402 



NA TURE 



[Marc/i 8, 1877 



is, all lines belonging to the elements entering into the carbon 

 compound, 



Roimd the electrodes of carbon we oliserve during the disrup- 

 tive discharge an aureole, which indicates a continuous discharge. 

 The spectrum of the aureole depends on the nature of the medium 

 in which the discharge passes ; in nitrogen we find, the blue and 

 violet groups which characterise cyanogen ; in hydrogen it is the 

 spectrum of the hydrocarbons i which we observe ; in oxygen we 

 get the spectrum which a Geissler tube, filled, with carbonic oxide, 

 shows. 



The shaded bands of cyanogen which are situated in the blue 

 and violet part of the spectrum, are also seen if the spark passes 

 the luminous part of a gas flame, or in the voltaic arc between 

 the carbon electrodes of a powerful battery. In the spectrum 

 of the voltaic arc, however, the brilliant lines of cyanogen are 

 mixed with those of hydrocarbons, the splendour of which is 

 still more magnificent. 



After these observations we may consider it to be a demon- 

 strated fact that the aureole gives respectively the spectra of cyano- 

 gen, hydrocarbon, oxide of carbon, or carbonic acid, according 

 as the gas which surrounds the electrodes consists of nitrogen, 

 hydrogen, or oxygen. The most natural supposition is, there- 

 fore, that the spectra belong really to the compound bodies, 

 which is the more probable as the general appearance of these 

 spectra suggests at once an origin of compound bodies rather 

 than of elementary bodies. 



It is well known that carbonic acid is decomposed by the 

 electric current, and that the spectrum which is observed belongs 

 exclusively to carbonic oxide, which is formed. One might 

 therefore imagine that carbonic acid would not have any spec- 

 trum of its own. If, however, carbonic acid is formed, as, for 

 instance, while cyanogen burns, it appears probable that lines 

 belonging to carbonic acid can appear, and this opinion has been 

 confirmed by an observation of Pliicker. He has found that the 

 shaded red bands of cyanogen burning in air or in oxygen be- 

 come stronger and wider as the combustion becomes stronger. 

 An experiment made by us with a spark passing in cyanogen 

 gas, circulating in a glass tube and freed therefrom by degrees of 

 every trace of oxygen, has taught us that these red bands only 

 extended to the first band of hydrocarbon, and even vanished 

 during some instants of the experiment. The probable cause of 

 the appearance of the spectrum of hydrocarbon in this case must 

 be looked for in the impossibility of drying the gas completely, 

 if it is prepared with cyanide of mercury. 



It seems to us that it is much more difficult to explain the 

 appearance of the spectrum of hydrocarbons in the combustion 

 of any compound of carbon and hydrogen, and also, according 

 to Mr, Attfield, in the flame of carbon disulphide. Though this 

 spectrum was considered by some observers to be due to carbon, 

 we cannot accept this view, and for this reason : If we employ a 

 condensator the spectrum of coal gas shows not only the spectrum 

 in question, but also the lines of carbon and hydrogen. The 

 appearance of the shaded bands, being similar to those of cyano- 

 gen, shows at once, as we have repeatedly said, that the body is 

 compound. 



The difficulty, it seems to us, must in great part disappear if we 

 could show that the same chemical compound is always formed 

 in the combustion of any hydrocarbon. M, Berthelot has shown 

 this to be true. According to him acetylene is formed whenever 

 an incomplete combustion of any hydrocarbon, ether, &c. , takes 

 place, and even if the electric spark passes between carbon elec- 

 trodes in hydrogen gas. It is therefore very probable that the 

 spectrum which is formed for all carbon compounds is due to 

 acetylene. 



As far as the observation of Mr. Attfield is concerned, that 

 oxide of carbon gives the ordinary spectrum of the hydro- 

 carbons, we must observe that this does not agree with our own 

 experiments. In a Geissler's tube, containing carbonic acid or 

 carbonic oxide, one can certainly find traces of the spectrum of 

 hydrocarbon, as the gas is never altogether dry, but according to 

 Pliicker's observation, the particular spectrum of oxide of carbon 

 has no resemblance to it. 



To the left of the Fraunhofer line G one sees generally a very 

 strong line which really belongs to carbon. We find here the 

 same thing which we have mentioned speaking of the spectra of 

 the metallic oxides, that often the spectrum of the oxide is mixed 

 with some of the lines of the elementary body. 



