TRANSACTIONS OF SECTION B. 587 



It was considered that the metameric esters would afford much information if 

 a sufficient numher 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 impurities. For instance, pyridine, an 

 invariable impurity in commercial ammonia, is present in the proportion of about 

 g^j^h. 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 material 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 quantities 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 under all circumstances 

 and under every reasonable condition could not possibly be the result of accident, 

 more particularly Lf it could be repeatedly obtained from different specimens 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 lass diactinic than methyl 

 alcohol, and both transmitted a spectrum nearly as long as that of water. This 

 was shown by Huntington and me when the usual 25 mm. of thickne.=!S 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 

 (0 : • OH). By the examination 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, no 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 spectra : First, those of substances the rays of 

 which are freely transmitted, the absorption being at the more refrangible end of 

 the spectrum, and the spectrum of which is readily increased in length by dilu- 

 tion ; secondly, those in which the sjjectra are of the same kind, but the absorptive 

 power is greater, so that they withstand dilution to a much greater extent ; 



