Page:Proceedings of the Royal Society of London Vol 1.djvu/451

Rh tion opposite to that of the sun, was estimated from the observations made two days preceding, When it measured about 9’ 40", and its length, consequently, 659,000 miles.

Dr. Herschel remarks, that the physical construction of this comet must have been extremely different from that of the former, approach- ing very nearly to the planetary condition, and having a diameter nearly one third that of the earth.

The light by which it was seen he also considers as planetary: that is to say, reﬂected from the sun, and not phosphorescent, like the preceding; for if this were self-luminous, says Dr. Herschel, we could hardly account for its little density, which Would scarcely bear to be magniﬁed even to 107 times, although the former was seen with a power of 600 even better than with one that was lower.

The chevelure, however, he conceives to consist of phosphoric matter suspended in an elastic atmosphere that surrounds the body of the comet; and he ascribes the faint appearance of this chevelure, according to the theory advanced in his late communication, to the existence of a very small quantity of nebulous matter, which had not been consolidated by passing through a perihelion. And‘hence, says the author, this last comet was but little more aﬁ'ected by a perihe- lion passage than a planet would have been.

In consequence of the discussion which has lately taken place concerning the nature of these acids, the author has been induced to repeat, with more perfect apparatus than he formerly possessed, a part of those experiments of which he published an account in the Philosophical Transactions for 1800, and to add others tending to elucidate the same subject. Those experiments in general related to the elevtrization of muriatic acid gas; but there was also one experiment in which he endeavoured and supposed that he had succeeded in extracting water from it, by means of muriate of lime, as sensible heat was evolved as soon as the muriate of lime was brought into contact with the gas. But he has since found that the evolution of heat occurs only when the muriate of lime has attracted moisture either from the atmosphere or from the mercury through which it is passed; for then it condenses a portion of the acid gas.

In his present experiments on electrization, Dr. Henry conﬁrms his former results with regard to the evolution of hydrogen by that means; and he observes, that when the electrization of muriatic acid gas is performed over mercury, the hydrogen evolved amounts to about one ﬁfteenth of the original quantity of gas employed. There appears, however, to be a contraction of volume, in consequence of the absorption of a part of the acid to form calomel. When the hydrogen amounts