January 14, 1892 



NATURE 



253 



THE SPECTRUM OF IRON AND THE 

 PERIODIC LA W. 



T N the course of a prolonged series of spectroscopic ob- 

 -*■ servations on iron containing y\j carbon, 08 sulphur, 

 •070 phosphorus, and about /^ per cent, of manganese, 

 and based on the previous investigations of Mr. Lockyer 

 on the iron spectrum at varying temperatures, I noted 

 some results summarized as follows : — 



(1) Iron heated i?i vacuo evolves a vapour showing the 

 H spectrum ; in addition, other lines are sometimes 

 visible. 



(2) I have found that iron kept in a vacuum slowly 

 evolves H at a temperature not exceeding 70^-80° ; this 

 continued for six months. Further, on applying heat, I 

 have observed a condensed sublimate at the sealed cold 

 end of the tube. 



Heated iron known to contain small quantities of some 

 of the more fusible metals evolves these bodies, and vice 

 versa absorbs them. Exhausted heated iron also absorbs 

 H in the same way, and most rapidly at an intense heat 

 approximating to fusion. 



(3) CJn iron being heated in the blow-pipe flame through 

 which the spark was passing, lines were detected in the 

 flame apart from the iron. 



(4) On heating iron electrodes, " varying the tension of 

 the spark,! also the flame temperature," according to 

 methods elsewhere given, I found it possible to obtain 

 iron spectra, varying roughly in accordance with the heat 

 of the flame and spark tension. Three nearly distinct 

 spectra have been observed : — (i) Lowest heat, a nearly 

 pure manganese spectrum. (2) Higher heat, manganese 

 lines ; other long lines appear, also the beginnings of a 

 short-line spectrum. (3) Highest heat, a complete iron 

 spectrum. 



As regards the first spectrum, manganese has been 

 identified by the ordinary method of chemical analysis. 

 The second group of long lines the chemist would say 

 were due to the presence of some body not identical with 

 either iron or manganese, but this problematic body has 

 not been identified or isolated ; the proof is wanting, 

 although it is a product, or function, of temperature, just | 

 as is the first or manganese spectrum. This spectrum I 

 may be due to dissociation of iron, and not to the vapor- j 

 ization of a foreign constituent. It is probable that iron 

 can be roughly split up into two bodies, one of which is 

 rnore volatile than the other, and that the relative quan- 

 tity of each present may not always be the same. At 

 any rate, it appears that by the simple heating of crude 

 iron its composition may be sensibly modified, and that, 

 even at a temperature as low as 70^-80'', slow dissociation 

 is going on, manifested by the evolution of hydrogen ; and 

 this continues, the rate of dissociation apparently broadly 

 corresponding to the heat applied. It follows that in 

 actual practice the chemical composition of iron may thus 

 be altered, such changes being probably so minute as to 

 escape recognition. 



These researches were made with the sole object of 

 utilizing the spectroscope as an aid to the ordinary 

 chemical analysis of iron, my previous experience having 

 taught me that an extension of the usual methods was 

 imperatively required. It was thought that by the 

 spectroscopic method some body or bodies as yet un- 

 recognized might be found ; in other words, I searched 

 for so-called impurities with laut scant success. 



Finally, I was forced to admit that I had exhausted the 

 purely analytical part of the inquiry, and must seek for 

 the solution of the many discrepancies observed in the 

 behaviour of iron and steel, and not comparable with its 

 chemical composition as determined by ordinary analysis. 

 Nothing was left for further study, with the exception of 

 the metal itself. It may be remarked, however, that 



' Suggested by Mr. Lockyer. 



NO. 1 1 59, VOL. 45] 



absolutely pure metal could not be obtained ; manganese, 

 for instance, seems always present, even after repeated 

 purifications. 



This led to a study of the periodic law as enunciated by 

 Mr. Crookes in his address on the genesis of the elements. 

 He advances the rational hypothesis that atoms are 

 formed from the original protyle or fire mist ; next, by a 

 series of atomic condensations, due to successive coolings, 

 the elements are formed. Mr. Lockyer, by somewhat dif- 

 ferent methods of research, appears to have come to the 

 same conclusion — viz. that temperature governs all ; and 

 tells us, " as the result of a long series of spectroscopic 

 observations," that an element is a very complex thing, 

 broken up— at higher temperatures— into simpler things. 

 Mr. Crookes, by a careful study of the periodic law, sup- 

 plemented by spectroscopic work, shows how elements 

 may be built up. Mr. Lockyer, pursuing the opposite 

 method, viz. by a study of the breaking up of the so-called 

 elements, and registering the results by means of the 

 spectroscope, appears to have experimentally proved the 

 same thing. 



It is quite obvious that an absolutely pure element can 

 only exist at a given temperature ; any deviation from this 

 — the critical temperature — must favour partial dissocia- 

 tion, and in this way it undergoes changes which may 

 veil its true atomic weight. As Mr. Crookes puts it, " of 

 a given mass of atoms, only a few may have the true 

 atomic weight, the others slightly varying from it." Grant- 

 ing a variation of atomic weight in the element for the 

 same reason, there may be a shifting of its spectral lines. 

 I submit also that the discrepancies in the position of 

 certain spectral lines may be due to divergence from the 

 j critical temperature, and not observational errors.^ 

 [ There appears, therefore, to be no necessity for the 

 ' use of such phrases as chemical affinity, cohesive force, 

 &c. : heat energy and the universal law of gravitation 

 seem the only factors controlling the genesis of the 

 elements— can we also say the genesis of known 

 chemical compounds ? 



We cannot well say how far the physical properties of 

 such a metal as iron are modified by temperature 

 variations ; yet we have seen that something like dis- 

 sociation is going on at 70^-80°, and that at a moderate 

 heat this is accentuated ; whilst at high temperatures the 

 spectrum of iron affords ample proof that such is the case. 

 Experiments have been made showing that even at the 

 bare fusion-point of iron matter is volatilized ; and at the 

 abnormally high temperature of the Bessemer blow — 

 " melting up lumps of cold steel plunged in, and weighing^ 

 2 or more cwt , like wax " — it is admitted that iron as 

 such is vaporized. It seems therefore, on the whole, that 

 even a stable body like iron, when heated, gives results 

 according with the periodic law ; and as regards other 

 bodies, from ice upwards, we do not need to be informed 

 that evaporation (dissociation) is constantly going on. 



It may not, however, be so well known that " on heat- 

 ing some of the more fusible metals in a vacuum," it is 

 possible to obtain almost invisible vapours, some of which 

 iron occludes or absorbs just as it does the gases hydrogen 

 and possibly carbon monoxide. 



Referring to the No. 2 spectrum indicating the probable 

 existence of an intermediate body betwixt iron and man- 

 ganese, not yet isolated, but which, nevertheless, is a 

 product or function of the temperature, just as manganese 

 is ; the supposition that this body is a constituent of iron 

 acquires force from the fact that recently it has been 

 shown that iron may be capable of assuming two forms — 

 the one termed a. or soft iron ; the other i3, or hard iron. 



' Or, possibly, at a given teiriperature a vapour may be evolved from one 

 body so nearly approximating in composition to that of another, the latter 

 not necessarily at the same temperature, as to be almost undistinguishable 

 from the other by the ordinary method of mic-ometric measurement. The 

 same difficulty in another form occurs in ordinary chemical work : bodies 

 are so clnsely allied in some instances as to render their separation and 

 identification very difficult. 



