142 
comet i‘eally was surrounded by an abnormally voluminous 
atmosphere, that it was subjected to sudden and violent 
thermal changes, and that it also exhibited the imposing 
aspects to which I have alluded, I think I am entitled to 
ask you to regard the phenomena in question as meteor- 
ological phenomena, and to regard the meteorological theory 
as affording a satisfactory explanation of the appearances 
presented at any rate by that particular body. 
The analyses of meteorites falling upon the surface of the 
earth from the regions of external space have not, it appears, 
led to the discovery of any new chemical body^', whilst the 
spectroscope has shown that chemical substances, common 
enough upon the earth, exist in the solar atmosphere, in the 
atmospheres of his attendant planets, in the fixed stars 
themselves, and in the far distant nebulse. In short, we 
have direct evidence that whatever quantitative differences 
there may be, qualitatively the community of matter 
extends throughout the fields of nature. 
This fact at once disposes of those cometary theories 
which assume the existence of some new chemical body as 
a necessary agent in the production of cometic phenomena, 
and it also disposes of those objections to other theories 
founded upon the supposition that the ordinary operation 
of physical laws as observed upon the earth may be ren- 
dered inoperative in the case of comets b}^ a non-community 
in their chemical nature. 
We have, it is true, very little special evidence as to the 
chemical constitution of comets. What little we have has 
been furnished by comparatively insignificant members of 
the family, in unfavourable circumstances of luminous in- 
tensity, and with indications as yet unsatisfactorily inter- 
preted, j* Evidence has however been adduced that comets 
for the most part consist of matter moving between us and 
Guillemin’s ^‘Heavens,” Eng\ trans., page i75j 
t See ^‘Nature/’ Jan, 8tli, 1874. 
