476 ON CONCRETIONS, SPICULES, [ch. 



external symmetry. Were the system actually composed of four 

 spherical vesicles in mutual contact, the outer margin of each of 

 the six interfacial planes would obviously be a circular arc; and 

 accordingly, at each angle of the tetrahedron^ we should expect 

 to have a depressed, or re-entrant angle, instead of a prominent 

 cusp. This is all doubtless due to some simple balance of tensions, 

 whose precise nature and distribution is meanwhile a matter of 

 conjecture. But it seems as though an extremely sim.ple explana- 

 tion would go a long way, and possibly the whole way, to meet 

 this particular case. In our ordinary plane diagram of three cells, 

 or soap-bubbles, in contact, we know (and we have just said) 

 that the tensions of the three partitions draw inwards the outer 

 walls of the system, till at each point of triple contact (P) we tend 



to get a triradiate, equiangular junction. But if we introduce 

 another bubble into the centre of the system (Fig. 230). then, as 

 Plateau shewed, the tensions of its walls and those of the three 

 partitions by which it is now suspended, again balance one 

 another, and the central bubble appears (in plane projection) as 

 a curvilinear, equilateral triangle. We have only got to convert 

 this plane diagram into that of a tetrahedral solid to obtain almost 

 precisely the configuration which we are seeking to explain. 

 Now we observe that, so far as our figure of Callimitra informs 

 us, this is just the shape of the little bubble which occupies the 

 centre of the tetrahedral system in that Radiolarian skeleton. 

 And I conceive, accordingly, that the entire organism was not 

 limited to the four cells or vesicles (together with the httle central 



