1896.] More about Argon. 7 



in the tubes. As the apparatus was at first arranged, this condition 

 was insufficiently satisfied. Although all the parts were carried upon 

 the walls of the room, frequent and somewhat sudden displacements 

 of the bands relatively to the web were seen to occur, probably in 

 consequence of the use of wood in some of the supports. The obser- 

 vations could easily be arranged in such a manner that no systematic 

 error could thence enter, but the agreement of individual measures 

 was impaired. Subsequently a remedy was found in the use of a 

 second system of bands, formed by light w^hich passed just above the 

 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 inches 

 (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 99-6 



0-01 1*77 



8-63 8-19 



Ratio = 0-961, 



signifying that 8 '53 inches of argon balanced 8*19 inches 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 respectively for the final ratio 0*962, 0*961, 0*961, 0*960, or as 

 the mean 



Eefractivity of argon 



Eefractivity of air 



= 0*961. 



The evidence from the refractivities, 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 N3. 



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

 to the most luminous region of the spectrum, say in the neighbour- 

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

 the two settings could be detected, the inference is that the dis- 

 persions 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 compensated in the above experiments amounted to about forty 

 bands, each band corresponding to about ^ inch (5 mm.) pressure of 

 mercury. 



Similar comparisons have been made between air and helium. 



