with water the soil admits the water of rains, etc., which bring with them _fertilizing gases from the atmosphere_, to be deposited among the absorbent parts of soil, and given up to the necessities of the plant. When this rain falls on lands already saturated, it cannot enter the soil, but must run off from the surface, or be removed by evaporation, either of which is injurious. The first, because fertilizing matter is washed away. The second, because the soil is deprived of necessary heat.