1880.] Prof. J. Dewar. Studies on the Electric Arc. 85 



III. " Studies on the Electric Arc." By Professor James Dewar, 

 M.A., F.R.S., Fullerian Professor of Chemistry at the Royal 

 Institution. Received January 8, 1880. 



In a former communication on the formation of hydrocyanic 

 acid in the electric arc, a more complete examination of the various 

 reactions taking place in the arc with poles of various kinds, and in 

 presence of different gaseous media, was promised. 



Various difficulties have impeded the satisfactory progress of the 

 investigation. During the course, however, of numerous experi- 

 ments, facts of interest have been recorded, which appear worthy of 

 embodiment in a short paper. 



Formation of Cyanogen Compounds. 



The influence of impurities in the carbon on the production of 

 hydrocyanic acid was first examined. For this purpose, some 

 drilled Siemens' carbons were placed in a porcelain tube, and treated 

 for several days at a white heat with a rapid stream of chlorine, until 

 the greater part of the silica, oxide of iron, alumina, &c, were 

 volatilised in the form of chlorides. Sometimes the carbons had a 

 subsequent treatment with hydrogen, or were directly treated with a 

 current of chlorine while the arc was in operation. 



Carbons treated in this way continued to yield hydrocyanic acid, 

 when a steady current of air was drawn through the positive pole as 

 formerly described, even when the same pole had several successive 

 treatments with chlorine during the discharge. Natural graphite poles 

 gave the same result. 



As it was evident from the foregoing experiments, that the elimina- 

 tion of a large portion of the impurities had little influence on the pro- 

 duction of the hydrocyanic acid, the only other explanation of its 

 formation appeared to be the presence of aqueous vapour, and organic 

 impurities in the air, or a direct formation of cyanogen from carbon 

 and nitrogen. An attempt was made to obtain a pure and dry atmo- 

 sphere in which such experiments could be carried out. The following 

 apparatus was devised for the purpose. 



A tin vessel, fig. 1, about two feet high and oue foot in diameter, 

 had an annular space, through which a constant stream of water was 

 kept flowing. This cylinder was placed upon a porcelain stand, having 

 a narrow groove filled with mercury, so as to make an air-tight joint. 

 The lamp was placed inside this vessel, the wires connecting it with 

 the machine being brought through the bottom of the stand. A tube 

 passed through the porcelain base, which allowed a current of dry air 

 to be forced through the vessel. A small aperture in the top of the tin 



