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Show 184 UNION OF AIR AND VAPOUR. the water has ascended the more minutely must it be divided, and the farther must all the parts be from each other. The action of the heat destroys the cohesion of the water with the pool, or the leaf, or other surface from which it rises, the very moment that it begins to ascend; and the cohesion of the ascending parts becomes less and less, even much faster than the diminishing size, because of the distance that they are apart, and that is the reason why vapour, which is so dense as not only to form a light floating rack, but castled clouds, with edges as well defined as if they were terrestrial solids, and even an entire covering, that extends over the whole visible heavens, and shadows the earth to very deep gloom, while yet that these formations ride buoyant on the air, and some of them are indications of dry weather. Thus though the air is the passage of the ascending water which is to maintain the springs, it is no more the cause of the ascent than the channel of a river is the cause why the tide of that river flows downwards, and the vapour, whether it be invisible or in clouds, is obeying the laws of its own nature, and in nowise under the control of or attracted by the air. Were there an attraction, and if the air and the water actually united in their ultimate particles, and formed a new substance, as an acid and an alkali do in the formation of a salt, we should soon, from the vast extent of surface at which they constantly meet each other (which may be said to be, considering how many moist substances stand surrounded by the air, and how often the face of the water is wrinkled with waves, equal to that of the whole globe )-if they acted chymica.lly upon each other we should very soon have neither air nor water; but a compound of the two: differing as much in its properties from either as the neutral salt does from the acid and the alkali. Common salt) which renders our food so savoury and so whole COLD FROM EVAPORATION. 185 some, is a compound of chlorine and hydrogen and soda, each of which is a poison, and when perfectly 1 pure a very deadly one, nor are we acquainted with any real chymical combination in which the properties of all the ingredients are not suspended and new ones produced, while the combination subsists. That, indeed, is just what is meant by a chymical combination. But the water, when in the most minute state of division, and ascending in air so very thin that the slightest cobweb would sink like a stone, is in every one of its little and invisible drops, as perfectly water as when it rolls in the flood of a river, or spreads in the ocean; and it is just as ready to obey all the laws of the water in the one situation as in the other. The evaporability of water is the principal reason why it, and substances that are wet with it, do not become so soon hot as substances that are dry. When it is ~xposed .to th_e a~r, it evaporates not only as long as 1t remams ltqmd, bnt even when it is frozen; although the ~vaporation of ice when it presents only one uniform surface to the air is slower than that of liquid water; because the heat has to melt the ice before it can turn the water of that ice into vapour. T~us the cooling influence of ice upon the atmosphere 1s much less and also more confined to the vicinity of the surface than that of water at the temperature of freezing, or even at a temperature a little higher than that, perhaps too as high as about forty-two degrees of the common thermometer, the temperature at which water has the greatest density, and at which, when it is all cooied down to it, the water in a pond or lake remains stationary, without any internal motion upwards or downwards. The slower evap~ration that takes place from ice than from water 1s the reason why, in walking abroad, one feels so much more warm and comfortQ. 2 |