1913] on The Spectroscope in Organic Chemistry 707 



belonging to the same division of organic chemistry snch as alcohols, 

 sugars, and fattv acids are either equally transparent to light, or only 

 cut off a portion of the extreme ultra-violet rays of the spectrum. 



If we now remove one atom of hydrogen from each of the two 

 end carbon atoms of hexane, these atoms are in a condition to unite 

 directly with each other, thus closing the chain. The substance so 

 formed belongs to the cyclic division of organic compounds. It is 

 known as cyclohexane, and has the formula CgH^o' ^ach carbon atom 

 having two hydrogen atoms attached to it. This substance resembles 

 hexane generally in its chemical properties, and behaves towards 

 light in the same way, that is to say, it is practically diactinic or 

 only cuts off some of the rays of light at the extreme ultra-violet end 

 of the spectrum. Speaking quite generally, it may be said that 

 bodies constituted like cyclohexane absorb more of the ultra-violet 

 rays than those constituted like hexane. 



But a wholly different condition is brought about if we suppose 

 one atom of hydrogen removed from each of the six carbon atoms 

 of cyclohexane. One linkage is thus set free in each of the six carbon 

 atoms, and we obtain benzene. How these linkages are actually 

 employed in benzene has never been determined with certainty. 

 Sometimes they are represented as mutually neutralizing one 

 another, sometimes as effecting a double link between the alternate 

 pairs of carbon atoms. However this may be, the structure which 

 bears the relation that I have indicated to the structure of hexane 

 and cyclohexane is characteristic of the large group of organic sub- 

 stances of which benzene is the type. It is to this division of the 

 cyclic compounds that the great majority of substances which show 

 selective absorption, i.e. produce breaks or dark bands in the 

 spectrum, belong. Here, then, we have a very important and a 

 very general relation between the structure of organic substances 

 and their absorption spectra. 



The difference in the behaviour of organic bodies towards the 

 ultra-violet rays, as exemplified in hexane and cyclohexane on the 

 one hand, and benzene on the other, is brought out very clearly 

 when w^e examine some of their derivatives. If we replace an atom 

 of hydrogen in hexane or cyclohexane by the monovalent group 

 hydroxyl, we get substances belonging to the class of alcohols, and 

 these substances are, like their parent substances, highly diactinic. 

 If, on the other hand, we replace an atom of hydrogen in benzene 

 by the same group we get carbolic acid or phenol, which, like 

 benzene, exercises selective absorption on the ultra-violet rays, but 

 gives a spectrum widely different from that of benzene. 



Having dealt with the most general relation that has been 

 observed between the structure of organic substances and their action - 

 on the ultra-violet rays, I propose to illustrate some of the more 

 special relations by examples from the phenomena of isomerism. By 

 replacing an atom of hydrogen in carbolic acid or phenol by the nitro- 



