Page:The story of the comets.djvu/71

III. exercised: the gravitational attraction varies with the mass or volume. If we consider the behaviour of very small particles, it follows that the attraction due to gravitation (depending on the volume of the particle) will diminish more rapidly than the repulsion due to light-pressure (depending on the surface of the particle) as we decrease continually the size of the particle, since its volume diminishes more rapidly than its surface. A limit therefore will be reached below which the repulsion will become greater than the attraction. Thus for particles less than the $1⁄25000$ part of an inch in diameter the repulsion of the Sun is greater than its attraction. Particles in the outer envelope of the comet below this size will be driven away in a continuous stream, and will form that thin, luminous fog which we see as the comet's tail."

The latest idea in tails is due to Barnard, who has suggested that there is evidence to show that the same causes (on the Sun?) which give rise to (?) auroral displays have some influence over the changes in the tails of Comets. He speaks of there being "some kind of disturbing medium in Space which can shatter and distort a tail when encountered by it."