CH.vi] OF SURFACE ENERGY 445 



morphology of the cell, and especially (so far as we have yet gone) 

 of the solitary cell or unicellular organism. 



But we also saw, according to Gauss's equation, that the potential 

 energy of the system will be diminished (and its diminution will 

 accordingly be manifested in work) if from any cause the specific 

 surface-energy be diminished, that is to say if it be brought more 

 nearly to an equality with the specific energy of the molecules in 

 the interior of the liquid mass. This latter is a phenomenon of 

 great moment in physiology, and, while we need not attempt to 

 deal with it in detail, it has a bearing on cell-form and cell-structure 

 which we cannot afford to overlook. 



A diminution of the surface-energy may be brought about in 

 various ways. For instance, it is known that every isolated drop 

 of fluid has, under normal circumstances, a surface-charge of 

 electricity: in such a way that a positive or negative charge (as 

 the case may be) is inherent in the surface of the drop, while a 

 corresponding charge, of contrary sign, is inherent in the imme- 

 diately adjacent molecular layer of the surrounding medium. Now 

 the effect of this distribution, by which all the surface molecules 

 of our drop are similarly charged, is that by virtue of the charge 

 they tend to repel one another, and possibly also to draw other 

 molecules, of opposite charge, from the interior of the mass; the 

 result being in either case to antagonise or cancel, more or less, 

 that normal tendency of the surface molecules to attract one 

 another which is manifested in surface-tension. In other words, 

 an increased electrical charge concentrating at the surface of a drop 

 tends, whether it be positive or negative, to lower the surface-tension. 



Again, a rfse of temperature diminishes surface-tension, and 

 consequently facilitates the formation of a bubble or a froth. It 

 follows (from the principle of Le Chatelier) that foam is warmer than, 

 the fluid of which it is made, and the difference is all the greater the 

 lower the concentration of the foaming (or capillary-active) substance*. 



But a still more important case has next to be considered. Let 

 us suppose that our drop consists no longer of a single chemical 

 substance, but contains other substances either in suspension or 

 in solution. Suppose (as a very simple case) that it be a watery 



* Cf. Fr. Schiitz, in Nature, April 10, 1937. In the case of 001 per cent, solution 

 of saponin, the temperature-diflFerence is no less than 3-3° C. 



