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SCIENCE 



[N. S. Vol. XXXII. No. 317 



gaseous envelope by expansion, thus increasing 

 its electrical conductivity ; the " ionic breeze " 

 becomes stronger and the electrical energy 

 more intense. Should the comet get in such 

 a position as to be under the influence of two 

 or more planets acting as cathodes at the same 

 time the streamer will spread out or may 

 divide into several " tails " stretching out to- 

 ward each cathode. The direction of the 

 " tail " will depend not only on the position 

 of the planet acting as the cathode, but it will 

 be modified by the influence of the magnetic 

 field it happens to be passing through at the 

 time. 



It is impossible, of course, to exactly repro- 

 duce the cometic conditions; but they can be 

 represented fairly well with a glass tube vacu- 

 ated, containing another glass tube enclosing 

 a highly attenuated gas. When this double 

 tube is brought within the range of an electric 

 disturbance such as Hertzian waves or Tesla 

 oscillations, the inner tube shows the cometic 

 glow. The effect is very well shown if the 

 tube is placed near the secondary of a fair- 

 sized induction coil giving a two- or three-inch 

 spark, the glow having, at the positive end of 

 the inner tube, the appearance of the " head " 

 of a comet. If the direction of the current in 

 the coil is reversed, the " head " in the inner 

 tube will reverse. No terminals are necessary 

 in either tube. Thus the conditions are the 

 same as in interstellar space, except the globule 

 of gas is enclosed in a glass tube and is of 

 uniform density throughout. 



The properties of the luminous gas in the 

 tube under the influence of Hertzian waves are 

 the same as those of a gas in an ordinary 

 Pliicker tube. The direction of the luminous 

 stream is changed when in a magnetic field 

 perpendicular to the lines of magnetic force. 

 If the cathode end of the tube be from you 

 and the north pole of the magnet on your left, 

 the stream is attracted. When the poles are 

 reversed, it is repelled. Some gases at certain 

 stages of rarefaction show marked stratifica- 

 tion and form brilliant nodes when the dis- 

 charge passes through them. This nodal effect 

 is greatly intensified in a magnetic field, espe- 

 cially if the gas be hydrogen, carbon monoxide 



or carbon dioxide. By pulling a magnet per- 

 pendicular to the direction of the discharge, 

 from one end of the tube to the other, the 

 stream of ions is considerably lengthened, 

 while the magnet makes little progress in pull- 

 ing the light electrons against the discharge 

 of the heavier ions. 



As the comet passes through the varying 

 magnetic fields during its flight, the effect will 

 be manifest in varying the direction, and 

 curvature of the " tail," and in forming nodal 

 effects which have been frequently noticed and 

 described as multiple " heads." 



Some observers have announced that the 

 light from certain comets is polarized. Zee- 

 man has shown that light, when it passes 

 through a magnetic field perpendicular to the 

 lines of force, is plane polarized, and if it 

 passes parallel to the lines of force it is partly 

 plane and partly circularly polarized. There- 

 fore it is natural to suppose that cometary 

 light would be polarized in a manner depend- 

 ing on the direction of the magnetic fields 

 through which it passes, and to an extent 

 conunensurate with the intensity of these 

 fields. 



The color of the light emitted by a comet 

 will depend on the constituents and on the 

 density of its gaseous envelope. The light 

 emitted by a gas in a Pliicker tube will change 

 color as the pressure decreases, generally be- 

 coming lighter. The brightness diminishes as 

 the voltage decreases. Thus as the comet re- 

 cedes from the sun all conditions serve to 

 diminish its luminosity, i. e., the gaseous en- 

 velope condenses and the electric energy weak- 

 ens until the comet ceases to emit light. 



Halley's comet was reported to have lost its 

 " tail " as it passed the earth last April. This 

 was probably only an apparent loss. When 

 the comet came under the dominating influ- 

 ence of , the earth acting as the strongest 

 cathode, the electrified stream was drawn di- 

 rectly toward us and we viewed it " end-on." 

 As the comet passed beyond us the more dis- 

 tant planets became the cathodes and the 

 " tail " reappeared. Wm. L. Dudley 



VANDEBBn.T UnIVEESITY, 



June, 1910 



