HISTORY OF COLD AND THE ABSOLHTE ZERO. 285 



iuiiu])L'i' of close coiiicidciici^s l)ctwoon the auroral rays and those 

 which arc ciiiittetl under electric exciteuieut by <i'as(\s of our atinos- 

 [)here almost constrains \if* to believe, what is indeed most probable on 

 other grounds, that the sun's coronal atmosphere is couiposed of the 

 same substances as the earth's, and that it is rendered luminous in the 

 same way — namely, by electric discharges. This conclusion has plainly 

 an important bearing- on the explanation which should be given of the 

 outburst of new stars and of the extraordinary and rapid changes in 

 their spectra. Moreover, leaving on one side tho question whether 

 gases ever become luminous ]>y the direct action of heat, apart from 

 such transfers of energy as occur in chemical change and electric dis- 

 turbance, it demands a revision of the theories wdiich attribute more 

 permanent diti'erences l)etween the spectra of diti'erent stars to difi'er- 

 ences of temperature, and a fuller consideration of the (juestion 

 whether they can not with better reason ])c explained ))}' dilferences 

 in the electric conditions which prevail in tlu^ stellar atmosphere. 



If we turn to the question what is th(^ cause of the electric discharges 

 which are generally believed to occasion auroras, but of which little 

 more has hitherto been known than that they arc^ comiected with sun 

 spots and solar eruptions, recent studies of electric discharges in high 

 vacua, with which the names of Crookes, Rontgen, Lenard, and J. J. 

 Thomson will always be associated, have opened the way for Arrhe- 

 nins to suggest a definite and rational ansvv(n'. He points out that the 

 frequent disturbances which we know to occur in the sun must cause 

 electric discharges in the sun's atmosphere far exceeding any that 

 occur in that of the earth. These will be attended with an ionisation 

 of the gases, and the negative ions will stream away through th(^, outer 

 atmosphere of the sun into the interplanetary space, becoming, as 

 Wilson has shown, nuclei of aggregation of condensaljle ^'apors and 

 cosmic dust. The liquid and solid particles thus formed will be (^f 

 various sizes; the larger will gravitate I)ack to the sun, whih^ those 

 with diameters less than one and a half thousandths of a millimeter. l)ut 

 nevertheless greater than a wave length of light, will, in accordance 

 with Clerk-MaxwelTs electro-magnetic theor}^, be driven away from 

 the sun l)y the incidence of the solar rays upon them, with v(docities 

 which may beconn^ enormous, until they meet other cel(\stial bodies, 

 or increase tluMr dimensions by picking u]) more cosmic dust or 

 diminish them by e\'aporation. The earth will catch its shar(> of such 

 particles on the side wdiich is turned toward the sun, and its upper 

 atmosphere will thereby become negativtdy el(>ctritied until the 

 potential of the charge reaches such a point that a discharge occurs, 

 which will be repeated as more charged particles reach the earth. 

 This theory not oidy accounts for the auroral discharges, and the 

 coincidence of their times of greatest frequency with thosi^, of the 

 maxima of sun spots, but also for the minor maxima and minima. 

 The vernal and autunuial maxima occur when the line throut>-h the 



