VII] OF THE PARTITIONING OF SPACE 555 



Eupatorium have a similar average configuration, with the same 

 predominance of pentagonal facets*. 



We learn, in short, from Lewis and from Marvin that the 

 mechanical result of mutual pressure, even in an assemblage of 

 co-equal spheres, is more varied and more complex than we had 

 supposed. The two simple and homogeneous configurations — the 

 rhombo-dodecahedral and tetrakaidekahedral assemblages — are 

 easily and commonly produced, the one by the compression of solid 

 spheres in ordinary close-packing, the other when a liquid system 

 of spheres or bubbles is free to slide and glide into a packing which 

 is closer still. Between these two configurations there is no othei 

 symmetrical or homogeneous arrangement possible; but random 

 packing and degrees of compression leave their random effects, 

 among which are traces here and there of regular shape and 

 symmetry. 



As a froth has its histological lessons for us, throwing light on 

 the structure of a parenchyma, so may we draw an illustration 

 or two from the analogous characteristics of an emulsion. Both 

 alike are "states of aggregation"; both are "two-phase systems," 

 one phase being dispersed and the other the medium of dispersion. 

 Both phases are liquid in the emulsion, in the froth the dispersed 

 phase is a gas; our living tissue is, so far, more likely to be an 

 emulsion than a froth. The concept widens. A colony of bacteria, 

 the blood corpuscles in their plasma, the filaments of an alga, the 

 heterogeneous texture of any ordinary tissue, may all be brought 

 under the general concept of "phase systems," and share the 

 common character that one phase exposes a large "interface" to 

 the other. If we take milk as a simple emulsion, we see its hquid 

 oil-globules dispersed in a watery medium and rounded by surface 

 tension into spheres. The watery medium, as is usual in such 

 emulsions, contains dissolved substances which tend to lower the 

 interfacial tension; for were that tension high the globules would 

 tend to be larger and their aggregate surface less. Suppose the 

 "phase-ratio" to alter, the globules becoming more numerous and 

 the disperse medium less and less, the globules will be close-packed 



* J. W. Marvin, The shape of compressed lead-shot, etc.; Amer. Journ. of 

 Botany xxvi, pp. 280-288, 1939; Cell-shape studies in the pith of Eupatorium, 

 ibid. pp. 487-504. 



