THE measurement of fine particles is a subject of increasing importance in many industrial processes and in the study of natural phenomena concerning soils and dusts. It is now realized that the influence of very fine particles is so predominant that in many researches measurements would be desirable to a lower limit of one micron in diameter, though it is not often that this degree of accuracy can be attained. The above report contains an account of the activities of two research associations having the object of improving particle size measurements, and they may be complimented on the broad way in which the problem has been treated. Determination of Particle Size in Sub-Sieve Range A Report of Discussions. Pp. 69. (London: British Colliery Owners' Research Association and the British Coal Utilisation Research Association, 1944.)