i6o 



NATURE 



[JLNl- 13, 1S95 



a trial, fully expecting to obtain forthwith a \-alue in harmony with 

 that already affortle<l by the ammonia method. The result, 

 however, provcti otherwise. The g.-is obtained by the copper 

 mctho<l, as I may call it, proved to Ik; one-thousandth part heavier 

 than that obtained by the ammonia method : and. on rcixitition. 

 that difference was only brought out more clearly. This was 

 aliout three years ago. Then, in order, if possible, to gel 

 further light u|K)n a discrepancy which puzzled me ver)' much, 

 and which, at that time, I regarded only with disgust and 

 impatience, I published a letter in N'ATfRE inviting criticisms 

 frimi chemists who might be interested in such questions. I 

 •ibtainetl various useful suggestions, but none going to the root of 

 the matter. .Several jX-Tsons who wrote to me privately were 

 inclined to think that the explanation was to be sought in a 

 lartial dissociation of ihe nitrogen derived from ammonia. For, 

 before going further, I ought to explain that, in the nitrogen 

 oblainetl by the ammonia method, some — about a seventh i>art — 

 is derived from the ammonia, the larger part, however, t>eing 

 derived as usual from the atmosphere. If the chemically 

 tlerived nitrogen were partly dissociated into its comix)nent 

 atoms, then the lightness of the gas so prepared wculd be 

 explained. 



The next step in the inquir)' wiis, if possible, to exaggerate 

 the discrepancy, (ine's instinct at first is to tr)- to get rid of a 

 discrepancy, but I l>elieve that experience shows such an 

 endeavour to \k a mistake. What one ought to do is to magnify 

 a small discrejancy with a view to finding out the explanation : 

 and, as it appeared in the present case that the root of the dis- 

 cre|>ancy lay in the fact that i>art of the nitrogen prepareii by the 

 ammonia metho<l was nitrogen out of ammonia, although the 

 greater ]>art remained of common origin in both cases, the 

 application of the principle suggested a trial of the weight of 

 nitrogen obtamed wholly from ammonia. This could easily 

 Ik.- done by substituting pure oxygen for atmosijheric air in 

 Ihe ammonia mclhoil, so that the whule, instead of only a part. 

 of the nitrogen collected should Ik; derived from the ammonia 

 itself. The discre])ancy was at once magnified some five limes. 

 The nitrogen so obtained from ammonia proved to be about one- 

 holf [ler cent, lighter than nitrogen obtained in the orclinarj- way 

 from the atmosphere, and which I may call for brevity " atmo- 

 spheric " nitrogen. 



That result st<i<)d out pretty sharjily from the first ; but it was 

 necessar)- to confirm it by comjiarison with nitrogen chemically 

 dcrive<l in other ways. The table before you gives a summary 

 of such results, the numliers being the weights in grams actually 

 contained under standard conditions in the globe employed. 



.Vtmosphf.kic Nitrogen. 



By hot copper (1802) ... 



By hoi iron (1S93) 



Hy ferrous hyilrate (1894) 



... 2-3103 

 .. 2-3100 

 ... 2-3102 



Mean 2-3102 



... 2-3001 

 2-2990 

 2-2987 

 2-2985 

 2 2987 



Chemical Nitrockn. 



From nitric oxide 



From nitrous oxide 



From ammonium nitrite purified at a read lie;il 



From urea 



From ammonium nitrite purifie<l in the cold 



Mean 2-2990 



The difference is alxiul II milligrams, or about one-half per 

 rent. ; ami it was sufficient to prove conclii.sively that the two 

 kiniU of nitrogen — the chemically derived nitrogen ami the 

 almiispheric nitrogen — <liffered in weight, and therefore, of 

 CMurst-. in qu.-ility, for some rca.s«>n hitherto unknnwn. 



I nccil not s|x.-nd time in explaining the various precautions 

 lliat were neri-ssarj- in order to establish surely that conclusion. 

 One hail tn lie <in one's guard against impurities, e.s|x-cially 

 again.«l the pre.sence of hydrogen, which might .scriou.sly lighten 

 any gn.» in which it was contained. I lielicve, however, that the 

 precautions taken were suflicienl to exclude all (piestions of that 

 sort, and the reMill. which I published alniut this lime l.rsi year, 

 M.nid sharply out. thai the nitrogen obtained from chemical 

 iwmrces was /lifft-rcnt from the nitrogen obtained from the air. 



Well, that differcnrc, admitling it to Ije established, was 

 sufficient lo show ihal vmie hitherto unknown ga.s is involved in 

 Ihe mailer. Il might Ih.- that the new giLs w.is dis.socialed 

 nitrogen, contained in thai which was l<>o light, the chemical 



NO. 1337, vor. 52] 



nitrc^en — and at first that was the explanation to which I 

 leaned : but certain experiments went a long way lo discourage 

 such a SHp|X)sition. In ihe first place, chemical evidence — and 

 in this matter I am greatly dependent u|>on the kindness of chem- 

 ical friends — tends to show that, even if ordinary nitrogen could 

 he disstjciaietl at all into its com|xinent atoms, such atoms 

 would not be likely to enjoy any verj- long continued existence. 

