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POPULAR SCIENCE REVIEW. 
tween this evaluation and the “ few pounds or ounces ” which 
we sometimes hear of as a probable estimate of their weight. 
Let it be further remembered 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 acknowledge that 
the belief in the diminutive mass of comets rests upon insecure 
if not insufficient evidence. The important discovery 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 ex- 
ceed, 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 nebulae, it is next to be 
considered whether the cometary systems are capable of supply- 
ing the nebulous matter with 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 con- 
siderable number whose spectra have been examined. The 
great comet of the past year is one of those whose 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 analo- 
gous 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 analysed 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 earth have been found to 
contain a large number of terrestrial elements — iron, in a 
native, uncombined form, and in great quantity ; cobalt, nickel, 
sulphur, silica, in the form of augite ; molybdenum, tin, copper, 
* It is often stated that the material of comets yielding the carbon spec- 
trum must he a hydro-carbon ; but it is to be remarked that none of the lines 
of hydrogen have ever been seen, and the assumption is therefore an en- 
tirely gratuitous one, made to get over the difficulty of the refractory nature 
of carbon. 
