comets' tails, the corona, and the aurora. 185 



say with coiilideiu-t' that U can not he an atniosphor(\ tliat is, a con- 

 tinuous mass of elastic gas hold up by its own elasticity. " ■■■ * 

 What, tlien, is the i-oroua^ Probably dotaclu'd particles partially or 

 wholly vaporized by the intense heat to which they are exposed. * '^ '^' 

 The difficult cpiestion which we meet is. How are these particles held 

 up;' To this cpiestion only conjectural replies can be o-iven." 



Three conjccturesarc then mentioned, of which wc may note the lirst: 



""That the matter of the corona is in what we may call a state of 

 projection, being- constantly thrown up by the sun, while e;ich particle 

 thus projected falls down again according to the law of gravitation. 

 The difficulty we encounter here is that wo nmst suppose velocities of 

 projection rising as high as 2(H) miles per second constantly maintained 

 in ever}' region of the solar globe. 



"The prominences are of two classes — the cloud-like and the erup- 

 tive. The first class presents the a]3pearance of clouds floating in an 

 atmosphere: 1)ut as no atmosphere dense enough to sustain anything 

 can possibly exist there, we find the same difficulty in accounting for 

 them that we do in accounting for the susptMision of the matter of the 

 corona.'' 



Professor Young is frankly despairing: 



"I do not know what to make of tiie corona. * ■'" "* By what 

 forces the peculiar radiated structure of the corona is determined I 

 have no definite idea. Th(^ analogies of (■/>/iir/,s' fails avd (Uiroral 

 stre((mers both appear suggestive; but on the other hand, the spc^ctra 

 of the corona, the aurora borealis, the comets, and the nel)uhe are all 

 difi'erent, no two in the least alike. * " "' Nf)r have I any theory 

 to propose to account for the c<:)i<i!ii foniwrfion ht-tireoi (Usfurlxiiices 

 of tJie solar siD'facc and of frrrrsfrtal magio'tlxinS^ 



The words we have underlined in this passage have almost a Sopho- 

 clean irony to a reader acquainted with the further developments of 

 Arrhenius's theorv, to which we now turn. 



THE ZODIACAL EKJHT AND TUK ( iEOENSCHEIN. 



Not only is the sun the source of those eru})tions of ordinary matter 

 which form the ])rominences, ))ut we have every reason to be b(dieve 

 that he nmst emit strt'ams of electrically charged c()ri)uscles both 

 directly, as a hot 1)ody. and indirectly, since th(^ (dectrical dischai'ges 

 which, according to all terrestrial analogies, nmst accompany the 

 violent chemical actions going on near his surface, will, when they take 

 place in the higher and rarer regions of his afmosphei'e, give ris(^. to 

 kathode rays, and these, in turn, to Rontgen rays. As l*rofessor 

 Thomson says: '•As a very hot metal emits these cor])nscles, it does 

 not seem an imi)roba])le hypothesis that they are emitted by that very 

 hot l)ody, the sun.'' 



Now the negatively charged corpuscles are preeminently htted to 

 serve as nuclei for the condensation of the ordinary mattcM-. Hence 

 those particles of the latter which, having more than the ci'itical diam- 

 eter, fall back to th(^ sun, will carry back a negative charge to him; while 



