THE PARTICLES IN TOWN AIR.

PARTICLES CONSTITUTE AN IMPORTANT PART OF AIR POLLUTION, AND THEIR BEHAVIOUR WHEN SUSPENDED IN AIR IS VERY DIFFERENT FROM THAT OF GAS MOLECULES: in particular, the mechanisms by which they become deposited on surfaces are different, and consequently the methods normally used for removing particles from the air, either for sampling or for cleaning it, rely mainly on mechanisms that do not enter into the behaviour of gas molecules. These mechanisms are described, and the ways in which they affect the problems of air pollution and its measurement are discussed.