1905.] on the Development of Spectro-Chemistry . 125 



IV. 



§ 9. Shortly before 1880, when I was studying the literature of 

 chemical optics, a brief note published by Gladstone in the Journal of 

 the Chemical Society for May 1870, excited my attention and curiosity. 

 The author there discusses the exceptions to Landolt's rule of sum- 

 mation. He shows firstly that in all such cases the molecular 

 refraction is never found to be too small, but always too great. Then 

 he shows that whole classes of compounds behave in this abnormal 

 fashion. 



§ 10. All optically abnormal compounds proved to be rich in 

 carbon. Gladstone, therefore, examined the effect which a gradual 

 increase of carbon in the composition of a body exerted on its re- 

 fractivity. He found that there actually was an increase in the excess 

 of the experimental as compared with the calculated molecular re- 

 fraction, but the increase was not regular enough to explain the 

 anomalies. 



The saturated hydrocarbons, or "paraffins^ of the general com- 

 position C„H2tc + 2, showed normal molecular refraction. 



Also the olefines, containing two atoms less of hydrogen, were 

 found normal by Gladstone. 



On the other hand, tlie hydrocarbons containing six atoms less of 

 hydrogen, viz., the terpenes, gave molecular refractions about 3 units 

 larger than would correspond to their composition. 



With the aromatic hydrocarbons, such as benzene, toluene, etc., 

 containing eight atoms less of hydrogen, this abnormal excess 

 amounted to 6 units : — 



Paraffins .... (CnH2n + 2) Normal 



defines .... „ — H2 „ 



Terpenes .... „ — Hg „ -|-3 



Benzene and Derivatives „ — Hg „ -1-6 



With stiU further decrease in the quantity of hydrogen contained 

 (i.e. with further increase of carbon), there resulted greater and 

 greater refractive increments. 



The last member of the series, however — pure carbon without 

 any hydrogen, represented by the diamond — proved to be perfectly 

 normal in its optical properties. 



V. 



§ 11. Frangois Arago, when sketching in his celebrated " Eloges " 

 the life and work of Thomas Young, relates that the latter was led 

 to his fundamental discovery of the interference of light by nothing 

 more extraordinary than soap-bubbles. In this connexion Arago re- 

 marks that it is one of the most precious gifts to be able to wonder 

 at the right time. 



Gladstone's observations just mentioned had been known almost 



