3 88 SCIENCE PROGRESS 



centration, temperature, and viscosity of the solutions and the 

 presence of colloids exert an influence on the intensity of the 

 light emitted. In the third paper, the relation of tribo-lumin- 

 escence to crystallo-luminescence is considered. All substances 

 possessing the latter property also possess the former, but not 

 vice versa. Further, the nature of the light emitted by such 

 substance c as arsenious oxide and alkali sulphates during 

 grinding is the same as that emitted during rapid crystallisa- 

 tion. In each case the effect is ascribed to the re-formation of 

 molecules broken up by crushing and solution respectively. 

 The intensity of the light, as might be expected, varies greatly 

 with the nature of the substance. 



So far as kathodo- and photo-luminescence is concerned, 

 the main result of numerous investigations {e.g. Wiedemann and 

 Schmidt, Ann. Phys., 56, 208, 1895 ; Lenard and Klatt, 

 ibid., 15, 225, 425, 603, 1904) has been to establish that the 

 phenomenon is due to the presence of a small amount of im- 

 purity, supposed by Schmidt (Die Kathodenstrahlen, 99, 1907) 

 to be in solid solution. 



According to E. L. Nichols and H. L. Howes (Phys. Rev., 11, 

 327, 1918; 12, 351, 1918 ; Proc. Nat. Acad. Sci., 4, 305, 1918), 

 the luminescence of calcite from Franklin Furnace, N.J., is 

 due to the presence of oxide of manganese, while prepared 

 impure calcium carbonate behaves in the same way. It is 

 suggested that the luminous centres may be calcium oxide 

 in the neighbourhood of which the impurity is located. From 

 the evidence of curves showing the decay of the luminosity, 

 two types of phosphorescence are recognised, one in which the 

 slope of the decay curve diminishes with time, and another in 

 which it increases. In the case of natural calcite, cathodo- 

 luminescence belongs to the latter type, and photo-luminescence 

 to the former. The effects of temperature on both types and 

 on different parts of the spectrum were also investigated. 



E. MacDougall, A. W. Stewart, and R. Wright (Jour. Chem. 

 Soc, 111, 663, 191 7. Cf. J. W. T. Walsh, Proc. Roy. Soc, 93A, 

 550, 191 7) have made a detailed investigation of the factors 

 influencing the phosphorescence of zinc sulphide. The effects, 

 due to the nature of impurities, temperature, and so forth, 

 on the luminescence of zinc sulphide under the excitation 

 of kathode rays and rays from radio-active materials have 

 been examined, and it is found that the results obtained in the 

 two cases are by no means concordant. The view that the 

 maximum luminescence occurs when the sulphide is in a state 

 of incipient crystallisation, in preference to the amorphous 

 and truly crystalline states, is verified. C. Waggoner (Phys. 

 Rev. 9, 175, 191 7), on the other hand, maintains that the 

 luminescent spectra of zinc salts are independent of the mode 



