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THE POPULAR SCIENCE MONTHLY.

any or not. If he concludes to have the work done, he may have it inspected, when it is completed, or nearly so, and obtain a Certificate as to the sanitary condition of his premises. The second year's inspection is a simpler matter than the first year's, for it is guided by the results of the previous inspection, and has to be only comparative. With respect to the efficiency of this system, Professor Jenkin says that it has been shown that the required services can be rendered in a thorough and efficient manner by one resident engineer, for four hundred and fifty or even five hundred houses in one year. Twelve hundred houses have been put in order in Edinburgh, and there has not been during three years one case of complaint that the houses were not thoroughly examined, or that the reports were not sufficiently detailed; but, at the annual meetings of the society, member after member has arisen to express his satisfaction at the work.

 

Dust, the Nucleus of Fog.—According to the researches of Mr. John Aitken, as described in a paper read by him before the Royal Society of Edinburgh, the formation of fogs and clouds is dependent on the presence of dust in the atmosphere. His view was illustrated by an experiment in which steam was mixed with air in two large glass receivers, one of which was filled with common air, the other with air that had been filtered. Clouds appeared in the former vessel, while the air in the other one remained perfectly transparent. Similar results attended an experiment with the air-pump, in the receiver of which a little water was placed to saturate the air. On removing a part of the pressure, a fogginess appeared, or nothing was visible, according as the air in the receiver was unfiltered or filtered. From these and other similar experiments, Mr. Aitken has concluded that, whenever water-vapor condenses in the atmosphere, it always does so on some solid nucleus; that dust-particles in the air form such nuclei; that if there were no dust, there would be no fogs, no clouds, no mists, and probably no rain; that the supersaturated air would convert every object on the surface of the earth into a condenser on which it would deposit; and that our breaths, when they became visible on a cold morning, and every puff of steam as it escapes into the air, show the impure and dusty condition of the atmosphere. Experiments with other vapors than that of water showed that their condensation is governed by a similar rule. The condensation is not produced by any particles which we can see, or even by those which are revealed by the sunbeam, for these may be driven off by heat and the fogs still be visible, but by vastly more numerous, infinitesimally small, and invisible particles which heat will not drive away. These particles may be furnished by the spray from the ocean, by meteoric matter, by the operation of almost every force. The products of all kinds of combustion give rise to them. The use of purer forms of coal, or even of gas, does not avoid them, nor even appear to diminish their number. Common salt is one of the most active fog-producers, but the products of burned sulphur exceeded in this respect all the other substances experimented upon. The density of the fog depends on the amount of fine dust in the air. If only a few particles are present, only a few fog-drops form, and they are heavy and fall like rain; if there are many, the more dust the finer are the fog-particles, and the longer they remain suspended in the air. Though the use of more perfect forms of combustion is not likely to prevent the generation of fogs, it will, by preventing the accumulation of smoke which now comes down into fogs and mixes with them, remove the cause which makes them so dark and extremely annoying.

 

Electric Lights for the French Coasts.—M. E. Allard, Director of the Central Lighthouse Service, has submitted to the French Minister of Public Works propositions for lighting the coasts of France with the electric light. He would begin by substituting the electric light for the present oil-lights in forty-two of the principal lighthouses, and adding sound-signals in twenty of them. The mean range of visibility of the present oil-lights is twenty-two miles on the ocean-coast and twenty-seven miles on the Mediterranean coast. Within these radii they can be depended upon as signals during one half of the year; during the other half of the year