296 Prof. W. J. Sollas. 



"On the Intimate Structure of Crystals. Part III. Crystals 

 of the Cubic System with Cubic Cleavage." By W. J. 

 SOLLAS, LL.D., D.Sc., F.R.S., Professor of Geology in the 

 University of Oxford. Received March 8, Read March 17, 

 1898. 



The remaining metals and diatomic compounds which crystallise 

 in the cubic system and possess cubic cleavage are few in number ; 

 some of them form the subject of the present communication. Tri- 

 atomic compounds fulfilling these conditions are left for later con- 

 sideration. 



Ammonium chloride. M. w., 53*506 ; sp. gr., 1'52 (Schroder) ; m. v., 

 35-201. Volume of four molecules, 35'2 x 4 = 140'8. 



Edge of cubelet, or sum of the diameters of one mole- 

 cule of NH 4 and one atom of 01., V140'8 5'204 



Diameter of one atom of Cl ........................ 2'4954 



...................... 27134 



Gross volume of KE 4 , 2'7134 3 = 19'98 ; volume of molecular sphere 

 of NH 4 , 19-98 Xi7r = 10-46. 



Galena, PbS.M. w., 238'97 ; sp. gr., 7'25 to 7'77 (7'513 taken); 

 m. v., 31-78; volume of four molecules, 31- 78x4 = l27'12. 



Edge of cubelet or diameter of Pb+ S . . . . 5'028 



Diameter of one atom of Pb ............ 2'625 



S ............ 2-408 



Lead enters into combination without change of volume. Gross 

 atomic volume of Pb, 2'625 3 = 18' 1 ; volume of atomic 

 sphere, IS-lx-^ = 9'481. Gross volume of S, 2'4t)8 3 = 

 13-96. 



Lead selenide, PbSe. M. w., 286'01 ; sp. gr., 8'154 (Little) ; m. v., 

 35-076. Volume of four molecules, 35'076 x4 = 140*305. 



Edge of cubelet or diameter of Pb + Se . . 5'1962 



Diameter of one atom of Pb ............ 2*625 



Se ............ 2-571 



Gross volume of Se, 17'0 ; volume of atomic sphere, 8'9. 



Of the oxides which crystallise in the cubic system with cubic 

 cleavage only three are sufficiently well known to afford data for 



