682 Professor Dewar [June 10, 



photographs of the spark in tetrachloride simultaneously with a 

 cyanogen flame, the latter being thrown in by reflection in the usual 

 way. 



Not one of many photographs so taken showed any traces of the 

 cyanogen bands. The general character of the violet part of the 

 spectrum of the spark in carbon tetrachloride taken without a con- 

 denser (but not the exact position to scale of wave-lengths of all the 

 lines) is shown at B in Fig. 2. At C of the same diagram are 

 shown the brightest of the additional lines which come out with the 

 use of a condenser. Photogra|)hs of sparks taken in hydrochloric 

 acid showed a precisely similar group of ultra-violet lines, so that the 

 three lines which our photographs show amongst the five ultra-violet 

 nitrocarbon bands are clue to chlorine. 



Having satisfied ourselves by repeated trials that pure carbon 

 tetrachloride or trichloride, if free from nitrogen, does not give any of 

 the bands we ascribe to nitrocarbon compounds, our next step was to 

 determine whether the addition of nitrogen would bring them out, and 

 if so, what quantity of nitrogen would make them visible. 



For this purpose we introduced a minute fragment of bichromate 

 of ammonia, carefully weighed, and wrapped in j)latinum foil, into the 

 neck of one of the sparking tubes containing carbon tetrachloride, 

 connected the tube to the Sprengel pump, and removed the air as before. 

 The spark examined in the tube showed no trace of any nitrocarbon 

 band. A pinch-cock was now put on the rubber tube, and the bichro- 

 mate heated by a spirit-lamp to decomposition (whereby it is resolved 

 into nitrogen, water, and oxide of chromium). On now passing the 

 spark the six violet bands were well seen. There was no change in the 

 condition of the coil or rheotome, so that the spark was of the same 

 character as it had been before when no nitrocarbon bands were 

 visible, and the change in the spectrum cannot be attributed to any 

 change in the spark. The weight of the bichromate was between 

 •0005 and -0006 grm. ; and the nitrogen this would evolve would fill 

 just about ^Q of a cubic centimetre at atmospheric pressure. The tube 

 held 30 cub. centims., so that vapour of carbon tetrachloride when 

 mixed with -g-i^ part of its volume of nitrogen, gives under the action 

 of the electric spark the nitrocarbon bands distinctly. Other similar 

 experiments confirmed this result. It is worthy of remark that the 

 nitrocarbon bands were not seen instantaneously on the admission of 

 nitrogen into the tube, but were gradually developed, as if it was 

 necessary that a certain quantity of nitrocarbon compound should be 

 formed under the influence of tlie electric discharge and accumulated 

 before its spectrum became visible. 



A tube, containing naphthaline, previously well washed with dilute 

 sulphuric acid, dried and resublimed, was attached to the Sprengel 

 pump, and treated as the tubes with tetrachloride had been. The 

 spark in this tube likewise showed no nitrocarbon bands. After a 

 tmie the tube cracked, and then the nitrocarbon bands made their 

 appearance, and on setting the pump going a good deal of gas was 



