PROGRESS IN PHYSICAL CHEMISTRY. 241 



make it possible to calculate the elastic properties of glass 

 to within 8 per cent, from a knowledge of its composition. 



Valuable information regarding the stoichiometry of 

 crystalline substances has been obtained by Tutton (67). 

 He has studied the isomorphous series consisting of potas- 

 sium rubidium and caesium sulphates. Both as regards the 

 angular measurements of the crystals, and the development 

 of the faces, and also as regards their optical behaviour, 

 molecular volume, etc., the properties of the rubidium salt 

 stand between those of the potassium and caesium salts, and 

 approximate more nearly to those of the potassium salt. 



That simplicity in the chemical structure of a substance is 

 correlated with its crystalline form, according to van't Hoff 

 (68) was pointed out by Buys- Ballot in 1846. Retgers (69) 

 has summarised the nature of the evidence, and finds that 85 

 per cent, of the elements, 88 of the diatomic compounds, and 

 53 of the triatomic crystallise in the regular and hexagonal sys- 

 tems ; whereas the majority of complex organic and inorganic 

 compounds crystallise in the rhombic and monoclinic systems. 



Surawicz (yo) states that the hydration of a substance is 

 usually attendedbya fall in thesymmetryof its crystalline form; 

 its specific gravity and hardness diminish at the same time. 



Phosphorescence. — The phosphorescence of solids at low 



temperatures has been studied by Pictet and Altschul and 



by Dewar. The former (71) find that in the case of 



sulphides of the alkaline earths phosphorescence ceases at 



67°. Dewar (72) finds that, in general, substances which 



feebly phosphoresce at ordinary temperatures become more 



active at low temperatures. The substances were cooled 



by immersion in boiling liquid air, all organic substances 



being solid under these conditions. They were then 



exposed for a short time to the light of the electric arc, and 



viewed in a darkened room. At low temperatures the 



substances seem capable of absorbing in an increased 



degree light energy which is emitted as phosphorescent 



light as the temperature rises. Amongst inorganic 



substances the platino-cyanides, and amongst organic 



substances ketonic compounds, exhibit the most marked 



phosphorescence. 



J. W. Rodger. 



