262 Proceedings of Royal Society of Ediiiburgli. [sess. 
had plenty of nuclei to give a dense form of fogging, this receiver 
was disconnected from the other one, and the air in it expanded a 
second time, but the second time the expansion was made very rapidly. 
The result was a condensation as dense as that given by the products 
containing the sulphur impurities. But it was only at the moment 
of complete quick expansion that the products from the lamp 
gave as dense a fog as the other ; in a few seconds it began to clear, 
owing to differentiation setting in and rapidly reducing the number 
of drops. The upper limit of the fog could also be seen to be 
descending, the air in the upper part of the receiver being quite clear 
of fog, and in a few minutes all the fog had gone, having rained itself 
to the bottom of the receiver. While these changes were taking place 
in the one receiver, the fog formed in the impure air of the other 
had cleared but little, and it showed no tendency to fall, the fog- 
ging extending to the upper part of the receiver, quite filling it. This 
fog remained thick for an hour, and could be detected hours after- 
wards. The fog in this receiver illustrated the characteristics of a 
town fog, and that in the other the characteristics of a country fog. 
It is concluded that it is not so much the number as the 
composition of the particles produced by combustion which causes 
the density and persistence of town fogs. A large number of particles 
without affinity for water vapour give a dense form of fogging 
only when the condensation is rapid, more rapid than it generally 
is in nature; and even when they do form a dense condensa- 
tion it rapidly becomes clearer by differentiation. It would thus 
appear that if we wish to reduce the density of town fogs, it is not 
so much a question of perfection of combustion, as of changing the 
nature of the products of combustion. Perfect combustion will 
reduce the amount of smoke, but it will not reduce the density of 
the fogging proper. To effect this we must change the nature of the 
sulphur and other products, and convert them into products which 
have no affinity for water vapour. It is hoped that some substance 
may be found which will admit of being burned along with the coal, 
or of being added to the air, which will change the water-attracting 
particles of town air into particles indifferent to vapour, so as to 
form fog particles having a tendency to clear. 
The influence of the products of combustion of a few substances 
in producing fogs is referred to. Alcohol burned at a plantinum 
