584 REPORT— 1903. 



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 7,800 ten-millionths 

 of a millimetre, or Angstrom units ; but these rays extend as far as wave-length 

 12,000, while JBecquerel has measured lines in the spectra of metals of as low a 

 refrangibility as wave-length 18,000. 



Abney and Testing (1881) investigated the influence of atomic groupings in the 

 molecules of organic substances bj- 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 compounds, but they are absent from 

 carbon compounds, containing no hydrogen, and do not all appear in some of the 

 hydrogen compounds. The lacts observed led to the conclusion 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 indi- 

 cating 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 anpear in the solar spectrum which correspond with those of ben- 

 zene (1881). 



Julius (189.3) 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 diflfraction 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, the emission spectrum of hydrogen burning in air corresponds 

 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 Barnes 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 hydrocarbonp. As 

 early as 1888 Julius, in an Inaugural Dissertation, quoting Tyndall, recognised that 

 the absorption and emission of rays measured with the thermopile were manifes- 

 tations of the molecular vibrations. 



The various absorption spectra examined included those of the alcohols, such 

 as isopentylic, isobutylic, normal butylic, propylic, ethylic, and methylic, as well 

 as hydrocarbons, 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 

 pages 391 to 40J of his work ' Heat as a Mode of Motion.') 



Puccianti (1900) has examined the infra-red absorption spectra of liquids, in- 

 cluding aromatic compounds and alkyl derivatives, while Donath has examined in 

 the same region various essential oils. Carbon combined with hydrogen shows 

 a maximum of absorption with a wave-length about (1'71 fimm.) 17,100 Angstrom 

 units. 



Benzene and pyridine have two other maxima of absorption 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 atmosphere ; 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 



