i 7 8 THE POPULAR SCIENCE MONTHLY, 



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 photo- 

 graphs 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 or, again, such a source of supply of heat may be withdrawn by 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 an- 

 other. 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 pe- 

 riodical 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 par- 

 ticles of which may be probably thirty, forty, or fifty miles apart." 

 These swarms of dust undergo condensation by attraction or gravi- 

 tation ; they will become hotter and brighter as their volume de- 

 creases, and we shall pass from the nebulae 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 



