Aquatic nymphs! you lead with viewless march The winged vapour up the aerial arch.

CANTO III. l. 13.

I. The atmosphere will dissolve a certain quantity of moisture as a chemical menstruum, even when it is much below the freezing point, as appears from the diminution of ice suspended in frosty air, but a much greater quantity of water is evaporated and suspended in the air by means of heat, which is perhaps the universal cause of fluidity, for water is known to boil with less heat in vacuo, which is a proof that it will evaporate faster in vacuo, and that the air therefore rather hinders than promotes its evaporation in higher degrees of heat. The quick evaporation occasioned in vacuo by a small degree of heat is agreeably seen in what is termed a pulse-glass, which consists of an exhausted tube of glass with a bulb at each end of it and with about two thirds of the cavity filled with alcohol, in which the spirit is instantly seen to boil by the heat of the finger-end applied on a bubble of steam in the lower bulb, and is condensed again in the upper bulb by the least conceivable comparative coldness.

2. Another circumstance evincing that heat is the principal cause of evaporation is that at the time of water being converted into steam, a great quantity of heat is taken away from the neighbouring bodies. If a thermometer be repeatedly dipped in ether, or in rectified spirit of wine, and exposed to a blast of air, to expedite the evaporation by perpetually removing the saturated air from it, the thermometer will presently sink below freezing. This warmth, taken from the ambient bodies at the time of evaporation by the steam, is again given out when the steam is condensed into water. Hence the water in a worm-tub during distillation so soon becomes hot; and hence the warmth accompanying the descent of rain in cold weather.

3. The third circumstance, shewing that heat is the principal cause of evaporation, is, that some of the steam becomes again condensed when any part of the heat is withdrawn. Thus when warmer south-west winds replete with moisture succeed the colder north-east winds all bodies that are dense and substantial, as stone walls, brick floors, &c. absorb some of the heat from the passing air, and its moisture becomes precipitated on them, while the north-east winds become warmer on their arrival in this latitude, and are thence disposed to take up more moisture, and are termed drying winds.

4. Heat seems to be the principal cause of the solution of many other bodies, as common salt, or blue vitriol dissolved in water, which when exposed to severe cold are precipitated, or carried, to the part of the water last frozen; this I observed in a phial filled with a solution of blue vitriol which was frozen; the phial was burst, the ice thawed, and a blue column of cupreous vitriol was left standing upright on the bottom of the broken glass, as described in note XIX.

II. Hence water may either be dissolved in air, and may then be called an aerial solution of water; or it may be dissolved in the fluid matter of heat, according to the theory of M. Lavoisier, and may then be called steam. In the former case it is probable there are many other vapours which may precipitate it, as marine acid gas, or fluor acid gas. So alcaline gas and acid gas dissolved in air precipitate each other, nitrous gas precipitates vital air from its azote, and inflammable gas mixed with vital air ignited by an electric spark either produces or precipitates the water in both of them. Are there any subtle exhalations occasionally diffused in the atmosphere which may thus cause rain?

1. But as water is perhaps many hundred times more soluble in the fluid matter of heat than in air, I suppose the eduction of this heat, by whatever means it is occasioned, is the principal cause of devaporation. Thus if a region of air is brought from a warmer climate, as the S.W. winds, it becomes cooled by its contact with the earth in this latitude, and parts with so much of its moisture as was dissolved in the quantity of calorique, or heat, which it now looses, but retains that part which was suspended by its attraction to the particles of air, or by aerial solution, even in the most severe frosts.

2. A second immediate cause of rain is a stream of N.E. wind descending from a superior current of air, and mixing with the warmer S.W. wind below; or the reverse of this, viz. a superior current of S.W. wind mixing with an inferior one of N.E. wind; in both these cases the whole heaven becomes instantly clouded, and the moisture contained in the S.W. current is precipitated. This cause of devaporation has been ingeniously explained by Dr. Hutton in the Transact. of Edinburgh, Vol. I, and seems to arise from this circumstance; the particles of air of the N.E. wind educe part of the heat from the S.W. wind, and therefore the water which was dissolved by that quantity of heat is precipitated; all the other part of the water, which was suspended by its attraction to the particles of air, or dissolved in the remainder of the heat, continues unprecipitated.

3. A third method by which a region of air becomes cooled, and in consequence deposits much of its moisture, is from the mechanical expansion of air, when part of the pressure is taken off. In this case the expanded air becomes capable of receiving or attracting more of the matter of heat into its interstices, and the vapour, which was previously dissolved in this heat, is deposited, as is seen in the receiver of an air-pump, which becomes dewy, as the air within becomes expanded by the eduction of part of it. See note VII. Hence when the mercury in the barometer sinks without a change of the wind the air generally becomes colder. See note VII. on Elementary Heat. And it is probably from the varying pressure of the incumbent air that in summer days small black clouds are often thus suddenly produced, and again soon vanish. See a paper in Philos. Trans. Vol. LXXVIII. intitled Frigorific Experiments on the Mechanical Expansion of Air.

4. Another portion of atmospheric water may possibly be held in solution by the electric fluid, since in thunder storms a precipitation of the water seems to be either the cause or the consequence of the eduction of the electricity. But it appears more probable that the water is condensed into clouds by the eduction of its heat, and that then the surplus of electricity prevents their coalescence into larger drops, which immediately succeeds the departure of the lightning.

5. The immediate cause why the barometer sinks before rain is, first, because a region of warm air, brought to us in the place of the cold air which it had displaced, must weigh lighter, both specifically and absolutely, if the height of the warm atmosphere be supposed to be equal to that of the preceeding cold one. And secondly, after the drops of rain begin to fall in any column of air, that column becomes lighter, the falling drops only adding to the pressure of the air in proportion to the resistance which they meet with in passing through that fluid.

If we could suppose water to be dissolved in air without heat, or in very low degrees of heat, I suppose the air would become heavier, as happens in many chemical solutions, but if water dissolved in the matter of heat, or calorique, be mixed with an aerial solution of water, there can be no doubt but an atmosphere consisting of such a mixture must become lighter in proportion to the quantity of calorique. On the same circumstance depends the visible vapour produced from the breath of animals in cold weather, or from a boiling kettle; the particles of cold air, with which it is mixed, steal a part of its heat, and become themselves raised in temperature, whence part of the water is precipitated in visible vapour, which, if in great quantity sinks to the ground; if in small quantity, and the surrounding air is not previously saturated, it spreads itself till it becomes again dissolved.