1902.] On the Measurement of Temperature. 489 



manipulate than liquid air. As its latent heat of vaporisation is very 

 high, little loss is entailed through cooling apparatus, previously- 

 cooled in liquid air, to its boiling point. Further, the liquid shows no 

 tendency to become superheated, and boils steadily, even under re- 

 duced pressure. 



Dewar* has obtained the following values for the boiling point of 

 hydrogen on the constant- volume hydrogen scale : -253 o, 03, -253°*37, 

 -252°'81, -250 o, 35; the pressure on the gas at the ice-point being 

 287 mm., 270 mm., 739 mm., and 127 mm. respectively. On the scale 

 of a thermometer filled with helium containing 7 per cent, of neon, at 

 a pressure corresponding to 728 mm. of mercury at the ice-point, he 

 found the temperature to be 252°-68 and 252 0, 84 C. In calculating 

 the temperatures on both thermometers he employed Chappuis' coeffi- 

 cient of expansion for hydrogen, 0*00366254. 



In our experiments we have employed the three small thermometers 

 which we used to determine the boiling point and vapour pressures of 

 liquid oxygen. The thermometers were filled with pure hydrogen or 

 helium, obtained by the methods described in Appendices II and III 

 to this paper. The small bulb communicating with a manometer, 

 which had in the former experiments contained pure liquid oxygen 

 for the measurement of the vapour pressure, contained, in these ex- 

 periments, pure hydrogen from palladium. This method of measuring 

 the vapour pressure was essential to the accuracy of the experiments, 

 for it appeared that the vapour pressure of the pure hydrogen, and of 

 the hydrogen in the vacuum vessel surrounding the thermometer, 

 always differed slightly, probably owing to the presence of impurities 

 dissolved in the latter. The agreement between the results obtained 

 with different thermometers containing different samples of gas is in- 

 dicated in the following table : — 



I. Hydrogen Scale. 



Temperature. 



Vapour pressures of , A N 



Thermometer. liquid hydrogen. Found. From smoothed error. 



A (12 c.c.) 757-2 mm. 20°-17 20°-21 



B (26 c.c.) 766-6 „ 20-28 20-25 



II. Helium Scale. 



A (12 c.c.) 765-0 mm. 20°-42 20°'44 



759-2 „ 20-41 20-41 



B (26 c.c.) 770-0 „ 20-43 20-46 



C (26-7 c.c.) ... 749-0 „ 20-36 20-36 



The vapour pressures of liquid hydrogen were measured on the hydro- 

 gen scale between the boiling point and a pressure of 100 mm. of mercury, 



* ' Eoy. Soc. Proe.,' vol. 68, Feb. 1901, p. 40. 



2 L 2 



