278 A NOTE ON ADSORPTION [ch. 



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

 potential energy of the system will be diminished (and its diminu- 

 tion 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 modern 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. 



In various ways a diminution of the surface energy may be 

 brought about. 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 

 immediately 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 

 this 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. 



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 

 fluid, exposed to air, and containing droplets of oil : we know that 

 the specific surface tension of oil in contact with air is much less 

 than that of water, and it follows that, if the watery surface of 

 our drop be replaced by an oily surface the specific surface energy 

 of the system will be notably diminished. Now under these 

 circumstances it is found that (quite apart from gravity, by which 

 the oil might float to the surface) the oil has a tendency to be 

 drawn to the surface ; and this phenomenon of molecular attraction 



