V] OF SURFACE TENSION ' 205 



a piece of hard glue, when we throw it into water and see it expand 

 as it sucks the water up, are not nearly so regular or so beautiful 

 as are those which appear when we blow a bubble, or form a 

 drop, or pour water into a more or less elastic bag. For these 

 curving contours depend upon the properties of the bag itself, 

 of the film or membrane that contains the mobile gas, or that 

 contains or bounds the mobile liquid mass. And hereby, in the 

 case of the fluid or semifluid mass, we are introduced to the 

 subject of surface tension : of which indeed we have spoken in 

 the preceding chapter, but which we must now examine with 

 greater care. 



Among the forces which determine the forms of cells, whether 

 they be solitary 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. 



Surface tension 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 like circumstances*. It is a property of liquids 

 (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 liquid, a solid or a gas. We note here 

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

 wherever Ave have solid particles imbedded in a fluid, wherever 

 we have a non-homogeneous fluid or semi-fluid such as a particle 



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

 De figuris superficierum fluidarum, in Comment. Soc. Boy. Gottiiigen, 1751, p. 301. 

 Hooke, in the Micrographia (1665, Obs. viii, etc.), had called attention to the 

 globular or spherical form of the httle 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 Ambient and included Fluid, which so. acts and modulates each other, that 

 they acquire, as neer as is possible, a spherical or globular form...." 



