134 Professor Julius WiJhehn Brilhl [May 26, 



bility. Among them are, for instance, the so-called " tautomeric " 

 compounds, hydrogen peroxide, and many other unstable compounds. 

 Substances of this kind are of a very special interest, for in conse- 

 quence of their tendency to change, they are the principal cause of 

 metamorphoses, the unceasing circulation of matter, the eternal birth 

 and decay that goes on in nature. 



Eesearch into the atomic structure of such bodies by purely chemical 

 methods is often very difficult, and not seldom impossible, because, 

 owing to their sensitive organisation, chemical interference leads 

 either to changes in the grouping of the atoms, which cannot always 

 be controlled, or even to total decomposition. 



In such cases it is of course of the greatest value to be able to 

 examine the constitution of the bodies without affecting them chemi- 

 cally ; and spectrochemistry, as we have seen, gives us the means of 

 doing so. By observing the behaviour of light on its passage through 

 the various substances, we gain an insight into their structure without 

 in any way distm-bing it 



§ 25. In the last ten years the spectro-chemistry of the nitrogen 

 compounds has also made remarkable progTess. Nitrogen is of the 

 greatest importance as an essential constituent of the proteids, the 

 alkaloids, and many other animal and vegetable products. But its 

 high valency and the extraordinary variety of combinations into 

 which it can enter with other elements, suiTOund it with special com- 

 plications. Regardless of these, however, the spectro-chemical 

 examination of nitrogen compounds has already yielded useful results, 

 especially in the study of the alkaloids. It is to be expected that 

 this optical method will also be of use in the chemistry of the 

 albuminoids, the study of which is now being prosecuted with so 

 much vigour. 



§ 26. One class of substances of increasing importance both to 

 science and to chemical industry is that constituted by the natural 

 and artificial perfumes. An overwhelming majority of them consists 

 of derivatives of the terpenes. We have already mentioned that 

 Gladstone, in this subject also a pioneer, was the first to study the 

 optical belDaviour of the terpenes. Since then the explanation of the 

 stnicture of these bodies and of a large number of rich natural per- 

 fumes derivable therefrom has been rendered easier by the use of 

 spectro-chemical methods. Similar assistance has been rendered to 

 the synthetic preparation of valuable scents, such as ionone, the 

 artificial scent of violets. In every scientific laboratory and in every 

 rationally conducted chemical factory where work is being done on 

 perfumes, the spectrometer is now an indispensable testing instrument, 

 and hence also an implement in industrial production. 



IX. 



§ 27. When scientific research opens up new metliods of observing 

 nature,.it is generally not long before a use is found for these methods 



