ON FOG. 
31 
during a thick fog, the degree of humidity was only 80 per cent 
of saturation. The same phenomena had been observed by Mr. 
Glaisher in his balloon ascents, the hygrometer passing through 
cloud or fog often showing the air to possess considerable dry- 
ness. In an ascent from Wolverhampton in 1862, at a height 
of 9882 feet, in passing through a cloud so dense that the 
balloon could not be seen from the car, the dry-bulb thermometer 
read 37°’8 F., and the wet-bulb 30°*2, indicating a dew-point 
17° '9 below the air temperature. On the 30th July of the 
same year, at an altitude of 6466 feet, while the balloon was 
passing between the Crystal Palace and Gravesend through c a 
great mist/ the dew-point was 12 0, 7 F. below the temperature 
of the air. A table of sixteen such observations is given, in 
which, taking 100 as saturation, the real condition rises only 
once to 87, sinks as low as 46, and averages between 50 and 60, 
or rather more than half. It is thus evident, he observes, that 
air closely surrounding the spherules of water in a fog is some- 
times far from containing its full proportion of watery vapour ; 
‘ although, as is well known to persons occupied with gas-analy- 
sis, when a perfectly dry gas is admitted into a moist eudiometer 
it very rapidly assumes the volume indicating saturation, not- 
withstanding that the proportion of water- surface to volume of 
gas is obviously far less than that afforded to the interstitial air 
of a fog/ 
One experiment was made to test the latter fact, and it was 
found that air dried over calcic chloride became completely 
saturated in less time than l m 50 s , when passed into a moist 
glass tube, J inch in diameter. Dr. Frankland, in seeking for 
an explanation of the anomalous behaviour of the watery vapour 
in the two cases above, thought of some experiments made, 
many years ago, by Mr. Spence of Manchester, showing 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. Mr. Spence in 
this way 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, diluted, sulphuric 
acid, by the lesser escape of steam from the surface of the hot 
liquid. 
He saw in this simple process a condition of things under 
which the so-called ‘ Dry Fog ’ might be produced. e From our 
manufactories and domestic fires/ he says, ‘ 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 the 
latter with an oily film, which would retard the evaporation of 
the water, and the consequent saturation of the interstitial air.’ 
