1896.] On some Physical Properties of Argon and Helium. 205 



tubes, to which, instead of to the web, the movable bands were referred. 

 The coincidence of the two systems could be observed with accuracy, 

 and was found to be maintained in spite of movements of both rela- 

 tively to the web. 



In the comparisons of argon and air (with nearly the same re- 

 fractivities) the changes of pressure employed were about 8 in. 

 (20 cm.), being deductions from the atmospheric pressure. In one 

 observation of July 26, the numbers, representing suctions in inches 

 of mercury, stood 



Argon. Air. 

 8*54 9'96 

 0-01 177 



8-53 8-19 

 Ratio = 0-961, 



signifying that 8*53 in. of argon balanced 8' 19 in. of dry air. Four 

 sets, during which the air and argon (from the globe as last filled for 

 weighing) were changed, taken on July 17, 18, 19, 26. gave respec- 

 tively for the final ratio 0'902, 0*961, 0'961, 0"960, or as the mean 



Refractivity of argon 0'961 



Refractivity of air 



The evidence from the ref ractivities, as well as from the weights, 

 is very unfavourable to the view that argon is an allotropic form of 

 nitrogen such as would be denoted by N 3 . 



The above measurements, having been made with lamp-light, refer 

 to the most luminous region of the spectrum, say in the neighbourhood 

 of D. But since no change in the appearance of the bands at the 

 two settings could be detected, the inference is that the dispersions 

 of the two gases are approximately the same, so that the above ratio 

 would not be much changed, even if another part of the spectrum 

 were chosen. It may be remarked that the displacement actually 

 conlpensated in the above experiments amounted to about forty bands, 

 each band corresponding to about \ in. (5 mm.) pressure of mercury. 



Similar comparisons have been made between air and helium. 

 The latter gas, prepared by Professor Ramsay, was brought from 

 London by Mr. W. Randall, who further gave valuable assistance in 

 the manipulations. It appeared at once that the refractivity of 

 helium was remarkably low, 13 in. pressure of the gas being balanced 

 by less than 2 in. pressure of air. The ratios given by single com- 

 parisons on July 29 were 0147, 0"146, 0'145, 011-6, mean 0-146 ; and 

 on July 30 0-147, 0°147, 145, 0-145, mean 0'146. The observations 

 were not made under ideal conditions, on account of the smallness of 



