THE CHEMISTRY OF A COMET. 
401 
any thickness of its substance, say 100,000 miles of it, proves 
to be quite transparent to the light of the stars, and has not, 
therefore, the opacity of a puff of steam or smoke. This ap- 
parent anomaly in its properties as matter is seen in a more 
exaggerated form in the fact that a comet has not a dark side 
to it — it presents no phases : measuring even hundreds of mil- 
lions of miles in a line from the sun, the rays of this luminary 
pass right through such a vast extent of matter without being 
extinguished, and illuminate its remote parts ; hundreds of 
millions of miles, therefore, of matter, in the condition to reflect 
light, is yet nearly or, for aught we know, quite transparent. 
Another great difiiculty lies in the apparent motions of the 
tail of a comet. In receding from the sun, as is well known, the 
tail of a comet is in advance of the head, although in advancing 
to it the tail is — in the position it ought to have to be called a 
tail — behind the head. The tail, therefore, swings round the 
head at the time of its passing through its perihelion. In the 
case of the great comet of 1843, its tail, many millions of miles 
long, swung round half a circle in little more than two hours ! 
Can we help feeling the incredibility that a material tail could 
have acquired and move with such a velocity, and this not by the 
aid of, but in opposition to, the attraction of the sun ? Indeed, 
it is impossible to admit both the fact of such a motion and the 
materiality of the tail, without attributing to the matter of the 
tail peculiar and improbable properties not possessed by ter- 
restrial matter. 
The apparent motion of the substance of the tail into and out 
of the head of the comet is hardly less difficult to understand, 
if the tail has really substance. For example, according to 
Newton, the great comet of 1680 shot out, after passing through 
its perihelion, a tail in two days, sixty millions of miles long I 
Can we admit, without the most conclusive evidence, that that 
which, starting from a state of relative rest, as regards, that is, 
the head of the comet, acquired a velocity wffiich carried it such 
a distance in such a time, was matter as we know it here ? Even 
if we can, we must suppose an almost equally inconceivable re- 
pulsive force to drive it out in such a Vv^ay ; so that we are little 
better off. Kepler, we believe, advanced an h}^pothesis of a re- 
pulsive force to account for the growth of the tail, and Sir John 
Herschel has also expressed his conviction that some such force 
must exist in the sun for the matter of the tail. He has 
suggested that the sun’s rays chemically decompose some of the 
matter of the head of a comet into gravitating and levitating 
matter — that is, matter attracted by the sun and matter re- 
pelled by it. Indeed, he has even said of comets, that among 
the uses they have served, is that of having “ furnished us with 
a demonstration of the existence of a repulsive force, directed 
