May b, 1913] 



NATURE 



257 



study of absorption spectra and the origin of colour, 

 an interesting branch of the subject with which, 

 however, it is impossible for me to deal within the 

 limits of this discourse. 



In the nitrophenols we have an example of what 

 is known as structural isomerism, or position iso- 

 merism, because the phenomenon depends upon differ- 

 ences in the position or arrangement of the atoms 

 within the molecule— in other words, in the internal 

 structure of the molecule. But it is possible to have 

 two substances of the same composition and structure 

 not identical, but related to one another as an object 

 is to its mirror-image. Substances so related are 

 termed optical-isomers or stereo-isomers. The spectra 

 of isomers of this class, unlike those ot structural 

 isomers, do not differ. This leads to an important 

 application of absorption spectra in chemical investiga- 

 tions. If two substances have the same composition 

 hut different spectra, we know that they must be 

 structurally different; if, on the other hand, they 

 have the same composition and the same spectra, 



The structure of methyl-isatin and of methyl-pseudo- 

 isatin has been determined by chemical methods, but 

 the structure of the parent substance isatin cannot be 

 determined in this way. Is it constituted like methyl- 

 isatin or like methyl-pseudo-isatin? Inspection of the 

 photographs of the spectra of the three substances 

 shows that while there is a wide difference between 

 the spectra of isatin and methyl-isatin, the spectra 

 of isatin and methyl-pseudo-isatin are almost iden- 

 tical, as we should 'expect them to be on the view 

 that thev are constructed alike. 



This phenomenon, which is known as tautomerism, 

 is due to the fact that some substances contain an 

 atom of hvdrogen, or it may be a hydroxyl group, 

 which readilv shifts its position within the molecule, 

 leaving its union with one atom to attach itself to 

 another. Another example of this is afforded by 

 cotarnine, a substance found in opium. The molecule 

 of cotarnine possesses an atom of carbon which is 

 directly combined with an atom of nitrogen, and has 

 also united to it a hydroxyl group. Under the influ- 



and yet are not identical, there is a strong probability 

 although not a certainty, that they are optical-isomers. 



The study of absorption spectra has proved of 

 special value in the investigation of substances capable 

 of existing in two forms which may pass the one 

 into the other. It is rarely the case that both forms 

 of such substances are stable, and it is often ex- 

 tremely difficult, or altogether impossible, on account 

 of this instability, to determine by the ordinary 

 chemkal processes wdiich of the two possible forms 

 the substance as we know it possesses. Such sub- 

 stanci s, however, frequently give rise to two series of 

 stable isomeric methyl- or ethyl-derivative-., the 

 structure of which can be ascertained by chemical 

 investigation. The parent substance, if not a mixture 

 of the two forms, must correspond in structure with 

 one or other of these derivatives, because it is a well- 

 established fact that the introduction of the methyl- 

 or ethvl-group into a substance in place of an atom 

 of hvdrogen does not appreciablv alter the spectrum. 



Aii example of this is afforded by the three -in- 

 stances isatin, methyl-isatin, and methyl-pseudo-isatin. 



ence of certain reagents the hydroxyl group leave- the 

 carbon atom and attaches itself to the nitrogen atom, 

 but can readilv, bv an alteration of the conditions, 

 be enticed back again to the carbon atom. The 

 shifting of the position of the hydroxyl group is 

 accompanied bv other changes which, however, it is 

 not necessarv that we should take into account for 

 our present "purpose. In this case both the tauto- 

 meric forms are, under certain conditions, stable. The 

 form in which the hvdroxyl is attached to the carbon 

 is colourless, while the fo'rm in which it is attached 

 to the nitrogen is vellow. The two forms have totally 

 distinct absorption spectra. When one of the forms 

 passes into the other under the influence of the 

 appropriate reag.ent, the amount of change is propor- 

 tional to the quantity of reagent added. It is pos- 

 sible, therefore, bv taking photographs after the addi- 

 tion 'of each successive quantity of reagent, to trace 

 the orogress of the chance through all its phases, and 

 to ascertain how much of each form is present at anv 

 time. This is done bv comparison with a series of 

 reference plates prepared bv photographing mixtures 



NO. 2271, VOL. QI J 



