380 



Mr. C. Wesendonck. 



[June 16, 



IV. " Note on the Spectrum of Carbonic Acid." By Charles 

 Wesendonck (Berlin). Communicated by Professor Helm- 

 holtz, For. Mem. R.S. Received May 16, 1881. 



The well-known spectrnm first observed by Swan at the inner 

 cone of a number of flames produced by burning liquid and gaseous 

 combinations of carbon, has been ascribed by Messrs. Attfield* and 

 Wattsf to the element carbon itself ; while several other natural philo- 

 sophers, amongst whom I only mention Messrs. Thalen and Angstrom, | 

 P. Smyth, Liveing§ and Dewar, || believe it to be due to some com- 

 bination of carbon with hydrogen. The fact that the bands and 

 lines of Swan's spectrum are seen even when oxide or binoxide of 

 carbon is made luminous in a Geissler tube by means of an electric 

 current, the last-named gentlemen explain as being caused by the 

 gases not having been entirely deprived of all moisture when intro- 

 duced into the vacuum tube. I therefore believed it to be not 

 altogether without interest to make some experiments on carbonic 

 acid dried with the utmost care. For this purpose I used a mercurial 

 pump, as invented by Professor Toepler, with some slight changes in 

 the original construction indicated by Dr. Hagen,^[ which enables us 

 not only to obtain the most perfect vacua, but also to avoid every 

 cock or junction by slided pieces, which must be greased in order to 

 be hermetic, so that no vapours and gases containing carbonic matters 

 arising from the grease, either by increase of temperature or by the 

 electric current when passing the vacuum tube, would render impure 

 the carbonic acid, as is the case with the common Greissler pumps. 

 Carbonic acid was produced in the well-known manner from marble 

 and diluted muriatic acid in a Kippian apparatus, well washed with 

 water in a large Woolf's bottle, and then sent through concentrated 

 sulphuric acid, three tubes of \J form containing chloride of calcium, 

 and a double tube half filled with anhydride of phosphoric acid. To 

 this last tube were fastened, by means of sealing, two Geissler glass 

 cocks, being on the other side fastened by sealing to a valve of sul- 

 phuric acid, that prevented all vapours coming from the cocks from 

 entering the vacuum tube. After all air had been driven and pumped 

 out of the whole apparatus, the spectrum tube was evacuated to the 

 highest possible degree, in order to free it from all moisture adherent 

 to the sides of the glass. This, however, was found to cause some 



* "Phil. Trans.," vol. 152, 1862. 

 t " Phil. Mag.," Ser. 4, vol. 38, 1869. 

 X " Nova Acta. Upsal.," Ser. 3, vol. 9. 

 § "Phil. Mag.," (5), 8, 1879. 

 || " Proc. Roy. Soc," vol. 30, 1880. 

 % "Wiedemann's Annalen," JSeue Folge, Bd. 12, 1881. 



