Chemistry and Physics. 307 



double effect is visible, for liquid air is produced both in the 

 inside and on the outside of the tube — in the one case by the 

 melting of the solid, and in the other by condensation from the 

 atmosphere. A tuft of cotton-wool soaked in the liquid and then 

 held near the pole of a strong magnet is attracted, and it might 

 be inferred therefrom that liquid hydrogen is a magnetic body. 

 This, however, is not the case : the attraction 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 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 be made to pass only with the greatest 

 difficulty. Another 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 constituents 

 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 further separation will be effected, 

 everything that is less volatile than hydrogen being condensed 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 hydrogen under the air-pump we can reach 

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

 progress is barred. This gap of 13 degrees might seem at first 

 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 remainino- deg-rees 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 material. The application 

 of the methods used in the liquefaction of gases becomes con- 

 tinually harder and more troublesome as the working tempera- 

 ture 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 sepa- 

 rates 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 



