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.