45 6 



NA TURE 



[Sept. 4, 1884 



coherence of the parts of a comet arising from their nun nil 

 gravitation is an extremely feeble force. Each particle of the 

 comet is directly solicited by the sun to pursue a path of its own, 

 and if the forces of coherence be not adequate to overcome this 

 tendency the comet must undergo a gradual degradation into 

 separate parts. As the periodic time of the orbit of each part 

 will vary, it will follow that the comet will be spread out in 

 fragments along its path. It would seem that these small 

 fragments constitute the meteors. 



It is often supposed that meteorites, or solid bodii 

 actually tumble down on the earth, are connected with shooting 

 stars, and hence it has been asserted, and even by very ■ ■ id 

 authority, that meteorites are connected with comets, if not 

 actually parts of comets. I merely mention this view for the 

 purpose of saying that to me it seems quite unsupported by the 

 facts. There is no reliable evidence, or indeed no evidence at 

 oil, that meteorites are connected with the periodic shov 

 shooting stars which alone are connected with the comets. ! his 

 would not be the occasion to discuss the interesting question as 

 to ill,- origin oi meteorites, but all the available facts seem to me 

 (o point to 111 origin on some body far more closely n 

 planet than a comet. 



It is now about sixteen years since Dr. Huggins first turned 

 his spectroscope upon these bodies, and showed that certain lines 

 in the spectra of the comet of 1868 were identical with certain 

 of the lines of carbon. Since then many comets have been 

 observed and much valuable spectroscopic work has been dine. 

 This has been so often and so fully discussed that I do nol now 

 propose to dwell on the subject at length. It is, however, quite 

 impossible to avoid a brief reference to one of the lair,! 

 of Dr. Huggins' marvellous skill. He has succeeded in inducing 

 a comet to depict with absolute fidelity its spectrum on the 

 photographic plate. That photograph has not only shown the 

 lines which could be seen with the spectroscope, but it In, also 

 exhibited many other lines in the invisible part of the spectrum. 

 The discussion of this photograph and of the bright lines and the 

 dark lines it contains is full of interest, though here I shall only 

 remark that it contains convincing evidence of the presence of 

 carbon in this comet. 



That a comet's tail should be directed away from the sun is a 

 very remarkable and characteristic feature of this group of bodies. 

 At the first glance it seems at variance with every received 

 doctrine of astronomy. The great law of Nature which regulates 

 the movements of the heavenly bodies is the law of ultra, tion. 

 The very movement of a comet in an elliptic path around the 

 sun is in itself a demonstration that the comet is attracted by 

 the sun. 



While the comet as a whole is amenable to the law of 

 gravitation, il is obvious that the materials, whatever they may 

 be, which constitute the tail of the comet must be repelled by 

 some force of an exceptional character. This force must some- 

 times be of very great intensity. Cases are not wanting where 

 a comet, after darting in close to the sun, has actually whirled 

 round the sun with the stupendous velocity of 300 miles a 

 second, and in a few Injurs has commenced its outward journey. 

 During this appalling swoop what has been the conduct of the 

 tail of the comet? It seems necessary to believe that at the 

 commencement the tail was streaming away for millions of miles 

 on one side of the sun, while in a few hours the tail has 

 crone completely round, so as to be extending for millions of 

 miles in the opposite direction. No known laws of mechanics 

 allow us to believe that the same tail is seen under circumstances 

 so divers,-. We .ue compelled to believe that the tail is constantly 

 dissipated ami constantly renewed. It would, in fact, seem that 

 the tail of a comet was in some respects like the column of sue ,ke 

 ascending from a chimney — the column remains, but the particles 

 of which that column consists are in perpetual transition. 



In the study of this subject we have to make use oi the 

 interesting labours of Prof. Bredichin of Moscow. This accom- 

 plished astronomer has devoted himself for many years to the 

 collection and to the discussion of all the known phenomena of 

 comets' tails, and he has succeeded, I believe, in taking a con- 

 siderable step in the solution of the problems involved. In the 

 first place he has shown that there are different types of comets, 

 and he has proceeded to classify them. There are, first of all, 

 the comets with very long and very straight tails, such, for 

 instance, as the comet of 1874, and many others. The next 

 class included the tails of a scimitar shape. These are often of 

 very great splendour, though not so long or so straight as those 

 of the first type. The great comet of 1858 may be cited as an 



illustration of this class. The third and last class of comets' 

 tails are very short and curved. It is to be observed that these 

 tails sometimes exist in combination, so that a comet is often 

 decorated with two tails of different types. 



