The blue of the ocean varies in different parts from a pure blue somewhat lighter than that of the sky, as seen about the northern tropic in the Atlantic, to a deep indigo tint, as seen in the north temperate portions of the same ocean: owing, probably, to differences in the nature, quantity, and distribution of the solid matter which causes the colour. The Mediterranean, and the deeper Swiss lakes, are also a blue of various tints, due also to the presence of suspended matter, which Professor Tyndall thought might be so fine that it would require ages of quiet subsidence to reach the bottom. All the evidence goes to show, therefore, that the exquisite blue tints of sky and ocean, as well as all the sunset hues of sky and cloud, of mountain peak and Alpine snows, are due to the finer particles of that very dust which, in its coarser forms, we find so annoying and even dangerous. But if this production of colour and beauty were the only useful function of dust, some persons might be disposed to dispense with it in order to escape its less agreeable effects. It has, however, been recently discovered that dust has another part to play in nature; a part so important that it is doubtful whether we could even live without it. To the presence of dust in the higher atmosphere we owe the formation of mists, clouds, and gentle beneficial rains, instead of water spouts and destructive torrents. It is barely twenty years ago since the discovery was made, first in France by Coulier and Mascart, but more thoroughly worked out by Mr. John Aitken in 1880. He found that if a jet of steam is admitted into two large glass receivers,--one filled with ordinary air, the other with air which has been filtered through cotton wool so as to keep back all particles of solid matter,--the first will be instantly filled with condensed vapour in the usual cloudy form, while the other vessel will remain quite transparent. Another experiment was made, more nearly reproducing what occurs in nature. Some water was placed in the two vessels prepared as before. When the water had evaporated sufficiently to saturate the air the vessels were slightly cooled; a dense cloud was at once formed in the one while the other remained quite clear. These experiments, and many others, show that the mere cooling of vapour in air will not condense it into mist clouds or rain, unless _particles of solid matter_ are present to form _nuclei_ upon which condensation can begin. The density of the cloud is proportionate to the number of the particles; hence the fact that the steam issuing from the safety-valve or the chimney of a locomotive forms a dense white cloud, shows that the air is really full of dust particles, most of which are microscopic but none the less serving as centres of condensation for the vapour. Hence, if there were no dust in the air, escaping steam would remain invisible; there would be no cloud in the sky; and the vapour in the atmosphere,