THE CHEMISTRY OF A COMET. 
4i)5 
passage, enough in what, after this treatment, was left in the 
connecting piece, to form a cloud in the light of the electric- 
lamp, which at the end of fifteen minutes discharged a body of 
light that, considering the amount of matter involved in its 
production, was simply astounding.” With this property of 
reflecting light, it possessed that of being apparently perfectly 
transparent. Thus, a page of print illuminated by it lost none 
of its distinctness by being viewed through the cloud itself. 
We are now prepared to consider Dr. Tyndall’s interpretation 
of the nature of cometary matter. According to him, a visible 
comet is formed of parts of a mass of gas or vapour of extreme 
tenuity, which become visible to us through a chemical change 
induced in them by the sun’s rays, this change having amongst its 
products at least one substance incapable of preserving the state 
of vapour at the temperature of the comet, and, therefore, pre- 
cipitating as a cloud of liquid spherules in the rest of the mass 
of vapour. Sir John Herschel estimates the weight of a great 
comet as lying between a few ounces and a few pounds, and Dr. 
Tyndall feels satisfied, by the experiment we have described, in 
which an inappreciable trace of vapour gave a white luminous 
cloud, by the light of which a printed page could be read, that a 
few ounces of one of the substances he used (the iodide of allyl), 
converted into vapour and sufficiently attenuated, would be quite 
enough to furnish in the sunbeams a cloud of the magnitude 
and luminosity of Donati’s comet — that is, about thirty millions 
of miles long and about ninety thousand miles thick. Like the 
luminous matter of a comet, too, this actinic cloud would be 
transparent, and allow, therefore, of the stars being seen 
through it. 
Let us first employ this theory to explain the formation of 
the head of a comet, or, in the case of the comets without tails, 
of the entire visible comet itself. Conceive the cometary mass 
as a vast body of exceedingly attenuated vaporous matter, which 
becomes denser towards some more or less central point, but still 
not to such an extent as to deprive it of its highly attenuated 
character. Such a condition may be the result of the gravitat- 
ing action of the small solid nucleus which some comets have 
appeared to possess, or it may be the result of a slight attraction 
existing between the particles of such highly attenuated vapour. 
As the sun’s rays pass through this mass of vapour, the}^ effect 
a chemical change, the products of which are visible as a cloud 
at the denser part of it, because the cloud-forming portion of 
them is here in greater quantity than can exist in the state of 
vapour at the temperature of the comet. In the rarer, outer 
parts of the mass the chemical products are not in greater 
quantity than can maintain there their vaporous condition. 
Now, with regard to that marvellous phenomenon, a comet's 
