482 Mr. Gr. Gladstone on the 



of an observation taken of a solution of fluoride of potassium. 

 He gave it, however, with all reserve, remarking at the same 

 time that u fluor-spar and kryolite gave very small values for 

 fluorine, or rather indicate that this body has scarcely any 

 influence on the rays of light." In the same paper he gives 

 the specific refractive energy as 0*073, which corresponds to 

 the refraction-equivalent given above. As it stands, it is by 

 far the lowest on the list, none of the other elements having a 

 specific refraction of less than 0*1 ; but had it been calculated 

 from the results given by the minerals above mentioned, the 

 specific refraction would have been as low as 0*016. 



As the discrepancy then noted was so very wide, and special 

 interest attaches to fluorine in this connection on account of 

 its singularly small refractive power, I have thought it worth 

 while to review all the evidence upon the subject, and to 

 obtain new observations of a solution of fluoride of potassium. 

 Brewster gives 1*344 and 1*349 as the index of refraction 

 of kryolite. The mean of these will give 24*63 as the refrac- 

 tion-equivalent for Na 3 Al 2 F 6 . Deducting 13*2 for the sodium 

 and 9*7 for the aluminium, which is the most recent value 

 assigned to this metal, there will only be 1*73 left for six 

 atoms of fluorine. Thus fluorine equals 0*29. 



Of fluor-spar there are several independent observers. 

 Brewster gives 1*436 for the index of refraction for the bright 

 part of the spectrum; and Wollaston 1*433. Fizeau gives 

 1*435 for the line D ; Stephan, in 1871, gives 1*4339 ; and 

 Kohlrausch, in 1878, gives 1*4324 and 1*4342 for the same. 

 The average of these is 1*4341, which, taking the specific 

 gravity at 3*183, according to Landolfs tables, will give a 

 refraction-equivalent of 10*64 for CaF 2 . Deducting 10*0 for 

 the calcium, fluorine will equal 0*32. 



I am indebted to my brother for a redetermination of the 

 aqueous solution of fluoride of potassium, which gives 8*15 as 

 the refraction-equivalent for the line A ; deducting 7*85 for 

 the potassium, the fluorine equals 0*30. 

 These are all closely accordant. 



Messrs. Topsoe and Christiansen have taken the indices of 

 refraction and the specific gravities of a series of crystalline 

 fluorides of uniform composition. They are biaxial crystals ; 

 and the observations are given by them for the lines 0, D, 

 and F of the spectrum, both of the ordinary and extraordinary 

 ray. In the following table I have taken the mean of the two 

 rays for F, and give also the mean value for A calculated 

 from the differences between the indices for the lines C and 

 F. The refraction-equivalent of the substance is given for 



