RECENT ADVANCES IN SCIENCE 387 



Schubert (Ann. Phys., 50, 283, 339, 1916 ; 55, 397, 577, 1918) 

 on the role of water in the alums. 



The determination of optical properties by means of the 

 microscope is becoming increasingly important in connection 

 with the identification of artificial chemical substances as well 

 as minerals. The determination of refractive indices is dis- 

 cussed by F. E. Wright (Jour. Wash. Acad. Set., 6, 465, 1916 ; 

 Jour. Amer. Chem. Soc, 38, 1647, I 9 I 7)> J- W. Evans (Min. 

 Mag., 18, 45, 52, 1916 ; 18, 130, 191 7), and S. Tsuboi (Jour. 

 Geol. Soc. Tokyo, 25, 38, 191 8). The dispersion phenomena 

 of minerals, particularly the felspars, is treated in detail in a 

 series of papers by S. Kozu (Min. Mag., 17, 189, 191 5 ; 17, 

 2 37, 2 53> 191 6 ; Bull. Soc. franc. Min., 40, 36, 1917 ; Jour. 

 Geol. Soc. Tokyo, 25, 31, 43, 191 8). E. T. Wherry (Jour. Wash- 

 ington Acad. Sci., 8» 277, 319, 191 8 ; Amer. Min., 3, 134, 191 8) 

 ha9 investigated the relations between the optical properties 

 and crystalline form of a number of tetragonal organic sub- 

 stances. It is found that the ratio of the refractivities obtained 

 by substituting the " ordinary " and " extraordinary " refrac- 

 tive indices in the Lorentz-Lorenz formula is the inverse of the 

 crystallographic axial ratio. The same author (Jour. Amer. 

 Chem. Soc, 40, 1852, 191 8) gives an account of the optical 

 properties of a-J-lyxose, a-d-xylose and yS-d-arabinose, and 

 L. Delhaye (Bull. Soc. franc. Min., 41, 80, 191 8) has made a 

 detailed study of the optical properties of sodium chromate 

 tetrahydrate. 



The fact that crystals under certain conditions emit light rays 

 has long been known, and the phenomena have been differen- 

 tiated into various types according to the nature of the exciting 

 agent (Wiedemann, Ann. Phys., 34, 446, 1888). Thus when 

 the luminescence is due to light or cathode rays, the phenom- 

 enon is photo- or cathodo-luminescence respectively ; tribo- 

 luminescence appears when the material is crushed, crystallo- 

 luminescence during crystallisation, and so forth. The terms 

 fluorescence and phosphorescence are used accordingly as the 

 emission of light ceases when, or continues after, the exciting 

 agent is removed. Cf. S. E. Sheppard Ilium. Eng. 10, 178, 

 191 7 ; Sci. Als., 20A, 471, 191 7. In many cases the phenomena 

 seem to be due to the combination of ions or to other chemical 

 reactions, and this view is adopted by H. Weiser (Jour. Phys. 

 Chem., 22, 439, 480, 576, 191 8) in a recent examination of 

 several substances showing crystallo- and tribo-luminescence. 

 He concludes that, so far as the former is concerned, in the 

 case of such alkali haloids, sodium and potassium chlorides, 

 and potassium bromide and iodide, as show it, the reaction re- 

 sponsible for the effect is the combination of the ions to form 

 the undissociated salt. It is found that factors such as con- 



