RECENT ADVANCES IN SCIENCE 563 



were in general very inert, chemically speaking. Thus, ordinary 

 ammonium chloride — to take a particular case — which is not 

 dried in the extreme sense of the term, on being vaporised 

 is very largely dissociated into ammonia and hydrochloric 

 acid. If, on the other hand, the solid is dried with extreme 

 care, the vapour does not dissociate sensibly, as Baker showed 

 experimentally. (The presence or absence of dissociation is 

 shown by the value of the vapour density.) Johnson, in 1908, 

 measured the vapour pressure of the undried and dried ammo- 

 nium chloride and found that the vapour pressures are the 

 same, from 212 to 333 C, but that the vapour density of the 

 undried material at 333 C. corresponded to 98 per cent, dis- 

 sociation, whilst that of the dried material at the same tem- 

 perature and exerting the same pressure corresponds to only 

 8 per cent, dissociation. It follows that the partial pressure 

 of ^dissociated ammonium chloride vapour in equilibrium 

 with the undried solid must be small, whilst that in equilibrium 

 with the dried solid accounts for practically the whole observed 

 pressure. 



Abegg was the first to call attention to the existence of 

 this anomaly, namely, the discrepancy between the two values 

 of the partial pressure of the undissociated molecules. If the 

 solid material, dried or undried, were the same substance it 

 follows necessarily that the vapour pressure of the undissociated 

 molecules should be identical likewise. Since this is not the 

 case there must be some fundamental difference between the 

 two solids. Wegscheider suggested that we are really dealing 

 with two allotropic forms of solid ammonium chloride, one of 

 them, the unstable one, having a considerably greater pressure 

 than the stable one at the same temperature. In the absence 

 of water, i.e. in the dried material, according to Wegscheider, 

 the transition from one form to the other cannot take place. 

 On Wegscheider's view it is a pure accident that the pressure 

 of the undissociated vapour over the dried material is numeri- 

 cally the same as the total pressure due to NH4CI, NH3, and 

 HC1, which exist over the stable form. 



Scheffer, in 191 5, actually discovered two allotropic forms 

 of solid ammonium chloride, with a transition point at i84'5°C. 

 At first sight this looks greatly in favour of Wegscheider's 

 view. Scheffer, however, determined the heat of transition, 

 and the specific heats of the two forms, and found that the 

 heat effect was too small to account for the facts on Weg- 

 scheider's hypothesis. (There is a well-known relation con- 

 necting the magnitude of the heat effect with the vapour 

 pressure.) Wegscheider's hypothesis loses, therefore, a great 

 deal of its probability. Recently Smith, Eastlack, and Scatch- 

 ard (/. Amer. Chem. Soc, 41, 1961, 1919) have cast still further 



