Apkil 27, 1900.] 



SGIENGE. 



649 



temperature its changes in resistance be- 

 come relatively small — the curve having 

 become practically asymptotic to the axis 

 of temperature? That is the most prob- 

 able supposition, and it further explains 

 the fact that the temperature of boiling hy- 

 drogen 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 hydro- 

 gen 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 by the evaporation of liquid hy- 

 drogen under exhaustion. From analogy, 

 it is probable that the practicable lowering 

 of temperature to be obtained by evapora- 

 ting liquid hydrogen under pressures of a 

 few mm., cannot amount to more than 10° 

 to 12° C, and it may be said with certainty 

 that, assuming the boiling point 35° abso- 

 lute 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 tem- 

 perature of— 256.8° on the same scale, or 

 6.4 platinum degrees from the point at 

 which the metal would theoretically become 

 a perfect conductor. The effect of cooling 

 platinum from the boiling point of liquid 

 oxygeu to that of liquid hydrogen is to 

 diminish its resistance to one-eleventh. 



The difiSculties in liquefying hydrogen 

 caused by the presence of air in the gas 

 have been referred to, * and later experi- 



* 'Proceedings,' 1898, 14, 130. 



ments had for their object the removal of 

 this fruitful source of trouble. This is by no 

 means an easy task, as quantities amount- 

 ing to only a fraction of one per cent, ac- 

 cumulate in the solid state, and eventually 

 choke the nozzle of the apparatus, necessi- 

 tating the abandonment of the operation. 

 Later experiments enabled me to procure 

 a larger supply of liquid hydrogen with 

 which the determination of certain ph3'sical 

 constants has been continued. The first 

 observations made with a pure platinum- 

 resistance thermometer had given — 238° as 

 the boiling point. A new thermometer, 

 constructed of platinum from a different 

 source, gave pi-actically the same value. As 

 these results might be affected by some con- 

 stant error, the determination was checked 

 by employing a thermometer constructed 

 from an alloy of rhodium and platinum, 

 containing 10 per cent, of the former. Al- 

 loys had been shown by Professor Fleming 

 and the author to diifer from pure metals 

 in showing no sign of becoming perfect con- 

 ductors at the absolute zero of temperature, 

 and a study of the rhodium-platinum alloy 

 had shown that the change in conductivity 

 produced by cooling from 0° to the boiling 

 point of liquid air is regular and may be 

 represented by a straight line. As deter- 

 mined by the rhodium-platinum thermom- 

 eter, the boiling point of hydrogen was 

 found to be — 246° or some 8° lower than 

 the platinum thermometer gave. Two 

 ways of explaining the discrepancy be- 

 tween the two values suggested themselves. 

 Pure platinum, although its resistance may 

 be represented by a straight line almost 

 down to the solidifying point of air, shows 

 signs of a departure from regularitj' at 

 about this point, and the curve may become 

 asymptotic at lower temperatures. On the 

 other hand, the resistance of the rhodium- 

 platinum alloy diminishes less rapidly at 

 these lower temperatures and is much 

 higher than that of pure platinum under 



