Abstract
By the application of Poiseuille's law to the rate of flow of water through collodion membranes, it is calculated that the membranes used had pore radii of the order of 0.3 to 2 x 10(-6) cm. On the same basis the number of pores per sq. cm. appears to vary from 270 x 10(10) to 7 x 10(10), decreasing with increase in pore size. Reasons are given for preferring these figures for the radii to figures, 100 times as large, which were calculated by others. Microscopic examination of the membranes, with dark-field illumination, indicates that they are made up of solid granules or filaments of collodion much less than 1 x 10(-4) cm. in thickness.