IX] OF THE NASSELLARIAN SKELETON 719 



As to the former, their arrangement is such as would result if 

 deposition or soKdification had proceeded in waves, starting inde- 

 pendently from each of the three boundary-edges of the Httle 

 partition- wall, and something of this kind is doubtless what actually 

 happened. We are reminded of the wave-hke periodicity of the 

 Liesegang phenomenon, and especially, perhaps, of the criss-cross 

 rings which Liesegang observed in frozen gelatine {supra, p. 663). 

 But there may be other explanations. For instance the film, hquid 

 or other, which originally constituted the partition, might conceivably 

 be thrown into vibrations, and then (hke the dust upon a Chladni 

 plate) minute particles in or on the film would tend to take up 

 position in an orderly way, in relation to the nodal points or fines 

 of the vibrating surface*. Some such hypothetical vibration may 

 (to my thinking) account for the minute and varied and very 

 beautiful patterns upon many diatoms, the resemblance of which 

 patterns to the Chladni figures (in certain of their simpler cases) 

 seems here and there striking and obvious. But I have not attempted 

 to investigate the many special problems suggested by the diatom- 

 skeleton. 



The cusps at the four corners of the tetrahedral skeleton are a 

 marked peculiarity of our Nassellarian shell, and we should by no 

 means expect to see them in a skeleton formed at the boundary- 

 edges of ai simple tetrahedral pyramid of four bubbles or cells. But 

 when we introduce another bubble into the centre of a system of 

 four, then, as Plateau shewed, the tensions of its walls and of the 

 surrounding partitions so balance one another that it becomes a 

 regular curvifinear tetrahedron, or, as seen in plane projection 

 (Fig. 337), a curvilinear, equilateral triangle, with prominent, not 

 re-entrant angles. A drop of fluid tends to accumulate at each 

 corner where four edges meet, and forms a bourreletf ; it is drawn 

 out in the directions of the four films which impinge upon it, and 



* Cf. Faraday's beautiful experiments, On the moving groups of particles found 

 on vibrating elastic surfaces, etc., Phil. Trans. 1831, p. 299; Researches, 1859, 

 pp. 314-358. 



t The bourrelet is not on4y, as Plateau expresses it, a "surface of continuity," 

 but we also recognise that it tends (so far as material is available for its production) 

 to further lessen the free surface-area. On its relation to vapour-pressure and to 

 the stability of foam, see FitzGerald's interesting note in Nature, Feb. 1, 1894 

 {Works, p. 309); and on its effect in thinning the soap-bubble to bursting-point, 

 see Willard Gibbs, Coll. Papers, i, p. 307 seq. 



