On Taracyanogen. 437 



less intense ; and, consequently, the horizontal reflection is generally 

 stronger than that in any other plane. But further than this, many 

 familiar facts shew that the horizontal vapours (or opaque particles, 

 of whatever kind they be, which occur in air), are, like a -sheet of 

 paper, capable of receiving light and of emitting it, not necessarily in 

 the plane of reflection ; and such light, after several reflections nearly 

 parallel to the horizon and completely encircling it, — as we often see 

 it do when a slight whitish haze is visible all round, — reaches the 

 eye, much enfeebled, no doubt, by numerous reflections, but more in- 

 tensely polarized on that account, and reinforced by the number of 

 the reflecting particles. The lights thus irregularly reflected or 

 emitted by the horizontal strata of air, and again regularly reflected 

 by other particles in the same strata, will come to the eye more or 

 less polarized in planes parallel to the horizon. A similar action v. ill 

 produce M. Babinet's second neutral point towards sunset ; and Mr 

 Forbes has remarked generally, after sunset, that the planes of pola- 

 rization no longer converge accurately to the luminary, but are more 

 or less twisted into a forced parallelism to the horizon. 



February 15. — Dr Abercromby, V.P. in the Chair. The 

 following communications were read :— 



1. Farther Researches on the Voltaic Decomposition of Aque- 



ous and Alcoholic Solutions. By Professor Connell. 



2. On the Preparation of Paracyanogen, and the Isomerism 



of Cyanogen and Paracyanogcn. By Samuel Brown, M.D. 

 Communicated by Dr Christison. 



After a short statement of the discovery of paracyanogen by Pro- 

 fessor Johnston, and of its leading properties, the author proceeded 

 to shew how, with certain precautions, cyanogen may be converted 

 entirely, or nearly so, into the isomeric form paracyanogcn. This 

 lie accomplished by exposing bicyanide of mercury suddenly to the 

 temperature most favourable to the production of paracyanogen, which 

 he found to be a low red heat, and employing also pressure, by con- 

 fining the cyanogen gas which is at first expelled. By these means 

 he succeeded in resolving the salt almost entirely into mercury and 

 paracyanogen, the latter of which amounted in some trials to more 

 than nine-tenths ofthecyfoogen contained in the bicyanide. 'I he pres- 

 sure required was not quite two atmospheres, namely 1.74. 



The author further stated, that he had succeeded in proving that 



