must be hemispherical; for if it were merely a band of light, all in
the same plane, its phases must have varied like the ring of Saturn.
This cap must also be comparatively thin, since the parts at a distance
from its edge, which were therefore seen transversely, appeared dark
in comparison to the circumference, where a greater quantity of luminous matter was seen by oblique vision. And it is to the same
cause that the comparative brightness of the edges of the tail is
ascribed by Dr. Herschel.
With respect to the production of some of the cometic phenomena, the author conjectures, that the light is of a phosphoric nature; that the luminous matter of the head, being expanded on one side by the action of the sun, occupies more space, and consequently occasions the planetary body to appear eccentric; that part of this matter, being greatly rarified, ascends in the cometic atmosphere till it occupies the surface of that medium on the side towards the sun, and forms the hemispherical part of the envelope. He next supposes a further attenuation and a decomposition of this matter, till its particles are sufficiently minute to receive a slow motion from the impulse of the solar beams, and consequently gradually to recede in a direction towards the region of the fixed stars, to the distance of 100,000,000 miles.
From the escape of such a quantity of light, and probably of other subtile elastic matters, in consequence of the comet's near approach to the sun, Dr. Herschel infers that a greater consolidation of the remaining solid matter of the comet takes place at the time of its perihelion passage. He further thinks it not unlikely that the matter they contain is derived from nebula, which they meet with in the extensive orbits they describe; that in their course they visit other suns beside our own; and at each successive approach to these various centres they undergo progressive condensation; from which we may conceive how other planetary bodies may begin to have existence.
Although it has been asserted by Messrs. Gay-Lussac and Thenard, and also by Mr. Murray, that carbonic acid and chlorine have no action upon each other, Mr. J. Davy has observed the contrary to be the case. A mixture of equal parts of these gases, previously dried over mercury, being exposed to bright sunshine for about one quarter of an hour, lost all colour of the chloric gas, and were found condensed into half their former volume. The smell of this gas was more suffocating than that of chlorine. It occasioned a very sensation in the eyes; it reddened litmus paper; it combined with ammonia, forming a salt perfectly neutral and dry, but deliquescent by attracting moisture from the atmosphere. This salt was decomposed by sulphuric, nitric, and phosphoric acids, and also by liquid muriatic
