SOLID HYDROOEN. 



253 



g-en itself. Hero are two vessels of the same shape as A, B, tig. 5. 

 The first contains helium, showing- no condensation when the part B 

 is cooled; the second is filled with hydrogen, which equally shows no 

 change of state under the conditions of the experiment. It is easy, 

 however, to make the hydrogen vessel show liquefaction. For this 

 purpose the experiment with the hydrogen is repeated, onl}^ before 

 doing so the part A is heated to about Hon C. over a Bunsen burner, 

 in order to increase the pressure of gas in the interior to above two 



atmospheres. Now, liquefaction is seen to take place with great 

 facility. No change is produced by similarly increasing the pressure 

 in the helium vessel. 



The extraordinary command liquid liydrogen gives us over the 

 transition of state in matter may be ])est illustrated by the use of a new 

 kind of cryophorus. Wollaston's celebrated instrument operates by 

 forcing the evaporation of water in a closed vessel by condensing its 

 vapor in a part of the receiver at a distance from the fluid, thereby 

 causing a lowering of temperature in the latter until freezing takes 

 place. Hence, the name cryophorus or cold-})earer. Instead of using 

 water we may now show that the same principle may be applied to the 

 solidification of nitrogen at a distance instead of water. The sole dif- 

 ference in this case is that the licjuid nitrogen must be isolated from 



