Index of Refraction of Liquefied Gases. 353 



compression to the Cailletet apparatus, because the heat generated in 

 becoming liquid lasts longer than with the other method, where it 

 was always kept ready in liquid state. Another disturbing influence 

 is exerted by some residual air in contact with liquid carbonic acid ; 

 discordant results led me to the fact that near to the critical tempera- 

 ture it becomes a very good solvent to air, and the index is then 

 much lowered. The gas was evolved in the tube from a mixture of 

 ammonia bicarbonate and strong nitric acid, and had to pass through 

 calcium chloride before entering into the glass cell. I expelled the 

 air as far as possible with an air-pump and allowing a little reaction 

 between the ingredients a certain quantity of carbonic acid gas was 

 liberated, which, pouring out from the capillary tube at the end of the 

 cell, filled remaining interstices and expelled remanent air, while the 

 tube was sealed. 



From numerous series of observations obtained during the last 

 years, I communicate the following numbers taken with different 

 tubes : 



D 



millims. 



I 15-98 



II 15-60 



III 16-05 



I has been taken with daylight, the two other observations with 

 the natrium light. 



To illustrate the effect of the residual air, I can present the 

 following experiments : 



I. II. 



millims. millims. Temp. 



D 16 -01 15 -98 



n 1 -193 1 -199 12-5 



1-175 1-184 17-5 



1-160 1-173 24 



It is evident that tube I having some air included, and II being 

 nearly free from air, the difference between the values at the same 

 temperature is increasing as this becomes higher (the difference is 

 greater than the probable error). It also appears in this table how 

 the index is very sensibly affected by changes of temperature ; this 

 influence is already marked in the second decimal, and the more as 

 the critical temperature is approached.* 



* The influence of the temperature on the index of refraction of liquefied car- 

 bonic acid, and the changes occurring near and above the critical temperature, will 

 probably form the subject of my next communication. 



