34 REPOKT— 1902. 



On the removal of the loose plug of cotton-wool used to cover the 

 mouth of the vacuum vessel in which it is stored, the action is followed 

 by a miniature snowstorm of solid air, formed by the freezing of the 

 atmosphere at the point where it comes into contact with the cold vapour 

 rising from the liquid. This solid air falls into the vessel and accumulates 

 as a white snow at the bottom of the liquid hydrogen. When the 

 outside of an ordinary test-tube is cooled by immersion in the liquid, 

 it is soon observed to fill up with solid air, and if the tube be now lifted 

 out a double efiect 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 afibrd a simple 

 means of producing vacua of veiy 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, &c., 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 

 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 material. 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 hydrogen — 

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



