Feb. 2 1, 1889] 



NA TURE 



401 



attraction and repulsion balance each other, or nearly so, and 

 the tails of this class are plumy and gently curved. In this 

 case the particles which go to form the tail leave the head with a 

 velocity of about 3000 feet per second. Tails of the third type 

 are short and strongly bent, the repellent energy being only about 

 (ine-fifth of the attractive energy of the sun, and the velocity of 

 the particles leaving the head is only about icxx) feet per second. 



Many comets exhibit tails of more than one type, and it was 

 conjectured long ago that such tails were composed of different 

 kinds of matter. 



Bredichin went further, and defined the composition of the 

 (lifterent kinds of tails which he had classified, by referring to the 

 molecular weights of the materials which would give the relative 

 values of the repulsive and attractive forces necessary for tails of 

 the different types. He thus found that the long straight tails of 

 the first type would be probably formed by hydrogen, since this 

 substance, on account of its exceeding lightness, would be little 

 influenced by gravity, while at the same time strongly influenced 

 by the solar repulsion. The second type of tails he considered 

 to be made of hydrocarbons, since hydrocarbons have a 

 molecular weight such that the repellent and attractive forces of 

 the sun upon their particles may be nearly equal. Iron, on the 

 other hand, would be more subject to the action of gravity, on 

 account of its greater weight, and was therefore taken as adapted 

 to tails of the third type. 



There is nothing extravagant in these suppositions, for we 

 know that all the substances in question do exist in comets, and 

 it is evident that much is to be learnt from a continuation of the 

 inquiry, but at the outset we can see that iron vapour cannot in 

 space remain as vapour to form a tail. 



We know that the short-period comets get less brilliant with 

 every approach to perihelion, and that some do not even throw 

 out a tail, and we can easily ascribe both these results to the 

 fact that after several such appulses the vapours liable to be 

 driven out of the meteorites by temperature get less and less. 



If this be so, we may regard a comet with many tails as one 

 which for the first time undergoes perihelion conditions. 



If it be conceded that the tails of comets are in part composed 

 of hydrogen and compounds of carbon with gases such as 

 oxygen, an explanation seems to be suggested of many recorded 

 phenomena, while at the same time it seems more probable 

 that the repulsive force would act continuously upon permanent 

 gases rather than on condensable vapours, such as iron vapour, 

 to take an instance. 



Suppose that the sun has been formed by the coming together 

 of meteors, whether brought by comets or not, it is obvious that 

 with equal temperatures of the sun the repellent action would be 

 the same on the permanent gases given off by the meteorites, 

 whether in large or small groups. In the larger groups there 

 would be possibly more collisions, and therefore greater 

 possibilities of higher temperature of the meteorites. 



This action would surround the sun, as it were, by a cordon, 

 inside which, to take instances, neither hydrogen nor oxycarbon- 

 compounds could enter. Hence we should have a sun without 

 hydrogen, carbon, or oxygen. 



But while, as demanded on this view, the quantity of carbon 

 and oxygen is extremely small, even if the latter exists at all, the 

 quantity of hydrogen is enormous. 



This difference can, however, be accounted for by the idea which 

 has been suggested on several other grounds, that the hydrogen 

 which plays such an important part in the sun's economy and in 

 the economy of all stars hotter than the sun is really produced 

 locally by the dissociation of the vapours of the chemical elements 

 which form the sun and the meteoritic constituents which still 

 reach it in the shape probably of iron and silicates. 



We know perfectly well (from Duner's work chiefly, in stars of 

 the class \\\.b) that when the sun gets cooler its atmosphere will 

 consist almost exclusively of carbon compounds ; and indeed 

 one of the last scenes in the drama of world-formation seems to 

 be the gradual approach of the " cordon " to which I have 

 referred, as the radiant energy of the star is diminished, thereby 

 enabling all the permanent gases in the system to gradually 

 approach the primary ; and it is not impossible that the great 

 differences of density of the interior and exterior planets may be 

 connected with this state of things. 



Before passing on, it is well to recur to the question. Why 

 should not vapours be also repelled from the cometary nucleus 

 and its envelopes } 



No doubt they are ; but it is straining the facts to suppose that 

 they would not be condensed by the cold of space before they 



had been repelled any great distance ; the enormous lengths of 

 some comets' tails would seem to negative any such possibility. 

