173 



2,5 



Dr= -jl~{d' — d) -I- d. 



As the specific gravity of the sohition must be determined for the 



viscosity measurements, it is easy to deduce the specific gravity of 



the particles. We find for molybdenum blue 1,83 to 1,93 ; for iron 



hydroxide 1,66 to 1,8. In this manner too, it becomes clear, that the 



particles suspended in the liquid cannot consist only of dissolved 



substance, the density of which is 3 or 4, but must also contain 



water. As has already been pointed out, the qualitative value of 



these conclusions is not attacked, if it .sliould appear, that instead of 



v' V 



2,5-, for instance, 3- must be written. Neither would this be the 



V V 



case, when we introduce into Einstein's formula the second power of 



v' 



-, as is necessary for the more concentrated solutions. 



I' 



I liope to discuss later from the Hlaiid|iuint taken in this paper, 



the viscosity measurements previously published by other observers. 



A preliminary investigation already led to remaikable results, but 



a great part of what is known, cannot serve my purpose ; I propose 



to fill up this lacuna by new determinations, and to discuss then at 



length the many questions, wliicii arise in this field. 



Inory. Chem. Lab. 

 Lfniversiti/ of Amsterdam. 



Physics. — "So7n(' RcnKirks o» //x' ('a/)il/<irüj/ Theonj of the Cnj-stal- 

 Uae Form". By Prof. V. Ehrenkkst. (Communicated i>y Prof. 



H. A. LORENTZ). 



(Communicated in the meeting of May 29, 191.5). 



^ 1. As is known, W. Giubs') and P. Curie") have set forth the 

 following view, and given further thermodynamic grounds for it. 

 A crystal in a solution is in thermodynamic equilibrium only when 

 it has that shape in which its surface energy has a smaller value 

 than for any other shape with the same content. That this equilii)iium 



1) W. GiBB.?: Theimodyn. Studiën p. 320. 



2) P. Curie: Bull, de la See. Min. de France 8 (1885) p. 145 of Oeuvres p. 153. 

 Gf. for the relations between tlie theories of Gibbs and Curie : 



J. J. P. Valeton: Kristalvorm en oplosbaarheld. Proefschr. Amsterdam 1915. 

 Ber. d. Sachs. Ges. d. Wiss. 67, (.1915). 



