Dec. 30, 1880] 



NATURE 



197 



and make them purer and whiter, by preventing the 

 smoke which at present mixes with our town fogs and 

 aggravates their character, and prevents them dissolving 

 when they enter our rooms. Smoke descends during a 

 fog, because the smoke particles are good radiators, and 

 soon get cooled and form nuclei on which the water 

 vapour condenses. The smoke thus becomes heavier 

 and falls. This explains why falling smoke is often a 

 sign of coming rain. It indicates a saturated condition 

 of the atmosphere. 



Sulphur when burned has been shown to be an intensely 

 actiA-e fog-producer. Calculation shows that there are 

 more than 200 tons of sulphur burned with the coal 

 every winter day in London, a quantity so enormous as 

 quite to account for the density of the London fogs. It 

 is suggested that some restriction ought to be put on the 

 amount of sulphur in the coal used in towns. 



Before utterly condemning the smoke and the sulphur, 

 it was pointed out that it would be necessary thoroughly 

 to investigate and fully to consider the value of smoke as 

 a deodoriser, and also the powerful antiseptic properties 

 of the sulphurous acid formed by the burning sulphur. 

 The air during fogs is still and stagnant. There is no 

 current to clear away the foul smells and deadly germs 

 that float in the air, which might be more deadly than 

 they are, were it not for the suspended soot and burned 

 sulphur. We must therefore be on our guard lest we substi- 

 tute a great and hidden danger for an evident but less evil. 



ON THE SPECTRUM OF CARBON 

 A LTHOUGH fifteen years have passed since the 

 -^*- possibility of one substance possessing more than 

 one spectrum was first suggested by Pliicker and Hittorf, 

 the- question of the existence of double spectra cannot 

 yet be considered as decided. One of the elements to 

 which multiple spectra have been attributed is carbon, 

 which was at one time supposed to possess four different 

 spectra : of these one has been shown to be due to oxide 

 of manganese, a second to oxides of carbon, the origin of 

 a third (obtained only from oxides of carbon) has hardly 

 been discussed (though it may prove to be one of the true 

 carbon spectra), and the other "carbon'' spectrum- -the 

 best known of all — is the one first attributed to carbon by 

 Attfield, but ascribed to acetylene by Angstrom. 



In a paper read before the Royal Society, and of which 

 an abstract is given in NATURE, vol. xxii. p. 620, Pro- 

 fessors Liveing and Dewar describe experiments to prove 

 that this spectrum is that of a hydrocarbon, and not of 

 carbon itself; and also that certain blue bands, best seen 

 in the flame-spectrum of cyanogen, are due to compounds 

 of carbon and nitrogen, and not to carbon itself They 

 attribute to hydrocarbon (amongst others) the yellowish- 

 green group, which we w-ill call 7, of wave-lengths from 

 about 5635 to 547S, and the emerald-green group, which 

 we will call 5, of wave-lengths from about 5165 to 50S2; and 

 they attribute to nitro-carbon the two blue groups of wave- 

 lengths 4600 to 4502 and 4220 to 4158, which we w-ill call 

 6 and f respectively. 



As these result are directly opposed to my own experi- 

 ence, I have thought it necessary to repeat two of the 

 experiments described in my paper on the carbon spectra 

 in the P/u'losophical Magazine for October, i86g, under 

 such conditions as to exclude (as far as lay in my power) 

 all trace of hydrogen in the one case, and of nitrogen in 

 the other. 



The difficulty of supposing carbon to be present in the 

 state of vapour at any temperature which we can com- 

 mand seems to be the chief reason why so many investi- 

 gators think it necessary to attribute the spectrum in 

 question (with experimental evidence or without it) to 

 compounds of carbon. I am not aware that Angstrom 

 ever gave any experimental proof of his assertion that this 

 spectrum was caused by acetylene. 



On the other hand, the evidence "that the spectrum is 

 due to carbon is that first stated by Attfield, that if these 

 lines "arc absent inflames in which carbon is absent, 

 and present in flames in which carbon is present," 

 if they are " observable equally in the flame of the oxide, 

 sulphide, and nitride as well as in the hydride of carbon," 

 and if " present whether the incandescence be produced 

 by the chemical force, as in burning jets of the gases in 

 the open air or by the electric force, as when hermetically- 

 sealed tubes of the gases are exposed to the discharge ot 

 a powerful induction-coil," then they "must be due to 

 incandescent carbon vapour" ; and if this is borne out by 

 experiment the conclusion that the lines are due to carbon 

 (as gas, liquid or solid) cannot be resisted, whatever may 

 be the apparent impossibility of volatilising or even liqui- 

 fying carbon, even by the most powerful current of 

 electricity directed through it. 



Wc must bear in mind how very small a quantity of 

 a gas is often sufficient to give us a spectrum, and when 

 the carbon spectrum is obtained by the decomposition of 

 defiant gas or cyanogen by passing sparks through the 

 gas, the carbon cei'tainly exists as gas in the compound 

 which is decomposed, and before the liberated atoms 

 unite together to form the molecules of the solid, there is 

 surely no impossibility in their existing for the moment 

 as gas — as gaseous carbon. 



On an examination of Professors Liveing and Dewar's 

 paper to ascertain the experimental evidence upon which 

 the bands y and S are attributed to hydrocarbon and not 

 to carbon itself, we find it stated that " the green and 

 blue bands characteristic of the hydrocarbon flame seem 

 to be always present in the arcs, whatever the atmo- 

 sphere. This is what we should expect if they be due, as 

 Angstrom and Thalen suppose, to acetylene, for the car- 

 bon electrodes always contain, even when they have been 

 long heated in chlorine, a notable quantity of hydrogen." 



Since then it is impossible to completely expel hydro- 

 gen from the carbon-poles, we must reject all the experi- 

 ments in which the electric arc was observed in atmo- 

 spheres of dift'erent gases, although " the green and blue 

 hydrocarbon bands were seen more or less in all of 

 them." 



Turning then to other methods of producing the spec- 

 trum, we find it stated that in the flame of carefully- 

 dried cyanogen " the hydrocarbon bands were almost 

 entirely absent " (they should have been entirely absent) ; 

 "only the brightest green band was seen, and that 

 faintly." Hence we are to infer, I suppose, that the 

 bands 7 and S, so brilliant in the flame of cyanogen in 

 air or oxygen, are due to the accidental presence of 

 hydrogen (see the extract from IMorren's paper. Nature, 

 vol. xxii. p. 7. Dibbits also speaks of this spectrum as 

 "by far the most magnificent " he has seen). 



Next we have the experiment of burning hydrocyanic 

 acid, in which, as we have hydrogen present, w-e expect 

 to find the hydrocarbon bands brilliantly developed. But 

 we find the result stated as " very much the same as that 

 of cyanogen." The flames of hydrogen and sulphide of 

 carbon, and of hydrogen and carbonic oxide, do not give 

 the hvdrocarbon bands (their spectra being continuous) ; 

 a mixture of hydrogen and carbon tetrachloride gives 

 them faintly, and a mixture of hydrogen and chloroform 

 gives them strongly. 



In all this we have no proof of the point in question, 

 nor even any special probability that the bands are 

 due to hydrocarbon ; and yet, in the face of experiments 

 in which the spectrum is obtained from cyanogen, when 

 care has been taken to exclude hydrogen, we are asked 

 to attribute the bands to the hydrocarbon formed by 

 combination with some trace of hydrogen (as water or 

 otherwise), supposed to be present as impurity. In the 

 same way the presence of the bands 6 and f obtained 

 under circumstances when nitrogen has been intentionally 

 excluded, is to be explained by " the extreme difficulty of 



