ARRANGEMENT OF ATOMS IN A CRYSTAL. 137 



in the tetragonal system. Here the atomic distribution 

 adopted is as follows : — 



H K 



CI O 



H— CI— Cu— CI— H H— CI— Cu— CI- H 



CI O 



H K 



Horizontal Plan. Vertical Elevation. 



This gives v 1 + v 2 = 132*9 ; a number almost exactly equal 

 to that obtained from the formula of the whole salt. 



Now the crystals are markedly dichroic, the ordinary 

 ray being green, and the extraordinary blue; it is suggested 

 that this corresponds to the above arrangement ; for the com- 

 position along the vertical line is Cu r O K or Cu 3 ; and 



that along the horizontal line Cu r C^ or Cu CL The 



6 2 



former is more of a cupric nature than the latter, and the 

 cupric salts are characteristically blue as contrasted with the 

 green cuprous salts. 



This suggestion of course opens up a most interesting 

 series of questions as to the relative distribution of ferrous 

 and ferric constituents in the pleochroic compounds of iron. 

 But we must here leave the bold and ingenious speculations 

 of Sollas in order to glance at the manner in which the same 

 problem has been approached by other investigators. 



DETERMINATION OF TOPIC AXES. 



Muthmann {Zeitschr. f. Krystallographie, xxii., p. 497), 

 and, independently (at the suggestion of Becke), Tutton 

 {J 02ti'7i. Chemical Soc, lxv., p. 628), have endeavoured, by 

 extremely accurate determinations of the angular relations 

 and of the specific gravity of certain substances, to deter- 

 mine the relative distances of the crystal molecules from 

 each other along different directions in a series of iso- 

 morphous salts. The argument is this : If A x B x Q be the 



