COMETS' TAILS. 265 



COMETS' TAILS, THE CORONA AND THE AUEOEA 



BOREALIS. 



By Professor JOHN COX, 



MCGILL UNIVERSITY. 



^r^HERE is undeniable fascination about a theory which includes 

 -'- within its sweep the time-honored problems of astronomy con- 

 nected 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 nebula 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 'Physi- 

 kalische 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 excep- 

 tion, are explained at length in that article, and are now very gen- 

 erally accepted. We may therefore say that the theory is based on 

 'verse causse,' 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 (Rontgen 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. 



