determined by Hydrogen and Helium Gees Thermometers. 49 



immersed. The gold thermometer had been previously tested and 

 found to give correct indications of temperature down to temperatures 

 not only well below the point in question, but lower than those obtain- 

 able by any other metal thermometer. In the result the oxygen ther- 

 mometer gave. - 189°"62, and the gold thermometer - 189°*68, as the 

 temperature of that particular sample of air boiling at atmospheric 

 pressure. 



For another method of comparison this oxygen thermometer was 

 partially discharged (No. 8) until its initial pressure was nearly the 

 same as that in the first hydrogen thermometers. In this state it gave 

 the boiling point of oxygen as - 182° '95, establishing again the reli- 

 ability of the method. All the boiling points of the liquid gases were 

 made on samples produced at different times. 



As an extreme test of the method, I charged the thermometer No. II 

 with carbonic acid (No. 11) at an initial pressure again a little less than 

 one atmosphere, and used it to determine the boiling point of dry CO* ; 

 the result was - 78°'22, which is the correct value. 



Hence it appears that either a simple or a compound gas at an initial 

 pressure somewhat less than one atmosphere, may be relied on to deter- 

 mine temperatures down to its own boiling point, in the constant 

 volume gas thermometer. 



Another thermometric substance at our disposal, as suitable for 

 determining the boiling point of hydrogen as hydrogen had been in 

 determining that of oxygen and other gases, is helium. The early 

 experiments of Olszewski and my own later ones showed that pure 

 helium is less condensible than hydrogen, and that the production of 

 liquid or solid products by cooling Bath helium to the temperatures of 

 boiling and solid hydrogen was only partial, and resulted from the 

 presence of other gases undefined at the time the experiments were 

 made. The mode of separating the helium from the gases given off' by 

 the King's Well at Bath is fully described in my paper on " The Lique- 

 faction of Air and the Detection of Impurities."* 



If the neon, present as impurity in the Bath helium which was used, 

 should reach its saturation pressure about the boiling point of hydro- 

 gen, the values given by this thermometer for the boiling point of 

 hydrogen would be too low. In order to avoid this, the crude helium 

 extracted from the Bath gas was passed through aU-tube cooled by liquid 

 hydrogen to condense out the known impurities —oxygen, nitrogen, and 

 argon. In my paper " On the Application of Liquid Hydrogen to the 

 production of High Vacua/'f it was shown that at the temperature of 

 boiling hydrogen, oxygen, nitrogen and argon have no measurable ten- 

 sion of vapour, and that the only known gases uncondensed in air after 

 such cooling were hydrogen, helium, and neon. This same neon material 



* 'Chem. Soc. Proc.,' 1897. 

 f 'Boy. Soc. Proc.,' 1898 (vol. 64, p. 231). 

 VOL. LXVIII. E 



