LORD KELVIN 307 



the gases is so arranged that the gas cooled after passing the 

 constriction flows back over the pipes carrying the gas arriv- 

 ing at the constriction, and thus cools it. In this way, the 

 fall of temperature continually increases if sufficient time is 

 allowed, until the critical temperature ( — 1 19° C in the case of 

 oxygen, — 146° C for nitrogen) is reached and passed, and 

 from this point the apparatus begins to deliver a continuous 

 stream of liquid air, it being of course necessary to continually 

 replace the liquid drawn off by a corresponding amount of 

 gaseous air. One litre of liquid air requires about one cubic 

 metre of gaseous air, which fact gives us a direct idea ©f the 

 comparatively great molecular distance in the gaseous state 

 as compared with the liquid state. The success of this 

 method is one of many examples of the fact that every 

 natural phenomenon, even though at first it appears in so 

 minute a form as hardly to be perceptible, can nevertheless 

 be raised in its effects to any degree, as soon as the laws of 

 it are known. 



After air, hydrogen and the most hardly liquefiable 

 gas of all, helium, have been liquefied, and finally also 

 solidified. Although the difficulty was very great, no new 

 fundamental knowledge was gained by the process. How- 

 ever, the liquefaction of these gases gave us the means of 

 carrying out entirely novel investigations at the lowest tem- 

 perature; for if liquid helium, for instance, is being used, we 

 are certain of maintaining steadily a temperature which 

 cannot be greater than the critical temperature of this sub- 

 stance — 268°C, that is only 5°C above absolute zero, below 

 which the temperature scale does not extend, since at that 

 temperature the motion of the molecules ceases, and their 

 kinetic energy, which is the measure of their temperature, 

 becomes zero. 



