488
S. G. CHAUDHURY
REFERENCES
(1) ADAIRAND ADAIR:Biochem. J. 28, 199, 1231 (1934). Cf.ADAIR: Trans. Faraday Soc. 33, 1106 (1937). (2) BJERRUM: Z . physik. Chern. 110, 656 (1924). (3) CHAUDHURY: J. Proc. Inst. Chern. 13, 62 (1941);The Donnan Membrane Equilibrium, p. 52, A. P. Bhattacharyya, Santragachi, Howrah, India (1945). (4) DONNAN AND GUGQENHEIM: Z . physik. Chem. 162, 346 (1932).
COR/IMUNICATION TO THE EDITOR ON ISOTROPIC CRYSTALS I n an article entitled “Definition of a crystal” the author (J. Phys. Chem. 48,95 (1944)) upheld the view that there can be no isotropic cystals because they are anisotropic by the very definition. This view was criticized by A. C. Shumnn (J. Phys. Chem. 48,425 (1944)),who expressed the opinion that crystals belonging to the cubic system are optically isotropic. This, in his view, is SUEcient justification for applying the term “isotropic crystals” to the cubic system. The following facts indicate, however, that there can be no optically isotropic crystals in the proper sense of the word. In cubic crystals the velocity of light, is the same along the three principal axes, but it cannot be the same along the diagonal. In traversing the crystal in this direction, light meets particles of the substance situated at longer distances from one another. Therefore the refractive index will be different. The same applies to magnetic properties. Formerly, cubic crystals were believed to be magnetically isotropic, but owing to researches of Pierre Weiss (G. Foex: “L’oeuvre scientifique de Pierre Weiss,” Ann. phys. 20, 111 (1945)), it is known that this is not the case. Magnetization changes with direction of the field and only an some axes of symmetry does the change not occur. In experiments of Ehrenhaft (Compt. rend. 222, 1100, 1345 (1946); see also numerous communications in the Bulletin of the American Physical Society), confirmed by Cotton (“Sur l’experience d’Ehrenhaft,” Ann. phys. 20,228 (1945)) and Tauzin (“Recherches de M. Ehrenhaft,” Ann. phys. 20, 557 (1945); “Les mouvements des particules ultremicroscopiques en suspension dans l’air,” Cahier de physique, No.15, August, 1913, SOC.Franc. Phys. Section Mediterranee; Cahier No.19, March, 1944),an intimate connection between magnetic and optical phenomena is made evident. Thus, if a crystal is not isotropic magnetically, it must be optically anisotropic also. GEORGE ANTONOFF. Department of Chemistry Fordham University New York, New York August, 1946
