Page:Popular Science Monthly Volume 37.djvu/190

178 facts, there is a limit to physical observation, and in some cases we can do no more than glance into the possible or probable source of natural phenomena.

This has been done, as to the origin of the universe, by Prof. Norman Lockyer, in his article on the History of a Star. The author proposes there to clear in our imagination a limited part of space, and then set possible causes to work: that dark void will sooner or later be filled with some form of matter so fine that it is impossible to give it a chemical name; but the matter will eventually condense into a kind of dust mixed with hydrogen gas, and constitute what are called nebulae. These nebulae are found by spectrum analysis to be made up of known substances, which are magnesium, carbon, oxygen, iron, silicon, and sulphur. This dust comes down to us in a tangible form—dust shed from the sky on the earth, and large masses, magnificent specimens of meteorites, which have fallen from the heavens at different times, some of them weighing tons. There are swarms of dust traveling through space, and their motion may be gigantic. From photographs taken of the stars and nebulae, we are entitled to conclude that the swarms of dust meet and interlace each other, becoming raised by friction and collision to a very high temperature, and giving rise to what looks like a star. The light would last so long as the swarms collide, but would go out should the collision fail; or, again, such a source of supply of heat may be withdrawn by the complete passage of one stream of dust-swarms through another. We shall, therefore, have various bodies in the heavens, suddenly or gradually increasing or decreasing in brightness, quite irregularly, unlike those other bodies where we get a periodical variation in consequence of the revolution of one of them round the other. Hence, as Mr. Lockyer expresses it, "it can not be too strongly insisted upon that the chief among the new ideas introduced by the recent work is that a great many stars are not stars like the sun, but simply collections of meteorites, the particles of which may be probably thirty, forty, or fifty miles apart." These swarms of dust undergo condensation by attraction or gravitation; they will become hotter and brighter as their volume decreases, and we shall pass from the nebulæ to what we call true stars. Mr. Lockyer imagines such condensed masses of meteoric dust being pelted or bombarded by meteoric material, producing heat and light, the effect continuing as long as the pelting is kept up. To this circumstance is due the formation of stars like suns. Our earth originally belonged to that class of heavenly bodies, but from a subsequent process of cooling assumed its present character.

The dust scattered everywhere in the atmosphere, which is lighted up in a sunbeam or a ray from the electric lamp, is of