As a second example Messrs, Angstrom and Thalen take the 



I Messrs Angstrom and Thalen call Swan's spectrum of the candle the 

 spectrum of hydrocarbons. 



spectrum of nitrogen. The so-called line spectrum of nitrogen 

 was first observed by Angstrom, while v. d. Willigen observed a 

 difterent spectrum seen in the aureola when the discharge takes 

 place in air. The two spectra which Pliicker observed were 

 therefore not new, though he gave a new way of obtaining them. 

 The following is a translation of the author's remarks on the 

 origin of the two spectra : — 



As to the interpretation of these two spectra we think that 

 they only depend on the way in which the electric discharge 

 takes place, and belong to two diff'erent bodies. The spectrum 

 of lines caused by the disruptive discharge must be attributed to 

 nitrogen as it appears in Geissler's tubes under the same circum- 

 stances, which accompany the disruptive discharge, but the 

 shaded bands belong doubtless to some combination of nitrogen 

 formed by the discharge of quantity or by conduction. 



In the aureole at the positive pole we find a great number of 

 shaded bands in which we distinguish two different series, one 

 situated in the least refrangible part of the spectrum, and 

 another in the green, blue, and violet parts. The appearance of 

 these two series is different and gives rise to the suspicion that 

 they belong to two different bodies. Whether this be true or not 

 it is certain that their intensity varies much according to circum- 

 stances and in different ways. 



At the negative pole we observe a bluish violet sheet the spec- 

 trum of which, situated in the green, blue, and violet, does not 

 change with the nature of the electrodes. 



In Geissler's tubes, containing rarefied nitrogen, we find for 

 the continuous discharge the same spectra as in the aureoles in 

 the atmosphere. But the positive light, which is very intense, 

 is not only seen near the pole but also in the capillary parts of 

 the tube. At the negative pole the bluish violet sheet gets 

 larger and more brilliant as the exhaustion proceeds. 



We now ask which combinations of nitrogen can cause the 

 spectra of the continuous discharge ? As -far as the negative light 

 is concerned we are in complete ignorance on the subject. As 

 to the gas which is found at the positive pole one can prove by 

 means of a solution of sulphate of iron that nitrogen dioxide is 

 formed. It is well known that the electric spark passing through 

 air produces the red fumes which indicate the existence of nitrous 

 acid. It follows that nitrogen combines under these circumstances 

 with oxygen. The only question therefore is, where does the oxygen 

 come from in a tube filled with nitrogen ? We must remember 

 that in making nitrogen we can never entirely get rid of air, or 

 at least there will always be a trace of aqueous vapour present, as 

 is shown by the hydrogen line C. This fact sufficiently explains 

 the possibility of the presence of a compound of nitrogen and 

 oxygen. As the luminous spectrum bears no resemblance to the 

 absorption spectrum of nitrous acid fumes, we conclude that the 

 dioxide of nitrogen causes the shaded bands at the positive pole 

 or in the aureole, and in the capillary part of Geissler's tube 

 containing nitrogen. Several experiments are mentioned which 

 have been made by the authors. Those on the spectrum of 

 carbon run as follows : — 



1. Spark between carbon electrodes in oxygen with condensator. 

 The lines of oxygen and carbon are seen. 



2. Spark between platinum electrodes, 35 mm., apart in a 

 current of carbonic acid. Two jars used as condenser. The 

 revolving mirror showed that the spark was instantaneous. The 

 lines of platinum, carbon, and oxygen, were seen, 



3. Same as 2, Distance of electrodes 5 mm. The attreole 

 gave the spectrum of carbonic acid, 



4. Spark between aluminium electrodes 10 mm. apart, in a 

 current of coal gas. The lines of hydrogen, carbon, and the 

 bands of carburetted hydrogen are seen. 



5. Spark without condenser in a current of cyanogen. The 

 lines of hydrogen, nitrogen were seen, besides the binds of 

 carburetted hydrogen, and some bands of cyanogen. 



The experiments on the spectrum of nitrogen have been made 

 with atmospheric air. A solution of sulphate iron was used to 

 show the presence of dioxide of nitrogen. The appearance of 

 Geissler's tubes at various pressures are given. 



Exact measurements of all the spectra discussed in the paper 

 are given, not only for the more intense lines or bands, but exact 

 micrometer measurements of some of the band. The names of 

 the authors are a sufficient guarantee of the accuracy of these 

 measurements. Excellent plates with drawings of the spectra 

 are added, A copy of the measurements will be found in the 

 first number of the Beibldtter zii Poggendo7-ff' s Annakn. 



Arthur Schuster 



