Index of Refraction of liquefied Gases. 355 



confirmed actually by experiment as 9'2. In 1882 Cailletet* con- 

 structed a special apparatus to prepare liquid ethylene in great 

 quantities, and pointed it out as a means of obtaining a much lower 

 degree of cold than was possible before, and this opened the way for 

 the liquefaction of oxygen and hydrogen. 



For my purpose I prepared readily a sufficient quantity of the 

 liquefied gas with the ordinary compression pump of Cailletet, taking 

 care to use a very pure gas obtained in the usual way. I collected 

 the liquid in a tube divided in calibrated parts, and could thus 

 determine the specific gravity at different temperatures after the 

 method described by Ansdell, f and applied by him to liquid 

 hydrochloric acid and acetylene. My experiments gave at 8 0'335, 

 at 6, 0-361, at 3, 0'386. For acetylene Ansdell observes that its 

 specific gravity being 0'450 at 0, it is the lightest fluid substance 

 known ; it is surpassed in this regard by liquid ethylene, that has 

 now to be considered as such. 



A second tube provided at one end with the parallel- sided vessel 

 already described, at the other end with a specially constructed iron 

 connecting piece,! an( ^ stopcock cemented on the tube, served for 

 observation of the index of refraction. After the column of mercury in 

 the pump had driven the liquefied gas into the vessel, the stopcock 

 was firmly screwed down, and now the tube with its contents could 

 be removed from the apparatus, and was always ready for observation. 

 Determinations of the index of liquid ethylene could only be made of 

 course when the temperature of the surrounding atmosphere was 

 below 9"2 ; I had them in free air, but the winter being very mild 

 this year, I had no opportunity to observe below 5. The effect of a 

 slight variation of temperature on the refraction is much more 

 marked than with carbonic acid, as was to be expected, and renders 

 an exact result rather difficult. I obtained under satisfactory condi- 

 tions .= 1'180 for daylight at 6. 



V. Conclusions, 



Some investigations of a mathematical character have been pub- 

 lished in later years that treated of the relation of the refractive power 

 of the same substance considered in different states. The theoretical 



* Cailletet : " Comptes Eendus," 1882, t. 94. He mentions on p. 1224 as the 

 critical temperature 13, and Sarrau (p. 846) nearly 8 or 9. I found the same 

 value as gi?en by v. d. Waals. 



t Ansdell : " Proc. Roy. Soc.," vol. 29, 1879, p. 221. 



J Such a connecting piece is a most useful appendix to the compression pump, 

 and is constructed also by Ducretet, at Paris. It is very accurately wrought. I 

 kept for more than ten days the liquid ethylene at a pressure from sixty to seventy 

 atmospheres without perceptible leakage, in a tube having the stopcock turned 

 down. 



