[ 328 j 



• 



XXXTT. On the Refractive Indices of Liquid Nitrogen and 

 Air. By Professors Liveing and Dewar *. 



IN describing the spectrum of liquid oxygen (Phil. Mag. 

 August 1892) we gave a determination of the refractive 

 index of that liquid, and of those of some other liquefied 

 gases, for the D ray. The difficulty of such measurements 

 was mainly the mechanical one of making a hollow prism 

 with truly wrought faces which would stand the extreme 

 cold, and not convey heat from outside to the liquid within 

 so fast as to keep it in rapid ebullition. The dispersion of 

 the light by the bubbles passing through the liquid is suffi- 

 cient to blur the image of the source of light, so much as to 

 render an accurate measurement of the dispersion almost 

 impossible. And though by diminishing the pressure, and 

 thereby causing a rapid evaporation, the liquid oxygen can 

 be reduced to —200° C. and then remains quite tranquil, 

 this was found to be of little avail ; for it involved the use 

 of a hollow prism capable of holding rather a large bulk of 

 liquid, and of sustaining variation of pressure as well as of 

 temperature without leaking. We had therefore to turn our 

 attention to other methods of determining refractive indices. 

 Of these the method used by MM. Terquem and Trannin 

 seemed to be most free from difficulty, and had been success- 

 fully employed in the determination of the refractive index 

 of liquid oxygen by Olszewski and Witrowski, whose value of 

 the index agreed with ours found with a prism [Bull, de 

 V Academie de Cracovie, 1891). This method consists in 

 suspending in the liquid two plates of glass with a thin layer 

 of air between them, and measuring the angle of incidence 

 at which the chosen ray suffers total reflexion at the surface 

 of the air. 



In this method it is not necessary that the vessel containing 

 the liquid should have truly wrought surfaces, and the 

 problem seemed to be simplified by the discovery of Professor 

 Dewar that the amount of heat communicated to the vessel 

 of liquid by radiation is trifling compared with that com- 

 municated by convection. He has found that in a double 

 glass vessel of any form, where the outer and inner vessels 

 are separated by even a narrow vacuous space, liquid oxygen 

 can be kept for a great length of time open to the air. Such 

 a liquid, filtered through ordinary filter-paper to remove 

 solid carbonic acid, remains quite tranquil and beautifully 

 transparent of pale blue tint, but of course evaporating 



* Communicated by the Authors. 



