On the densities of particles in smokes

Abstract

All methods of determining the radii of ultra-microscopic particles which involve the use of a resistance law based on Stokes’ or analogous equations require a knowledge of the density. This is usually assumed to be the same as that of the substance in bulk, an assumption which is most probably valid for liquid and homogeneous solid particles, but can only lead to erroneous results in the case of aggregates of solid particles formed as a result of coagulation. It has been shown previously that the very large complexes in cadmium, zinc, and magnesium oxide smokes possess a loose structure, being made up of a large number of units, and for these, at all events, it would clearly be wrong to assume a normal density in deducing the size from the rate of fall. Further, it seems probable that the smaller particles of so-called ultra-microscopic size in these smokes also possess a similar structure, since they are formed in the same way and consist most likely of a number of primaries. For this reason the attempt has been made to determine the densities of these ultra-microscopic particles. (For the purposes of this paper the density of a complex particle is defined as that of a sphere of the same mass which falls through air at the same rate.) Theoretical considerations of packing render it highly improbable that the density of these aggregates can be much greater than half the normal density, whilst it may be considerably less. Experimental evidence bearing on this point is furnished by the work of Kohlschutter and Tuscher on the mean density in bulk of electrically precipitated smokes. These smokes were formed by volatilisation of the metal from the electric arc in air and the density of the powder collected in the electrical precipitator was determined. Table I contains the collected results.