U8 ON CONCRETIONS, SPICULES, ETC. [ch. 



cells^ emerges as an alternative (and, as it seems to me, a highly 

 preferable alternative) to Dreyer's conception of an accumulation 

 under mechanical pressure in the vacant spaces left between one 

 cell and another. 



But a purely chemical, or purely molecular adsorptioD, is not 

 the only form of the hypothesis on which we may rely. For 

 from the purely physical point of view^ angles and edges of contact 

 between adjacent cells will be loci in the field of distribution of 

 surface-energy, and any material particles whatsoever will tend 

 to undergo a diminution of freedom on entering one of those 

 boundary regions. In a very simple case, let us imagine a couple 

 of soap bubbles in contact with one another. Over the surface 

 of each bubble there glide in every direction, as usual, a multitude 

 of tiny bubbles and droplets ; but as soon as these find their way 

 into the groove or re-entrant angle between the two bubbles, 

 there their freedom of movement is so far restrained, and out of 

 that groove they have httle or no tendency to emerge. A cognate 

 phenomenon is to be witnessed in microscopic sections of steel or 

 other metals. Here, amid the "crystalline" structure of the 

 metal (where in cooling its imperfectly homogeneous material has 

 developed a cellular structure, shewing (in section) hexagonal or 

 polygonal contours), we can easily observe, as Professor Peddie 

 has shewn me, that the little particles of graphite and other 

 foreign bodies common in the matrix, have tended to aggregate 

 themselves in the walls and at the angles of the polygonal 

 cells — this being a direct result of the diminished freedom 

 which the particles undergo on entering one of these boundary 

 regions*. 



It is by a combination of these two principles, chemical adsorp- 

 tion on the one hand, and physical quasi-adsorption or concentration 

 of grosser particles on the other, that I conceive the substance 

 of the sponge-spicule to be concentrated and aggregated at the 

 cell boundaries; and the forms of the triradiate and tetractinellid 

 spicules are in precise conformity with this hypothesis. A few 

 general matters, and a few particular cases, remain to be con- 

 sidered. 



It matters little or not at all, for the phenomenon in question, 



* Cf. again Benard's Tourhillons ceUulaires, Ann. de Chimie, 1901, p. S-l. 



