The Movements of Floating Particles

Abstract

WILL any physicist be good enough to explain the following to an ignorant amateur? If a clean saucer be half-filled with a decoction of tea on the surface of which bubbles or unwetted shreds of ash (as from the consumed paper round the lighted end of a cigarette) are floating but not in a continuous layer, then if the decoction, after coming to rest, be gently rocked the floating particles will partake of its perpendicular, but little, if at all, of its lateral motion. Sunken particles, on the other hand, will partake of the lateral motion. Again, if the saucer be gently tilted the fluid will flow away, but each floating particle will remain stationary, and will be deposited under its original position.