206 THE FORMS OF CELLS [ch. 



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 tensions, not only 

 on the exterior of the mass but also throughout its interstices, 

 wherever like meets unhke. 



Surface tension is due to molecular force, to force that is to 

 say arising from the action of one molecule upon another, and it 

 is accordingly exerted throughout a small thickness of material, 

 comparable to the range of the molecular forces. We imagine 

 that within the interior of the liquid mass such molecular inter- 

 actions negative one another: but that at and near the free 

 surface, within a layer or film approximately equal to the range 

 of the molecular force, there must be a lack of such equilibrium 

 and consequently a manifestation of force. 



The action of the molecular forces has been variously explained. 

 But one simple explanation (or mode of statement) is that the 

 molecules of the surface layer (whose thickness is definite and 

 constant) are being constantly attracted into the interior by those 

 which are more deeply situated, and that consequently, as 

 molecules keep quitting the surface for the interior, the bulk of 

 the latter increases while the surface diminishes ; and the process 

 continues till the surface itself has become a minimum, the surface- 

 shrinkage exhibiting itself as a surface-tension. This is a sufficient 

 description of t|;ie phenomenon in cases where a portion of liquid 

 is subject to no other than its oivn molecular forces, and (since the 

 sphere has, of all solids, the smallest surface for a given volume) 

 it accounts for the spherical form of the raindrop, of the grain 

 of shot, or of the living cell in many simple organisms. It accounts 

 also, as we shall presently see, for a great number of much more 

 complicated forms, manifested under less simple conditions. 



Let us here briefly note that surface tension is, in itself, a 

 comparatively small force, and easily measurable: for instance 

 that of water is equivalent to but a few grains per linear inch, 

 or a few grammes per metre. But this small tension, when it 

 exists in a curved surface of very great curvature, gives rise to a 

 very great pressure directed towards the centre of curvature. We 

 can easily calculate this pressure, and so satisfy ourselves that, 

 when the radius of curvature is of molecular dimensions, the 



