328 



moderate strength, an ordinary haloid salt when dissolved, decomposes 

 the water of the alcohol. 



The conducting power of solutions is, generally speaking, favoured 

 by the chemical changes which take place in them under galvanic 

 action. Salts act by being resolved into their constituent acid and 

 alkali. Acids and alkalies, in their separate state, act by being trans- 

 ferred to their respective poles in their aqueous solutions. Bromine 

 and iodine are not so transferred ; and when they promote the con- 

 ducting power of their solutions, they act by uniting with the hydro- 

 gen of decomposed water. 



2. On the Prepai'ation of Paracyanogen, and the Isomerism 

 of Cyanogen and Paracyanogen. By Samuel Brovpn, M.D. 

 Communicated by Dr Christison. 



After a short statement of the discovery of paracyanogen by Pi-o- 

 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 paracyanogen. This 

 he 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 of the cyanogen contained in the bicyanide. The pres- 

 sure required was not quite two atmospheres, namely 1.74. 



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

 paracyanogen once formed cannot be again converted into cyanogen. 

 Professor Johnston supposed the contrary, because he obtained cyano- 

 gen gas by exposing paracyanogen to a strong heat. But the author 

 found that this arose from the latter having retained some cyanogen 

 by absorption, that after the absorbed gas is removed heat subse- 

 quently expels nitrogen only, and that the same result is obtained 

 from the first by using pure paracyanogen, prepared by dissolving the 

 impure substance in concentrated sulphuric acid, and separating it in 

 a state of purity by leaving the acid exposed to the air so as to attract 

 humidity. 



To these facts the author added some views as to the composition 

 of cyanogen and paracyanogen, and their relation to one another. In 

 order to account for the exceeding difference in properties prevailing 

 between these two bodies, which appear nevertheless to consist of 



