

J. jDewar — Boiling Point of Liquid Hydrogen. 295 



and as a further precaution the gas used in the thermometer 

 was the vapor 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 pres- 

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

 critical pressure of oxygen was taken about 51 atmospheres, 

 and that of the hydrogen about 18 atmospheres. There are 

 good reasons for believing that the critical pressure of hydro- 

 gen is more likely to be about 11 or 12 atmospheres. In the 

 event of the lower value being eventually found the more cor- 

 rect, 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°'29, thus corroborating in a satisfactory manner the 

 reliability of the method of determining the boiling point of 

 hydrogen. 



The question still remained, how far is a gas thermometer 

 to be trusted at temperatures in the neighborhood of the boil- 

 ing point of the gas with which it is filled? 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 immersed. The gold ther- 

 mometer 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 obtainable 

 by any other metal thermometer. In the result the oxygen 

 thermometer gave — 189°*61, 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 practically 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 0, 95, 

 establishing again the reliability of the method. 



As an extreme test of the method, I charged the thermom- 

 eter No. II with carbonic acid (No. 11) at an initial pressure 

 again a little less than one atmosphere, and used it to deter- 

 mine the boiling point of dry C0 2 ; 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 determine temperatures down to its own boiling 

 point. 



