I 



J. Dewar— Boiling Point of Liquid Hydrogen. 291 



Art. XXIII. — The Boiling Point of Liquid Hydrogen, deter- 

 mined by Hydrogen and Helium Gas Thermometers;* by 

 James Dewar, M.A., LL.D., Professor of Chemistry at the 

 Royal Institution. 



[Read before the Royal Society of London, February 7, 1901.] 



In a former paperf it was shown that a platinum-resistance 

 thermometer gave for the boiling point of hydrogen — 238°4 

 C, or 34r°'6 absolute. As this value depended on an empirical 

 law correlating temperature and resistance which might break 

 down at such an exceptional temperature, and was in any case 

 deduced by a large extrapolation, it became necessary to have 

 recourse to the gas thermometer.. 



In the present investigation the advantage claimed for the 

 constant-pressure gas thermometer over the constant-volume 

 thermometer is absent. The effect of high temperature com- 

 bined with large increase of pressure does not occur in these 

 experiments, where only very low temperatures and a maxi- 

 mum range of presssure of less than one atmosphere were 

 encountered. At the same time, before dispensing with the 

 effect of pressure upon the capacity of the reservoir of the 

 thermometer, it was carefully estimated and found that it could 

 not affect the volume of the reservoir by as much as l/60,000th 

 part. This being determined a particular advantage results 

 from the use of the constant-volume form, because in its case 

 it is unnecessary to know the actual volume of the reservoir, 

 and of the " outside " space. It is only necessary to know the 

 ratio of these two volumes, and as this ratio appears only in the 

 small terms of the calculation it is not a serious factor in the 

 estimation of such low temperatures. 



Two constant-volume thermometers (called No. I and No. 

 II) were employed, in each of which the volume of the reservoir 

 was about 40 c.c, and the ratio of the outside space to the 

 volume of the reservoir was 1/50 and 1/115 respectively. A 

 figure of the apparatus is shown in fig. 1 (p. 293), where A is the 

 thermometric bulb covered with a vacuum vessel to hold the 

 liquid hydrogen, and be exhausted when necessary; B is the 

 manometric arrangement for adjusting the mercury at C to 

 constant volume, and D is the barometer. The readings were 

 made on a fixed scale by means of a telescope with cross wires 

 and lever attached. A similar telescope was permanently fixed 

 on the mark to which the volume had to be adjusted. It was 

 found convenient to use both telescopes on the same massive 



* From an advance proof sent by the author 



fOn the Boiling Point of Liquid Hydrogen under Reduced Pressure, Roy. 

 Soc. Proc, 189S. 



