1905.] on the Development of Spectro-Chemistnj. 127 



However, I did not allow myself to be discouraged ; and, behold ! 

 my expectations were confirmed by the very first experiment. The 

 define examined not only proved to be optically abnormal, but gave 

 the predicted refractive increment of 2 units, corresponding to the 

 presence of one double carbon bond. 



Gladstone, therefore, as I had supposed, was mistaken in this case. 

 Further experiments proved that not one of the olefines was optically 

 normal. Without exception they gave the refractive increment of 

 2 units, one-third of that of benzene. 



I next proceeded to examine the diolefines — substances which con- 

 tain two double carbon bonds. Here also, in conformity with ex- 

 pectation, a constant refractive increment was found, double as large 

 as that of the olefines and two-thirds of that of benzene : — 



Paraffins .... (CnH2TO + 2) Normal 



Olefines .... „ — Hg „ + 2 



Diolefines .... „ - — H4 „ + 4 



Benzene compounds . „ — Hg „ -|- 6 



The dimensions of our subject this evening prevent the detailed 

 demonstration of these important facts by experiment. I will only 

 show you that the spectrum of a saturated hydrocarbon (a paraffin) is 

 distinguishable at a glance from that of a substance containing double 

 bonds. 



On this screen we project the electric spectrum of metallic calcium. 

 First we cause the rays of light to pass through a prism filled with 

 paraffin-oil. Then we exchange this prism for another, filled with a 

 substance containing atoms linked by double bonds. (Experiment.) 



In the second case you observe, firstly, a much greater deviation 

 of the whole spectrum, i.e. greater refraction, and secondly, far wider 

 intervals between the coloured lines of the spectrum, i.e. greater 

 dispersion, which is usually correlative to the refraction. 



§ 13. Thus quantitative experimental confirmation was obtained 

 for the view that abnormal refractive increments which increase with 

 the diminution of hydrogen contained in the substances, are caused 

 by the presence of double carbon bonds. 



At the same time, however, the experiments yielded a second 

 result of fundamental importance. The olefines contain 2, and the 

 diolefines 4 atoms of hydrogen less than the paraffins. Similarly the 

 refractive increment of the olefines is 2, and of the diolefines 4. 



Benzene, CsHg, contains 8 atoms of hydrogen less than the cor- 

 responding paraffin, hexane, C6H14. The increment of benzene,, 

 however, amounts not to 8, but to 6 ! Thus in the formation of ben- 

 zene from hexane, 2 atoms of hydrogen have been eliminated without 

 influence on the refractive increment of the product. 



But in the formation of benzene from hexane, 2 atoms of hydro- 

 gen have been employed to close the ring (see diagram on p. 126). 



