The nutrition of vegetables furnishes a beautiful and simple example of the use of water in nature. The body of the vegetable, the proper wood, may be considered as being composed of water and of charcoal; and hence, when we heat a piece of wood until we decompose it, the water is expelled, and carbon or charcoal remains behind. In order to grow, a plant must therefore get water and charcoal in a form fit for its use, that is, in such a form as it can make food of, and digest them. For this, the carbon is supplied in the carbonic acid which the air contains, and the water in the state of vapour which the air contains also, and which is continually descending under the form of dew and rain to moisten the leaves and the roots of the plants, when it has been absorbed into the ground. All the water which is absorbed by plants is not assimilated, or digested; a great part is again thrown out by the surface of the leaves; for, precisely as the air which an animal expires from the lungs in breathing is loaded with vapour, so is there a process of perspiration from the surface of the leaves, which are the lungs of plants. For the formation of substances which are peculiar to certain plants, other substances are required as food, thus, most plants require nitrogen, which is accordingly furnished abundantly in atmospheric air; others must have access to sulphur, in order to flourish; but this depends, as it were, upon particular branches of manufacture in which the plant is engaged; for its own support, for making wood, and the tissue of its leaves and vessels, it uses only water and carbonic acid. The conversion of water into steam or invisible vapour by boiling, is one of the best known facts in science; but by a little attention we can observe that this change takes place at almost all temperatures, although much less rapidly. Thus, if a little water be laid in a plate, it is soon dried up, and wet clothes, by being hung up in the air, are very soon completely dried. Even below the temperature at which water freezes, it still evaporates; and thus, when a fall of snow is succeeded by a continued frost, the snow gradually disappears from the fields without having melted, evaporating while yet solid. From the surface of all the water of the globe, therefore, there is continually ascending a stream of watery vapour; but as the proportion of sea is so much greater than that of land, we may look upon the ocean as being the source of the watery vapour of the air upon the large scale.