1905.] on the Development of Spectro-Chemistry. 131 



diamond, that a progressive removal of hydrogen and increase of 

 carbon need not lead to the slightest optical anomaly. 



At the same time there arises here a case of the optical influence 

 of isomerism, for hexylene, which has already been mentioned, with 

 the same formula (CgHis) as hexamethylene, but in stracture an 

 define : — 



CI12 — CH • CH2' CH2' CH2' CH3 



possesses the familiar refractive increment of 2 units. This example 

 again shows how the spectro-chemical behaviour of a body discloses its 

 chemical structure by enabling us to distinguish with certainty 

 between an optically normal cycloid (or ring-substance), and an 

 isomeric open-chain olefinoid formation, which is optically abnormal. 



VI. 



§ 17. Carbon can thus act variously upon light according to the 

 manner in which its atoms are combined. We can therefore transfer 

 the refractive increment of the double bond to the atom itself. 



In the diamond, and in all paraffinoid carbon compounds, the 

 atomic refraction of carbon equals 5 ; it is therefore equal to 10 for 

 two carbon atoms. The double bond increases the refraction by 2, 

 so that for two carbon atoms with a double bond the refraction 

 amounts to 12. The atomic refraction of one carbon atom with a 

 double bond is therefore equal to 6, i.e. 20 per cent, greater than that 

 of the atom with the sins^le bond : — 



1 Carbon atom C (diamond and paraffins) 



2 Carbon atoms 2C (diamond and paraffins) 

 Double bond ._ . 



2 Carbon atoms with a double bond C — C 



1 Carbon atom with a double bond C — 



Atomic 

 Refraction. 



5 



10 



2 



12 



§ 18. Carbon, being a quadrivalent element, can also appear with 

 triple bonds : — 



R.C=C.R 



Experiment has shown that carbon with a triple bond also acquires a 

 special atomic refraction. 



Thus it becomes possible to establish the presence of this kind of 

 bond in substances, and to distinguish it from the double and simple 

 bonds — a further criterion of structure. 



§ 19. In consequence of these discoveries it became highly pro- 

 bable that all multivalent elements, such as carbon, possessed an 

 atomic refraction varying with the kind of bond, while the univalent 

 elements, such as hydrogen, display constant optic values because 

 atoms such as theirs can only be linked with a simple bond. 



K 2 



