622 



NATURE 



\Oct. 28, 1 88c' 



sparking tubes containing carbon tetrachloride, the tube con- 

 nected to tbe Sprengel pump, and the air removed as before. On 

 examination of the spark with the spectroscope no trace of any 

 cyanogen band could be detected. A pinch-cock was now put 

 on the rubber tube, and the bichromate was 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 cyanogen bands 

 w'ere visible, and the change in the spectrum cannot be attributed 

 to any change in the spark. Tlie weight of the bichromate was 

 between '0005 and 'oooS grm., and the nitrogen this would 

 evolve would fill just about ^ of a cubic centimetre at atmo- 

 spheric pressure. The tube held 30 cub. centims., so that vapour 

 of carbon tetrachloride wlien mixed with -^^ part of its volume 

 of nitrogen, gives under the action of the electric spark the 

 cyanogen bands distinctly. Other similar experiments confirmed 

 this result. 



Similar experiments with carbon bisulphide, benzol, and well- 

 purified naphthaline, gave like results when care was taken to 

 remove air completely. 



As Watts laid much stress on the occurrence of the^cyanogen 

 bands in the spectrum of the spark taken in carbonic oxide at 

 atmospheric pressure (though they do not appear in carbonic 

 oxide at reduced pressures), as a proof that these bands were 

 due to carbon only, the authors made a series of careful experi- 

 ments with carbonic oxide at atmospheric pressures. 



In the first experiments with this substance the gas was made 

 by the action of sulphuric acid on dried formiate of sodium, and 

 it was found that the cyanogen bands disappeared as air was 

 expelled from the apparatus, reappearing brightly when air, 

 not exceeding ^3 of the whole gas in the apparatus, was 

 admitted. 



Carbonic oxide was next generated by. heating, in a tube of 

 hard glass in an ordinary combustion furnace, a mixture of pure 

 and dry potassium oxalate with one quarter of its weight of 

 quicklime, the mixture having been previously heated for some 

 time so as to expel traces of ammonia. The tube was connected 

 with a Sprengel pump, and the air exhausted before heating the 

 oxalate. The distant end of the tube v\ ith the oxalate was then 

 heated, and the whole apparatus filled with carbonic oxide ; it 

 was then again exhausted with the pump, refilled by heating 

 more oxalate, and the gas allowed to stream out through the 

 pump for some time. 1'he heat was then low'cred, sparks were 

 passed, aud the spectrum observed. No trace whatever of the 

 cyanogen bands could be detected, how-ever the spark might be 

 varied. The pump was now set going again, and the pressure 

 of the gas reduced to one inch of mercury, while the spectrum 

 was observed from time to time. Still no trace of the cyanogen 

 bands could be detected. More of the oxalate was next heated, 

 and the observations repeated again and again, always with the 

 same result. The conclusion was that carbonic oxide, if quite 

 free from nitrogen, does not give, at the atmospheric or any less 

 pressure, the cyanogen bands. 



From Dr. Watts's account of his experiments, it appeared 

 that he had used carbonic oxide prepared by the action of 

 sulphuric acid on ferrocyanide, and it was probable that it 

 might have been contaminated with nitrogen, or with nitrogenous 

 compounds, from the ferrocyanide. The authors accordingly 

 repeated their experiments with carbonic oxide so prepared, 

 and found that the cyanogen bands were then always distinctly 

 seen. 



They have also repeated Angstrom and Thalen's experiments 

 with the spark between carbon poles in nitrogen and carlionic 

 acid gas. They observed that in nitrogen the cyanogen bands 

 were plainly visible through a great range of variations of the 

 character of the spark ; even the use of a condenser of moderate 

 size did not diminish them. Photographs were taken with and 

 without the use of the condenser, and these showed the violet 

 and idtra-violet cyanogen bands, including those near N and I'. 

 The nitrogen was then swept out by a current of carbonic acid 

 gas, and on now passing the spark the cyanogen bands could no 

 longer be detected, and photographs taken as before showed no 

 trace of any of them. 



Other experiments showed the sensitiveness of the spectro- 

 scopic tests for compounds of carbon with nitrogen, and that all 

 traces of W'ater can hardly be removed from apparatus and 

 reagents which do not admit of being Iieated red hot. 



