490 



On the Measurement of Temperature. [June 19, 



and on the helium scale between the boiling point and a pressure of 

 50 mm., by a method which is described in detail in the full paper. 

 The temperatures on the helium and hydrogen scales were found 

 to differ to a greater extent than at the temperature of liquid oxygen. 

 The difference, as the following table shows, is from 0°*19 to 0°*21 

 over the range of temperature investigated. Considering that the 

 critical .point of hydrogen lies about 35° Abs., while that of helium 

 is probably in the neighbourhood of 10° Abs., this difference is not 

 surprising. 



The Vapour Pressures of Liquid Hydrogen. 



Pressure in Temperatures on Temperatures on 



millimetres. the hydrogen scale. the helium scale. 



800 20°-41 20°'60 



760 20-22 20-41 



700 19-93 20-12 



600 19-41 19-61 



500 18-82 19-03 



400 18-15 18-35 



300 17-36 17-57 



200 16-37 16-58 



100 14-93 15-13 



50 — 14-11 



Appendix I — The melting point of hydrogen was found to be 14°-1 

 on the helium scale; the temperature given by Dewar* in 1901 is 

 16°, but an earlier measurement by himf gives the melting pressure 

 as 55 mm. The details of the experiments cannot be entered into in 

 this abstract. 



Appendix II. — The pure hydrogen used in our thermometers, &c, 

 was obtained by means of spongy palladium. The method of purify- 

 ing the gas is given in detail. 



Appendix III. — The gas from the Bath wells is not a good source of 

 helium for thermomebric purposes since it contains much neon, and the 

 latter, as we shall presently show, has a considerable vapour pressure 

 at the temperature of liquid hydrogen, and cannot be completely 

 separated from the helium. 



Pure helium is most readily obtained from clevite gas, which appears 

 to contain only helium, argon, and a trace of krypton. The gas used 

 in our experiments was passed through a glass coil immersed in liquid 

 hydrogen boiling under normal pressure in one case (thermometer A), 

 and under a pressure of 110 mm. of mercury in another (thermometers 

 B and C). This helium was probably very pure. 



* 'Boy. Soc. Proc.,' vol. 68, p. 360. 

 t ' Nature,' Sept. 21, 1899. 



