SOLID HYDROGEN. 



253 



gen itself. Here are two vessels of the same shape ;is A. B, fig. 5. 

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

 is cooled; the second is tilled 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, only before 

 doing so the part A is heated to about 300 C. over u Bunsen burner, 

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



atmospheres. Now, liquefaction is ^acn to take place with great 

 facility. No change is produced by similarly increasing the pressure 

 in the helium vessel. 



The extraordinary command liquid hydrogen gives us over the 

 transition of state in matter may be best 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-bearer. 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 liquid nitrogen must be isolated from 



