September 17, 1903] 



NATURE 



473 



\ 



Everyone is more or less familiar with the subject of 

 spectrum analysis. This was defined by Tait as an optical 

 method of making a diagnosis of the chemical composition 

 of either (a) a self-luminous body, or (6) an absorbing 

 medium, whether self-luminous or not. It has now become 

 necessary to enlarge this definition, and I would suggest 

 that it is the study of the composition and the constitution 

 of matter by means of radiant energy, and recording in 

 the order of their refrangibilities the rays emitted and 

 absorbed by matter. By this modified statement the infra- 

 red or so-called " invisible heat rays," the visible or 

 "colour rays," and the ultra-violet or "chemical rays" 

 are included. 



Spectra are of two kinds, emission and absorption spectra. 

 It will be convenient if the latter are considered first. 



Absorption Spectr-a. 

 The Infra-red Region. 



Abney (1880) by the preparation of a particularly sensitive 

 form of collodion emulsion containing silver bromide was 

 successful in obtaining very extraordinary results. Such 

 films as he prepared were so sensitive to invisible radiations 

 of long wave-length as to be capable of forming a repre- 

 sentation of even a kettle of boiling water, standing in an 

 absolutely dark room. This picture could not of course 

 be properly referred to as a photograph, though the process 

 by which it was obtained was such as we are accustomed 

 to term a photographic process. It may with greater 

 propriety be termed an actinograph, the result not of light, 

 but of dark rays. The least refrangible of the visible rays 

 lies^about wave-length 7800 ten-millionths of a millimetre, 

 or Angstrom units ; but these rays extend as far as wave- 

 length 12,000, while Becquerel has measured lines in the 

 spectra of metals of as low a refrangibility as wave-length 

 18,000. 



Abney and Festing (1881) investigated the influence of 

 atomic groupings in the molecules of organic substances 

 by measuring their absorption in the infra-red region of the 

 spectrum. 



They studied such simply constituted substances as water, 

 hydrochloric acid, chloroform, carbon tetrachloride, and 

 cyanogen, besides many hydrocarbons with their hydroxyl, 

 haloid, and carboxyl derivatives. Characteristic groups of 

 lines or very narrow bands were observed in carbon com- 

 pounds, but they are absent from carbon compounds con- 

 taining no hydrogen, and do not all appear in some of the 

 hydrogen compounds. The facts observed led to the con- 

 clusion that they belonged to hydrogen, but are subject to 

 some occasional modifications. Oxygen in hydroxyl, for 

 instance, modifies two of the lines, since it obliterates by 

 absorption the rays which lie between them. Oxygen in 

 aldehyde, or when it forms part of the carbon nucleus of 

 some such compound, presents bands which are bounded 

 by well-defined lines, or are inclined to be linear. These 

 appear to be characteristic bands indicating the carbon 

 nucleus of a series of substances. Alkyl radicals, such as 

 ethyl, exhibit absorption bands, and so does the benzene 

 nucleus. It is a remarkable fact that bands appear in the 

 solar spectrum which correspond with those of benzene 

 (1881). 



Julius (1803) has investigated the absorption in the infra- 

 red caused by many carbon compounds by means of the 

 bolometer, combined with a prism, and also with a diffrac- 

 tion grating. He showed that the molecules of compound 

 substances absorbed the rays which were emitted at the 

 time of their formation. Thus, to take the simplest case, 

 tha emission spectrum of hydrogen burning in air corre- 

 sponds with the absorption bands of water vapour, that is 

 to say, the absorption spectra of the compounds are the 

 counterpart of the emission spectra of the flames which 

 yield these compounds during combustion. The emission 

 spectrum of carbon dioxide is found in the spectrum of 

 burning carbon monoxide, cyanogen, methane, and carbon 

 disulphide ; and that of water-vapour in various hydro- 

 carbons. As early as 1888 Julius, in an Inaugural Disser- 

 tation, quoting Tyndall, recognised that the absorption and 

 emission of rays measured with the thermopile were mani- 

 festations of the molecular vibrations. 



