68
vital air, oxygen ; and as these terms are now well known, theyshall be used in what follows. Now, the carbon which coloursvegetable mould, seems to be insoluble or very refractory; and wehave no evidence that vegetables can detach aeriform fluids fromtheir connexion with the soil, or decompose and modify them totheir increase.
Since the soil alone, however well adapted to contribute itspart to the production of vegetables, does not furnish them withfood, from whence is this food derived l The atmosphere hasbeen held to contribute considerably to the nourishment of plants;but the atmosphere is not under human controul, and comesnot within our province. Water is indispensable in vegetation.Without its presence, no seed can germinate, and no plant cangrow. As it is a necessary concomitant of the soil in all itsfunctions—a principal agent both in the formation and decom-position of vegetables—and proves the nourisher or destroyer ofall land plants, according to the measure and manner of its ap-plication, it merits here particular examination. Water is not,as it had long been esteemed, a simple element, but a compoundof two aeriform bodies, oxygen and hydrogen, as chemical ana-lysis has now fully demonstrated. According to Lavoisier , 8.5parts of oxygen, and 15 parts of hydrogen, make 100 parts ofwater. To form a proper judgment of its agency in vegetation,we must view it in its different aptitudes, or laws, to which it issubjected.
1st, The first and most obvious of these, is the general law ofgravitation, by which it continues in a. mass, all its parts pressingtowards the centre, and forming a level surface, which it still pre-serves, unless disturbed by foreign agents. In this state, it is af-fected by heat, as other bodies are, the volume enlarging as thetemperature is increased; but with this peculiarity, that the vo-lume is also enlarged by a great degree of cold, bursting the ves-sels in which it is confined, when it freezes.
2d, The affinity of aggregation, by which its particles adhere,and follow one another, as it were, by mutual affection. But theinfluence of this and the former law is slender, and is perpetuallycounteracted by those which follow, which are directly the inverseof the foregoing,
3d, Evaporation, by which it ascends in minute particles, andtningles with the atmosphere. This is always greater as the tem-perature is higher, but perhaps is never suspended, a great eva-poration being sometimes observable during a clear frost.
4th, Expansion, when its particles quit their aggregate state,and are diffused through the empty pores of dry substances, »few drops moistening all around to a considerable distance. Inobedience to this law, it sometimes mounts to a considerableheight. When a wall of porous materials is built on a moist foun-dation, the marks of the damp may be seen from 2 to 3 feet absvvthe surface*