( 583 ) 



prevent decomposition on exposure to air: smtiragdine amorphous 

 plates about 3 ni.m. thick. 



At 18 light band 673,7—681,4; vague indistinct band 695,3—736,9. 

 At — 193° light band 672,6— 680,8 ; dimly defined band 692,3—747,6. 



Natural cinenild. [Be, Al, (SiOs)J ; hexagonal, coloured by a few 

 permilles Ci\0^ ; sensibly dichroitic (grassgreen-seagreen). Worthless 

 light green specimen not c^uite transparent, provided at the laboratory 

 with parallel facets, thickness 6 mm. 



In the ordinary spectrum at 18^ rather strong band 679,0— 

 680,7; somewhat stronger band 682,4—685,0; at —193° strong 

 band 678,2—679,5, and still stronger band 681,8—683,4. 



In the extraordinary spectrum the bands were much paler but 

 at the same places, their relative widths being interchanged. 



§ 6. Ruf)//. [Al, O,] ; rhombohedral, solid solution of a little CrjO,; 

 dichroitic (purplered-brickred). By the kindness of Mr. M. A. Wolff — 

 UE Beer, manager of the Amsterdam diamond factory, several natural 

 and several artificial rubies were placed at our disposal. The last- 

 mentioned rough material is imported from Paris in the form of 

 cones ') ; by cleaving them we got pieces of about uniform colour 

 and crystallographic orientation, as is to be observed by means of 

 the dichroscope. With carborundum a square plate (7 X 7 X 3 mm.) 

 was ground and polished at the laboratory, a side of which con- 

 tained the direction of the axis ; most of the experiments were m^de 

 with it. There is no reason to suppose that natural ruby w^oiild 

 show other properties than the artificial material. 



This stone absorbs green and yellow light. In the above investi- 

 gations of J. Becquerel we only found it briefly stated that "a group 

 lying between 657 and 676 disappears in liquid air, that the band 

 at 697 becomes thinner and the band at 705 broader and intenser 

 than at ordinary temperature;" no mention is made of any magnetic 

 influence '). 



Further ruby was more closely investigated by Miethe, who found 

 the two principal absorption bands at 694 and 696, with a breadth 

 of about 0.4 i^qi, besides 6 bands of less importance. Moreover he 

 described the remarkable fluorescence spectrum; the latter we have 

 no further examined, as Prof. Miethe intended to proceed with his 

 experiments on this subject "). 



1) M. DuBOiN, Compt. Rend. 134 p. 840, 1902 ; A. Verneuil, ibid. 135 p. 79 J , 1902. 



2) J. Becquerel. Physik. Zeitschr. 8 p. 932, 1907 (Se,.t\ 



•^) A. Miethe, Verb. D. physik. Ges. 9 p. 715, J 907 (Nov.) 



40 



Proceedings Royal Acad. Amsterdam. Vol. X. 



