450 



NA TURE 



[September io, 1903 



Bredechin, by his amazing industry, has made his own pro- 

 perty, aad that any stranger casually passing by and taking 

 a random shot should receive the severe penalty awarded to 

 poachers in this country. Bredechin has dealt unmercifully 

 — I do not say unjustly — with the author of at least one such 

 random theory. 



It is therefore with the greater diffidence and more urgent 

 plea for forbearance that I venture to draw certain parallels 

 and hazard certain suggestions which I admit freely have not 

 reached a stage at which detailed comparisons with known 

 comets are possible. 



It does not seem possible now to contemplate the 

 phenomena of the comet, of the divided tails, of their 

 tenuity and transparency, of the pale luminosity, partly 

 reflected solar light, partly light as from a glowing gas ; of 

 the gradual wearing out and disappearance of those comets 

 which constantly pay visits to solar regions, with all the 

 mysteries of radium now so much in evidence without tracing 

 the features in which they resemble one another. By radium, 

 of course, I mean any material with the remarkable radio- 

 active properties that radium exhibits with such pre-eminent 

 splendour, whether known in the laboratory or not. 



How many physicists have been peering at comets through 

 radium spectacles, or how many astronomers detect the 

 sparkle of radium in the fairy tresses of their hirsute stars I 

 know not. One writer, however, T. C. Chamberlin, so 

 long ago as July, 1901, looked upon a connection between 

 radio-active materials such as were then known and comets 

 as at least worth considering. Chamberlin 's paper in the 

 Astrophysical Journal was mainly on the tidal disruption of 

 gravitating bodies and the possible evolution of comets, 

 nebulae and meteorites, and he did not pursue this consider- 

 ation in any detail ; indeed, the enormous accumulation of 

 new properties of radium was not then available. 



Whatever may be imagined as to the constitution of a 

 comet, difficulties still remain. All I suggest now is that 

 the curious properties of radium and of similar bodies should 

 be kept in mind. Radium at least supplies the means by 

 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-rays, 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 be sufficient for about 400 

 millions of Rutherford's o-ray particles, an advantage surely 

 where diffuseness 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 forty 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-thousandth 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 arith- 

 metical 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 1000 miles. An electrostatic field 

 of under t\yo 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 thirt\ 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 light if it were not 

 for the change in properties of matter which supervenes 

 when any such velocity is nearly reached. Thus, according 

 to the ratio of charge to mass, particles such as Rutherford's 

 o-rays would be sent away each with its limiting velocity, 

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



NO. 1767, VOL. 68] 



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 negatively charged rays such as radium is 

 said to give out should form a tail directed towards 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 in- 

 dependent 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 elec- 

 trified 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 dis- 

 sipated into regions where far less acceleration becomes pos- 

 sible, the radiation falls off 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 have 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 pro- 

 duce 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 identified. 

 Whether this is so or not, radio-active bodies and their emana- 

 tions can produce light independently of such action ; and now 

 these observers have found that in the case of radium in air 

 this light gives the spectrum, line by line, of nitrogen. Is it 

 possible that the enveloping nitrogen has had its atoms so 

 harried by the activity of the radium as to give a response 

 hitherto only awakened by electric discharge? The ability to 

 obtain such a response opens up a new possible interpreta- 

 tion of these spectra, which hitherto have been assumed, with 

 our laboratory experience only to guide us, to have required 

 for their production temperature above a red heat. li 

 further observation should confirm this, the hydrogen, the 

 hydrocarbon, and possibly even the sodium or iron spectrum 

 that has been observed, may have come from cold atoms ; 

 and it is not even quite beyond the limits of imagination to 

 picture, not from the comet matter itself, but from loose 

 residual and highly attenuated matter through which the 

 comet is passing. 



There is one other feature of this remarkable observation 

 of equal interest. The lines of the spectrum were not 

 exactly in their proper place, but were all shifted towards the 

 red end of the spectrum about twice the distance between 

 the D lines. If only one or two lines had been so observed 

 a different origin might well have been suspected ; but when 

 the whole series are faithfully reproduced it is reasonable to 

 look upon the spectrum as modified to that extent as though 

 the works of the nitrogen atom had not only been set in 

 movement, but had been loaded with the radium emanation. 

 Before dismissing these random speculations on the pos- 

 sible connection between radio-activity and comets I would 

 ask your leave to refer once more to Bredechin 's conclusions. 

 He has found that it is merely necessary to postulate three 

 kinds of matter, issuing from the nucleus with three initial 

 velocities, and subject to repulsion from the sun with three sets 

 of forces of repulsion — i.e. as compared with ordinary gravita- 

 tive attraction — for the whole of the phenomena of all sorts 

 of comets to be very completely accounted for. His highest 

 initial velocity is only about five miles a second, and his 

 lowest about a quarter of a mile a second. His highest 

 repulsion, after deducting gravitative attraction, is only 

 eleven times gravity, and his lowest only a fifth of gravity. 

 If, then, with such'velocities and forces the phenomena can 

 be exactly accounted for, it would seem futile to consider 

 the possibility of initial velocities from 4000 to 80,000 times 

 as great and effective repulsions of a corresponding order 

 being able to produce effects with anything in common. 

 This is not necessarily the case, for with the comparatively 

 slow separation of the atoms of Bredechin 's matter from 

 the nucleus, each one describing its own hyperbola convex 



