422 



Profs. G. D. Liveing and J. Dewar. [Dec. 21, 



the substance at the relatively low temperatures of the arc and flame. 

 The test experiments made on this hypothesis are recorded in the 

 paper entitled " General Observations on the Spectrum of Carbon and 

 its Compounds." It is there shown that some seven of the marked 

 ultra-violet spark lines of carbon occur in the spectrum of the arc 

 discharge, although one of the strongest lines, situated in the visible 

 portion of the spectrum at wave-length 4266, could not be found. 

 Further, it is proved that the strongest ultra-violet line of carbon does 

 occur in the spectrum of the flame of cyanogen fed with oxygen. 

 Thus it seems probable that the same kind of carbon molecule exists, 

 at least in part, in the arc and flame, as is found to be produced by the 

 most powerful electric sparks, taken between carbon poles or in carbon 

 compounds. 



Now the spark gives us the spectrum which is associated with the 

 highest temperatures, and therefore it is assumed that this spectrum 

 is that of the simplest kind of carbon vapour. If that be the case, we 

 cannot avoid inferring that denser forms of carbon vapour may exist 

 in arc and flame, emitting, like other complex bodies, a fluted, in 

 contrast to a line, spectrum ; or rather that the two distinct kinds of 

 spectra may be superposed. Such considerations showed that a series 

 of new experiments and observations must be made with the special 

 object of reaching a definite conclusion regarding the origin of the 

 flame spectrum, and the following paper contains a summary of the 

 results of such an inquiry. 



Vacuous Tubes. 



We have heretofore laid little stress on observations of the spark in 

 vacuous tubes on account of the great uncertainty as to the residual 

 gases which may be left in them. The film of air and moisture 

 adherent to the glass, the gases occluded in the electrodes, and minute 

 quantities of hydrocarbons of high boiling-point introduced in sealing 

 the glass, may easily form a sensible percentage of the residue in the 

 exhausted tube, however pure the gas with which it was originally 

 filled. The excessive difficulty of removing the last traces of moisture 

 we learnt when making observations on the water spectrum, and the 

 almost invariable presence of hydrogen in vacuous tubes is doubtless 

 due in great measure to this cause. Wesendonck (" Proc. Roy. Soc," 

 vol. 32, p. 380) has fully confirmed our observations as to this diffi- 

 culty. By a method similar to that employed by him, we have, how- 

 ever, succeeded in so far drying tubes and the gases introduced into 

 them that the hydrogen lines are not visible in the electric discharge. 

 For this purpose the (Pliicker) tube was sealed on one side to a tube 

 filled for some six or eight inches of its length with phosphoric anhy- 

 dride through which the gas to be observed was passed, and on the 

 other side to a similar tube full of phosphoric anhydride, which was in 



