43 
ditions under which their materials exist. Now from the 
motion of comets we know that their heads follow the 
same laws of motion and gravitation as all other matter, and 
therefore we have good evidence, so far as it goes, that 
comets and planets are similarly constituted as regards 
matei’ials. And since the appearance of a comet changes 
very much as it passes round the sun, any assumptions 
with regard to the material of comets in order to account 
for their difference from planets would not account for the 
variety of appearance the same comet presents at differ- 
ent times. On the other hand the conditions of comets 
and planets must necessarily be very cliffei’ent, from the 
extreme difference in the shapes of the orbits they describe. 
Each planet remains nearly at a constant distance from the 
sun (whatever that distance may be), so that the heat or 
any physical effect the sun may have upon it will also be 
constant ; on the comets its action must change rapidly from 
time to time, particularly when the comet is in certain 
parts of its orbit. Hence we may say that the temperature 
and general physical condition of planets is nearly constant, 
and that of comets for the most continually varying. 
There is, too, a very remarkable connection between the 
appearance of the comet and the rate at which the sun’s action 
on it changes. Herschel says : — “ Sometimes they first 
make their appearance as faint and slow moving objects, 
with little or no tail, but by degrees accelerate, enlarge, and 
throw out from them this appendage, which increases in 
length and brightness till (as always happens in such cases) 
they approach the sun and are lost in' his beams. After a 
time they again emerge on the other side, receding from the 
sun with a velocity at first rapid, but gradually decaying. 
It is, for the most part, after thus passing the sun that 
they shine forth in all their splendour, and their tails acquire 
their greatest length and development; thus indicating 
plainly the sun’s rays as the exciting cause of that extra- 
