2S6 



NATURE 



[May 8, 1913 



and fatty acids, are either equally transparent to light, 

 or only cut off a portion of the extreme ultra-violet 

 rays of the spectrum. 



ff 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 

 C f H,„, each carbon atom having two hydrogen atoms 

 attached to it. This substance resembles hexane 

 generally in its chemical properties, and behaves to- 

 wards 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 spec- 

 trum. 



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 



group hydrcxyl, 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 ravs, but gives a spectrum widely different 

 from that of benzene. 



Having dealt with the most general relation that 

 has been observed but ween the structure of organic 

 substances and their action on the ultra-violet rays, 

 I propose to illustrate some of the more special rela- 

 tions by examples from the phenomena of isomerism. 

 By replacing an atom of hydrogen in carbolic acid or 

 phenol by the nitro-group we obtain three distinct 

 nitrophenols. The ultimate particles or molecules of 

 these nitrophenols are all composed of the same 

 elements — carbon, hydrogen, oxygen, and nitrogen- - 

 and of the same number of atoms of each element. 

 Such substances are said to be isomeric, i.e. they are 

 made up of equal parts, although they do not possess 



Spark spectrum of 



of salicylic acid frorr 



are actually employed in benzene has never been 

 determined with certainty. Sometimes they are re- 

 presented as mutually neutralising one another, some- 

 times 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 indi- 

 cated to the structure of hexane and cyclohexane is 

 characteristic of the large group of organic substances 

 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. pro- 

 duce 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 one hand, and benzene on the 

 other, is brought out very clearly when we examine 

 some of their derivatives. If we replace an atom of 

 hydrogen in hexane or cyclohexane by the monovalent 



NO. 22/1, VOL. Ql] 



the same properties. The difference between them 

 lies in the arrangement of the parts relatively 

 to each other ; in this case in the position 

 of the nitro-group in relation to the hydroxyl group. 

 On comparing the spectra of the three nitrophenols 

 we find that they differ in quite a marked manner 

 from one another, and afford an illustration of the 

 important general rule that substances which have 

 the same composition but different spectra differ in 

 structure. 



It will have been noticed that the substitution of 

 the nitro-group for hydrogen in phenol has the effect 

 of shifting the absorption band nearer to the visible 

 region. One of the three nitrophenols has a yellow 

 colour, and in this case the gap in the spectrum cuts 

 a little way into the violet end of the visible region. 

 By the addition of soda to the solution the colour is 

 changed to red, and on examining the spectrum of 

 this solution we see that the gap now extends far 

 into the visible region. This example will serve to 

 illustrate the close connection that exists between the 