 Even ozone goes slowly lack lo the more normal slate of oxygen -. 

 and it was thought that dis-sociated nitrogen would have even a 

 greater tendency to revert to the normal condition. The ex- 

 periment suggeste<l hy ihat remark was as follows — lo keep 

 chemical nitrogen — the loo light nilrt)gen which might be sup- 

 posed lo contain dissociated molecules — for a good while, and lo 

 examine whether il changed in density. Of course it would be 

 useless to shut up gas in a globe and weigh il, and then, after an 

 interval, lo weigh il again, for there would be no t>i>porlunity 

 for any change of weight lo occur, even although the gas within 

 the globe had inidergone some chemical alteration. It is 

 necessar)' to re-establish Ihe standard conditions of temperature 

 and pressure which are always understood when we speak ol 

 filling a globe with gas. for I need hardly say that filling a globe 

 w ilh gas is but a figure of speech. Kverylhing dejicnds upon the 

 lem|X-rature and pressure at which you work. However, that 

 obvious [wint being Imrnein mind, il was proved by experiment 

 that the gas did not change in weight by standing for eight 

 months — a result lending 10 show that ihe abnorntal Hghmess 

 was not the consequence of dissociation. 



l-'urther exiierimeiUs were tried U))on the action of the silent 

 electric discharge l«)lh upon the atmospheric nitrogen an<l upon 

 the chemically derived nitrogen — but neither of Ihem .seemed 10 

 be sensibly aflecled by such Irealment ; .so that, altogether, ihe 

 Ixilance of evidence seemed to incline against the hypothesis of 

 abnormal lighlness in Ihe chemically derived nitrogen being due 

 li> dissociation, anil lo suggest strongly, as ahnosl the only 

 jiossible alternative, that there nuisi Ije in atmospheric nitrogen 

 some conslilueni heavier than true nitrogen. 



\\ lhal point the tpieslion arose, What was the evidence that 

 all the so-called nitrogen of the atmosphere was of one <|uality? 

 -Vnd I remember — I think it was about this time la.sl year, or a ■ 

 little earlier — pulling the question lo my colle;igue I'rof. Dewar. 

 His answer was lhal he doubled whether anything material h.ad 

 been done upon the matter since the lime of Cavendish, and 

 thiit I hail belter refer lo t'avendish"s original pa|Kr. That 

 advice 1 quickly followe<l, and I was rather surprised to find lhal 

 Cavendi.sh hail himself put this question (|uile as .sharply as I 

 could put it. Translated from the old-fashioned phraseology- 

 connected with the theor)- of phlogiston, his cpieslion was 

 whether the inert ingredient of the air is re.ally all of one kind : 

 whether all the nitrogen of the air is really the same as Ihe nitro- 

 gen of nitre. Cavendish not only a.sked himself this question, 

 bul he enileavoured lo answer il by an ajipeal In ex|K-riment. 



I should like to show you Cavendish's experiment in some- 

 thing like its original form. He inverted a C lube filled with 

 mercury, the legs standing in two separate mercury cups. He 

 then i)a.ssed up, so as to stand above the mercur)-, a mixture of 

 nitrogen, or of air, and nxygen : anil he caused an electric 

 current from a frictional electrical machine like the one I have 

 before me lojuss fmm the mercur)- in the one leg lo ihe mercury 

 in Ihe other, giving sparks across the intervening colunm of air. 

 I (111 not propose lo use a frictional machine to-night, liul I will 

 subslilule for il one giving electricity of Ihe same quality of the 

 construction introduced by Mr. Wimshurst, of which we have a 

 fine specimen in the Institution. Il stands just outside the door 

 of the theatre, and will supply an eleclric current along insulateit 

 wires, leading In the mercury cups ; and, if we are successful, we 

 shall cause sparks lo |)ass through ihe small length of air included 

 above Ihe columns of mercur)-. There they are ; and after a 

 lillle time you will notice that the mercury ri.ses, indicating lhal 

 the gas is sensibly absorbed under ihe influence of ihe sparks 

 and of a piece of pol.ash floating im the mercury. Il was by 

 that means lhal Cavemiish established his great discovery iif ihe 

 iialure of ihe inert ingredient in the atmosphere, which we now 

 call nilriigen -. and, as I have viid, C"avendish himself proposed 

 the question, as distinctly as we can do. Is this inert ingredient 

 all of one kind? and he proceeded to lest lhal question. He 

 found, after days anil weeks of prolracled experiment, that, for 

 the most pari, the nitrogen of the atmosphere absorbed in this 

 manner, was converted into nitrous acid : but that there was a 

 small residue remaining after prolonged treatment wilh sparks, 

 and a final absnrption of ihe residual oxygen. That residue 



