io6 SCIENCE PROGRESS 



connected with the increased attention that is now being paid to 

 the important part played by the solvent in determining ionisa- 

 tion, etc. (cf. Lowry, Science Progress, October 1908, p. 202). 

 Most of the solvents used in such researches are gases at ordinary 

 temperatures or very volatile liquids, amongst the most important 

 being ammonia, sulphur dioxide and hydrogen cyanide. 



The first-mentioned of these is of special interest owing to 

 the fact that solutions in liquid ammonia show many interesting 

 analogies with aqueous solutions. This is in accordance with 

 the close parallelism that exists between many of the physical 

 properties of ammonia and of water. Thus, the boiling-point 

 of water is abnormally high, when compared with that of 

 sulphuretted hydrogen and other non-associated bodies and is 

 probably accounted for by water being polymerised in the liquid 

 state. The boiling-point of ammonia (— 33*5° C.) is also some- 

 what high when compared with the boiling-points of substances 

 such as phosphine, arsine, sulphuretted hydrogen and hydrogen 

 chloride. Among the properties of water which are most 

 noteworthy are its high specific heat, its high latent heat of 

 vaporisation and of fusion, its high critical temperature and 

 pressure, its power to unite with salts as water of crystallisa- 

 tion and its low boiling-point-elevation constant. All these 

 properties are shown in a marked degree by ammonia also. 

 Indeed, its specific heat and latent heat of fusion are greater 

 than the corresponding constants for water, whilst its boiling- 

 point-elevation constant is the lowest of any known liquid. 

 Ammonia readily unites with salts as ammonia of crystallisation 

 and although inferior to water as a solvent for salts, excels it 

 in its power of dissolving carbon compounds. Lastly, ammonia 

 is a fairly good ionising solvent but in comparison with water 

 both its dielectric constant and its factor of molecular association 

 are low. It has been noticed that the ionising power of a 

 solvent seems to run parallel to the possession of a high 

 dielectric constant and to the existence of polymerised mole- 

 cules. The behaviour of ammonia seems to be against this 

 view but on the other hand it seems probable that the ionising 

 power and other exceptional properties of water and ammonia 

 are connected with the presence of residual affinity in the 

 molecules. In the case of the former, divalent oxygen is capable 

 of acting as a tetrad and in that of the latter, triad nitrogen can 

 become pentad. 



