OCTOBKE 10, 1902.] 



SCIENCE. 



575 



of the solid, and in the other by condensa- 

 tion from the atmosphere. A tuft of cot- 

 ton-wool soaked in the liquid and then held 

 near the pole of a strong magnet is at- 

 tracted, and it might be inferred therefrom 

 that liquid hydrogen is a magnetic body. 

 This, however, is not the case : the attrac- 

 tion is due neither to the cotton-wool nor 

 to the hydrogen — which indeed evaporates 

 almost as soon as the tuft 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 pro- 

 ducing vacua of very high tenuity. When 

 one end of a sealed tube containing ordi- 

 nary 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 be made to 

 pass only with the greatest difficulty. An- 

 other important application of liquid air, 

 liquid hydrogen, etc., is as analytic agents. 

 Thus, if a gaseous mixture be cooled by 

 means of liquid oxygen, only those con- 

 stituents will be left in the gaseous state 

 which are less condensable than oxygen. 

 Similarly, if this gaseous residue be in its 

 turn cooled in liquid hydrogen a still fur- 

 ther separation will be effected, everything 

 that is less volatile than hydrogen being 

 condensed to a liquid or solid. By pro- 

 ceeding 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 evapora- 

 tion of solid hydrogen under the air-pump 

 we can reach within 13 or 14 degrees of 

 the zero, but there or thereabouts our pro- 

 gress is barred. This gap of 13 degrees 

 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 dif- 

 ferent 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 ac- 

 complished 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 con- 

 tinually harder and more troublesome as 

 the working temperature is reduced; thus, 

 to pass from liquid air to liquid hydrogen 

 —a difference of 60 degrees— is, from a 

 thermodynamic point of view, as difficult 

 as to bridge the gap of 150 degrees that 

 separates liquid chlorine and liquid air. 

 By the use of a new liquid gas exceeding 

 hydrogen in volatility to the same extent 

 as hydrogen does nitrogen, the investigator 

 might get to within five degrees of the 

 zero; but even a second hypothetical sub- 

 stance, 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 ab- 

 solute zero, provided that its parts were 

 highly transparent to all kinds of radia- 

 tion, otherwise it would be affected by the 

 radiation of the sun, and would therefore 

 become heated. But supposing all difficul- 

 ties to be overcome, and the experimenter 

 to be able to reach within a few degrees 

 of the zero, it is by no means certain that 

 he would find the near approach of the 

 death of matter sometimes pictured. Any 

 forecast of the phenomena that would be 

 seen must be based on the assumption that 

 there is continuity between the processes 

 studied at attainable temperatures and 

 those which take place at still lower ones. 

 Is such an assumption justified ? It is true 



