Dispersion of Fluorhenzene and Allied Compounds. 5 



employed the old formula 



P- 



P, we have calculated it also 



according to Lorenz's formula ( 2,9 ) t! tn i s gives the 



atomic refraction of fluorine at 0'92 for the line A, and 0'84 

 for the line H, showing again a minus dispersion. 



No minus dispersion-equivalent of the kind has hitherto 

 been met with, unless it be in some of the thin metallic prisms 

 examined by Kimdt,and more recently by du Bois and Rubens. 

 It is true that in iodine vapour, as in fuchsin and a few other 

 substances, the irrationality of the spectrum is so great that 

 the order of certain parts is actually reversed, but there is not 

 the regular progressive reversion exhibited in the case of 

 fluorine. 



The refraction -equivalent of fluorine had been already 

 determined by Mr. George Gladstone at from 0'29 to 0-32 

 from observations on metallic fluorides, and at from 0*59 to 

 0*84 from observations on fluosilicates. It became of interest 

 to see whether the same fluorides would indicate this reversed 

 dispersion. 



Fluor-spar. — Among the many determinations that have 

 been made of the refraction of fluor-spar there are three which 

 will answer our purpose, as they give the index of several 

 lines. Not one of them gives the density of the particular 

 specimen ; but we shall not be far wrong in taking 3*18 as 

 the specific gravity. 



Bailie (Annales du Conservatoire, vii. 1867) gives the 

 refractive indices for the lithium-line, C, D, F, and a blue 

 copper-line. From these we may calculate the molecular 

 refraction ; and from the number so obtained, by subtracting 

 the values for calcium, we get the presumed values for two 

 atoms of fluorine for different parts of the spectrum. 





Lithium red. 



! 1 

 O. D. F. 



i 



Copper blue. 



CaF 2 



Ca 



10-61 

 1007 



10-61 



10-08 



10-63 

 1015 



10-70 

 10-33 



10-77 

 10-51 



F 2 



•54 



•53 



•48 



•37 



•26 



From the figures given by Stefan ( Wiener Ber. lxiii . 1871) 

 the following may be deduced ; — - 