The first point the authors had before them in these investi- 



gations is whether the groups of shaded bands seen in the mor' 

 refrangible part of the spectrum of a cyanogen flame are due to 

 the vapour of carbon uncombined, or, as they conclude, to f 

 compound of carbon with nitrogen. ' 



Now the evidence that carbon uncombined can take the stat 

 of vapour at the temperature of the electric arc is at present ver ' 

 imperfect. Carbon shows at such temperatures only incipieii' 

 fusion, if so much as that, and that carbon uncombined shouU 

 be vaporised at the far lower tem; erature of the flame o 

 cyanogen is so incredible an hypothesis that it ought not to bi 

 accepted if the phenomena admit of any other probable expW 

 nation. On the other hand it has been shown that cyanogen oj 

 hydrocyanic acid is generated in large quantity in the electrii 

 arc taken in nitrogen, and Eerthelot has shown that hydrocyanii' 

 acid is produced by the spark discharge in a mixture of acetylen* 

 and nitrogen, so that in the cases in which these bands shine ou'' 

 with the greatest brilliance, namely, the arc in nitrogen and thr 

 cyanogen flame, we know that nitrocarbon compounds arf 

 present. Further, the authors have shown that these band:^ 

 fade and disappear in proportion as nitrogen is removed fron 

 the arc. Angstrom and Thalen had previously shown the sami 

 thing with regard to the spark discharge between carbon elec 

 trodes ; and the conclusion to which they have come wouU' 

 probably have commanded universal assent if it had not been foi 

 tlie fact that these bands had been seen in circumstances when 

 nitrogen w as supposed to be absent ; but where, in reality, thtj, 

 difficulty of completely eliminating nitrogen, and the extreme 

 sensibility of the spectroscopic test, had been inadequatelj* 

 apprehended. 



To clear up the question from this point of view, the experi' 

 ments last described have been made, and they appear to the' 

 authors quite conclusive. Were the evidence less conclusive'' 

 than it is, it would still be as rash and as illogical to conclude' 

 from the appearance of the cyanogen bands in a case where' 

 nitrogen was presumed, not proved, to be absent, that they werf 

 not due to a compound of carbon with nitrogen, as it would b< 

 to deny that the well-known yellow lines were due to sodium< 

 because they had been seen in cases where sodium was supposed 

 to be absent. The argument of the authors is an induction froir 

 a very long series of ohservations which lead up to one conclusior>,i 

 and hardly admit of any other explanation. But Mr. Lockyei 

 attempts to explain the disappearance of the bands when nitro- 

 gen is absent by the statement "that the tension of the current 

 used now brings one set of flutings into prominence, and now 

 another." This is no new observation. It is well known that 

 variations in the discharge produce variations in the relative in- 

 tensities of ditferent parts of a spectrum. Certain lines of mag- 

 nesium, cadmium, zinc, and other metals, very brilliant in the 

 spark, are not seen, or are barely seen, at all ;in the arc. His 

 remark might be applied to the spectra of compounds as well as 

 to those of elements. Variation in the discharge accounts very 

 well for some of the variations of intensity in the bands if they 

 be due to a compound of carbon with nitrogen ; it will not, how- 

 ever, account for the fact that the bands, or those of them which 

 have the greatest emissive power, and are best developed by the 

 particular current used, come out on the_ addition of a minute 

 quantity of nitrogen, when there is every reason to think that no 

 variation of the current occurs. 



Much the same may be said with regard to the changes of the 

 spectrum produced by changes of temperature. We cannot infer 

 from any of these changes that the spectrum is not due to a com- 

 pound. The bands in question are singularly persistent through 

 a great range of temperatures, from the temperature of a cyano- 

 gen flame cooled by admixture with carbonic acid gas, as related 

 liy Watts {Phil. Mag , 1869, p. 258), to that of the spark of an 

 induction coil with condenser. 



But again, Mr. Lockyer attempts to get over the difficulties of 

 his case by the supposition that "the sets of carbon flutings 

 represent different molecular groupings of carbon, in addition to 

 that or those which give us the line spectrum." 



Now, until independent evidence that carbon can -exist at all 

 in the state of vapour uncombined at the temperature of a 

 cyanogen flame can be adduced, and further independent 

 evidence of the existence of different groupings in such vapour, 

 the hypothesis here enunciated is a gratuitous one, so long as 

 any other hypothesis for which independent evidence can be 

 adduced, as is true of the existence of nitrocarbon compounds in 

 the flame, arc, and spar!;, will sufficiently explain tbe facts. 



The authors have not expressed any opinion whether or no the 

 cyanogen bands are visible in the solar spectrum. The observa- 



