344 MIJ. CLIVE CUTHBERTSON ON THE 



Work on Fluorine. 



In a letter to ' Nature,' of November 13th, 1902, it was pointed out that, if there 

 were any simple relation between the refractivities of the elements of the different 

 chemical groups, it should be found to exist also in the case of fluorine, whose 

 refractivity should bear to that of chlorine the same ratio of 1 to 4 which the 

 refractivity of neon bears to that of argon. 



With the object of testing this prediction I have spent some time in attempting 

 to measure the refractive index of fluorine in the gaseous state ; but I regret that the 

 experimental difficulties due to the active nature of this element have not yet been 

 completely overcome. Sufficient experience of the nature of these difficulties has, 

 however, been gained to promise the hope of success in the near future. 



All that can at present be said is that the results given by two of the most 

 promising series of experiments are not inconsistent with the value predicted. 



In February, 1904, I visited Paris, and attempted to measure the index of the gas 

 by letting it flow through a hollow prism of copper, provided with plates of fluor 

 spar. By the kindness of M. MOISSAN, of whose great hospitality and generosity I 

 desii'e to render the most cordial acknowledgment, I was able to command a stream 

 of fluorine, prepared for me by the skill of his coadjutor, M. VITEAUX, to whom also 

 I owe my most sincere thanks. The gas was allowed to displace the air in the prism 

 till the refractive index reached a minimum, and the deflection of the image of a 

 bright slit was read on a micrometer placed in the focus of a lens of long focal length. 

 The results obtained were not always concordant, and, in the light of subsequent 

 experiments, indicate that the air was never wholly displaced. However, the best 

 experiments gave two concordant values of p = 1 '000226 and 1 "000225, and, since the 

 index of F is certainly lower than that of all other gases except H, He, and Ne, it 

 seems probable that this value is somewhat higher than the index of the pure gas. 



Experience shows that a fair guess can be made at the refractive index of an 

 element by comparing the index of a gaseous compound which contains it with that 

 of another compound in which an element of known refractivity is substituted. 

 With this object I measured the index of methyl fluoride and compared it with that 

 of methane. I have to thank Professor NOKMAN COLLIE for the loan of a specimen 

 of methyl fluoride of very great purity. Its index, determined in a JAMIN'S refracto- 

 meter, was ascertained to be fi 1 '000449. 



The index of CH 4 , as determined by M. MASCAUT, is /u, = T000441. 



Thus, the substitution of an atom of fluorine for one of hydrogen has raised the 

 index by '000008, and the inference is that the refractivity of F 2 is probably about 

 16 higher than that of H 2 . Taking the latter as equal to 139, we obtain for F 2 , 

 approximately, p. = 1 '0001 55. 



If these inferences are justified, we may believe that the index of F 3 is somewhere 

 between 1 '000226 and 1 '0001 56; a value which is, at least, not inconsistent with 



