42 
Now, I think the answer must be in the affirmative, and 
that we may leave out the terms conceive and conceivable. 
For electricity is a well known state, and gases are well 
known substances ; and when electricity under certain con- 
ditions, as in Dr. Geissler’s tubes, is made to traverse 
exceedingly rare gas, the appearance produced is similar to 
that of the comets’ tails; the rarer this gas is, the more 
susceptible is it of such a state, and so far as we know 
there is no limit to the extent of gas that may be so illu- 
minated. Hence we may suppose the exciting cause to be 
electricity, and the material on which it acts and which fills 
space to have the same properties as those possessed by gas. 
What is more, we can conceive the sun to be in such a 
condition as to produce that influence on this electricity 
which should cause the tail to occupy the direction it does. 
For such an electric discharge will be powerfully repelled 
by any body charged with similar electricity in its neigh- 
bourhood. 
The electricity would be discharged by the comets on 
account of some influence which the sun may have on 
them, such an influence being well within the limits of our 
conception. 
The appearances of the comet in detail, such as the 
emission of jets of light towards the sun and the form of 
the illuminated envelope are all such as would necessarily 
accompany such an electrical discharge. 
In fact, if the possibility of such a discharge is admitted, 
I believe it will explain all the phenomena of comets. 
As to the possibility, or even the probability of such a dis- 
charge, I think it may be established on very good grounds. 
The tails of comets may or may not be one with their 
heads; but whichever is the case, it is certain that the dif- 
ference in the appearance of comets and of planets indicates 
some essential difference either in the materials of which 
these bodies are respectively composed, or else in the con- 
