Refraction and Dispersion of Gaseous Compounds. 601 



Element. 



Position of 



absorption band 

 observed. 



Wave-length of 



free vibrations 



(calculated). 





3580-3020 T7fin 



Iodine 



f visible -3832 ) 

 12500-limitsof ob. j 



5000 



3352 

 4560 



begins to be flatter than that which the simple theory 

 demands a long way from the free frequency. It is unfor- 

 tunate that we have no determinations of the coefficients of 

 absorption in the case of these gases, by the use of which a 

 closer approximation might be obtained. 



In the case of the sulphur compounds H 2 S and S0 2 we 

 find that the evidence points in the same direction. Here,, 

 also, we have an element of high refractive, dispersive, and 

 absorptive power united to one of low refractive and dis- 

 persive power, and free from absorption; and here again the 

 resulting compound shows a decrease of refraction, a dis- 

 persive power intermediate between those of the two com- 

 ponents, and a disappearance of the absorption band charac- 

 teristic of the sulphur molecule. The same hypothesis as in 

 the case of the halogens will account for the direction of 

 these changes ; but it must be admitted that it is not so 

 successful in accounting for their magnitude. The ex- 

 pressions which I obtain for the atomic refraction of one 

 atom of sulphur are 



3-4168 xlO 27 , 2*7780 x 10 27 



an( 



i7078xl0 27 -n 2 7384xl0 27 -^' 



when calculated from H 2 S and S0 2 respectively. But there 

 is room for many important disturbing factors in the com- 

 plexity of the molecules both of sulphur and of the compounds. 

 A comparison of the cases of NO, N 2 and NH 3 shows that 

 the number of atoms in the molecule has a considerable effect 

 on the interatomic portion of the ref ractivity. 



Turning now to the group of compounds in which the 

 refractivity is higher than the sum of the refractivities of its 

 constituents, we observe that they are composed of elements 

 like hydrogen, nitrogen, and oxygen whose refractive and 

 dispersive power is small, so that it might be expected, on 

 our hypothesis, that, when their molecules break up, their 

 loss of these powers would be slight, while, when they unite 



