474 



NATURE 



[September 17, 1903 



related to the chemical constitution of organic substances, 

 it is necessary to examine a wider range of spectrum than 

 that included in the merely visible region, and this may 

 be done by extending the observations into the ultra-violet. 



The Ultra-violet Region. 

 Stokes in preparing his experiments for a Friday evening 

 discourse at the Royal Institution observed that the spec- 

 trum of electric light when a prism and lenses of quartz 

 were used extended no less than six or eight times the 

 length of the visible spectrum. In 1862 he studied the 

 ultra-violet spectra of metals and executed drawings of the 

 lines exhibited by aluminium, zinc, and cadmium. He 

 discovered the fact that certain solutions show light and 

 dark bands in the spectrum of rays transmitted by them, 

 the solutions being colourless ; the bands are invisible unless 

 they fall on a fluorescent screen. It was under such con- 

 ditions that they exhibited light and darkness. The screen 

 used was of plaster of Paris saturated with a fluorescent 

 substance, such as uranium phosphate. 



William Allen Miller in 1863, simultaneously with Stokes, 

 described his method of examining the photographic trans- 

 parency of various saline solutions and organic substances 

 and of depicting metallic spectra. A sensitised photographic 

 plate was used for the reception of the rays of the spectrum, 

 so that they were made to register their own position and 

 intensity. L. Soret invented the fluorescent eye-piece for 

 the purpose of investigating the ultra-violet rays and ascer- 

 tained the best media for the transmission of rays of high 

 refrangibility. Colourless fluor-spar, a rare mineral, was 

 found to answer best, and quartz lenses were achromatised 

 with this. Iceland spar was found to absorb some of the 

 more refrangible rays, and a pure spectrum was difficult to 

 obtain with quartz prisms owing to double refraction, which 

 caused the lines in metallic spectra' to be duplicated. 

 Struck by the fact that Miller had examined many organic 

 substances without obtaining evidence of a connection 

 between their constitution and their absorption spectra — 

 the actual words used by Miller were, " I have not been 

 able to trace any special connection between the chemical 

 complexity of a substance and its diactinic power " (Journ. 

 Chem. Sac, vol. xi. p. 68) — it appeared to me desirable 

 that this point should be systematically reinvestigated. 

 L. Soret had already proceeded with work in this direction, 

 by examining and drawing a great variety of organic sub- 

 stances and diagrams of absorption curves. But it was 

 deemed necessary to make a large number of examinations 

 of substances of a comparatively simple constitution, and 

 according to theory closely related, and afterwards gradually 

 to proceed to the study of substances of greater complexity. 

 For such purposes a photographic method alone appeared 

 a practicable one, particularly when comparisons had to be 

 made between substances observed at diff'erent times, for 

 the reason that none but photographic records could be 

 absolutely relied upon and stored away for future reference.^ 



^ Clerk Maxwell had calculated for Miller the best focal length of lenses 

 of quartz which would give an approximately flat field. His computation 

 made this something over a leng'h of three feet. All Miller's photographs 

 were taken with the plate placed normal f. the axis of the lens, but Stokes 

 bad. shown that the locus of the foci of the difl^erent rays formed an arc of 

 a curve or nearly a straight line, lying very obliquely to the axes of the 

 pencils coming through the lens. 



It was obvious from Miller's pho'ographs that only one or two rays on 

 each plate were even approximately in focus. To obtain spectra in focus 

 from end to end it was evidenth necessary to incline the plate so that the 

 end ijpon which the red rays would fall, which are of longest wave-leng'h, 

 should be farther off than that upon which the ultra-violet fall, which are of 

 shortest wave-length. It w.asalsn found experimentally that lenses of much 

 shorter focal length (ten or twelve inches) could be used, giving perfect 

 definition, and, what is still more important, it was found a po.sitive 

 advantage not to have them corrected by fluor-spar or calcite. The plate 

 carrier was adjusted at an inclination of approximately 22° to the normal ; 

 in such a position the rays from the vellow sodium I'ne to the ext>eme ultra- 

 violet of the spark spectrum of cadmium were simultaneously in focus on a 

 plane surface. 



