190 JOURNAL OF THE, SOCIETY OF COSMETIC CHEMISTS Other devices have been based on the fact that a suspension has effectively a density somewhat ,higher than the pure fluid, and which can be measured by means of a hydrometer. As the suspension settles out its density falls off and so gives a corresponding change in the hydrometer reading. Unfor- tunately, the hydromeier has to be removed from the suspension between readings to avoid deposition of particles on the shoulder of the bulb. Such disturbances of the suspension are not desirable. There is also the difficulty of surface tension effects where the hydrometer breaks the surface. A better alternative is the use of small divers in the suspension. Several divers of different densities are required. They are streamlined in shape and the falling particles do not settle on them. As the suspensions are usually opaque the divers may be fitted with a ferrous core so that they may be brought to the side of the column by means of a magnet. A later refinement of this has been detection ,electrically of the position of the divers (with permalloy cores) by means of a thin search ring around the outside of the column. Mariometric measurements of changing density of. a suspension have been used. Two tubes fitted with pure liquid are immersed in the suspension at different levels. The slight difference in heights of the manometers is pro- portional to the mean difference in density between the two levels. The manometric readings are made with a travelling microscope. Probably the most attractive modification of the simple Andreasen type is the photo-extinction method. Here a narrow beam of light is directed through the suspension at a set level below the surface of' the suspension and the fraction of the light which passes through is measured by a photocell system. As the particles settle out less light is obstructed and the increasing light intensity picked up by the photocell gives a measure of the particle range. This method has the great virtue that the suspension is not disturbed during the course of the determination. It can be made more rapid than the pipette method by moving the light beam to higher levels for finer particles. Much experimental work has been done on this method since it was first examined some twenty years ago. The theory of the method is very com- plex. Its range is about 25 microns--1 micron. The amount of sample examined is about 0.01 gin. Difficulties in interpretation occur on account of light scattering by the particles and the transparency of some materials at low particle sizes. , . PERMEABILITY METHODS We have considered the movement of particles against a moving stream of fluid '(elutriation), and the movement of particles in a standing column of fluid (sedimentation). The so-called permeability methods first investigated by Carman make use of movement of fluid through a standing column of

FINE PARTICLES IN THE COSMETIC INDUSTRY 191 particles. As air is forced through a bed of powder the viscous drag, which is related to the surface area of the particles, causes a differential pressure to be set up across the bed of particles. The difference in pressure is correlated with the total surface of the particles forming the bed and so gives a single figure of specific surface (cms. per gramme). It is very attractive to be able to express the size of a powder as a single figure but for many purposes it does not give enough information. Since the method was first devised twenty years ago, it has been developed into a very simple apparatus easy to use eve• in unskilled hands. It gives repeatable results for given materials and by carefully standardised forms of procedure has become a standard method in some industries where the particle range of powders is small for example, the .cement industry. Despite much work on the theory of the process there is uncertainty about the absolute values for particle surface obtained by this method. This difficulty turns on what is meant by surface. Some surfaces are very much more fissured than others. There is some difficulty in assessing if the method applies to the gross surface or includes also some or all of the finer structme of the surface. OTHER •½[ETHODS Attempts have been made to measure the surface of a bed of powder by gas adsorption by which it is possible to differentiate between the outer surface and the fine structure surface. The sample to be treated is out- gassed to a pressure of 10 -4 m.m., and the adsorbant gas is then admitted to the powder from a gas burette until saturated. In the same way solutes may be adsorbed from solutions by particles to give a measure of surface area. The powder is immersed in a suitable dye solution of known concentra- tion, stirred until a state of equilibrium is reached, then the strength of the residual dye solution determined by suitable titration. The relation between amount adsorbed and the equilibrium concentration is complex but has been worked out to express the monolayer capacity and hence the specific surface of the powder. This demands a knowledge of the molecular dimen- sions and orientation of the adsorbate, and this is rarely known with accuracy. Since dyestuff molecules are comparatively large they will not penetrate the finer pores. Iodine has a much smaller molecule and so can penetrate further than a dye molecule. It is found, in fact, that estimation of surface by means of iodine solution will give higher results than with a dye solution. A further method Of this type is measurement of heat of wetting which will be proportional to the surface area of the powder. Here again the sample must be freed from any adhering gases by high vacuum before being introduced into the wetting liquid in a calorimeter. Of these three methods the dyeing technique is very simple to perform. The gas adsorption method will yield absolute values, the other two methods somewhat more relative values.