14 
Proceedings of the Royal Society 
BUSINESS. 
Dr T. A. Wise and Mr Thomas Gray were balloted for, and 
declared duly elected Fellows of the Society. 
Monday, 20 tli December 1880. 
Sir WYYILLE THOMSON, F.R.S., Vice-President, in 
the Chair. 
The following Communications were read : — - 
1. On Dust, Fogs, and Clouds. By Mr John Aitken. 
{Abstract). 
Dust, fogs, and clouds seem to have but little connection with 
each other, and we might think they could be better treated of 
under two separate and distinct heads. Yet I think we shall 
presently see that they are more closely related than might at first 
sight appear, and that dust is the germ of which fogs and clouds are 
the developed phenomena. 
This was illustrated by an experiment in which steam was mixed 
with air in two large glass receivers; the one receiver was filled with 
common air, the other with air which had been carefully passed 
through a cotton-wool filter and all dust removed from it. In the 
unfiltered air the steam gave the usual and well-known cloudy form 
of condensation, while in the filtered air no cloudiness whatever 
appeared. The air remained supersaturated and perfectly trans- 
parent. 
The difference in the behaviour of the steam in these two cases 
was explained by corresponding phenomena, in freezing, melting, and 
boiling. It was shown that particles of water vapour do not com- 
bine with each other to form a cloud -particle, but the vapour must 
have some solid or liquid body on which to condense. Vapour in pure 
air therefore remains uncondensed or supersaturated, while dust- 
particles in ordinary air forms the nuclei on which the vapour con- 
denses and forms fog or cloud-particles. 
This represents an extremely dusty condition of the air, as every 
