224 HISTOEY OF COLD AND THE ABSOLUTE ZERO. 



is taken out of the liquid — but to the oxygen of the air, which is 

 well known to be a magnetic body, frozen in the wool by the extreme 

 cold. 



The strong condensing powers of liquid hydrogen afford a simple 

 means of producing vacua of very high tenuity. When one end of a 

 sealed tube containing ordinary air is placed for a short time in the 

 liquid, the contained air accumulates as a solid at the bottom, while 

 the higher part is almost entirely deprived of particles of gas. So 

 perfect is the vacuum thus formed, that the electric discharge can ])e 

 made to pass only with the greatest difficulty. Another important 

 application of liquid air, liquid hydrogen, etc., is as analj'tic agents. 

 Thus, if a gaseous mixture be cooled b}^ means of liquid oxygen only 

 those constituents will be left in the gaseous state which are less con- 

 densable than oxygen. Similarl}^, if this gaseous residue be in its 

 turn cooled in liquid hydrogen a still further separation will be 

 effected, ever3^thing that is less volatile than hydrogen being con- 

 densed to a liquid or solid. By proceeding in this fashion it has 

 been found possible to isolate helium from a mixture in which it is 

 present to the extent of only one part in one thousand. By the 

 evaporation of solid h3^drogeu under the air pump we can reach 

 within 13° or 14° of the zero, but there or thereabouts our progress 

 is barred. This gap of 13° might seem at first sight insignificant in 

 comparison with the hundreds that have already been conquered. 

 But to win one degree low down the scale is quite a different matter 

 from doing so at higher temperatures. In fact, to annihilate these few 

 remaining degrees would be a far greater achievement than any so 

 far accomplished in low-temperature research. For the difficulty is 

 twofold, having to do partly with process and partly with mate- 

 rial. The application of the methods used in the liquefaction of 

 gases becomes continually harder and more troublesome as the working 

 temperature is reduced; thus, to pass from liquid air to liquid hydro- 

 gen — a difference of 60° — is, from a thermodynamic point of view, as 

 difficult as to bridge the gap of 150° that separates liquid chlorine and 

 liquid air. By the use of a new liquid gas exceeding hydrogen in vola- 

 tility to the same extent as hydrogen does nitrogen, the investigator 

 might get to within 5° of the zero; but even a second hypothetical 

 substance, again exceeding the first one in volatility to an equal extent, 

 would not suffice to bring him quite to the point of his ambition. 

 That the zero will ever be reached by man is extremely improbable. 

 A thermometer introduced into regions outside the uttermost confines 

 of the earth's atmosphere might approach the absolute zero, provided 

 that its parts were highly transparent to all kinds of radiation; other- 

 wise it would be affected b}^ the radiation of the sun, and would there- 

 fore become heated. But supposing all difficulties to be overcome, 

 and the experimenter to be able to reach within a few degrees of the 



