41 
behind the comet, this would represent in some degree the 
conception such a phenomenon irresistibly calls up. But 
this is not all. Even such an extraordinary excitement of 
the aether, conceive it as we will, will afford no account of 
the projection of lateral streamers, of the effusion of light 
from the nucleus of the comet towards the sun; and its 
subsequent rejection of the irregular and capricious mode in 
which that effusion has been seen to take place, none of the 
clear indications of alternate evaporation and condensation 
going on in the immense regions of space occupied by the 
tail and coma — none, in short, of innumerable other facts 
which link themselves 'with almost equally irresistible 
cogency to our ordinary notions of matter and force.” 
There can be no doubt that if these tails are matter 
moving with the comet, this matter must be endowed with 
properties such as we not only have no experience of, but of 
which we can form no conception. This alone would seem 
a sufficient reason for rejecting the first hypothesis. More- 
over, on the second hypothesis there is no difficulty in the im- 
mense velocity with which these tails are projected from 
the head or whirled round when the comet is in perihelio. 
For to take the “negative shadow” as an illustration, here 
we should have a velocity of projection equal to that of 
light, and the only effect of the whirling would be a slight 
lagging in the extremity of the tail, causing curvature simi- 
lar to that which actually exists. And whatever the action 
may be, if its velocity of emission or transmission be suffi- 
ciently great, this effect will be the same ; but whether this 
hypothesis is to be rejected because involving assumptions 
beyond conception or contrary to experience, must depend 
on the answers to the following question — Do we know, or 
can we conceive any physical state into which any substance 
which can be conceived to occupy the space traversed by 
comets could possibly be brought so as to make it present 
the appeai’ance exhibited by comets ? 
