194 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS as the particle which falls at the same speed as the particle. (This is the "diameter" used in Stokes Law above.) Correlation of these different descriptions of particle ,size have been made in some cases. They involve the establishment of shape factors for the particles. For instance, the total iurfaces of stones have been measured by wax dipping and their volumes by displacement in a fluid. In this way correlation may be obtained between an equivalent surface diameter and an equivalent volume diameter. This ratio may be considered to be constant for a given material in that it tends to fracture in a set way. For large particles of the size of pebbles the ratio does hold over quite a range of size. But whether this can be extended down to particles of 20 microns or less is debatable. Uneven particles have been classified to simulate a small number of geometrical shapes and this offers some means of assessing shape factors. AGGLOMERATION AND DISPERSION Particulate matter, particularly of 5 microns and finer, when placed in a liquid medium, may coalesce into agglomerates of particles. Where this occurs in sedimentation methods a clump of particles will behave as if it were one large particle, giving completely erroneous results. Checks should be made that the material does not coalesce in the liquid selected., Parallel trials should be made using a series of liquids and the one settling most slowly taken as free from agglomeration. Many effective dispersing agents have been used to combat agglomeration and references are dotted about through the literature. They seem to be specific for some materials. There does not seem to have been any systematic investigation of this phenomenon. It would seem that at least two factors are involved--wetting of the particles and electrostatic charges. Perhaps in some cases a wetting agent is also a good dispersing agent. There is no universal dispersing agent although sodium hexametaphosphate and pyrophosphate (0.001 to 0.01 per cent weight/volume of suspension) are useful in many instances. Other dispersing agents in common use are such compounds as sodium silicate, potassium citrate, sodium oxalate. Failing these there are many suitable organic liquids which are listed in the literature for specific materials. Some variation in dispersibility may occur in the case of natural minerals which may contain small, variable amounts of soluble salt occlusions, and in materials •vhich are themselves slightly soluble in the liquid selected. In conclusion, I would say that the work on particle size analysis has been increasing over the past ten years, as indicated by the symposia and conferences organised on the subject, notably in this country by Society of Chemical Industry, Institute of Mines and Metallurgy, and most recently by the Institute of Physics. The latter conference produced some clarifying concepts of the relative motion of particles and fluids, on light scattering by particles .and on particle shapes. It was also shown that the tiring work

FINE PARTICLES IN THE COSMETIC INDUSTRY 195 of microscope technique of counting and measuring fine particles could now be handed over to electronic scanning instruments. For anyone wishing to pursue this subject further the following publica- tions should be consulted. They all carry extensive references to the litera- ture on the subject, both academic and applied. Heywood, Trans. Institution Mech. Engineers, London (1933). Heywood, ibid. (1938). Am. Soc. for Testing Matehals. Symposium on Particle Size Deter- mination (1941). B.C.U.R.A. and B.C.O.R.A., London, Report on Determination of Particle Size (1944). Inst. Chem. Engineers, London. Symposium on Particle Size Analysis (1947). Dalla Valle, Micromeritics (1948). Brit. Journal Applied Physics, Suppl. 3 (1954). Finally, I must thank Dr. H. Heywood of Imperial Institute, London, for slides used to illustrate this lecture, and Miss L. P. Torry, a fellow member, for help in preparing the paper. Erratum. In the article on "Smell-Threshold Concentration," by A. W. Middleton, B.Sc., Ph.D., F./•.I.C., which appeared in Volume VIII, No. 1, of the Journal of the Society of Cosmetic Chemists, the word "basis" appearing in line 3, paragraph 4, page 41, should read "hairs."