10 Professor Dewar [Jan. 20, 



vapours are drawn over the outside of the hydrogen vacuum vessel, 

 aud this helps to isolate the liquid from the couvective currents of gas. 

 To effect proper isolation, the whole apparatus ought to be immersed 

 in liquid air under exhaustion. Arrangements of this kind add to the 

 complication, so in the first instance the liquid was used as described. 

 The liquid hydrogen evaporated quietly and steadily under a di- 

 minished pressure of about 25 mm. Naturally the liquid does not 

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

 reduction of pressure began, the resistance of the thermometer was 

 0*131 ohm. This result compares favourably with the former obser- 

 vation on the boiling point, which gave a resistance of 0* 129 ohm. On 

 reducing the pressure, the resistance diminished to * 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 one degree lower on 

 its own scale, than the boiling point at atmospheric pressure, whereas 

 the temperature 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 curve of the relation of tempera- 

 ture and resistance for No. 7 thermometer is shown on the accom- 

 panying diagram (Fig. 3). As a matter of fact, however, this platinum 

 thermometer was, when placed in liquid hydrogen, cooled at starting 

 below its own temperature of perfect conductivity, so that no exhaus- 

 tion was needed to bring it to this point. The question arises then 

 as to what is the explanation of this result ? Has the platinum resist- 

 ance thermometer arrived at a limiting resistance about the boiling 

 point of hydrogen, so that at a lower temperature its changes in re- 

 sistance become relatively small — the curve having become practically 

 asymptotic to the axis of temperature ? 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 enough. 



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, 

 however, that we can reach from 14° to 15° absolute by the evapo- 

 ration of liquid hydrogen under exhaustion. From analogy, it is 

 probable that the practicable lowering of temperature to be obtained 

 by evaporating liquid hydrogen under pressures of a few mm. cannot 

 amount to more than 10° to 12° C, and it may be said with cer- 

 tainty that, assuming the boiling point 35° absolute to be correct, no 

 means 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 thermometer had a zero point of — 263 * 2 platinum 

 degrees, and when immersed in boiling liquid hydrogen, indicated a 

 temperature of — 256*8° on the same scale, or 6*4 platinum degrees 



