August 26, 1910] 



SCIENCE 



287 



the distance from the center of gravity of the 

 comet, like the atmosphere surrounding the 

 earth; but much more rarefied than the earth's 

 atmosphere, since the mass of the comet is 

 much less than that of the earth. The com- 

 position of the gaseous envelope of different 

 comets varies. Cyanogen, hydrocarbons and 

 other gases have been detected in the few 

 comets which have been examined by modern 

 spectrographs. 



A comet, then, is a globule of gas, with a 

 more or less dense nucleus, floating in almost 

 vacuous space. During the greater part of 

 its journey it is between the sun and one or 

 more planets. 



It is well known that there are electric dis- 

 charges constantly passing between the sun 

 and the planets. The difference in potential 

 between the earth and the sun is a billion 

 volts. The sun is positive, while the earth and 

 the other planets are negative. Thus we have 

 in our solar system one large anode and nu- 

 merous cathodes — great and small. The elec- 

 trons are expelled in steady streams from the 

 earth and other planets toward the sun "with 

 ever-increasing speed, acquiring almost the 

 velocity of light as they near the end of their 

 journey. According to Arrhenius, the sun 

 drains the space surrounding it of all of its 

 free electrons as far out as one sixtieth of the 

 distance of the nearest fixed star. The near- 

 est fixed star (a Centauri) is 4.35 light-years 

 distant from the sun, and one sixtieth of this 

 distance would be about 152 times the mean 

 distance of Neptune from the sun. In the 

 meantime the sun is expelling positively 

 charged atoms — ions — carrying the same 

 amount of charge as the electrons, but of 

 opposite character. These ions are about one 

 thousand times as heavy as the electrons and 

 having the same amount of electric charge, 

 they move at about one thousandth the veloc- 

 ity, which, however, is still prodigious. The 

 velocities of groups of positive ions have been 

 measured by Wein, who found one group to 

 have one one-thousandth the velocity of elec- 

 trons, another one two-thousandth, and still 

 another one one-millionth of their velocity. 



When the electrons approach the sun they 



discharge the ions and cause condensation of 

 the gases. These condensed particles are then 

 exposed to the radiation of the sun, which, 

 according to Maxwell and Bartoli, is suflicient 

 to balance the weight of a drop 8 X 10" cm. 

 in radius, having the density and capacity of 

 water, i. e., a radiation pressure of about 2.5 

 times solar gravity. Therefore, these particles 

 are expelled from the sun in enormous num- 

 bers with great velocity. 



Thus we have the comet — a gaseous body 

 with a more or less dense nucleus — a con- 

 ductor, in a constant electric stream which 

 increases in energy as the comet approaches 

 the sun and diminishes as it recedes. The 

 anode is always the same, but the cathode is 

 changing from time to time as the comet pro- 

 ceeds in its orbit ; each cathode and the comet 

 constantly shifting their relative positions. 



As the comet approaches the sun it en- 

 counters, more and more, the resistance of the 

 positive discharge, and this " ionic breeze " 

 forces the comet's gaseous envelope away from 

 the solar anode. This positive discharge con- 

 sisting of material one thousand times as 

 heavy as electrons and traveling at a tremen- 

 dous velocity would afford considerable re- 

 sistance. 



The comet, being between an anode and a 

 cathode, becomes electrically polarized, the 

 portion of the nucleus, or " head," nearest 

 the sun becomes negative and the opposite 

 side, positive, discharging through the gaseous 

 envelope, brushed back more or less by the 

 " ionic breeze " from the sun, toward the 

 strongest cathode. In fact, if there were no 

 brushing back of the gaseous envelope the 

 electrical disturbance would be in the direc- 

 tion of the cathode and the illumination 

 would take place only in the ionized path, thus 

 giving the effect of a " tail " or streamer. 

 The brushing back of the envelope lengthens 

 the streamer, and since the effect of the " ionic 

 breeze " becomes greater as the comet ap- 

 proaches the sun, the streamer therefore be- 

 comes progressively longer. As the comet 

 moves toward the sun the " tail " not only 

 becomes longer but brighter, because the in- 

 creasing heat more and more attenuates the 



