From every natural fact invisible relations radiate, the apprehension of which imparts a measure of delight; and there is a store of pleasure of this kind ever at hand for those who have the capacity to turn natural appearances to account. It is pleasant, for example, to lie on one’s back upon a dry green slope and watch the clouds forming and disappearing in the blue heaven. A few days back the firmament was mottled with floating cumuli, from the fringes of which light of dazzling whiteness was reflected downwards, while the chief mass of the clouds lay in dark shadow. From the edge of one large cloud-field stretched small streamers, which, when attentively observed, were seen to disappear gradually, and finally to leave no trace upon the blue sky. On the opposite fringe of the same cloud, and beyond it, small patches of milky mist would appear, and curdle up, so as to form little cloudlets as dense apparently as the large mass beside which they were formed. The counter processes of production and consumption were evidently going on at opposite sides of the cloud. Even in the midst of the serene firmament, where a moment previously the space seemed absolutely void, white cloud-patches were formed, their sudden appearance exciting that kind of surprise which might be supposed to accompany the observation of a direct creative act.

These clouds were really the indicators of what was going on in the unseen air. Without them no motion was visible; but their appearance and disappearance proved not only the existence of motion, but also the want of homogeneity in the atmosphere. Though we did not see them, currents were mingling, possessing different temperatures and carrying different loads of invisible watery vapour. We know that clouds are not true vapour, but vapour precipitated by cold to water. We know also that the amount of water which the air can hold in the invisible state depends upon its temperature; the higher the temperature of the air, the more water will it be able to take up. But, when a portion of warm air, carrying its invisible charge, is invaded by a current of low temperature, the chilled vapour is precipitated, and a cloud is the consequence. In this way two parcels of moist air, each of which taken singly may be perfectly transparent, can produce by their mixture an opaque cloud. In the same way a body of clear humid air, when it strikes the cold summit of a mountain, may render that mountain ‘cloud-capped.’

An illustration of this process, which occurred some years ago in a Swedish ball-room, is recounted by Professor Dove. The weather was clear and cold, and the ball-room was clear and warm. A lady fainted, and air was thought necessary to her restoration. A military officer present tried to open the window, but it was frozen fast. He broke the window with his sword, the cold air entered, and it snowed in the room. A minute before this all was clear, the warm air sustaining a large amount of moisture in a transparent condition. When the colder air entered, the vapour was first condensed and then frozen. The admission of cool air even into our London ball-rooms produces mistiness. Mountain-chains are very effective in precipitating the vapour of our south-westerly winds; and this sometimes to such an extent as to produce totally different climates on the two sides of the same mountain-group. This is very strikingly illustrated by the observations of Dr. Lloyd on the rainfall of Ireland. Stations situated on the south-west side of a mountain-range showed a quantity of rain far in excess of that observed upon the north-east side. The winds in passing over the mountains were drained of their moisture, and were afterwards comparatively dry.

Two or three years ago I had an opportunity of witnessing a singular case of condensation at Mortain in Normandy. The tourist will perhaps remember a little chapel perched upon the highest summit in the neighbourhood. A friend and I chanced to be at this point near the hour of sunset. The air was cloudless, and the sun flooded the hillsides and valleys with golden light. We watched him as he gradually approached the crest of a hill, behind which he finally disappeared. Up to this point a sunny landscape of exquisite beauty was spread before us, the atmosphere being very transparent; but now the air seemed suddenly to curdle into mist. Five minutes after the sun had departed, a dense fog filled the valleys and drifted in fleecy masses up the sides of the hills. In an incredibly short time we found ourselves enveloped in local clouds so dense as to render our retreat a matter of some difficulty.

In this case, before the sun had disappeared the air was evidently nearly saturated with transparent vapour. But why did the vapour curdle up so suddenly when the sun departed? Was it because the withdrawal of his beams rendered the air of the valleys colder, and thus caused the precipitation of the moisture diffused through the air? No. We must look for an explanation to a more direct action of the sun upon the atmospheric moisture. Let me explain. The beams which reach us from the sun are of a very composite character. A sheaf of white sunbeams is composed of an infinitude of coloured rays, the resultant effect of all upon the eye being the impression of whiteness. But though the colours, and shades of colour, which enter into the composition of a sunbeam are infinite, for the sake of convenience we divide them into seven, which are known as the prismatic colours.

The beams of the sun, however, produce heat as well as light, and there are different qualities of heat in the sunbeam as well as different qualities of light—nay, there are copious rays of heat in a sunbeam which give no light at all, some of which never even reach the retina at all, but are totally absorbed by the humours of the eye. Now, the same substance may permit rays of heat of a certain quality to pass freely through it, while it may effectually stop rays of heat of another quality. But in all cases the heat stopped is expended in heating the body which stops it. Now, water possesses this selecting power in an eminent degree. It allows the blue rays of the solar beam to pass through it with facility, but it slightly intercepts the red rays, and absorbs with exceeding energy the obscure rays; and those are the precise rays which possess the most intense heating power.

We see here at once the powerful antagonism of the sun to the formation of visible fog, and we see, also, how the withdrawal of his beams may be followed by sudden condensation, even before the air has had any time to cool. As long as the solar beams swept through the valleys of Mortain, every particle of water that came in their way was reduced to transparent vapour by the heat which the particle itself absorbed; or, to speak more strictly, in the presence of this antagonism precipitation could not at all occur, and the atmosphere remained consequently clear.[43] But the moment the sun withdrew, the vapour followed, without opposition, its own tendency to condense, and its sudden curdling up was the consequence.

With regard to the air, its temperature may not only have remained sensibly unchanged for some time after the setting of the sun, but it may have actually become warmer through the heat set free by the act of condensation. It was not, therefore, the action of cold air upon the vapour which produced the effect, but it was the withdrawal of that solar energy which water has the power to absorb, and by absorbing to become dissipated in true vapour.

I once stood with a friend upon a mountain which commands a view of the glacier of the Rhone from its origin to its end. The day had been one of cloudless splendour, and there was something awful in the darkness of the firmament. This deepening of the blue is believed by those who know the mountains to be an indication of a humid atmosphere. The transparency, however, was wonderful. The summits of Mont Cervin and the Weisshorn stood out in clear definition, while the mighty mass of the Finsteraarhorn rose with perfect sharpness of outline close at hand. As long as the sun was high there was no trace of fog in the valleys, but as he sloped to the west the shadow of the Finsteraarhorn crept over the snow-fields at its base. A dim sea of fog began to form, which after a time rose to a considerable height, and then rolled down like a river along the flanks of the mountain. On entering the valley of the Rhone, it crossed a precipitous barrier, down which it poured like a cataract; but long before it reached the bottom it escaped from the shadow in which it had been engendered, and was hit once more by the direct beams of the sun. Its utter dissipation was the consequence, and though the billows of fog rolled on incessantly from behind, the cloud-river made no progress, but disappeared, as if by magic, where the sunbeams played upon it. The conditions were analogous to those which hold in the case of a glacier. Here the ice-river is incessantly nourished by the mountain snow: it moves down its valley, but does not advance in front. At a certain point the consumption by melting is equal to the supply, and here the glacier ceases. In the case before us the cloud-river, nourished by the incessant condensation of the atmospheric vapour, moved down its valley, but ceased at the point where the dissipating action of the sunbeams equalled the supply from the cloud-generator behind.