1896.] On some Physical Properties of Argon and Helium. 201 



May 22 , 3-2710 



June 4 3-2617 



June 7 3 2727 



June 13 32652 



June 18 32750] 



June 25 3 2748 > 3*2746 



July 2 3 2741 J 



The results here recorded are derived from the comparison of 

 the weighings of the globe " fall " with the mean of the preceding 

 and following weighings " empty," and they are corrected for the 

 errors of the weights aud for the shrinkage of the globe when 

 exhausted, as explained in former papers. In the last series, the 

 experiment of June 13 gave a result already known to be too low. 

 The gas was accordingly sparked for fourteen hours more. Between 

 the weighings of June 18 and June 25 there was niue hours' spark- 

 ing, and between . those of June 25 and July 2 about eight hours' 

 sparking. The mean of the last three, viz., 3*2746, is taken as trie 

 definitive result, and it is immediately comparable with 2"6276, the 

 weight under similar ch'cumstances of oxygen.* If we take 2 — 16, 

 we obtain for argon 



19-940, 



in very close agreement with Professor Ramsay's result. 



The conclusion from the spectroscopic evidence that the gases 

 isolated from the atmosphere by magnesium and by oxygen are 

 essentially the same is thus confirmed. 



The Befractivity of Argon and Helium. 



The refractivity of argon was next investigated, in the hope that 

 it might throw some light upon the character of the gas. For this 

 purpose absolute measurements were not required. It sufficed to 

 compare the pressures necessary in two columns of air and argon of 

 equal lengths, in order to balance the retardations undergone by 

 light in traversing them. 



The arrangement was a modification of one investigated by 

 Fraunhofer, depending upon the interference of light transmitted 

 through two parallel vertical slits placed in front of the object 

 glass of a telescope. If there be only one slit, and if the original 

 source, either a distant point or a vertical line of light, be in focus, 

 the field is of a certain width, due to " diffraction," and inversely 

 as the width of the slit. If there be two equal parallel slits whose 

 distance apart is a considerable multiple of the width of either, the 



* ' Boy. Soc. Proc.,' vol. 53, p. 144, 1893. 

 VOL. LIX. P 



