As the _proximate principles_ of vegetables are chiefly carbon, hydrogen, and oxygen, it will be useful to inquire how vegetables obtain these materials. Water, which is composed of hydrogen and oxygen, is a ready source whence both its constituents may be obtained; and it is concluded that it is decomposed in the glands of vegetables, assisted by solar light, and becomes fixed in them in the state of _oil_, _extract_, _mucilage_, &c. The greatest part, however, of vegetables consists of _carbon_, or, to make ourselves more intelligible, _pure charcoal_; the carbon, notwithstanding its solidity in the shape of charcoal, most readily combines with oxygen, and hence it forms, as carbonic acid, a small portion of atmospheric air, from which source the carbon of plants is in part at least derived. Another source from which plants derive their carbon is the earth, and decaying vegetable matters; the dung of animals supplies also some of the constituents of vegetables. Indeed, in the application of dung and other matters, so as to promote the healthy and vigorous growth of vegetables, does the science of agriculture chiefly consist. It appears, however, that nourishment is received principally, if not entirely, by plants in a liquid or gaseous form. It should be noticed too, that few, if any, healthy vegetables will grow any where except in _light_, a powerful stimulant at all times, not only to plants but to animals; such are its effects, that many _dyes_ in cloth are materially altered, nay, sometimes destroyed by it. _Animal substances_ thus differ from vegetables: they afford a considerable quantity of _ammonia_, (which is, it is now known, a compound body consisting of _hydrogen_ and _nitrogen_), and very fetid products, either by the action of fire, or by the putrid fermentation. They also putrify more readily and speedily than vegetables, and give out a very disagreeable smell. They also contain a considerable quantity of _nitrogen_, the presence of which constitutes the most striking peculiarity of animal compared with vegetable bodies; but as some vegetables contain nitrogen, so there are certain animal principles into the composition of which nitrogen does not enter. The chief _ultimate principles_ then of animal matter are carbon, hydrogen, oxygen, and nitrogen; but _phosphorus_ and _sulphur_ are also often contained in it. _Lime_ also exists in animal bones and shells in considerable quantity, usually, however, in combination with the _phosphoric_ and the _carbonic acid_. The chief _proximate principles_ of animal matter are _blood_, _albumen_, _gelatine_, _colouring matter_, _milk_, _bile_, _lymph_, _urine_, _skin_, _muscle_, _horn_, _hair_, _fat_, _cerebral substance_, _shell_, and _bone_, &c.