168 Proceedings of Royal Society of Edinburgh. [sess. 
Tlie rapidity with which the products of combustion are cleared 
out of a room is a subject well worth investigation, and the dust- 
counting apparatus promises to be of considerable assistance in the 
investigation. Apart altogether from the question of ventilation by 
means of openings, there will be in most rooms a certain amount of 
circulation through the plaster of the walls and ceilings, and it 
might not be a bad plan to make the plaster of our houses as 
porous as possible. If this were done, and the space behind the 
plaster were connected with the open air, an insensible circulation 
of pure air would be secured. 
The air of our smoking-rooms will probably contain a very great 
number of particles. As yet, however, I have made no actual 
determination of the number. I may, however, mention that I find 
a cigarette smoker sends 4,000,000,000 particles, more or less, into 
the air with every puff he makes. As this smoke is not very hot, 
it does not tend to keep near the ceiling like the products of com- 
bustion, and must therefore make the air breathed by the smokers 
very full of particles. 
It will be observed that the smallest number of particles entered 
in the table is 500 per c.c. This is about the lowest yet observed 
in nature, though in some tests made in an agricultural part of 
Dumfriesshire a number slightly less was obtained. When testing 
such pure air as this, it does not require to be mixed with filtered 
air ; the receiver is filled entirely with the air to be tested. When 
the particles are so few they are just sufficiently far apart for easy 
counting, and all of them fall with one expansion. Though one 
expansion is sufficient to bring down all the dust particles in pure 
country air, we must not imagine that therefore a fog could not be 
formed in air with so few particles. We must not suppose that, 
because they are so few, they would form rain, and all of them 
fall as in the test-receiver. The conditions are very different in the 
two cases. The high expansion used in the testing apparatus gives 
rise to a considerable cooling, and consequent supersaturation; 
whereas in nature the load of vapour tending to condense might 
be small, and each particle only get enough to make it visible. 
Five hundred fog particles per c.c. are quite enough to make a 
fog, and as these particles may not be heavy, they may float and 
fog the air. 
