Index of Refraction of Liquefied Gases. 331 



was done by Faraday, as these always exploded. The compression 

 pump of Cailletet, therefore, was extremely useful in this case, and I 

 applied it with its glass tube modified as described in the former 

 chapter. Enough liquid was produced to fill the cell at the top 

 completely at a pressure of nearly sixty atmospheres ; a series of 

 observations taken in 1879 led to D = 16'34 millims., with natrium 

 light cZ=2-64 millims., =M93 at 16, with daylight =M96, also 

 at 16. Later on, in 1883, when controlling my former experiments 

 with another glass cell, I found D = 15'56 millims., with natrium light 

 cZ=2'53 millims., and n=T194 at 15, with daylight w=l'198. 

 These results are almost identical. 



5. Phosphoretted Hydrogen (PH 3 ). 



The liquefaction of this gas was also readily effected with the aid 

 of the Cailletet pump. It was prepared from a mixture of phos- 

 phonium iodide and a weak solution of potash,* evolving by their 

 reaction a thoroughly pure gas ; at a pressure of about thirty atmo- 

 spheres a perfectly clear liquid filled the glass cells. 



The experiments gave D = 15'll millims., with natrium light 

 <2=3'64 millims., w=l'317, at the temperature 17'5 with daylight 

 w=l-323 at 11. 



6. Sulphuretted Hydrogen (H 2 S). 



A series of observations was taken in 1879 with, the aid of the 

 compression pump, and the following values on experimenting with 

 the liquefied substance ensued: D = 15'25 millims., with natrium 

 light cZ=4'19 millims., w=l'380 at a temperature of 12 0< 5. 



Later on I repeated the experiments, using the faradaic tube, and 

 liquefied the gas by its own pressure. I obtained D = 15 77 millims., 

 _rf=4'38 millims., with natrium light w=l'384 at 18'5, with daylight 

 =1'390 at the same temperature. Instead of leaden disks tinfoil 

 was applied to obtain a -perfectly closed vessel, and to prevent any 

 chemical reaction. 



7. Hydrochloric Acid (HC1). 



This gas was evolved in a suitable bent glass tube from a mixture 

 of ammonia chloride and strong sulphuric acid; the very little gas 

 bubbles (evolved under increasing pressure exerted by the gas itself) 

 had to pass, of course, through a long column of this acid, and were 

 thus deprived of any moisture. The liquefied gas did not at all 

 attack the leaden disks used between the glass plates of the cell 



The observations led to D = 14'04 millims., with the natrium light 

 r/=2'81 millims., w=l'252 at 16'5. Another set of observations 



* This method was first proposed by Hofmann, " Ber. Chem. Gesellsch.," 1873, 

 I3d. iv, S. 100. 



