Page:Popular Science Monthly Volume 60.djvu/273

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COMETS' TAILS.

265

COMETS' TAILS, THE CORONA AND THE AURORA BOREALIS.

By Professor JOHN COX,

MCGILL UNIVERSITY.

THERE is undeniable fascination about a theory which includes within its sweep the time-honored problems of astronomy connected with comets' tails and the reason why they point away from the sun; the solar prominences and the corona; the source of the light by which the nebulæ shine; the origin and structure of meteor-swarms; and the aurora borealis; besides solving incidentally half a dozen minor outstanding mysteries of the heavens.

Such a theory has been advanced by Sweden's distinguished chemist and physicist, Svante Arrhenius, in a paper published in the 'Physikalische Zeitschrift' for November, 1900. Its main points were briefly mentioned with approval by no less an authority than Professor J. J. Thomson at the end of his captivating article on 'Bodies smaller than Atoms' in the August number of The Popular Science Monthly. All the physical principles on which Arrhenius relies, with one exception, are explained at length in that article, and are now very generally accepted. We may therefore say that the theory is based on 'veræ causæ,' and its accordance with known facts is so impressive when the comparison is made in detail that I venture to think the readers of Professor Thomson's article will be interested in a more complete statement of Arrhenius' views than time permitted him to give.

Let us begin by taking stock of the physical principles already to hand. We know (Professor Thomson's paper) that corpuscles, about 1,000 times smaller than hydrogen atoms, and each bearing a charge of negative electricity, are discharged with high velocity:

(1) from the negative electrode in a Crookes tube (kathode rays).

(2) from objects struck by kathode rays (Röntgen rays).

(3) from hot bodies, such as glowing metals.

(4) from cold metals under the influence of ultra-violet light.

(5) from the radio-active substance radium.

Again we know that these corpuscles, or ions, in passing through a gas produce other ions by collision with the molecules of the gas, and that the negatively charged ions are capable of serving as nuclei for the condensation of ordinary matter.