Liquefaction of Gases. 191 



the boiling-points of ethane (17), propane (17), and hydrogen 

 selenide (19), as well as their critical temperatures and 

 pressures. The following liquids were solidified for the first 

 time by me: — methyl (14), ethyl (14), amyl alcohol (10), 

 ethyl ether (10), carbon bisulphide (14), and phosphorous 

 chloride (PC1 3 ) (14). 



In order to measure very low temperatures, I used exclu- 

 sively a hydrogen thermometer : the bulb was plunged in 

 the liquefied gas itself; only exceptionally a few not very low 

 temperatures were measured with the carton-bisulphide 

 thermometer. Wishing to ascertain how far gas-thermo- 

 meters maybe used to measure very low temperatures, 1 com- 

 pared thermometers filled with different gases, and especially 

 the hydrogen thermometer with the nitrogen, oxygen, and 

 nitric-oxide thermometers, immersing them in liquid ethylene, 

 gradually cooled to —151° G. It was proved by this that the 

 three last-mentioned thermometers indicated temperatures not 

 very different from those indicated by the hydrogen thermo- 

 meter, even at temperatures much lower than the critical 

 temperatures of the corresponding gases. 



This experiment proved at the same time that nitric oxide 

 does not change its molecular weight, corresponding to the 

 formula "NO, even at a temperature so low as —147° C. 

 Profs. Victor Meyer and Daccomo, disregarding the results of 

 my investigations, performed a similar experiment (Liebig's 

 Ann. d. Chem. cexl. p. 326), but they cooled the nitric oxide 

 with solid carbon dioxide and ether down to —70° C. only. 



As, according to my experiments on the liquefaction of 

 hydrogen, its critical temperature lies below —220° C, it 

 may be admitted that its coefficient of expansion does not, 

 even at —220° C, differ much from the coefficient of gases 

 at ordinary temperatures, and that hydrogen is the only body 

 which can be used in a thermometer for measuring very low 

 temperatures. The determinations of the temperatures by 

 measuring the quantity of heat taken away from a given 

 body, a silver ball for instance, is not precise ; for, as 

 Zakrzewski * showed, the specific heat of silver changes in 

 the interval of 0° to —100° C. by about 3 per cent. ; so that 

 the temperature which is thus determined must differ from the 

 true one to some not inconsiderable extent. Thermoelectric 

 thermometers, or thermometers based upon the variation of 

 the electric conductivity of metals at low temperatures, can 

 be used only in the limits between which they have been 

 compared with the hydrogen thermometer; every extra- 

 polation may lead to great mistakes. An excellent thermo- 

 * Bullet. Intern, of the Acad, of Cracow, April 1891, p. 146. 



