IN the course of some experiments recently carried out in these laboratories, a curious series of annular markings was observed in a glass tube through which air, carrying water-vapour and powered charcoal, was passing under diminished pressure. The accompanying photograph (Fig. 1) shows the regularity of the distribution of the rings, of which 48 occupied a length of one decimetre. The diameter of the tube was 4.5 mm., and the pressure at the time of the experiment was approximately 600 mm. of mercury.