LIFE AND ENERGY 31 



But an emulsoid system may also be formed by dispersion of 

 a solid in a watery phase, provided that this solid is one that 

 soaks up water by the process known as " imbibition." A well- 

 known case is that of gelatin (E., p. 177). Here the redistribution 

 of phases takes place merely on warming and cooling. A jelly 

 consists of droplets of a very dilute solution of gelatin encased in 

 chambers of the solid gelatin holding water in its substance by 

 imbibition. On warming, the more solid phase becomes internal, 

 particles surrounded by watery solution. Hence the system, as a 

 whole, becomes liquid. What the nature of imbibition is, is not 

 completely known. There is evidence that it is essentially an 

 adsorption of water by the surfaces of constituent elements of the 

 solid, owing to certain physical peculiarities of these surfaces ; but 

 the precise interpretation clearly depends on what these elements 

 are. 



Whatever may be the nature of imbibition, a fact of importance 

 in the physiological behaviour of emulsoids is that the amount of 

 water present may vary in its distribution between the two phases. 

 The change is produced especially by electrolytes (E., p. 177), not 

 in virtue of their electrical charges, but owing to the effect they 

 have on the properties of water (p., pp. 96, 97). The importance 

 of being able to extract water from a system in which chemical 

 reactions are taking place will become more evident when we 

 study the actions of enzymes. 



Remembering that a colloidal solution consists merely of a 

 substance very finely divided and dispersed in a liquid, we see at 

 once that the properties that distinguish it from those of a system 

 consisting of the same amount of material in a single lump immersed 

 in the liquid depend on the enormous extent of boundary surfaces 

 between liquid and solid phases, so that they may be regarded 

 as only differing in degree ; but there is a very great difference 

 in degree. The properties are, therefore, those which manifest 

 themselves at such interfaces. These are especially those dependent 

 on surface tension, electrical charges, etc. We expect to find 

 adsorption phenomena in a marked degree, and we shall see, 

 presently, the way in which electrical charges play their part. 

 From this point of view we may note again that we cannot make 

 any hard and fast line of distinction between coarsely heterogeneous 

 and colloidal systems, except in degree. On the other side, it is 

 difficult to say at what stage of subdivision the properties of 

 surface cease and molecular properties begin. As will be seen 

 presently, some molecules are large enough to show the properties 

 of surface when single, but in most cases, and especially in the 

 suspensoid colloids, the particles consist of a large number of 

 molecules. It is generally agreed, however, to call those solutions 



