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SCIENCE 



[N. S. Vol. XXXIV. No. 875 



eral nature of this phenomenon, but will 

 only state (as I fear for my purpose I 

 must) a few of its physical manifestations 

 or laws. Of these the most esential facts 

 for us are as follows: Surface-tension is 

 manifested only in fluid or semi-fluid 

 bodies, only at the surface of these : though 

 we may have to interpret surface in a lib- 

 eral sense in cases where the interior of the 

 mass is other than homogeneous. Sec- 

 ondly, a fluid may, according to the nature 

 of the substance with which it is in contact, 

 or (more strictly speaking) according to 

 the distribution of energy in the system to 

 which it belongs, tend either to spread 

 itself out in a film, or, conversely, to con- 

 tract into a drop, striving in the latter case 

 to reduce its surface to a minimal area. 

 Thirdly, when three substances are in con- 

 tact (and subject to surface-tension), as 

 when water surrounds a drop of proto- 

 plasm in contact with a solid, then at any 

 and every point of contact, certain definite 

 angles of equilibrium are set up and main- 

 tained between the three bodies, which 

 angles are proportionate to the magnitudes 

 of the sui'face-tensions existing between the 

 three. Fourthly, a fluid film can only re- 

 main in equilibrium when its curvature is 

 everywhere constant. Fifthly, the only 

 surfaces of revolution which meet this con- 

 dition are six in number, of Avhich the 

 plane, the sphere, the cylinder and the so- 

 called unduloid and catenoid are the most 

 important. Sixthly, the cylinder can not 

 remain in free equilibrium if prolonged 

 beyond a length equal to its own circum- 

 ference, but, passing through the unduloid, 

 tends to break up into spheres : though this 

 limitation may be counteracted or relaxed, 

 for instance, by viscosity. Finally, we 

 have the curious fact that, in a complex 

 system of films, such as a homogeneous 

 froth of bubbles, three partition- walls and 

 no more always meet at a crest, at equal 



angles, as, for instance, in the very simple 

 case of a layer of uniform hexagonal cells ; 

 and (in a solid system) the crests, which 

 may be straight or curved, always meet, 

 also at equal angles, four by four, in a com- 

 mon point. From these physical facts, or 

 laws, the morphologist, as well as the physi- 

 ologist, may draw important consequences. 

 It was Plofmeister who first showed, more 

 than forty years ago, that when any drop 

 of protoplasm, either over all its surface or 

 at some free end (as at the tip of the pseu- 

 dopodium of an Amosha), is seen to 

 ' ' round itself off, ' ' that is not the effect of 

 physiological or vital contractility, but is a 

 simple consequence of surface-tension — of 

 the law of the minimal surface ; and in the 

 physiological side, Engelmann, Biitschli 

 and others have gone far in their develop- 

 ment of the idea. 



It was Plateau, I think, who first showed 

 that the myriad sticky drops or beads upon 

 the weft of a spider's web, their form, their 

 size, their distance apart, and the presence 

 of the tiny intermediate drops between, 

 were in every detail explicable as the result 

 of surface-tension, through the law of min- 

 imal surface and through the corollary to 

 it which defines the limits of stability of 

 the cylinder; and, accordingly, that with 

 their production, the will or effort or intel- 

 ligence of the spider had nothing to do. 

 The beaded form of a long, thin pseudo- 

 podium, for instance of a Heliozoan, is an 

 identical phenomenon. 



It was Errera who flrst conceived the 

 idea that not only the naked surface of the 

 cell but the contiguous surfaces of two 

 naked cells, or the delicate incipient cell- 

 membrane or cell-wall between, might be 

 regarded as a weightless film, whose posi- 

 tion and form were assumed in obedience 

 to surface-tension. And it was he who first 

 showed that the symmetrical forms of the 

 unicellular and simple multicellular organ- 



