Forces which Produce the Tails of Comets. 93 
ture and origin of this force ; but they are all more or less 
inconsistent with known facts. 
One mode of accounting for the existence of such a force 
has, so far as I am aware, been hitherto overlooked. 
The nebulous substance of a comet appears to consist of 
matter in the state of smoke or mist: that is to say, of mi- 
nute solid or liquid visible particles, suspended in invisible 
vapour. If we suppose that, on the approach of the comet 
to its perihelion, each of these visible particles is partially 
volatilized by the sun’s heat, it follows that it will emit in- 
visible vapour, chiefly on the side nest the sun, and that, by 
the reaction of the vapour so emitted, the portion of the par- 
ticle which remains in the visible state will be propelled 
away from the sun, with a force depending on the rapidity of 
the evaporation. This force will be greatest in the super- 
ficial portion of the nebulous envelope, the internal parts 
being more or less protected from ‘the sun’s rays. The par- 
ticles thus propelled, after describing an orbit more or less 
elongated, will return towards the nucleus under the action 
of gravity. 
Such will be the ordinary action of the solar heat, accord- 
ing to the supposition now proposed, as displayed in the 
usual form and position of the tails of comets; but it may 
be modified by other forces, such as the action of jets of va- 
pour issuing from the nucleus, so as to give rise to those 
forked and oblique tails which have occasionally been ob- 
served. 
I do not propose this conjecture as a theory sufficient to 
account for all the phenomena of the tails of comets, but 
merely as a speculation, somewhat less visionary, and more 
in accordance with the known properties of matter, than 
those which have hitherto appeared on the same subject. 
EDINBURGH, September 1849. 
