October 17, 1903.] 



SCIENCE. 



627 



heat, apart from such transfers of energy 

 as occur in chemical change and electric 

 disturbance, it demands a revision of the 

 theories which attribute more permanent 

 dii¥erences between the spectra of different 

 stars to differences of temperature, and a 

 fuller consideration of the question wheth- 

 er they cannot with better reason be ex- 

 plained by differences in the electric condi- 

 tions which prevail in the stellar atmos- 

 phere. 



If we turn to the question what is the 

 cause of the electric discharges which are 

 generally believed to occasion auroras, but 

 of which little more has hitherto been 

 known than that they are connected with 

 sunspots and solar eruptions, recent studies 

 of electric discharges in high vacua, with 

 which the names of Crookes, Rontgen, Len- 

 ard and J. J. Thomson will always be asso- 

 ciated, have opened the way for Arrhenius 

 to suggest a definite and rational answer. 

 He points out that the frequent disturb- 

 ances 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 at- 

 tended with an ionization of the gases, 

 and the negative ions will stream away 

 through the outer atmosphere of the sun 

 into the interplanetary space, becoming, 

 as Wilson has shown, nuclei of aggrega- 

 tion of condensable vapors and cosmic dust. 

 The liquid and solid particles thus formed 

 will be of various sizes; the larger will 

 gravitate back to the sun, while those with 

 diameters less than one and a half thou- 

 sandths of a millimeter, but nevertheless 

 greater than a wave-length of light, will, 

 in accordance with Clerk-Maxwell's elec- 

 tromagnetic theory, be driven away from 

 the sun by the incidence of the solar rays 

 upon them, with velocities which may be- 

 come enormous, until they meet other celes- 

 tial bodies, or increase their dimensions by 

 picking up more cosmic dust or diminish 



them by evaporation. The earth will catch 

 its share of such particles on the side which 

 is turned towards the sun, and its upper 

 atmosphere will thereby become negatively 

 electrified until the potential of the charge 

 reaches such a point that a discharge oc- 

 curs, which will be repeated as more 

 charged particles reach the earth. This 

 theory not only accounts for the auroral 

 discharges, and the coincidence of their 

 times of greatest frequency with those of 

 the maxima of sunspots, but also for the 

 minor maxima and minima. The vernal 

 and autumnal maxima occur when the line 

 through the earth and sun has its greatest 

 inclination to the solar equator, so that 

 the earth is more directly exposed to the 

 region of maximum of sunspots, while the 

 twenty-six days' period corresponds closely 

 with the period of rotation of that part of 

 the solar surface where faculse are most 

 abundant. J. J. Thomson has pointed out, 

 as a consequence of the Richardson obser- 

 vations, that negative ions will be con- 

 stantly streaming from the sun merely re- 

 garded as a hot body, but this is not incon- 

 sistent with the supposition that there will 

 be an excess of this emission in eruptions, 

 and from the regions of faculee. Arrhe- 

 nius' theory accounts also, in a way which 

 seems the most satisfactory hitherto enun- 

 ciated, for the appearances presented by 

 comets. The solid parts of these objects 

 absorb the sun 's rays, and as they approach 

 the sun become heated on the side turned 

 towards him until the volatile substances 

 frozen in or upon them are evaporated and 

 diffused in the gaseous state in surround- 

 ing space, where they get cooled to the 

 temperature of liquefaction and aggregated 

 in drops about the negative ions. The 

 larger of these drops gravitate towards the 

 sun and form clouds of the coma about the 

 head, while the smaller are driven by the 

 incidence of the sun's light upon them 

 away from the sun and form the tail. The 