The prism was of quartz cut on Cornu's plan, the method of construction 

 designed to get rid of all double refraction being communicated to me by 

 M. Lornu in a very kindly written letter. The first instrument was con- 

 structed in 1878 and the description of it published in i88i. It has been the 

 model for several others. One with two prisms and lenses of 12 inches 

 focus was exhibited by me in the Inventions Exhibition in 1882. At the 

 Jubilee meeting of the British Association at York the .spark spectra of 

 iron, cobalt, and nickel, enlarged 1 1 twenty-five diameters and printed by the 

 Autotype Company, were exhibited. They are more than 8 leet in length, 

 and have proved very useful for reference. 1 he photographic proc»ss is a 

 point of great importance ; the then newly invented gelatine bromide films 

 made by Kennet were alone quite suitable. 



NO. 1768, VOL. 68] 



The plan of the proposed investigation was to photograph 

 the rays transmitted by molecular proportions of hydro- 

 carbons, alcohols, acids, and esters, either alone as vapour 

 or liquid, or dissolved in some neutral and, in comparison 

 with the substances to be examined, an optically non- 

 absorbent solvent. 



It was considered that the metameric esters would afford 

 much information if a sufficient number of them were 

 examined and their spectra compared, and if the acids 

 themselves were not responsive the sodium and potassium 

 salts in solution would serve the purpose, since the alkalies 

 did not affect the spectrum. The general deductions (1879) 

 are now well known, but two points not generally taken 

 into account were well established. First, the extraordinary 

 delicacy of the ultra-violet spectrum in detecting traces of 

 im.purities. For instance, pyridine, an invariable impurity 

 in commercial ammonia, is present in the proportion of 

 about I /30000th. It was proved that the absorption spectra 

 of the normal paraffins prepared with the greatest care by 

 Schorlemmer contained traces of impurities which could not 

 be separated. Secondly, some of the normal alcohols could 

 not be rendered pure by the ordinary methods employed, and 

 great care was necessary in their preparation. It may well 

 be asked that, if such were the case, upon what grounds 

 was it concluded that impurities were present? How was 

 it possible to distinguish between a normal and an abnormal 

 absorption spectrum when no standards of comparison 

 existed? It may be of interest if this question be now 

 answered, as no adequate account of it has been made 

 public. All the substances in any one homologous series 

 were shown to vary in the extent to which the rays at the 

 more refrangible end of the spectrum were absorbed, and 

 the different terms of the series differ solely by the number 

 of CH, groups in the molecule ; and the greater the number 

 of these the greater the absorption. The extent of the 

 absorption should be proportional to the molecular weight 

 of the substance. Accordingly if repeatedly purifying and 

 fractionally distilling a considerable quantity of materia! 

 failed to give spectra which were constant and identical, 

 but gave instead spectra which were variable, even in a 

 slight degree, it was evident that the absorption due to the 

 molecule of the substance was interfered with by some 

 impurity. 



When, however, it became evident that successive quanti- 

 ties of methylic alcohol, for example, prepared in a certain 

 manner yielded spectra which were practically identical under 

 different conditions, such as thickness of liquid, and that 

 they differed but slightly from that of pure water after the 

 type of which the alcohol is constituted, the conclusion was 

 inevitable that we were dealing with a pure preparation. 

 In short, the longest spectrum obtained in all circum- 

 stances and under every reasonable condition could not 

 possibly be the result of accident, more particularly if it 

 could be repeatedly obtained from different speciinens of 

 the same substance. The same reasoning applies to the 

 acids and their salts in the investigation of which similar 

 difficulties arose. 



Soret and Rilliet pointed out that in the rectification and 

 prolonged desiccation of the alcohols there is often slight 

 oxidation which leads to the production of impurities which 

 affect the spectra transmitted by them. 



They found that ethyl alcohol is not appreciably less 

 diactinic than methyl alcohol, and both transmitted a spec- 

 trum nearly as long as that of water. This was shown by 

 Huntington and me when the usual 25 mm. of thickness of 

 the layer of liquid were tested. By taking columns of 

 liquid 100 mm. in length the differences are greater, and 

 they increase with columns of increased length. 



The influence of each additional CH^ in the molecule 

 causes a shortening of the spectrum. This was shown to 

 be due to the carbon atoms and not to the hydrogen. The 

 acids, containing the same number of carbon atoms as the 

 alcohols, have a much greater absorptive power, which is 

 due to the carboxyl group (C : O ■ OH). By the examin- 

 ation of a number of various substances, such as poly- 

 hydric alcohols, as glycol, glycerol, mannitol, and various 

 sugars, it was found that, no matter what its complexity, 

 m) open-chain compound causes selective absorption, i.e. 

 absorption bands. 



Shortly it may be stated that in the examination of 

 organic substances we have three variations in absorption 



