6o PHYSICAL SCIENCE 



From the practical side, however, difficulties 

 accumulate and increase as the absolute zero is 

 approached. As Sir James Dewar remarked, 

 "the step between the liquefaction of air and 

 that of hydrogen is, thermodynamically and 

 practically, greater than that between the lique- 

 faction of chlorine and that of air." The boiling- 

 points of chlorine, air, and hydrogen under the 

 atmospheric pressure are — 33°f "193°) ^^^ 

 — 253° C. respectively. If we express these 

 temperatures on the absolute scale, they become 

 240°, 80°, and 20°. The interval between the 

 boiling-points of chlorine and air is 160°, but 

 the ratio of the absolute temperatures is 240 : 80, 

 or 3:1. On the other hand, while the interval 

 between air and hydrogen is only 60°, the ratio 

 of the absolute temperatures is 80 : 20, or 4:1. 

 The difficulty of the transition from one to the 

 other temperature is much more nearly pro- 

 portional to the ratio than to the difference 

 between them. 



The absolute boiling-point of hydrogen is, 

 as we have said, about 20^ and at present this 

 temperature is the lowest which we can con- 

 veniently maintain in an ordinary laboratory. 

 Any further advance towards the absolute zero 

 must be made by the help of helium. By the 

 sudden expansion of gaseous helium at a pressure 

 of 100 atmospheres and at the temperature of 

 solid hydrogen, it was estimated that a transient 

 temperature of 9° or 10° absolute was reached. 

 When that gas was liquefied. Professor Onnes 

 found that its boiling-point under the normal 

 atmospheric pressure was about 4^.5 on the 

 absolute scale. This temperature is about one- 