 Some of these lengths may be given in miles : — 



Comet 1843 (I.) ... 

 „ 1680 



„ 1847(1.) ... 

 „ 1811 (I.) ... 

 „ i860 (III.)... 



198,800,000 miles 

 149,000,000 ,, 

 130,800,000 ,, 

 109,400,000 ,, 

 21,700,000 ,, 



With regard to the rate at which the tails are th rown out it 

 may be stated that, in the case of Donati's comet, between 

 September 23 and October 10 the tail had increased from 

 15,000,000 to 55,000,000 miles, or, speaking roughly, the tail had 

 increased by 2,coo,ooo miles a day. 



If we are justified in considering that the materials of the comet 

 thus repelled to form the tail are non-condensable gases, such as 

 the hydrogen and the carbon compounds which are actually found 

 in meteorites, we have in this fact probably the vera causa of the 

 so-called occlusion of these gases by meteorites. That is, one 

 set of meteorites — a comet — may be giving off these gases, while 

 other meteorites, which have never been members of such a large 

 swarm, may occupy regions of space swept over by the gases 

 repelled from the comet. 



But if it be agreed that it is not probable that, say, the vapours of 

 iron and magnesium could retain their vaporous condition so long 

 as the hydrogen and the carbon compounds — there can be no 

 doubt that they start on the common journey in consequence of 

 the repulsive action outside the track of the comet — then we shall 

 expect to find condensed particles of iron, nickel-iron, and mag- 

 nesium or their compounds ; and here again we have a vera 

 causa for the chondrites which enter so largely into the 

 composition of meteorites. 



The tail of a comet being thus formed at the expense of the 

 materials composing the head, the materials removed from the 

 head can never be returned to it because of its insufficient gravi- 

 tational power over them, and moreover they can no longer 

 traverse the same orbit as the comet to which they originally 

 belonged, because they have already been turned out of that 

 course by the forces attending the development of the tail. The 

 small tail-forming meteoric bodies thus become distributed 

 throughout the space occupied by our system, and give no further 

 trace of their existence, unless they happen to break into our 

 atmosphere and appear as shooting-stars. 



Comets must thus degenerate, so far at all events as their easily 

 volatilized constituents are concerned, with each perihelion 

 passage, bat as the majority of them only approach the sun at 

 long intervals of time they do not suffer much in this way. Some 

 of the short-period comets get less and less brilliant at each 

 successive perihelion passage, and others are then observed 

 entirely without tails, all the available tail-forming material 

 having been used up and dispersed into space. 



It is a fact well worthy of consideration that on many occa- 

 sions pulsations exactly resembling those observed in aurorae 

 have been observed in comets' tails. 



This subject is thus referred to in Guillemin's book on 

 comets : — 



" Kepler is the first observer who has made mention of the 

 changes. 'Those,' he says, 'who have observed with some 

 degree of attention the comet of 1607 (an apparition of Halley's 

 comet) will bear witness that the tail, short at first, became long 

 in the twinkling of an eye.' Several astronomers — Kepler, 

 Wendelinus, and Snell — saw, in the comet of 1618, jets of light, 

 coruscations, and marked undulations. According to Father 

 Cysatus, the tail appeared as if agitated by the wind ; the rays 

 of the coma seemed to dart forth from the head and instantly 

 return again. Similar movements were observed by Hevelius in 

 the tails of the comets of 1652 and 1661 ; and Pingre, describing 

 the observations of the comet of 1 769, made at sea, between 

 August 27 and September 16, by La Nux, Fleurien, and him- 

 self, thus describes the phenomenon of which he was a 

 witness : — ' I believe that I very distinctly saw, especially on 

 September 4, undulations in the tail similar to those which may 

 be seen in aurora borealis.' The stars which I had seen decidedly 

 included within the tail were shortly after sensibly distant 

 from it. 



" M. Liais has given the following account of the observations 

 made by him of the great comet of i860 :— ' On the evening of 

 July 5, whilst I was observing the comet at sea, I saw a rather 



