546 THE SCIENTIFIC MONTHLY
could slowly come together as a collection of small solid masses, such as seem to compose the nucleus of a comet. Such a nucleus could at- tract and attach to itself any dust particles and molecules coming within its sphere of attraction. These might well, and probably would, include a collection of finely divided matter that had already been driven off in the tails of comets which in earlier ages had visited the sun. The materials thus collected would be attracted by the sun, a few of the collections would eventually pass comparatively close to the sun, a few of the latter would be discovered as comets, and a part of the finely divided material contained in them would be driven off again as comets' tails into space, possibly to return many times in the bodies of comets coming later into the sun's neighborhood. Certain of these bodies would come so close to the planets as to have their orbits trans- formed from very long ellipses to very short ellipses. These comets would be disintegrated and their materials be widely scattered. We have seen that the earth has collided with such materials, and the earth is growing slowly, very slowly, through the deposition of the remains upon its surface. Probably a little of the same materials goes likewise to other planets of the solar system and adds slowly to their masses. However, an insignificant proportion of the materials scattered in this manner through the solar system is thus accounted for, and the re- mainder doubtless revolves around the sun in ellipses, probably con- tributing its share of reflected sunlight to the faint glow near the sun known as the zodiacal light.
We have seen that devoted students of comets have learned much concerning these interesting travelers. Many mysteries have been re- moved, but many questions remain for the astronomers of the future to answer. We should especially like to know more of the physical conditions existing in comets, more about their chemical contents, and more as to why and how they shine by their own light. Perhaps the most valuable result of cometary investigation has been the emancipa- tion of civilized peoples from unreasoning and groundless fears of these bodies, which come and go in obedience to the same simple laws that govern our every-day affairs.
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