240 



Prof. E. Frank] and on Dry Fog. [Jan. 9, 



Spence of Manchester, that the evaporation of saline solutions, kept 

 just "below their boiling point in open pans, can be almost entirely 

 prevented by covering the liquid with a thin stratum of coal-tar. It 

 was thus that Mr. Spence effected a considerable saving of fuel in that 

 part of the process of manufacturing alum in which burnt aluminous 

 shale is digested for many hours with hot dilute sulphuric acid; 

 much less fuel being of course required when the digestion was 

 carried on without evaporation, than when steam escaped from the 

 surface of the hot liquid. This simple though important technical 

 application suggested to me a condition of things under which the 

 existence of so-called " dry fog " would be possible. From our manu- 

 factories and domestic fires, vast aggregate quantities of coal-tar and 

 paraffin oil are daily distilled into the atmosphere, and condensing 

 upon, or attaching themselves to, the watery spherules of fog or cloud, 

 must of necessity coat these latter with an oily film, which would, in 

 all probability, retard the evaporation of the water, and the consequent 

 saturation of the interstitial air. 



The following experiments were made in order to test the validity 

 of this explanation : — 



I. Two platinum dishes, containing water and presenting equal 

 surfaces of liquid, were placed side by side in a moderate draught of air ; 

 the water in one being coated by a very thin film of coal-tar. By com- 

 paring the loss of weight in the two dishes, it was found that during 

 twenty-four hours the evaporation was reduced by the film of coal- 

 tar from 7'195 grms. to 1*124 grms. or 84*4 per cent. 



II. In a similar experiment, the evaporation during twenty-four 

 hours was reduced from 7'986 grms. to 1*709 grms., or 78*6 per 

 cent. 



In order to imitate more nearly the modus operandi of actual smoke 

 in foggy air, the smoke- from burning coal was in the next experiments 

 blown upon the surface of the water in one of the platinum dishes, 

 the dishes being placed as before in a draught niche. 



III. The evaporation during eighteen hours was reduced from 4" 26 

 grms. to *969 grm., or 77*3 per cent. 



IV. In another experiment, the evaporation during twenty-four 

 nours was diminished from 6'325 grms. to 1*173 grms., or 81'5 per 

 cent., and during forty-two hours from 10 '585 grms. to 2 '142 grms., 

 or 79*8 per cent. 



So far the experiments were made in a current of ordinary air of 

 varying humidity ; but they were afterwards repeated with the 

 following results, under a large bell- jar, in which the enclosed air was 

 continually dried by a large surface of concentrated sulphuric acid. 

 As in the last two trials, the film was produced by coal smoke. 



Y. During forty-eight hours, evaporation was diminished from 

 5*178 grms. to * 737 grm., or 85*8 per cent. 



