CHANGE OF STATE LIQUID V A POUfc. 



199 



plug experiment of Thomson and Joule (chap, xix.), and the cool- 

 ing is chiefly due to the conversion of molecular kinetic energy 

 into potential energy in the work done against the internal forces. 

 At the ordinary density of a gas so few of the molecules are at any 

 one instant near each other, that we may consider that all the 

 molecules have the potential energy of complete separation from their 

 fellows. But when the gas is at several hundred atmospheres, and 

 still more when it is cooled far on its way to the absolute zero, a very 

 large proportion of the molecxales are close together and are exerting 

 cohesive forces. If now expansion occurs, work has to be done in 

 separating them, or at any rate in separating a very large proportion of 

 them, and cooling results. 



In Joule and Thomson's researches hydrogen alone showed no certain 

 evidence of such cooling, but even hydrogen at the pressures used in the 

 regenerative method appears to cool with the rest when it is at a 

 sufficiently low temperature initially. 



Though hydrogen was not liquefied in the earlier experiments, 

 Olzewski was able to reduce it to a very low temperature, and to calcu- 

 late its boiling-point from its behaviour, and in 1898 Dewar succeeded 

 in obtaining and keeping it liquid by cooling it first to - 205 0. at a 

 pressure of 180 atmospheres, and then allowing it to issue from a nozzle 

 into a vacuum vessel.* In 1899 he succeeded in obtaining solid 

 hydrogen. A small vacuum vessel containing liquid hydrogen was 

 suspended in a larger one also containing the liquid, and on allowing 

 evaporation to take place below 60 mm., a slight air leak caused the 

 hydrogen to solidify as a white froth-like foam in the inner vessel 

 (Nature, lx., 1899, p. 514). 



The temperatures of the boiling points and melting points of these 

 ordinary gaseous substances are not found with perfect consistency 

 with different instruments, but the various experimenters obtain results 

 not very far apart. 



The following are some of these (Travers, Study of Gases, p. 247) : 



* Science Abstracts, i. 1898, p. 562. An account of the liquefaction of hydrogen 

 is given by Travers in the Phil. Mag., 1901, i. p. 411. 

 t Andrews. 