Once the form of the tail has been laid down, and the 

 perihelion distance of the comet given, then the investigation of 

 the forces adequate to the production of that tail is a problem 

 admitting of numerical solution. It can be demonstrated that 

 the straightest tail that ever streamed from a comet could be 

 produced by a repulsive force not more than twelve times as 

 great as the intensify of gravitation at the same distance. This 

 number twelve will be the characteristic of tails of the first type. 

 The tails of the second type vary within certain limits, but 

 speaking generally, the repulsive force adequate to their pro- 

 duction need not be more than about equal to the force of 

 I Isof the third type would lie explained 



if the repulsive force were only the fifth part of ■ 



The next question that arises is as to the physical explanation 

 of the repulsive force which produces these tails. We have to 

 find this force of three different intensities, one about twelve 

 times as greaf as gravity, one about equal to gravity, and one 

 about a fifth of gravity. Before we postulate the existence of 

 a new force of some unknown character, it is surely our duty to 

 inquire whether there may not be some force already known 

 which is competent to produce the phenomena. The best known 

 repulsive force is of course that with which every one is familiar 

 in connection with electricity. Electricity attracts electricity of 

 an opposite type, while it repels that of the same type. We 

 are also aware that in some mysterious manner the sun is con- 

 nected with electricity. We know that the phenomena of 

 terrestrial magnetism are connected with solar phenomena, and 

 hence wc are tempted to inquire whether the electricity of the 

 sun may not offer an adequate explanation of the phenomenon 

 of the comet's tail. 



Let us suppose that the sun is attracting a distant body by 

 virtue of gravitation, and at the same time repelling that body 

 in virtue of the fact that the sun and the body are both charged 

 with electricity of the same name. When the attracted body is 

 one of large dimensions, the attraction will vastly exceed the 

 repulsion, and indeed the latter may be entirely neglected in 

 most eases. There is, however, a radical difference between the 

 nature of the electrical forces and the nature of the gravitational 

 forces. The latter are proportional to the masses of the attract- 

 ing bodies, while the electrical forces are proportional to their 

 surfaces. The mass varies as the cube of the linear measure- 

 ments, while the surface only varies as the square. The relative 

 efficiency of the electric repulsion in comparison with the gravi- 

 tational attraction increases as the radius of the particle decreases. 

 It must thus necessarily follow that no matter how great maybe 

 the preponderance of the power of gravitation on masses of 

 finite dimensions, yet it must always be possible, other things 

 being equal, to have a particle so small that the electrical re- 

 pulsion shall exceed in any required ratio the intensity of the 

 attraction of gravitation. 



As the comet draws near the sun, the heat it experiences in- 

 creases, so that the materials of the comet begin to dilate, and 

 to be driven off into a vaporous condition. The matter is thus 

 resolved into a state of extreme subdivision. These separate 

 particles are charged with an electricity similar to that of the 

 sun, and in virtue of their minuteness the intensity of that re- 

 pulsion has become sufficient to sweep off the particles in a 

 stream, and thus generate the tail. 



Such is the modern view of the formation of comets' tails. 

 Professor Bredichin has given good reasons for thinking that 

 we can even discover the special ingredients which enter into 

 the formation of each of the three types of tail. It seems, from 

 the molecular nature of hydrogen, that this element is especially 

 suitable for the tails of the first type. The tails of the second 

 type seem to arise from some substances possessing the pro- 

 perties of hydrocarbons, while the tails of the third type contain 

 some elements which seem to have a high atomic weight. The 

 theory of Professor Bredichin is well illustrated by the comet 

 of 1858. This comet, besides the majestic curved tail, the ob- 

 ject of so much admiration, had a pair of long, faint, slender 

 tails, streaming straight from the head. These two objects were 

 doubtless the edges of a conical tail of the first type, too faint 

 to be visible throughout its entire extent. The great tail was 

 one of the second type. 



We have many reasons for believing that the masses of 

 comets are very much less than the masses of the planets. We 



