380 THE FORMS OF CELLS [ch. 



stable figure until it becomes longer than its own circumference, 

 and then the risk of rupture may be said to begin. But Rayleigh 

 shewed that still longer waves, leading to still greater instability, 

 are needed to break down material resistance*. For, as Plateau 

 knew well, his was a theoretical result, to be departed from under 

 material conditions; it is affected largely by viscosity, and, as in 

 the case of a flowing cylinder or jet, by inertia. When inertia plays 

 a leading part, viscosity being small, the node of maximum in- 

 stability corresponds to nearly half as much again as in the simple 

 or theoretical case: and this result is very near to what Plateau 

 himself had deduced from Savart's experiments on jets of water "j". 

 When the fluid is external >(as when the cyhnder is of air) the wave- 

 length of maximal instability is longer still. Lastly, when viscosity 

 is very large, and becomes paramount, then the wave-length between 

 regions of maximal instability may become very long indeed: so 

 that (as Rayleigh put it) ''long (viscid) threads do not tend to divide 

 themselves into drops at mutual distances comparable with the 

 diameter of the cylinder, but rather to give way by attenuation at 

 few and distant places." It is this that renders possible the making 

 of long glass tubes, or the spinning of threads of "viscose" and like 

 materials; but while these latter preserve their continuity, the 

 principle of Plateau tends to give them something of a wavy, 

 unduloid surface, to the great enhancement of their beauty. We 

 are prepared, then, to find that such cylinders and unduloids as 

 occur in organic nature seldom approach in regularity to those which 

 theory prescribes or a soap-film may be made to shew; but rather 

 exhibit all manner of gradations, from something exquisitely neat 

 and regular to a coarse and distant approximation to the ideal 

 thing {. 



The unduloid has certain peculiar properties as regards its limita- 

 tions of stabihty, but we need mention two facts only: (1) that 

 when the unduloid, which we produce with our soap-bubble or our 



* Rayleigh, On the instability of fluid surfaces, Sci. Papers, iii, p. 594. 



t Cf. E. Tylor, Phil. J^ag. xvi, pp. 504-518, 1933. 



X Cf. F. Savart, Sur la constitution des veines liquides lancees par des orifices, 

 etc., Ann. de Chimie, liii, pp. 337-386, 1833. Rayleigh, On the instability of 

 a cylinder of viscous liquid, etc., Phil. Mag. (5), xxxiv, 1892, or Sci. Papers, i, 

 p. 361. See also Larmor, On the nature of viscid fluid threads. Nature, July 11, 

 1936, p. 74. 



