RECENT ADVANCES IN SCIENCE 389 



of excitation. E. Karrer and E. H. Kabakjian {Jour. Franklin 

 Inst., 186, 317, 191 8) consider that the luminescence of radio- 

 active salts is due to changes in the crystalline state. M. B. 

 Hodgson (Phys. Rev., 12, 431, 191 8), in a paper on the physical 

 characteristics of the fluorescence produced by X-rays, finds 

 that the fluorescence varies with the nature of the crystal 

 structure so far as calcium tungstate is concerned. The 

 question of cathodo-luminescence is also considered by F. G. 

 Wick and L. S. McDowell {Phys. Rev., 9, 241, 191 7) and T. B. 

 Brown {ibid., 11, 38, 191 8). 



In a paper on " Radio-activity and the Coloration of 

 Minerals," E. Newbery and H. Lupton {Proc. Manchester Lit. 

 Phil. Soc., 62, Mem. 10, 191 8) come to the conclusion that the 

 colour and thermo-luminescence of many minerals are due to 

 the action of radio-active materials, either during formation or 

 subsequently. The colours produced by a, /3, and 7 rays may 

 be different, but in each case they are most probably due to 

 the dissociation of impurities present in minute quantities and 

 not to the dissociation of the main substance. This latter 

 point is regarded as still in doubt (cf. Goldstein, Nature, 494, 

 1 9 14), but the impossibility of a pure substance showing phos- 

 phorescence is considered to be established (cf. Lenard and 

 Klatt, loc. cit. ; G. Urbain, Ann. Chim. Phys., 8, 222, 289, 1909; 

 E. Tiede, Ber., 49, 1745, 1916). Similar methods have been 

 employed by B. Blount and J. H. Sequeira {Jour. Chem. Soc, 

 115, 705, 19 1 9), in an investigation of the colour of fluorspar. 

 It is concluded that the colour is due to the presence of traces 

 of organic material, not to radio-active effects. Since Lupton 

 and Newbery came to the opposite conclusion after an ex- 

 haustive examination of the same mineral, the question of the 

 origin of the colour must still be considered to be open. 



BOTANY. By E. J. Salisbury, D.Sc, F.L.S., University College, London. 



Anatomy and Cytology. — Further details regarding the struc- 

 ture of the stele of Platyzoma microphyllum are furnished by 

 McLean Thompson {Trans. Roy. Soc. Edin.), who finds 

 that the stele is devoid of perforations or leaf gaps, but that 

 considerable changes occur with respect to the medulla and the 

 internal endodermis. The latter was found in one specimen 

 to contract and finally disappear. Expansion of the pith was 

 accompanied by a reappearance of the endodermis, and the 

 same features were again repeated at a higher level in the 

 stem. 



The structure of Protoplasm formed the subject of Prof. 

 Harper's Presidential Address to the Botanical Society of 

 America {Amer. Jour. Bot.), in which he considers especially 



