138 LIQUID HYDROGEN. 



air under exhaustion. Arrangements of this kind add to the complica- 

 tion, so in the first instance the liquid was used as descril)ed. The 

 liquid h3^drogen evaporated quietly and steadily under a diminished 

 pressure of about 25 millimeters. Naturally the liquid does not last 

 long, so the resistance has to be taken quickly. Just before the reduc- 

 tion of pressure began the resistance of the thermometer was 0.131 ohm. 

 This result compares favorabl}- with the former observation on the 

 boiling point, which gave a resistance of 0.129 ohm. On reducing the 

 pressure the resistance diminished to 0.114 ohm, and kept steady for 

 some time. The lowest reading of resistance was 0.112 ohm. This 

 value corresponds to — 239.1° C, or only 1° lower on its own scale 

 than the boiling point at atmospheric pressure, whereas the tem])era- 

 ture ought to have been reduced at least 5° under the assumed 

 exhaustion according to the gas-thermometer scale. The position of 

 the observation on the i-urve of the relation of temperature and resist- 

 ance for No. 7 thermometer is shown on the acconq)anying diagram 

 (fig. 3). As a matter of fact, however, this platinum thermometer was, 

 when placed in liquid hydrogen, cooled at starting })elow its own tem- 

 perature of perfect conductivity, so that no exhaustion was needed to 

 bring it to this point. The question arises, then, as to what is the expla- 

 nation of this result. Has the platinum resistance thermometer ai'i-ived 

 at a limiting resistance about the boiling point of hydrogen, so that at 

 a lower temperature its changes in resistance become relatively small — 

 the curve having l)ecome practically asymptotic to the axis of tempera- 

 tnre? That is the most probable supposition, and it further explains 

 the fact that the temperature of boiling hydrogen obtained by the linear 

 extrapolation of the resistance temperature results in values that are 

 not low enouoh. 



As the molecular latent heats of liquids are proportional to their 

 absolute boiling points, the latent heat of liquid hydrogen will be 

 about two-fifths that of liquid oxygen. It will be shown later, how- 

 ever, that we can reach from 14'-' to 15° absolute b^^ the evaporation of 

 liquid hydrogen under exhaustion. From analogy it is probable that 

 the practicable lowering of temperature to be obtained by evaporating 

 liquid hydrogen under pressure of a few milimeters can not amount to 

 more than 10^ to 12° C, and it may be said with certainty that, assum- 

 ing the boiling point 35° absolute to l)e correct, no meai]S are at present 

 known for approaching nearer than 20° to 25° to the absolute zero of 

 temperature. The true boiling point is in reality about —252° C, in 

 terms of the gas-thermometer scale, and the latent heat of the liquid 

 is therefore about two-ninths that of an equal volume of oxygen, or 

 one-fourth that of liquid nitrogen. The platinum-resistance thermom- 

 eter had a zero point of —263.2 platinum degrees, and when immersed 

 in boiling liquid hydrogen indicated a temperature of —266.8° on the 

 same scale, or 6.4 platinum degrees from the point at which the metal 



