420 



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 lu- 

 minous 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 occu- 

 pies 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 par- 

 ticles are sufficiently minute to receive a slow motion from the im- 

 pulse 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 nebulae, 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 va- 

 rious centres they undergo progressive condensation ; from which we 

 may conceive how other planetary bodies may begin to have exist- 



On a gaseous Compound of carbonic Oxide and Chlorine. By John 

 Davy, Esq. Communicated by Sir 

 Sec. R.S. Read February 6, 1812. 



Davy, Esq. Communicated by Sir Humphry Davy, Knt. LL.D. 

 . [Phil. Trans. 1812, p. 144.] 



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 con- 

 densed into half their former volume. The smell of this gas was more 

 suffocating than that of chlorine. It occasioned a very painful sen- 

 sation in the eyes ; it reddened litmus paper ; it combined with am- 

 monia, 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 



