That such dust exists everywhere in the air, even up to great heights, is not a supposition but a proved fact. By exposing glass plates covered with glycerine in different places and at different altitudes the number of these particles in each cubic foot of air has been determined; and it is found that not only are they present everywhere at low levels, but that there are a considerable number even at the tops of the highest mountains. These solid particles also act in another way. By radiation in the higher atmosphere they become very cold, and thus condense the vapour by contact, just as the points of grass-blades condense it to form dew.
When steam is escaping from an engine we see a mass of dense white vapour, a miniature cloud; and if we are near it in cold, damp weather, we feel little drops of rain produced from it. But on a fine, warm day it rises quickly and soon melts away, and entirely disappears. Exactly the same thing happens on a larger scale in nature. In fine weather we may have abundant clouds continually passing high overhead, but they never produce rain, because as the minute globules of water slowly fall towards the earth, the warm dry air again turns them into invisible vapour. Again, in fine weather, we often see a small cloud on a mountain top which remains there a considerable time, even though a brisk wind is blowing. The mountain top is colder than the surrounding air, and the invisible vapour becomes condensed into cloud by passing over it, but the moment these cloud particles are carried past the summit into the warmer and drier air they are again evaporated and disappear. On Table Mountain, near Cape Town, this phenomenon occurs on a large scale, and is termed the table-cloth, the mass of white fleecy cloud seeming to hang over the flat mountain top to some distance down where it remains for several months, while all around there is bright sunshine.
Another phenomenon that indicates the universal presence of dust to enormous heights in the atmosphere is the blue colour of the sky. This is caused by the presence of such excessively minute particles of dust through an enormous thickness of the higher atmosphere—probably up to a height of twenty or thirty miles, or more—that they reflect only the light of short wave-length from the blue end of the spectrum. This also has been proved by experiment. If a glass cylinder, several feet long, is filled with pure air from which all solid particles have been removed by filtering and passing over red-hot platinum wires, and a ray of electric light is passed through it, the interior, when viewed laterally, appears quite dark, the light passing through in a straight line and not illuminating the air. But if a little more air is passed through the filter so rapidly as to allow only the minutest particles of dust to enter with it, the vessel becomes gradually filled with a blue haze, which gradually deepens into a beautiful blue, comparable with that of the sky. If now some of the unfiltered air is admitted, the blue fades away into the ordinary tint of daylight.
Since it has been known that liquid oxygen is blue, many people have concluded that this explains the blue colour of the sky. But it has really nothing to do with the point at issue. The blue of the liquid oxygen becomes so excessively faint in the gas, further attenuated as it is by the colourless nitrogen, that it would have no perceptible colour in the whole thickness of our atmosphere. Again, if it had a perceptible blue tint we could not see it against the blackness of space behind it; but white objects seen through it, such as the moon and clouds, should all appear blue, which they do not do. The blue we see is from the whole sky, and is therefore reflected light; and as pure air is quite transparent, there must be solid or liquid particles so minute as to reflect blue light only. In the lower atmosphere the rain-producing particles are larger, and reflect all the rays, thus diluting the blue colour near the horizon, and, by refraction and reflection combined, producing the various beautiful hues of sunrise and sunset.
This production of exquisite colours by the dust in the atmosphere, though adding greatly to the enjoyment of life, cannot be considered essential to it; but there is another circumstance connected with atmospheric dust which, though little appreciated, might have effects which can hardly be calculated. If there were no dust in the atmosphere, the sky would appear black even at noon, except in the actual direction of the sun; and the stars would be visible in the day as well as at night. This would follow because air does not reflect light, and is not visible. We should therefore receive no light from the sky itself as we do now, and the north side of every hill, house, and other solid objects, would be totally dark, unless there were any surfaces in that direction to reflect the light. The surface of the ground at a little distance would be in sunshine, and this would be the only source of light wherever direct sunlight was cut off. To get a good amount of pleasant light in houses it would be necessary to have them built on nearly level ground, or on ground rising to the north, and with walls of glass all round and down to the floor line, to receive as much as possible of the reflected light from the ground. What effect this kind of light would have on vegetation it is difficult to say, but trees and shrubs would probably grow laterally towards the south, east, and west, so as to get as much direct sunshine as possible.
A more important result would be that, as sunshine would be perpetual during the day, so much evaporation would take place that the soil would become arid and almost bare in places that are now covered with vegetation, and plants like the cactuses of Arizona and the euphorbias of South Africa would occupy a large portion of the surface.
Returning now from this collateral subject of light and colour to the more important aspect of the question—the absence of cloud and rain—we have to consider what would happen, and in what way the enormous quantity of water which would be evaporated under continual sunshine would be returned to the earth.
The first and most obvious means would be by abnormally abundant dews, which would be deposited almost every night on every form of leafy vegetation. Not only would all grass and herbage, but all the outer leaves of shrubs and trees, condense so much moisture as to take the place of rain so far as the needs of such vegetation were concerned. But without arrangements for irrigation cultivation would be almost impossible, because the bare soil would become intensely heated during the day, and would retain so much of its heat through the night so as to prevent any dew forming upon it.
Some more effective mode, therefore, of returning the aqueous vapour of the atmosphere to the earth and ocean, would be required, and this, I believe, would be done by means of hills and mountains of sufficient height to become decidedly colder than the lowlands. The air from over the oceans would be constantly loaded with moisture, and whenever the winds blew on to the land the air would be carried up the slopes of the hills into the colder regions, and there be rapidly condensed upon the vegetation, and also on the bare earth and rocks of northern slopes, and wherever they cooled sufficiently during the afternoon or night to be below the temperature of the air. The quantity of vapour thus condensed would reduce the atmospheric pressure, which would lead to an inrush of air from below, bringing with it more vapour, and this might give rise to perpetual torrents, especially on northern and eastern slopes. But as the evaporation would be much greater than at the present time, owing to perpetual sunshine, so the water returned to the earth would be greater, and as it would not be so uniformly distributed over the land as it is now, the result would perhaps be that extensive mountain sides would become devastated by violent torrents, rendering permanent vegetation almost impossible; while other and more extensive areas, in the absence of rain, would become arid wastes that would support only the few peculiar types of vegetation that are characteristic of such regions.
Whether such conditions as here supposed would prevent the development of the higher forms of life it is impossible to say, but it is certain that they would be very unfavourable, and might have much more disastrous consequences than any we have here suggested. We can hardly suppose that, with winds and rock-formations at all like what they are now, any world could be wholly free from atmospheric dust. If, however, the atmosphere itself were much less dense than it is, say one-half, which might very easily have been the case, then the winds would have less carrying power, and at the elevations at which clouds are usually formed there would not be enough dust-particles to assist in their formation. Hence fogs close to the earth's surface would largely take the place of clouds floating far above it, and these would certainly be less favourable to human life and to that of many of the higher animals than existing conditions.