_Carbonic acid_ gas (Carbon dioxide) is one of the components of the atmosphere,--a small one, ordinarily only about 1/2500 of its bulk,--sufficient for the supply of vegetation, but not enough to be injurious to animals, as it would be if accumulated. Every current or breeze of air brings to the leaves expanded in it a succession of fresh atoms of carbonic acid, which it absorbs through its multitudinous breathing-pores. This gas is also taken up by water. So it is brought to the ground by rain, and is absorbed by the roots of plants, either as dissolved in the water they imbibe, or in the form of gas in the interstices of the soil. Manured ground, that is, soil containing decomposing vegetable or animal matters, is constantly giving out this gas into the interstices of the soil, whence the roots of the growing crop absorb it. Carbonic acid thus supplied, primarily from the air, is the source of the carbon which forms much the largest part of the substance of every plant. The proportion of carbon may be roughly estimated by charring some wood or foliage; that is, by heating it out of contact with the air, so as to decompose and drive off all the other constituents of the fabric, leaving the large bulk of charcoal or carbon behind. _Nitrogen_, the remaining plant-element, is a gas which makes up more than two thirds of the atmosphere, is brought into the foliage and also to the roots (being moderately soluble in water) in the same ways as is carbonic acid. The nitrogen which, mixed with oxygen, a little carbonic acid, and vapor of water, constitutes the air we breathe, is the source of this fourth plant-element. But it is very doubtful if ordinary plants can use any nitrogen gas directly as food; that is, if they can directly cause it to combine with the other elements so as to form protoplasm. But when combined with hydrogen (forming ammonia), or when combined with oxygen (nitric acid and nitrates) plants appropriate it with avidity. And several natural processes are going on in which nitrogen of the air is so combined and supplied to the soil in forms directly available to the plant. The most efficient is _nitrification_, the formation of nitre (nitrate of potash) in the soil, especially in all fertile soils, through the action of a bacterial ferment. 450. =Assimilation= in plants is the conversion of these inorganic substances--essentially, water, carbonic acid, and some form of combined or combinable nitrogen--into vegetable matter. This most dilute food the living plant concentrates and assimilates to itself. Only plants are capable of converting these mineral into organizable matters; and this all-important work is done by them (so far as all ordinary vegetation is concerned) only