The entire mass of sawdust still weighs one pound; but its surface has been vastly enlarged. Indeed, the particles of sawdust, individually considered, may be said to consist of much surface and very little weight. If it were possible to take each granule of visible sawdust and subdivide it into invisible particles, a point would be reached where the pressure of light would exactly counterbalance the pull of gravitation, so that the particles would remain suspended in space, perfectly balanced in the scale of opposing cosmic forces. Finally, if the subdivision be continued beyond this critical point, the particles will be wrenched away from the grip of gravitation and hurled out into space by the pressure of light. So much has been discovered about the particles that compose a Comet’s tail that the more progressive scientists of our day have accepted this ingenious theory. Thus it has been decided by them that the delicate tresses of a Comet are to a large extent composed of fine particles of dust and soot. [Illustration: June 26.] [Illustration: June 28.] [Illustration: June 30.] [Illustration: July 1.] [Illustration: July 6.] [Illustration: July 8. CHANGES IN THE COMET OF 1863.] Before we can completely accept the view that light-pressure forms this train of soot we must ascertain whether the pressure of light is capable of accounting for the flash-like rapidity with which a Comet’s tail changes. A Comet may throw out a tail sixty million miles long in two days. Is it actually possible for light-pressure to accomplish that astonishing feat? Arrhenius has computed that 865,000 miles an hour is the speed of a light-flung particle of one-half the critical diameter. Because they are only one-eighteenth as large as this particle of critical diameter, the dust grains in a Comet’s tail would be propelled over the same 865,000 miles in less than four minutes. It follows that the solar radiation is amply strong enough to toss out a tail of sixty million miles in two days.