TRAJVSACTIONS OF SECTION B. 585 



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 Bays or Colour Region. 



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

 considered to be colourless in layers from 1-6 to 3-7 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 (1888) calculated the wave-lengths of these 

 bands, and it appears that the higher members of the homologous series have the 

 bands displaced towards the red end of the spectrum. Russell and Lapraik (1879) 

 made similar observations on columns of liquid from two to eight feet in length. 

 All the substances gave well-defined absorption bands lying between wave-lengths 

 6,000 and 7,000. 



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 CH3 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 luminous rays withdrawn by absorption is but a very small fraction of the whole 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 are partly physical and partly chemical, that is to say, they are caused 

 by changes in the constitution of the solution. Yogel 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 dissolved 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 

 Tarying degrees of dilution. 



In order to ascertain in what way absorption spectra are 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 spectrum 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, sdnc, 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 conditions 



