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Show 182 EVAPOHATION. abundantly does the air supply them with nature's most abundant, most refreshing, and most valuable production. If you would know the real :ralue of water, ask a man when he is stretched on his c.ou.ch in the heat of a fever, and when his throat IS mflamed and swollen so that it will not do its office; or if he cannot answer, then ask him who sinks do~n under the ardours of the mid-day heat, on the wide and bu.rning sand of Sahara, at many leagues' dista1~ce from the little dingy pool and the overshadowmg palms: question him as to the value of water, and, thoucrh the charter of the world's wealth were in his keeping, he would cheerft~ll:y give it for. one little cup or even that he were sittmg on the brmk of one of those stagnant ditches which we shun. As we do not see the partieles of the atmosphere as a whole, the particles of it~ two chief ingredients, the oxygen and the hydrogen, or the particles of water which it takes up in the process of evaporation we cannot know the nature of the agency by whi~h any of these are held togeth er. T~e ?o~esion of particles in the entire substance, as air, IS mdeed not only small, but absolutely negative, and entirely obedient to the action of heat; and 1 ot only that, but if a1r is let into a larger ~pace upon which. there · is no pressure it will expand; and cool, that IS, become senl::libly cold, or ab~tract heat from ot~er su~stances as it expands. And when the quc:ntity of II; in a close vessel is diminished by pumpmg a portion of it out and water is placed in the vessel, and some substa~ce is also placed in it which has .mo~e attraction for water than the air has, and which m consequence drinks up the v~p.our ?f .the water as soon as it is formed the remammg au m the vessel will become so cold that the water will be frozen into a cake of ice, even though the app;uatus be in a warm room. That simple experiment throws some light upon he very general and important process of evapo- ASCENT OF VAPOUR. 183 ration. It shows us that when water passes into a state of vapour, or becomes endowed with that i"persive motion of its particles which sends it invisibly through the air, it is really changed to a state very much resembling that of the air; and thus it may ascend among the particles of the air, in consequence of the dispersive motion which it itself acquires by being heated. So that, though the vapour is invisible~ we are not to suppose that it necessarily enters into chymical combination with the air, in such a manner as that the two form one ccmpound substance; but that it is only dispersed through the air mechanically, and rises by the general law of gravitation, just because the quantity of it which is contained in any given bulk, in a gallon for instance, is less in weight than the quantity of air contained in the sC~me. That this is actually the case is proved by the fact that in dense air, when that air is warmed, and consequently communicates heat, not only to the surface from which the water is evaporated but to the little drops as they ascend through it, the water rises in visible vapour: and as that vapour mounts into air, containing less and less water as it ascends, and receiving more and more heat in its progress, the little drops are subjected to continual division, so that long before they have risen so high as the top of an ordinary hill, they have become far too minute for observation, and are so dispersed that a gallon of the watery vapour may not weigh onetwentieth part of a gallon of the air through which it is ascending. In that state it is not only impossible that it cau fall down to the ground, but it must continue to ascend, and to C~scend rapidly, in proportion as, bulk for bulk, it is lighter than the air. And the farther that it ascends, too, it wiJl ascend the more freely and rapidly, and spreatl to the greater extent; because the rarer that the au is, the farther must its particles be asunder; and the higher that |