The purpose of the author was to urge that specific volume should precede density, because (1) the idea of specific volume is more important and fruitful than the idea of density; (2) the notion of specific volume is easier to acquire. Dealing first with the second reason, evidence was quoted from experienced science teachers and examiners to the effect that boys frequently failed to grasp the idea of density although they could formulate a correct definition, and carry out and describe a density determination with unimpeachable correctness. On the other hand the author had found that the idea of density had been readily acquired where the class had approached the subject through a preliminary lesson on specific volume. A short demonstration was then given of a method by which the relative specific volumes of substances or their comparative “roominess” (Raümigkeit), i.e. the volume occupied by 1 lb. of each, could be readily and effectively brought home to the class. (In subsequent discussion the chairman suggested the word ‘roomage’ in place of specific volume.)