VIl] 



OF HEXAGONAL SYMMETRY 



319 



lines of contact, representing surfaces of contact in the actual 

 spheres or cyUnders ; and the equal circles of our diagram will 

 be converted into regular and equal hexagons. The angles of 

 these hexagons, at each of which three hexagons meet, are of 

 course angles of 120°. So far as the form is concerned, so long as 

 we are concerned only with a morphological result and not with 

 a physiological process, the result is precisely the same whatever 

 be the force which brings the bodies together in symmetrical 

 apposition ; it is by no means necessary for us, in the first instance, 

 even to enquire whether it be surface tension or mechanical 



Fig. 121. Diagram of hexagonal cells. (After Bonanni.) 



pressure or some other physical force which is the cause, or the 

 main cause, of the phenomenon. 



The production by mutual interaction of polyhedral cells, 

 which, under conditions of perfect symmetry, become regular 

 hexagons, is very beautifully illustrated by Prof. Benard's 

 " tourhillons cellulaires^^ (cf, p. 259), and also in some of Leduc's 

 diffusion experiments. A weak (5 per cent.) solution of gelatine 

 is allowed to set on a plate «f glass, and little drops of a 5 or 

 10 per cent, solution of ferrocyanide of potassium are then placed 

 at regular intervals upon the gelatine. Immediately each little 

 drop becomes the centre, or pole, of a system of diffusion currents. 



