THE CHEMISTRY OF A COMET. 
407 
cloud. If so, this luould he visible in the sv/ii^s illuminating 
rays, and present the appearance of the comet’s tail. 
By the change which takes place in the relative position of the 
sun and the head of the comet, the heat-shadow moves over the 
surrounding cometary vapour, and, as it does so, leaves behind 
it parts that were just before within its area, and receives, as it 
advances, parts previously exposed to the heating action of the 
sun. Whilst, therefore, near the advancing side of the shadow, 
fresh cloud is being formed by the cooling of the previously 
heated vapour, the cloud left behind by the shadow is being 
dissipated by the action of the heating rays of the sun received 
by it. This formation and dissipation of the cloud being a 
process of time, the luminous cloud will lie a little behind the 
heat-shadoiu ; and as that part of the cloud just emerged from 
the shadow which is nearest the sun will first, and to the greatest 
degree, receive the heat, it will be the first dissipated, and thus 
give rise to a curve in the cloud. Hence, the observed facts 
that the tail of a comet is generally curved and inclined to the 
side of the region just left by the comet. At the period of 
passing through its perihelion, half the circumference of the 
cometary mass is traversed by the heat-shadow with all the 
velocity observed in a comet’s tail. When the volatility of the 
chemical product is very slight, and its quantity sufficient, this 
shadow may keep up the existence of a tail-cloud ; in other 
cases, the heat of the sun dissipates the tail entirely for the 
time. The enormous velocity with which, as the heat-deprived 
sunbeams are cast into fresh portions of vapour, the comet’s 
tail grows after the perihelion passage of the comet, is such as 
would be imitated by the growth of a chemical cloud under 
suitable conditions, according to Dr. Tyndall. Lastl}^ with 
regard to other peculiarities in the tails of comets, they would 
none of them probably afford any difficulty in accounting for 
them had we a little more intimate acquaintance with the con- 
stitution of individual comets. Thus, as the distinguished pro- 
pounder of the theory suggests, “ in the struggle for mastery of 
the two classes of rays [heating and chemical], a temporary 
advantage, owing to variations of density, or some other cause, 
may be gained by the actinic rays, even in parts of the cometary 
atmosphere which are unscreened by the nucleus. Occasional 
lateral streamers, and the apparent emission of feeble tails 
towards the sun, would be thus accounted for.” 
The theory we have now laid before our readers, that a comet 
is a chemical cloud, is at present without doubt open to material 
criticism, but we feel a strong conviction that its general sound- 
ness will be full}^ confirmed by time and further investigations. 
