The world-wide distribution of atmospheric dust is a remarkable phenomenon. As the blue colour of the sky is universal, the whole of the higher atmosphere must be pervaded by myriads of ultra-microscopical particles, which, by reflecting the blue rays only, give us not only the azure vault of heaven, but in combination with the coarser dust of lower altitudes, diffused daylight, the grand forms and motions of the fleecy clouds, and the 'gentle rain from heaven' to refresh the parched earth and make it beautiful with foliage and flowers. Over every part of the vast Pacific Ocean, whose islands must produce a minimum of dust, the sky is always blue, and its thousand isles do not suffer for want of rain. Over the great forest-plain of the Amazon valley, where the production of dust must be very small, there is yet abundance of rain-clouds and of rain. This is due primarily to the two great natural sources of dust—the active volcanoes, together with the deserts and more arid regions of the world; and, in the second place, to the density and wonderful mobility of the atmosphere, which not only carries the finest dust-particles to an enormous height, but distributes them through its whole extent with such wonderful uniformity.
Every dust-particle is of course much heavier than air, and in a comparatively short time, if the atmosphere were still, would fall to the ground. Tyndall found that the air of a cellar under the Royal Institution in Albemarle Street, which had not been opened for several months, was so pure that the path of a beam of electric light sent through it was quite invisible. But careful experiments show that not only is the air in continual motion, but the motion is excessively irregular, being hardly ever quite horizontal, but upwards and downwards and in every intermediate direction, as well as in countless whirls and eddies; and this complexity of motion must extend to a vast height, probably to fifty miles or more, in order to provide a sufficient thickness of those minutest particles which produce the blue of the sky.
All this complexity of motion is due to the action of the sun in heating the surface of the earth, and the extreme irregularity of that surface both as regards contour and its capacity for heat-absorption. In one area we have sand or rock or bare clay, which, when exposed to bright sunshine, become scorching hot; in another area we have dense vegetation, which, owing to evaporation caused by the sunshine, remains comparatively cool, and also the still cooler surfaces of rivers and Alpine lakes. But if the air were much less dense than it is, these movements would be less energetic, while all the dust that was raised to any considerable height would, by its own weight, fall back again to the earth much more rapidly than it does now. There would thus be much less dust permanently in the atmosphere, and this would inevitably lead to diminished rainfall and, partially, to the other injurious effects already described.
Atmospheric Electricity
We have already seen that vegetable organisms obtain the chief part of the nitrogen in their tissues from ammonia produced in the atmosphere and carried into the earth by rain. This substance can only be thus produced by the agency of electrical discharges, or lightning, which cause the combination of the hydrogen in the aqueous vapour with the free nitrogen of the air. But clouds are important agents in the accumulation of electricity in sufficient amount to produce the violent discharges we know as lightning, and it is doubtful whether without them there would be any discharges through the atmosphere capable of decomposing the aqueous vapour in it. Not only are clouds beneficial in the production of rain, and also in moderating the intensity of continuous sun-heat, but they are also requisite for the formation of chemical compounds in vegetables which are of the highest importance to the whole animal kingdom. So far as we know, animal life could not exist on the earth's surface without this source of nitrogen, and therefore without clouds and lightning; and these, we have just seen, depend primarily on a due proportion of dust in the atmosphere.
But this due proportion of dust is mainly supplied by volcanoes and deserts, and its distribution and constant presence in the air depends upon the density of the atmosphere. This again depends on two other factors: the force of gravity due to the mass of the planet, and the absolute quantity of the free gases constituting the atmosphere.
We thus find that the vast, invisible ocean of air in which we live, and which is so important to us that deprivation of it for a few minutes is destructive of life, produces also many other beneficial effects of which we usually take little account, except at times when storm or tempest, or excessive heat or cold, remind us how delicate is the balance of conditions on which our comfort, and even our lives, depend.
But the sketch I have here attempted to give of its varied functions shows us that it is really a most complex structure, a wonderful piece of machinery, as it were, which in its various component gases, its actions and reactions upon the water and the land, its production of electrical discharges, and its furnishing the elements from which the whole fabric of organic life is composed and perpetually renewed, may be truly considered to be the very source and foundation of life itself. This is seen, not only in the fact of our absolute dependence upon it every minute of our lives, but in the terrible effects produced by even a slight degree of impurity in this vital element. Yet it is among those nations that claim to be the most civilised, those that profess to be guided by a knowledge of the laws of nature, those that most glory in the advance of science, that we find the greatest apathy, the greatest recklessness, in continually rendering impure this all-important necessary of life, to such a degree that the health of the larger portion of their populations is injured and their vitality lowered, by conditions which compel them to breathe more or less foul and impure air for the greater part of their lives. The huge and ever-increasing cities, the vast manufacturing towns belching forth smoke and poisonous gases, with the crowded dwellings, where millions are forced to live under the most terrible insanitary conditions, are the witnesses to this criminal apathy, this incredible recklessness and inhumanity.
For the last fifty years and more the inevitable results of such conditions have been fully known; yet to this day nothing of importance has been done, nothing is being done. In this beautiful land there is ample space and a superabundance of pure air for every individual. Yet our wealthy and our learned classes, our rulers and law-makers, our religious teachers and our men of science, all alike devote their lives and energies to anything or everything but this. Yet this is the one great and primary essential of a people's health and well-being, to which everything should, for the time, be subordinate. Till this is done, and done thoroughly and completely, our civilisation is naught, our science is naught, our religion is naught, and our politics are less than naught—are utterly despicable; are below contempt.
It has been the consideration of our wonderful atmosphere in its various relations to human life, and to all life, which has compelled me to this cry for the children and for outraged humanity. Will no body of humane men and women band themselves together, and take no rest till this crying evil is abolished, and with it nine-tenths of all the other evils that now afflict us? Let everything give way to this. As in a war of conquest or aggression nothing is allowed to stand in the way of victory, and all private rights are subordinated to the alleged public weal, so, in this war against filth, disease, and misery let nothing stand in the way—neither private interests nor vested rights—and we shall certainly conquer. This is the gospel that should be preached, in season and out of season, till the nation listens and is convinced. Let this be our claim: Pure air and pure water for every inhabitant of the British Isles. Vote for no one who says 'It can't be done.' Vote only for those who declare 'It shall be done.' It may take five or ten or twenty years, but all petty ameliorations, all piecemeal reforms, must wait till this fundamental reform is effected. Then, when we have enabled our people to breathe pure air, and drink pure water, and live upon simple food, and work and play and rest under healthy conditions, they will be in a position to decide (for the first time) what other reforms are really needed.