Manchester Memoirs, Vol. liii. ( 1 909), No. 17. 3 



Now Moissan has shown that when sodium is heated 

 to temperatures between 340° C. and 430° C. some hydride 

 must be dissolved in the molten metal, since by the action 

 of liquid ammonia (in which sodium is soluble) at — 40°C., 

 a residue remains which consists of nearly pure hydride. 

 Troost and Hautefeuille from their observations, concluded 

 that sodium when heated to 380° C. for a long time 

 absorbed a volume of hydrogen consistent with the 

 formation of a compound of composition Na^H but that 

 this compound began to dissociate about 300° C, the 

 amount of dissociation rising rapidly with the temperature, 

 till at 421° C. the tension of the hydrogen in the com- 

 pound is 760 m.m. Hence, at this temperature, none of 

 the compound is formed unless the hydrogen in which the 

 sodium is heated has a pressure greater than that of the 

 atmosphere. 



It appears quite possible to co-ordinate the formation 

 of this supposed hydride Na^H of Troost and Hautefeuille 

 with the known compound NaH prepared by Moissan. 



When sodium is heated in a current of hydrogen at 

 atmospheric pressure combination begins at 270° C, the 

 hydride NaH being formed. 



Most of this dissolves in the excess of sodium, forming 

 a solution which may be conveniently regarded as sodium 

 dissolved in sodium hydride (or NaH.Na). The addition 

 of sodium to the hydride NaH will reduce its vapour 

 tension, and hence none of it sublimes. 



As the temperature rises the combination of the 

 sodium with the hydrogen becomes more rapid, and the 

 sodium dissolves more of the sodium-sodium hydride 

 solution, the ratio of sodium to the hydride decreasing. 

 But as the ratio decreases, the vapour tension of the 

 hydride becomes greater, until at length it begins to 

 sublime. 



