178 The Nebular Hypothesis : Its Present Condition. 



The only other comet that has given us a favorable opportunity of 

 weighing it against one of the planets is Lexell's, which in 1767 and 

 1779 must have approached very near to Juj^iter, without deranging 

 to any extent, so far as we are aware, his system of satellites. Laplace 

 has proved that this comet had certainly less than -j^^tli part of the 

 earth's mass, but there is a very considerable difference between this 

 evaluation and the " few pounds or ounces " which we sometimes hear 

 of as a probable estimate of their weight. Let it be further remem- 

 bered that Lexell's comet, like Encke's, was one of the comets of short 

 period, which are admittedly the least considerable of their class, and 

 we must acknoAvledge that the belief in the diminutive mass of comets 

 rests upon insecure if not insufficent evidence. The important dis- 

 covery that comets in their orbits are accompanied by streams of 

 meteors, and that they themselves are either meteors of very unusual 

 size, or a dense cluster of such bodies, proves that they must have a 

 mass at least comparable with, if they do not often exceed, that given 

 by Laplace as the maximum possible for Lexell's comet. 



Having to some extent removed these misconceptions as to the masses 

 of the comets and of the nebula?, it is next to be considered whether 

 the cometary systems are capable of supplying the nebulous matter 

 Avith the requisite material for the formation of new suns, and here our 

 imperfect knowledge of the constitution of the former acts as a serious 

 drawback. It would seem certain, however, that the composition 

 of comets is very various. One substance alone that is known to us 

 at present has been discovered, from spectroscopic analysis, as existing 

 in comets, and to form it would appear, in those cases in which it is 

 found, the sole constituent. This substance is carbon,* but it has been 

 identified only in three or four comets out of a considerable number 

 whose spectra have been examined. The great comet of the past year 

 is one of those Avhose spectrum has been thus identified. Others have 

 yielded spectra which, being simple in character and very similar in 

 appearance to that of carbon, may possibly belong to some substance 

 of analogous properties ; at least, it is likely that a single uncombined 

 element will be found, on increased knowledge, to form their principal 

 or sole constituent. Again, it is not improbable that other comets may 

 be of like composition to the meteors, many of which have been 

 analyzed by the ordinary methods of the chemist ; but whether this is 

 so or not, since these latter follow in the track of comets, they must 

 suffer the same catastrophes. The meteors that have fallen upon the 



* It is often stated that the material of comets yielding the carbon spectrum must be a 

 liydro-carbon ; but it is to be remarked that none of the lines of hydrogen have ever been seen, 

 and the assumption Is therefore an entirely gratuitous one, made to get over the difficulty of 

 the refractory nature of carbon. 



