286 Prof. W. J. Sollas. 



scratch the face of the crystal. It was found that if it required a 

 load of one to produce scratching parallel to the edge of a face of the 

 cube, a load of 1'3 was necessary parallel to a diagonal. Looked at 

 broadly, it will be seen by reference to the figure that the effect of a 

 force acting parallel to a diagonal should be to that acting parallel 

 to a side, as 1 : \/2, i.e., as 1 : 1'414. Considering how complicated 

 the problem actually is, this correspondence is quite as close as could 

 possibly be expected. 



By several distinct lines of argument, resting on the study of 

 molecular volumes, of the density of solutions, of the symmetry and 

 physical characters of crystals, we have been led to the same result ; 

 given the atomic volumes in the ratios we have assigned to them, no 

 other was possible ; and we may now with greater confidence 

 proceed to the investigation of other diatomic compounds, which 

 will be found to throw a surprising light on the molecular tactics of 

 crystals. 



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

 the Cubic System with Cubic Cleavage. Haloid Com- 

 pounds of Silver." By W. J. SOLLAS, LL.D.,D.Sc., F.R.S., 

 Professor of Geology in the University of Oxford. Re- 

 ceived January 27, Read February 3, 1898. 



The haloid compounds of silver offer more points of interest in the 

 study of crystal tactics than those of the alkalis, especially as they 

 include that remarkably anomalous substance iodide of silver. 



Silver itself crystallises in the cubic system, and possesses the 

 same kind of structure as all other metals which possess cubic 

 symmetry, i.e., its atoms are arranged on the plan of most open 

 packing: unlike the alkali metals it undergoes no change of volume 

 on entering into combination. Its atomic weight is 107'93 ; its 

 density has been differently determined by different observers, for 

 silver heated in vacuo Dumas found IO512, and Roberts- A usten 

 10*57; we take the mean of these numbers 10'541. The atomic 

 weight, 107-938, divided by 1O541, gives 10*233, the gross atomic 

 volume. From this the diameter of the atomic sphere is found to bo 

 2-17152, and its volume 5'3616. 



Both the chloride and bromide crystallise in the cubic system, 

 and are regarded from analogy as possessing the same crystalline 

 structure and symmetry as the corresponding salts of potassium and 

 sodium. 



The specific gravity of silver chloride is obtained from the solid 

 after fusion. The latest determinations give for silver chloride a 



