TRANSACTIONS OF SECTION A. 531 



which, if the increasing warmth or the tidal action of the sun should awaken its 

 activity, Rutherford's a-rays should be shot out at the speed that he has measured 

 of a thousand million inches a second, i.e. one-twelfth the velocity of light. These 

 a-ray particles, according to Rutherford, consist of helium ; they weigh each twice 

 as much as a hydrogen atom, and so the same weight of comet matter that would 

 make one of Nichols and Hull's best particles, i.e. one that would be just visible 

 with a microscope, would he sufficient for about 400 millions of Rutherford's a-ray 

 particles, an advantage surely where diftuseness seems so miraculous. 



These particles, shot out at a velocity one-twelfth that of light, go so fast that, 

 if they were to start horizontally on the surface of the earth, the gravitative 

 attraction of the earth would curve their path to the infinitesimal extent of a 

 curve with a radius of fort3'-thousand million miles. Yet so great is the electric 

 charge they carry that a visible curvature can be imposed upon them in a practicable 

 electrostatic field. 



Now imagine these transferred into space at a distance from the sun, for 

 instance, equal to that of Venus. Gravity there due to the sun is only one-thou- 

 sandth of what it is here, so gravity there would be, to the same extent, less 

 able to impose visible curvature on their paths. But their electric charges are 

 still available, and unless I have made an arithmetical blunder of a considerable 

 order, it would require no very heavy electrification of the sun to bend these rays 

 round in a curve with a radius of 1,000 miles. An electrostatic field of under 

 two ten-thousandths of a unit should be sufficient, a field which would be produced 

 if the sun were only charged with a surface density of one electrostatic unit on 

 every three square centimetres. 



Whether these figures are correct or not — and I know the risk of getting just 

 thirty-thousand-million times too large or too small a result — does not much matter. 

 An electrified sun, which after all others besides Arrhenius have postulated, would 

 be sufficient to turn the rays and send them away at rapidly increasing speed so as 

 to form the tail. The speed would in a short time reach the velocity of ligjit if it 

 were not for the change in properties of matter which supervenes when aiiy such 

 velocity is nearly reached. Thus, according to the ratio of charge to mass, particles 

 such as those in Rutherford's a-rays would be sent away each with its limiting 

 velocity, giving rise to streaks more or less well defined, and double, triple, or 

 multiple according to the number of kinds of ray which the various radio-active 

 materials were able to generate. 



Not only should streaks pointing away from the sun be formed, but any nega- 

 tively charged rays such as radium is said to give out should form a tail directed 

 towa'rds the sun. Perhaps this might be expected to be general, but while not 

 common one was described by Hind in the comet of 1823-24, and three or four 

 more have been observed. 



The head or coma would be the envelope of all the independent orbits, leaving 

 the nucleus in all directions — orbits which while their velocities are still of the 

 Rutherford order would be hyperbolas convex to the sun. 



If this should not appear to be absolute nonsense it would seem as if another 

 difficulty should become less than it has been. I refer to the visibility, luminosity, 

 and spectral character. 



Lodge, as an interpreter of Larmor, tells us that an electrified ion subject to 

 acceleration, whether transverse or in the line of motion, radiates energy. The 

 streamers from the nucleus subject to the greatest acceleration may be bright almost 

 as the nucleus itself; then, as they have become dissipated into regions where far 

 less acceleration becomes possible, the radiation falls oif and the tail is lost in 

 space. 



The observations made last month by Sir William and Lady Huggins of the 

 spectrum given by a piece of radium in the air may ha-\e some bearing upon the 

 luminosity of the comet. It is possible that the internal motions set up by the 

 separate parts, each pursuing its individual orbit, may produce collisions numerous 

 and violent enough to account for all the light that is seen, and for temperature 

 sufficient to bring out the spectral lines that have been,^(^entitied. Whether this 

 is so ox not, radio-active bodies and their emanations can produce light indepen- 



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