ATTAINMENT OF VERY LOW TEMPERATURES. 



The temperature in the last column was calculated by linear extra- 

 polation from the resistance of the coil at temperatures corresponding 

 to — i82.5°C. and — 2o8.5°C. 



According to Dewar, though the method by which the measurements 

 were made has not been published, the critical temperature of hydro- 

 gen is about — 244°C., and the critical pressure less than 15 atmos- 

 pheres. The boiling point of hydrogen according to Travers and 

 Jaquerod* is 20.55° on the helium scale and 20.36° on the hydrogen 

 scale. Callendarf has pointed out the recalculation of Olszewski's re- 

 sults by means of his dif5ference forinula leads to the conclusion that the 

 temperatures are about one degree higher than those given above ; at 

 the same time it is possible that the platinum thermometer may behave 

 erratically at low temperatures. This latter view is confirmed by 

 Dewar4 who obtained — 243.6° and — 237.9° for the boiling point 

 of hydrogen on two platinum thermometers ; one of these values is 

 almost identical with that obtained by Olszewski. 



These facts may be taken as sufficient evidence that Olszewski 

 obtained liquid hydrogen, first in the form of a mist or spray in a glass 

 tube, and afterwards in a steel vessel. In the latter experiment it is 

 probable that the liquid at its boiling point persisted for a sufficient 

 time to cool a platinum resistance thermometer to that temperature. 



III. Theoretical Discussion of the Foregoing. 



In discussing the principles on which the experiments of Cailletet. 

 Pictet, Wroblewski and Olszewski on the one hand, and of Dewar and 

 myself on the other hand, considerable confusion has arisen. Indeed 

 several well-known text-books of physics, and even certain works 

 devoted to the study of liquefied gases, attribute the cooling which takes 

 place in every case to adiabatic expansion. Dewar's early papers make 

 no mention of the fact that in his, as in my own experiments, the cooling 

 is due to the fact that when hydrogen is cooled to the temperature of 

 liquid air it, like air or carbonic acid, becomes cooled when allowed 

 to expand without performing external work. The cooling may be the 

 result of work done against internal forces. This matter is first dis- 

 cussed in my paper on the liquefaction of hydrogen published in 1901. 



In the earlier experiments referred to above, the gas was compressed 

 either into a tube over mercury, so that in opening a cock the mercury 



* Philosophical Transactions, 1902. 

 t Philosophical Magazine, 1899, 191. 

 t Bakerian Lecture, 1901. 



