We have, it may be stated, generally two processes going on. During the day the soil is engaged in absorbing its heat from the sun's rays; when night comes, and the sun goes below the horizon, the air is chilled below the temperature of the soil, which radiates out its stored-up heat into the air. The result is that the temperature of the soil is soon reduced below the temperature of the air, and the moisture, present in the air in the form of vapour, coming in contact with the cold surface of the earth, is condensed into dew, which is deposited, and is seen best early in the morning before the sun has had time to evaporate it again. Dew is most abundant in summer-time, for the reason that the difference in temperature of the day and night is then greatest. In winter-time it is seen as hoar-frost. _Heat of Soils._ The temperature of a soil, however, is due to other sources than the sun's rays. Whenever vegetable matter decays, there is always a certain amount of heat generated. Soils, therefore, in which there is a large amount of decaying vegetable matter, are certain to receive more heat from this source than soils of more purely mineral nature. _Heat in Farmyard Manure._ A good example of the amount of heat that accompanies fermentation, or decay of vegetable matter, is seen in the case of rotting farmyard manure. The danger of loss of the volatile ammonia from this cause is often great, and care must be taken to prevent fermentation going on too quickly, and the temperature from becoming too high.[44] The actual increase in the temperature of a soil effected by the addition of certain bulky organic manures, such as farmyard manure, may thus be considerable. In some experiments carried out at Tokio, Japan, it was found that the application of 20 tons of farmyard manure per acre increased the temperature of the soil to a depth of five inches, for a period of nearly a month, on an average, one and a half degrees Fahrenheit. The amount of water present in a soil, it may be noticed in passing, will have a considerable effect in regulating its temperature, a damp soil being, as a rule, a cold soil. _The Cause of the Heat of Fermentation._