(b) Settling velocity
The settling velocity is the terminal velocity of a particle falling under gravity in an infinite expanse of a medium at rest. This quantity can be measured, and it has a definite value for spheres and roughly spherical (isometric) particles. For non-isometric particles the settling velocity in the Stokes law range depends on their orientation to the direction of the gravitational force. We then have to assign an average settling velocity to the particle. It is measured in practice by settling tests. With Newtonian fluids the settling velocity depends on the viscosity and density of the fluid, so that its nature and temperature must both be specified. The fluid and particle mechanics of sedimentation are dealt with at length in Chapter 3.
(c) Optical measures of dispersity
If we wish to measure particles without interfering with them during a process (e.g. if a cloud of particles is passing through a duct) then the best method available is the light-scattering procedure. In this method the intensity of the scattered light emitted from an illuminated particle at a given angle to the direction of illumination (the angle of scatter) is a measure of dispersity. With spheres of diameters greater than 5 /xm and less than about 0.5 /xm this intensity is a monotonic function of their diameter. With non-spherical particles in this same size range and for a given angle of scatter the relation remains approximately monotonic. The extinction of a beam of light or X-rays by particles is used more for determining the cumulative size distribution than for measuring dispersity.
In ore dressing we are interested in separating minerals according to their chemical composition. The density serves to indicate the metallic content of an ore particle and is then a measure of dispersity.