RECENT ADVANCES IN SCIENCE 107 



recognised to be a variable, altering with the temperature. The 

 precise form of this variation has not been fully determined. 

 No one can read Mr. McDougall's contribution, however, with- 

 out being struck by the very extensive general applicability 

 of Dieterici's expression. 



A llotropy .—Some years ago the attention of chemists was 

 called to a remarkable chemical paradox in the behaviour of 

 ammonium chloride vapour when moist and when absolutely 

 dry as observed by Johnson. That there was an abnormality 

 at all in the behaviour of this vapour was first pointed out by 

 Abegg. The point was, that the vapour pressure exerted by 

 the ammonium chloride was the same whether the vapour was 

 moist or dry, although in the first case there was large dissocia- 

 tion into ammonia and hydrochloric acid gas, in the second 

 case no dissociation whatsoever. To get over the difficulty, 

 Wegscheider suggested that ammonium chloride existed in two 

 allotropic forms. Whilst this was shown to be sufficient to 

 account for the anomaly, it was, nevertheless, an hypothesis of 

 a rather ad hoc nature. Considerable interest attaches, there- 

 fore, to some recent measurements by F. E. C. Scheffer (Proc. 

 Akad. Wetenschap. Amsterdam, 13, 446, 191 5) which go to show 

 that two allotropic forms of ammonium chloride actually exist. 

 From heating and cooling curves it was inferred that a tran- 

 sition point between the two forms exists somewhere between 

 174 and i87°C. Using a catalyst (e.g. glycerine) the point 

 was fixed with greater precision in the region of 1 84* 5 C, a value 

 which is probably accurate to a few tenths of a degree. This 

 work naturally affords considerable evidence in favour of 

 Wegscheider's explanation of the anomalous behaviour of the 

 vapour. 



INORGANIC CHEMISTRY. By C. Scott Garrett. 

 Colour and Reactivity. — A series of papers on materials for 

 experimental dispersoidology has recently been published by 

 Weimarn in the Journal of the Russian Physical Chemistry 

 Society, which in the light of recent theories of constitution 

 and reactivity are of some considerable importance. Most 

 chemists who have followed the developments which have 

 resulted from the application of spectroscopic methods to the 

 phenomenon of colour — using the word in its broadest sense — 

 in chemical substances will be familiar with the theory recently 



