ON DYNAMIC ISOMERISM. 93 



a sharp distinction between those substances which produce only a 

 ' general ' absorption of light and those which give rise to definite 

 absorption ' bands.' It has even been suggested that this property 

 might be used to classify carbon compounds into two groups, one group 

 characterised by a ' fixed ' structure and giving rise to general absorp- 

 tion, the other group including ' labile ' compounds undergoing isomeric 

 change readily and giving rise to banded spectra. The impracticability of 

 tbis demarcation was shown in the report presented at Winnipeg in 

 1909, in which an account was given of several labile compounds which 

 gave ' continuous ' absorption curves, whilst certain compounds of 

 fixed structure were shown to produce ' banded ' absorption. 



• In the report presented at Sheffield in 1910 the origin of banded 

 spectra was considered, special attention being directed to those altera- 

 tions of molecular structure which have the effect of" increasing or 

 diminishing the intensity of the local absorption. It was found that by 

 reducing the residual affinity of the absorbing centres an absorption 

 band could be driven back almost to the extreme limit of oscillation- 

 frequency at which it can be photographed by ordinary methods, and 

 that any further weakening of the absorbing centres had the effect of 

 converting the banded into a general absorption. The view was there- 

 fore adopted that a curve of general absorption might be produced by a 

 band situated in such a position as to be inaccessible to ordinary 

 methods of observation, say at 1 /A4200 or beyond. 



During the past year attention has been directed to the study of 

 general absorption and attempts have been made to determine the 

 approximate positions of the inaccessible bands to which this type of 

 absorption curve appears to be due. The method adopted depends on 

 the well-known fact that most of the optical constants of a substance 

 increase with great rapidity when an absorption-band is approached 

 and appear to tend towards an infinite value in the case of a sharply 

 defined absorption-line. In applying this method to carbon compounda 

 the magnetic rotatory dispersion has been found to be a very convenient 

 property to discuss, and measurements of this kind have been used to 

 calculate the limit of transmission for a large number of substances — in 

 other words, the position of the heads of the inaccessible bands by 

 which the general absorption is produced. Typical values are as 

 follows : — 



\ l /A 



Methyl and ethyl alcohols 1300 7500 



Propyl alcohol, normal esters .... 1350 7400 



Fatty acids, higher ketones, glycol, glycerol . 1380 7200 



Higher alcohols (pr. and sec.) .... 1400 7100 



Water, acetone, ter. alcohols .... 1500 6700 



Allyl alcohol 1700 5800 



Phenyl ethyl carbinol 2000 4900 



Carbon disulphide 2200 4350 



The chief points to be noted are : — ■ 



(1) That the optical properties of the majority of saturated carbon- 

 compounds are determined mainly by an absorption in the far ultra-violet 

 at wave-length 1300 to 1400; this appears to represent the extreme 

 limit of transmission in the case of all compounds containing carbon, 

 hydrogen, and oxygen. 



