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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 sys- 

 tem. Professor Jenkin says that it has been 

 shown that the required services can be ren- 

 dered 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 exam- 

 ined, 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 Nnelcns 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 ex- 

 periments, Mr. Aitken has concluded that, 

 whenever water-vapor condenses in the at- 

 mosphere, 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 super- 

 saturated 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 es- 

 capes into the air, show the impure and 

 dusty condition of the atmosphere. Ex- 

 periments with other vapors than that of 

 water showed that their condensation is 

 governed by a similar rule. The condensa- 

 tion is not produced by any particles which 

 we can see, or even by those which are re- 

 vealed by the sunbeam, for these may be 

 driven off by heat and the fogs still be vis- 

 ible, but by vastly more numerous, infini- 

 tesimally 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 ex- 

 perimented 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, re- 

 move 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 propo- 

 sitions for lighting the coasts of France 

 with the electric light. He would begin by 

 substituting the electric light for the pres- 

 ent oil-lights in forty-two of the principal 

 lighthouses, and adding sound-signals in 

 twenty of them. The mean range of visi- 

 bility 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 dnring one half of the 

 year ; during the other half of the year 



