THE conception of molecule is essential in explaining the phenomena of both chemistry and physics. Porosity and compressibility point to the conclusion that matter does not entirely fill space, to account for the dispersion of light requires that matter should have a grained structure; these and countless other physical facts find an explanation in the conception of molecule. Moreover, from various observations, more especially on the properties of gases and the phenomena of surface tension, the size of molecules can be approximately calculated, and in terms of the idea of molecule deduced in ways such as these physical properties are explained. Lehrbuch der Allgemeinen Chemie. Von Dr. Wilhelm Ostwald. Band 1. 1891, Band II. Theil 1. 1893. Zweite Auflage. (Leipzig: Wilhelm Englemann.)