500 



Prof. A. Smithells on the 



even with a leyden-jar in circuit. This observation was 

 contradicted by Liveing and Dewar so far as the more re- 

 frangible groups were concerned, and these observers, as also 

 Kayser and Runge, Tietz and Hartley, assign the groups in 

 question to carbon in association with nitrogen. Wesendonck 

 again criticises Lockyer's precautions for the exclusion of 

 moisture as inadequate. 



According to Liveing and Dewar. though the Swan spectrum 

 was seen in carbon tetrachloride when the tube was not much 

 exhausted, at high exhaustions it gave a continuous spectrum. 

 With the actual spark between close electrodes in the satu- 

 rated vapour freed from moisture and air the candle spectrum 

 was always " more or less plainly seen " (elsewhere they say 

 " brightly"). 



Wesendonck, on the other hand, under the circumstances 

 last described, observes scarcely any sign of the Swan spec- 

 trum. At low pressure he observed both the oxycarbon and 

 the Swan spectrum ; at higher pressures he gets chlorine 

 lines and the bright green " flame line/' 



I have spent a great deal of time in examining the spectrum 

 of carbon tetrachloride, and have to thank Mr. Brereton 

 Baker for supplying me with a quantity of this substance 

 which had been treated by him for the removal of oxygen. 

 I regret that I cannot record any very positive results. The 

 vapour is so quickly decomposed by the electric discharge 

 that the maintenance of a steady pressure is impossible. The 

 spectrum is preeminently that of chlorine and carbon (line- 

 spectrum), and I have failed frequently to obtain a sign of 

 the Swan spectrum. At other times the' Swan spectrum 

 would appear, and occasionally also the oxycarbon spectrum, 

 showing that there was oxygen contamination. It is not 

 improbable that the liberated chlorine attacks the glass. 



Dr. H. A. Wilson, a former student of the Yorkshire 

 College, who has rendered me material assistance in this 

 work, made a number of observations on the spectrum of 

 chloroform vapour. The results which he obtained seemed 

 to show unmistakably that a tube containing this substance 

 did not yield the Swan spectrum when adequate precautions 

 had been taken to remove oxygen. The presence of a little 

 air or oxygen led to the development of the spectrum. 



I have not abandoned hope of obtaining more unequivocal 

 results from the examination of carbon tetrachloride. For 

 the present I cannot claim more than that the observations 

 hitherto recorded by spectroscopists with this substance are 

 incapable of furnishing a decision in one sense or the other 

 as to the origin of the Swan spectrum. 



