Apeil 27, 1900.] 



SCIENCE. 



647 



and that the critical pressure will probably 

 not exceed 15 atmospheres. 



If we assume the resistance reduced to 

 zero, then the temperature registered by the 

 electric thermometer ought to be —244° C. 

 At the boiling point of hydrogen, registered 

 by the electric-resistance thermometer, if 

 the law correlating resistance and temper- 

 ature can be pressed to its limits, a lowering 

 of the boiling point of hydrogen by 5° or 6° 

 C. , would therefore produce a condition of 

 affairs in which the platinum would have no 

 resistance, or would become a perfect conduc- 

 tor. Now we have every reason to believe 

 that hydrogen, like other liquids, will boil at 

 a lower temperature the lower the pressure 

 under which it is volatilized. The question 

 arises, how much lowering of the temper- 

 ature can we practically anticipate? For 

 this purpose we have the boiling point given 

 by the hydrogen gas thermometer, and critical 

 data available, from which we can calculate 

 an approximate vapor pressure formula, ac- 

 cepting 22° absolute as about the boiling 

 point, 33° absolute as the critical temper- 

 ature, and 15.4 atmospheres as the critical 

 pressure ; then, as a first approximation — 



1 a AM, 77.62 

 log. p = 6.410 = mm. 



(1) 



If, instead of using the critical pressure 

 in the calculation, we assume the molecular 

 latent heat of hydrogen to be proportional 

 to the absolute boiling point, then, from a 

 comparison with an expression of the same 

 kind, which gives accurate results for oxy- 

 gen tensions below one atmosphere, we 

 can derive another expression for hydro- 

 gen vapor pressures, which ought to be 

 applicable to boiling points under reduced 

 pressure. 



The resulting formula is — 



log.i3= 7.0808—^ mm. ... (2) 



Now formula (1) gives a boiling point of 

 14 . 2° absolute under a pressure of 25 mm., 



whereas the second equation (2) gives for 

 the same pressure 15 . 4° absolute. As the 

 absolute boiling point under atmospheric 

 pressure is about 22°, both expressions lead 

 to the conclusion that ebullition under 25 

 mm. pressure ought to reduce the boiling 

 point some 7° C. For some time experi- 

 ments have been in progress with the 

 object of determining the temperature of 

 hydrogen boiling under about 25 mm. pres- 

 sure, by the use of the platinum thermom- 

 eter ; but the difficulties encountered have 

 been great, and repeated failures very ex- 

 asperating. The troubles arise from the 

 conduction of heat by the leads ; the small 

 latent heat of hydrogen, volume for volume, 

 as compared with liquid air ; the inefficiency 

 of heat isolation ; and the strain on the 

 thermometer, brought about by solid air 

 freezing on it and distorting the coil of wire. 

 In many experiments, the result has been 

 that all the liquid hydrogen has evaporated 



Fig. 2. 

 before the pressure was reduced to 25 mm., 

 or the thermometer was left imperfectly 

 covered. The apparatus employed will be 

 understood from Fig. 2. The liquid hydro- 

 gen collected in the vacuum vessel A was 

 suspended in a larger vessel of the same 

 kind, B, which is so constructed that a 



