424 MR JOHN AITKEN ON THE 



shall find that the fogging will be very greatly increased. It almost looks as if this 

 might be a good test for sulphur products in our atmosphere, as the density of the 

 fogging given by any sample of air seems to depend very much on the proportion of 

 products of combustion from coal present. But as I have not yet found any sample of 

 unpurified air which, when saturated with vapour and a little ammonia added, did not 

 give without being expanded a few fog particles when carefully examined, I must wait 

 for opportunities of testing air purer than can be obtained in this district. We see 

 from these experiments with products from fires in which the combustion is perfect, that 

 something more is necessary to diminish the density and persistence of town fogs. To 

 effect this, as already indicated, we must change even the products of perfect combustion 

 of coal, and make them as harmless as the products from a paraffin-lamp. After that is, 

 done we may look for some alleviation of the miseries and discomforts of town fogs. 



Fog Particles at Temperatures below Freezing. 



Before concluding this paper I wish to refer shortly to some observations made on 

 fog particles when the temperature was below the freezing-point. In a paper read before 

 this Society in March 1887, "On Hoar-Frost," I attempted to explain why it is that 

 crystals of hoar-frost grow quickest in the direction from which the wind is blowing, if 

 one may use that term for the gentle air that moves on nights when hoar-frost is 

 deposited. The explanation offered was founded on the observations of Prof. Ramsay 

 and Dr Young.* Their experiments show that the vapour pressure of ice is less than 

 that of water at the same temperature. From this I concluded that, if the atmosphere 

 be saturated with vapour to the tension corresponding to the presence of water-drops at 

 temperatures below the freezing-point, that it will be supersaturated to ice surfaces. The 

 explanation of the ice crystals growing in the direction from which the air was coming 

 was, therefore, founded on the assumption that the fog particles in the air were in a 

 liquid condition though the temperature was below the freezing-point, that the air was 

 therefore supersaturated to ice surfaces, and that it unburdened itself of its superfluous 

 moisture on the first ice surface with which it came in contact. The hoar-frost crystals 

 thus grow quickest on the side first touched by the supersaturated air. It should be 

 here noticed that heavy deposits of hoar-frost always take place on foggy nights. 



At that time I had to assume that, when the hoar-frost crystals grew in the peculiar 

 way referred to, that the fog particles were liquid, but since then I have had oppor- 

 tunities of observing these particles on a number of occasions when the temperature was 

 below 32° Fahr., by means of the fog-particle counter, and on all occasions yet observed 

 the particles were liquid. The lowest temperature at which as yet observations have 

 been made is 27°, or five degrees below freezing. The night minimum was 24°, with a 

 temperature on the grass of 14 c . Under these conditions the drops seen falling on the 

 micrometer were liquid. Every precaution was taken to ensure that they were not 



* Phil. Trans., part ii., 1884. 



