431 



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 tunes, 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 affected by a perihe- 

 lion passage than a planet would have been. 



Additional Experiments on the Muriatic and Oxymuriatic Acids. By 

 William Henry, M.D. F.R.S. V.P. of the Lit. and Phil. Society, 

 and Physician to the Infirmary, at Manchester. Read March 19, 

 1812. [Phil. Trans. 1812,;?. 238.] 



In consequence of the discussion which has lately taken place con- 

 cerning 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 electrization of muriatic acid gas ; but there was also one experi- 

 ment 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 absorp- 

 tion of a part of the acid to form calomel. When the hydrogen amounts 



