I'.US] on The Spectroscope in Organic Chemistry 709 



from which we should infer that they are in all proljability stereo- 

 isomers, a conclusion which has been independently established by the 

 most satisfactory chemical evidence. 



In applying this reasoning in the investigation of bodies of un- 

 known structure, it has always to be borne in mind that while we may 

 safely infer from dissimilarity of spectra, dissimilarity of structure, we 

 cannot always infer similarity of structure from similarity of spectra. 



The study of absorption spectra has proved of special value in the 

 investigation of substances capable of existing in two forms which 

 may pass from the one into the other. It is rarely the case that 

 both forms of such substances are stable, and it is often extremely 

 difficult, or altogether impossible, on account of this instability to 

 determine by the ordinary chemical processes which of the two possible 

 forms the substance as we know it possesses. Such substances, how- 

 ever, frequently give rise to two series of stable isomeric methyl 

 or ethyl derivatives, whose structure 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, liecause it is a well-established fact that the introduction 

 of the methyl or ethyl group into a substance in place of an atom 

 of hydrogen does not appreciably alter the spectrum. 



An example of this is afforded by the three substances isatin, 

 methyl-isatin, and methyl-pseudo-isatin. 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 identical, as we should expect 

 them to be on the view that they are constructed alike. 



This phenomenon, which is known as tautomerism, is due to the 

 fact that some sul:)stances contain an atom of hydrogen, or it may be 

 a hydroxyl group, which readily shifts its position within the mole- 

 cule, 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 influence of certain 

 reagents the hydroxyl group leaves the carbon atom and attaches 

 itself to the nitrogen atom, but can readily, by an alteration of the 

 conditions, be enticed back again to the carbon atom. The shifting 

 of the position of the hydroxyl group is accompanied by other 

 changes which, however, it is not necessary that we should take into 

 account for our present purpose. In this case both of the tautomeric 

 forms are, under certain conditions, stable. The form in which the 

 hydroxyl is attached to the carbon is colourless, while the form in 



