Although the measurement of the surface area or particle size of a moderately fine powder is frequently an important procedure in industrial laboratories, it is seldom included in laboratory courses in physical chemistry in universities and technical colleges. Of the large number of methods available (/) some form of the permeability method based on the measurement of the rate of flow of air or other suitable fluid through a plug of the powder is, perhaps, the one most frequently employed industrially. A commercially made apparatus in common use, although very convenient for routine work in that the desired answer is given directly, is unsatisfactory for teaching purposes, since its operation requires no knowledge of the theory on which the measurement depends.
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