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Show the former case, wnen there was no dust in suspension, the air remained cleiir; in the Intter case, when the ordinary ordi-nary atmospheric dust was in the vessel, ves-sel, fog at once appeared. The invisible dust then is detected by the introduction introduc-tion of water vapor. Until very lately it was thought that particles of water vapor combined with each other to form a cloud particle, but it is now found that some solid body, however small, is required for this formation. In fact, when there are no dust particles on which the water vapor at a proper temperature and pressure can condense, there is at present no knowledge as to the point at which the change will take place. But the fine particles of dust in the air act as free surfaces on which the water vapor condenses con-denses into fog. When there is abundance abund-ance of dust in the air and little water vapor present there is an overpropor-tion overpropor-tion of dust particles, and the fog particles par-ticles i-.ro in consequence closely packed, but light in form and small in size, and take the lighter appearance of fog. Accordingly, Ac-cordingly, if the dust is increased in the air, there is a proportionate increase of fog. But on the other hand if the dust particles are fewer in proportion to the number of molecules of water vapor, each particle soon gets weighted, springs into visible existence as if by a creative hand, and falls in mist or rain. If the water vapor had no dust at all on which to settle, it would use the objects on the surface of the ground for the same end, as the grass, leaves, tree branches or house projections. Moisture would then be ever dripping. The occasional genial rain, though at times lashed np to a biting storm of sleet, would require to give place to a constant wetness on the roads and grass. Umbrellas would not be needed, but india rubber protectors for the feet and legs would be in constant requisition. Even the irritable housekeeper, ever annoyed an-noyed at the unaccountable appearance of dust in rooms which she left for clean, would prefer the old necessary evil in this dry dust form than see the walls dripping and the floor wet. Good Words. J THE VALUE OF DUST. lis Fai'tirlcs Form Free Surfaces for the Collection of Vapor. How can invisible particles he brought within the range of our vision? That was one of the first of the many marvelous marvel-ous discoveries of Mr. John Aicken. F. R. S., Falkirk, a distinguished physicist, whose remarkable work has revolutionized revolution-ized a branch of meteorology. Ho showed that without dust in the air there could be no fogs, no mist, no cloud and probably no rain. Tho particles of dust are the free surfaces, which, under certain conditions, attract the water vapor of tho atmosphere to form fug. Invisible before, they become visible when clothed all over with the moisture; unseen as dust, they become distinct as fog particles. This can be easily verified. If ordinary ordi-nary air be forced through a filter ol cotlon wool into a glass receiver, it is deprived of all its dust particles. Let steam be introduced into this receiver frum a boiler, no change will be observed; ob-served; the vessel is quite transparent. But if a jet of steam be introduced into a similar vessel containing ordinary air. it will be seen rising in a dense cloud; then a beautiful fog will be formed, so dense that it camot be seeo through. In |
