48 Prof. J. Dewar. The Boiling Point of Liquid Hydrogen, 



quantity too large to escape detection. This experiment gave the 

 boiling point of oxygen as - 182 0, 2, and that of hydrogen as 

 -253°-0. 



In the second experiment (No. 2) a new thermometer, No. II, was- 

 constructed with a much smaller value of x, and as a further protection 

 against the presence of impurities, palladium hydrogen was employed as 

 the source of the gas. A rod of palladium, weighing about 120 

 grammes, kindly placed at my disposal by Mr. George Matthey, 

 F.B.S., was charged with hydrogen in the manner described in my 

 paper " On the Absorption of Hydrogen by Palladium at High Tem- 

 peratures and Pressures,"* and subsequently used as the source of 

 supply to fill the thermometer. The initial pressure was slightly less 

 than that in the first experiment ; the corresponding results were 

 -182°-67 and -253°'37.f 



The new thermometer was filled afresh (No. 4) with palladium 

 hydrogen at an initial pressure rather less than one atmosphere, and 

 gave for the boiling point of hydrogen the temperature - 252 °'8. 

 This result is a confirmation of the absence of polymerisation. 



The next step was to compare these results with the results of 

 similar experiments made upon another gas whose boiling point fell 

 within the range of easily determined temperatures ; and as a further 

 precaution the gas used in the thermometer was the vapour rising from 

 the liquefied gas whose boiling point was to be determined. The gas 

 first selected was oxygen (No. 5), and as an additional condition to be 

 noted, the initial pressure was made slightly more than an atmosphere, 

 so that it would be in a Van der Waal's " corresponding " state with the 

 hydrogen in the first two experiments, namely, the initial pressure in 

 each case was about 1/50 of the critical pressure. The critical pressure 

 of oxj^gen was taken about 51 atmospheres, and that of the hydrogen 

 about 18 atmospheres. There are good reasons for believing that the 

 critical pressure of hydrogen is more likely to be about 11 or 12 atmo- 

 spheres. In the event of the lower value being eventually found the more 

 correct, the effect as between the oxygen thermometer and the hydrogen 

 thermometer will be to make the boiling point of hydrogen a little too 

 high. The result obtained from this experiment was to place the boiling 

 point of oxygen at - 182° '2 9, thus corroborating in a satisfactory 

 manner the reliability of the method of determiniug the boiling point 

 of hydrogen. 



The question still remained, How far is a gas thermometer to be 

 trusted at temperatures in the neighbourhood of the boiling point of 

 the gas with which it is filled 1 To answer this question the oxygen 

 thermometer was used to determine the boiling point of liquid air 

 (No. 7) in which a gold-resistance thermometer was simultaneously 

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



f This thermometer gave 99 c- 7 for the boiling point of water; 



