with Remarks on the Construction of its different Parts. 144 
inconceivable expansion and rarefaction of the luminous sub- 
stance of the comet about the time of its perihelion passage. 
It is admitted, on all hands, that the act of shining denotes 
a decomposition in which at least light is given out; but that 
many other elastic volatile substances may escape at the same 
time, especially in so high a degree of rarefaction, is far from 
improbable. 
Then, since light certainly, and very likely other subtile 
fluids also escape in great abundance during a considerable 
time before and after a comet’s nearest approach to the sun, 
I look upon a perihelion passage in some degree as an act of 
consolidation. 
If this idea should be admitted, we may draw some inter- 
esting conclusions from it. Let us, for example, compare the 
phenomena that accompanied the comet of 1807 with those of 
the present one. The first of these in its approach to the sun 
came within 61 millions of miles of it ; and its tail, when longest, 
covered an extent of 9 millions. The present one in its peri- 
helion did not come so near the sun by nearly 36 millions of 
miles, and nevertheless acquired a tail 91 millions longer than 
that of the former. The difference in their distances from the 
earth when these measures were taken was but about 2 mil- 
lions. 
Then may we not conclude, that the consolidation of the 
comet of 1807, when it came to the perihelion, had already 
been carried to a much higher degree than that of the present 
one, by some former approach to our sun, or to other simi- 
larly constructed celestial bodies, such as we have reason to 
believe the fixed stars to be ? 
And that comets may pass round other suns than ours, is 
