1879.] on the Absorption of the Ultra- Violet Rays, 235 



The various parts of the apparatus are screwed down so as to be 

 immovable after a proper adjustment. 



The Examination of Organic Substances. 



Dr. Miller failed to trace any connexion between the chemical com- 

 plexity of a substance and its actinic absorption. 



With the view of ascertaining whether any such connexion existed 

 an examination was made of the normal alcohols, the normal fatty 

 acids, and a series of ethereal salts. Great trouble was occasioned by 

 the interference of minute traces of otherwise undetected impurities, 

 the presence of which was often unaccountable. Four diagrams, show- 

 ing the relative transparency of different substances, illustrate this 

 part of the paper, and from the results obtained the following conclu- 

 sions have been drawn. 



(1.) The normal alcohols of the series C n H 2ll+ iOH are remarkable 

 for transparency to the ultra-violet rays of the spectrum, pure methylic 

 alcohol being as nearly so as water. 



(2.) The normal fatty acids exhibit a greater absorption of the more 

 refrangible rays of the ultra-violet spectrum than the normal alcohols 

 containing the same number of carbon- atoms. 



(3.) There is an increased absorption of the more refrangible rays 

 corresponding to each increment of CH 2 in the molecule of the alcohols 

 and acids. 



(4.) Like the alcohols and acids, the ethereal salts derived from 

 them are highly transparent to the ultra-violet rays, and do not exhibit 

 absorption-bands. 



In order to ascertain whether isomeric bodies exhibited similar 

 or identical absorption spectra a series of benzene derivatives 

 was examined. From the great absorptive power of this class of 

 substances it was found necessary to use very dilute solutions even 

 though the cells holding the liquids were not more than 0*75 inch in 

 thickness. Curves w^re plotted by taking the proportions of sub- 

 stances in solution as ordinates, and the position of absorption-bands 

 as abscissae, and these curves are highly characteristic features of very 

 many compounds. About twenty diagrams have thus been. made. 



The following is a summary of the chief points of interest apper- 

 taining to benzene and its derivatives. 



(1.) Benzene, and the hydrocarbons, the phenols, acids, ..and amines 

 derived therefrom, are remarkable firstly, for their powerful absorp- 

 tion of the ultra-violet rays ; secondly, for the absorption-bands made 

 visible by dissolving them in water or alcohol, and diluting ; and 

 thirdly, for the extraordinary intensity of these absorption-bands, that 

 is to say, their power of resisting dilution. 



(2.) Isomeric bodies, containing the benzene nucleus, exhibit widely 



s 2 