The various absorption spectra examined included those 

 of the alcohols, such as isopentylic, isobutylic, normal 



butylic, propyljc, ethylic, and methylic, as well as hydro- 

 carbons, chloroform, and benzene. The study of the 

 maxima of radiation and the maxima of absorption offers 

 us a means of arriving at a knowledge of a series of new 

 and valuable physical constants, namely, the vibration 

 periods characteristic of the molecules. (Tyndall discussed 

 this subject in his usually luminous style on pp. 391 to 402 

 of his work " Heat as a Mode of Motion.") 



Puccianti (1900) has examined the infra-red absorption 

 spectra of liquids, including aromatic compounds and alkyl 

 derivatives, while Donath has examined in the same region 

 various essential oils. Carbon conibined with hydrogen 

 shows a maximum of absorption with a wave-length about 

 (1-71 n mm.) 17,100 - ngstrom units. 



Benzene and pyridine have two other maxima of absorp- 

 tion in common. The alcohols have very similar maxima 

 of absorption at wave-length 21,000. 



The three isomeric xylenes show absorption spectra which 

 are almost identical. At or about wave-length 23,200 

 another maximum of absorption is shown. 



Julius refers to Langley's observation that at a wave- 

 length of 27,000 there is an abrupt termination to the solar 

 spectrum, probably caused by the water vapour in the atmo- 

 sphere ; but a band extends to 273,000, and at no very 

 great elevation above the earth's surface there are rays with 

 a wave-length of 45,700 Angstrom units. All radiations of 

 longer wave-length — and Julius has measured down to 

 149,000 Angstrom units — are likely to be absorbed by the 

 carbon dioxide in the atmosphere. 



The Visible Rays or Colour Region. 



J. L. Schonn (1879) examined the absorption spectra of 

 substances usually considered to be colourless in layers 

 from 1-6 to 37 metres in thickness and observed narrow 

 bands in the spectra of methyl, ethyl, and amyl alcohol, 

 lying in the red, orange, and yellow ; methyl alcohol showed 

 two bands, ethyl and amyl alcohol each three. Gerard 

 Kriiss (i888) calculated the wave-lengths of these bands, 

 and it appears that the higher members of the homologous 

 scries have the bands displaced towards the red end of the 

 spectrum. Russell and Lapraik (1879) made similar observ- 

 ations on columns of liquid from two to eight feet in length. 

 All the substances gave well-defined absorption bands lying 

 between wave-lengths 6000 and 7000. 



The bands of the different substances differed altogether 

 from the bands of water. Alcohols give a band which is 

 similar in different alcohols, but the higher the alcohol 

 stands in the homologous series, that is to say, the larger 

 the number of carbon atoms it contains, the nearer is the 

 band to the red end of the spectrum (1881). 



It was definitely established that for each CH, introduced 

 into a molecule of ammonia or benzene there is a shifting 

 of the absorption bands towards the red end of the 

 spectrum. 



It will, of course, be understood that the liquids examined 

 were perfectly colourless in the ordinary acceptation of the 

 term ; and that they appear so is owing to the bands of 

 absorption being very narrow, so that the percentage of 

 luminous rays withdrawn by absorption is but a very small 

 fraction of the whole spectrum emitted by a source of light 

 when viewed under ordinary conditions. 



Numerous observations were made by Melde. Burger, 

 Magnus, H. W. Vogel, and Landauer (1876-78), which 

 showed that changes in the absorption spectra of solutions 

 a-e partly physical and partly chemical, that is to say, they 

 a-e caused by changes in the constitution of the solution. 

 Vogel mentions cases where no chemical change was 

 believed to take place, as, for instance, where naphthalene 

 red shows different spectra according to whether it is dis- 

 solved in alcohol, water, resin, or is solid or used to colour 

 paper (1878). 



This points to some difference in the constitution of the 

 solution. A well-known instance is that of iodine in 

 alcohol, chloroform, or carbon disulphide. 



It must be observed that Vogel 's work referred merely 

 to phenomena observable in the visible spectrum, to small 

 thicknesses of the absorbing medium, and was not applied 

 quantitatively. Two solutions may give spectra which are 

 apparently identical at one concentration, but spectra quite 

 different when submitted to varying degrees of dilution. 



In order to ascertain in what way absorption spectra are 



NO. 1768, VOL. 68] 



