The moisture thus floating at all times in the air, serves for the support of vegetable life, even in countries where rain seldom if ever falls. It is absorbed by the leaves of living plants, which thus increase in weight even when suspended in the atmosphere and disconnected with the soil. During intense heats, and when the soil is parched and dry, we see the life of plants thus preserved until the earth is again refreshed with showers, and the roots supplied with their wonted moisture. _Clouds_, are produced when aqueous vapor returns to the state of water; and this process is called _condensation_. Whenever the temperature becomes lower than the constituent temperature, requisite for the maintenance of the vapory state, some of the vapor, or invisible steam, will be condensed, and become water. This may be seen illustrated in the condensation of the steam, as it issues from the spout of a tea-kettle. Clouds not only moderate the fervor of the sun, but they also check radiation from the earth, for we find that the coldest nights are those which occur under a cloudless winter sky. The use of clouds in the formation of rain, is too obvious to need pointing out more particularly. _Snow_ is frozen vapour aggregated by a confused action of crystalline laws, and _ice_ is water, solidified while in its fluid state, by the same crystalline forces. These are bad conductors of cold, and when the ground is covered with snow, or the surface of the soil, or if the water is frozen, the roots or bulbs of plants beneath are protected by the congealed water from the influence of the atmosphere, the temperature of which in northern winters, is usually very much below the freezing point; and this water becomes the first nourishment of the plant, in early spring. The expansion of water during its congelation, at which time its volume increases one twelfth, and its contraction in bulk during a thaw, tend to pulverize the soil, to separate its parts from each other, and to make it more permeable to the influence of the air. When ice changes to water, or water to steam, although at an invariable degree of temperature, yet the change is not sudden, but gradual. When the heat reaches the point, at which thawing or boiling takes place, the temperature makes a stand; a portion of it disappears, or becomes _latent_, as it is called; thus the temperature of ice cannot be raised, till the whole is thawed, nor that of boiling water, till it has all been converted into steam; all the heat that is applied being absorbed in producing these changes. Were it not for this law of latent heat, thaw and evaporation would be instantaneous, we should be overwhelmed with floods, at the first glow of warmth in the spring, and in heating water the whole would flash instantaneously into steam upon reaching the boiling point.