230 Professor Dewar on the Atmosphere. [April 11, 1902. 



Without stopping to discuss that question, it is certain that 

 changes in the character of the electric discharge produce definite 

 changes in the sj)ectra excited by them. It has long been known 

 that in many spectra the rays which are inconspicuous with an un- 

 condensed electric discharge become very pronounced when a Leyden 

 jar is in the circuit. This used to be ascribed to a higher temperature 

 in this condensed spark, though measurements of that temperature 

 have not borne out the explanation. Schuster and Hemsalech have 

 shown that these changes of spectra are in part due to the oscillatory 

 character of the condenser discharge, which may be enhanced by self- 

 induction, and the corresponding change of spectrum thereby made 

 more pronounced. 



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 sun- 

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

 vacua, with which the names of Crookes, Eontgen, Lenard, and 

 J. J. Thomson will always be associated, have opened the way for 

 Arrhenius to suggest a definite and rational answer. 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 the outer atmosphere of the sun into the interplanetary space, 

 becoming, as Wilson has shown, nuclei of aggregation of condensable 

 vapours 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 thousandths of a 

 millimetre, but nevertheless greater than a wave-length of light, will, 

 in accordance with Clerk-Maxwell's electromagnetic theory, be driven 

 away from the sun by the incidence of the solar rays upon them, with 

 velocities which may become enormous, until they meet other celestial 

 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 occurs, 

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



[J. D.] 



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