THE USE OF LIQUID AMMONIA AS A SOLVENT 109 



formation of a compound of sodium and ammonia, although 

 this was not the view taken by Seely himself. He passed dry 

 ammonia gas over sodium and noticed that the sodium lost its 

 metallic appearance, became pasty and then liquefied, forming 

 a mobile opaque liquid of a fine copper-red colour when viewed 

 by reflected light. As the ammonia continued to liquefy, it 

 mixed with this liquid, which, after a series of colour changes, 

 finally appeared of an intense blue colour when viewed either 

 by reflected or transmitted light. Seely sought to explain 

 these colour changes on purely physical grounds ; in order to 

 throw more light on the nature of the solution of the alkali 

 metals in ammonia, a fresh investigation was started by Joannis 

 in 1889 (6). He mixed one equivalent of sodium with twenty 

 equivalents of ammonia and found that the vapour pressure of 

 the solution rapidly diminished as the ammonia was allowed 

 to evaporate, until it became constant at a certain point cor- 

 responding to a liquid having the composition Na + 5*3 NH 3 . 

 The composition of the liquid was a function of the temperature 

 and hence it could not be regarded as a true chemical compound. 

 If more ammonia were caused to evaporate a solid separates 

 having the same colour as the liquid — namely an intense copper- 

 red. As more ammonia was removed the pressure remained 

 constant ; finally, when the mixture contained one equivalent of 

 sodium and one equivalent of ammonia, not a trace of liquid 

 was left. The solid thus obtained therefore had the composi- 

 tion NaNH 3 and must be regarded as sodium-ammonium — i.e. 

 ammonium in which one hydrogen atom has been displaced by 

 sodium. 



Further removal of ammonia caused the substance to de- 

 compose, sodium being left as the final product. As the pressure 

 remained constant during this " decomposition," it was thought 

 by Joannis that we are here dealing with a case of the true 

 dissociation of a compound. It is noteworthy that the vapour 

 pressure of the saturated solution is the same as the dissociation 

 pressure of the compound. For this reason it is extremely 

 difficult to determine the exact point when all the liquid has 

 vanished but the dissociation of the compound has not yet 

 begun. This fact explains how it was that Seely was led to 

 the opinion that no compound was formed at any stage of the 

 process of solution. In favour of Seely's views it may be 

 pointed out that the results obtained from measurements of 



