V] OF SURFACE TENSION 351 



Among the forces which determine the forms of cells, whether 

 they be sohtary or arranged in contact with one another, this force 

 of surface-tension is certainly of great, and is probably of paramount, 

 importance. But while we shall try to separate out the phenomena 

 which are directly due to it, we must not forget that, in each 

 particular case, the actual conformation which we study may be, 

 and usually is, the more or less complex resultant of surface-tension 

 acting together with gravity, mechanical pressure, osmosis, or other 

 physical forces. The peculiar beauty of a soap-bubble, solitary or 

 in collocation, depends on the absence (to all intents and purposes) 

 of these ahen forces from the field ; hence Plateau spoke of the films 

 which were the subject of his experiments as "lames fluides sans 

 pesanteur.'' The resulting form is in such a case so pure, and simple 

 that we come to look on it as wellnigh a mathematical abstraction. 



Surface-tension, then, is that force by which we explain the form 

 of a drop or of a bubble, of the surfaces external and internal of 

 a "froth" or collocation of bubbles, and of many other things of 

 like nature and in hke circumstances*. It is a property of Hquids 

 (in the sense at least with which our subject is concerned), and it 

 is manifested at or very near the surface, where the liquid comes 

 into contact with another hquid, a solid or a gas. We note here 

 that the term surface is to be interpreted in a wide sense; for 

 wherever we have solid particles embedded in a fluid, wherever we 

 have a non-homogeneous fluid or semi-fluid, or a "two-phase colloid " 

 such as a particle of protoplasm, wherever we have the presence of 

 "impurities" as in a mass of molten metal, there we have always 

 to bear in mind the existence of surfaces and of surface-phenomena, 

 not only on the exterior of the mass but also throughout its inter- 

 stices, wherever like and unlike meet. 



* The idea of a "surface-tension" in liquids was first enunciated by Segner, and 

 ascribed by him to forces of attraction whose range of action was so small "ut 

 nullo adhuc sensu percipi potuerat" {Defiguris super ficier um fiuidarum, in Comment. 

 Soc. Roy. GoUingen, 1751, p. 301). Hooke, in the Micrographia (1665, Obs. viii, 

 etc.), had called attention to the globular or spherical form of the little morsels 

 of steel struck off by a flint, and had shewn how to make a powder of such spherical 

 grains, by heating fine filings to melting point, "This Phaenomenon" he said 

 "proceeds from a propriety which belongs to all kinds of fluid Bodies more or less, 

 and is caused by the Incongruity of the Atnbient and included Fluid, which so 

 acts and modulates each other, that they acquire, as neer as is possible, a spherical 

 or globular form " 



