64 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



the fluid in which it is dissolved, so that the reaction towards other 

 colloids and towards electrolytes will vary. 



One feature of proteins of importance in this connection is that known 

 as the isoelectric point, at which the protein exists with a maximum of 

 electrically neutral molecules. This point is reached by adding acid to 

 a protein solution. The acid represses the dissociation of the protein 

 acting as an acid, and therefore diminishes the number of free hydrogen 

 ions ; and at the same time it combines with the NH 2 groups and neutral- 

 izes the basic characteristics. The alteration in electric charge thus in- 

 duced alters the water-absorbing powers of the protein and therefore 

 all of the properties which we have seen to be associated therewith 

 (page 63). 



SURFACE TENSION 



Before we consider a very important property of colloids known as 

 adsorption, by means of which they are able to perform many reactions 

 that do not conform with the laws of mass action, it will be w r ell to 



A. 



Fig. 17. Diagram to illustrate surface tension. The rings A and B inclose soap films in 

 which a very fine loop of silk is suspended. In A it is loose but in B, where the film inclosed 

 in the loop has been broken, it is drawn into a circle by the tension of the soap film. (From 

 Bayliss.) 



say a few words concerning the physical phenomenon upon which this 

 depends namely, surface tension. The creation of this force is due 

 to the fact that, whereas the molecules within a liquid are subjected to 

 equal forces of attraction on all sides, at the surface these forces act on 

 one side of the molecules only, and therefore tend to pull them inwards. 

 This causes the surface to pull itself together so as to occupy the least 

 possible area, and it is this force which constitutes surface tension. 

 The surface behaves as if stretched. There are various simple experi- 

 ments that reveal the presence of surface tension. If a film is made on 

 a loop of wire by dipping it in soap solution, a fine silk thread can be 

 floated in the film, so that it forms a loop that is quite loose. If the 

 portion of the film inside the loop is destroyed by touching it with filter 

 paper, the film will break in the loop, which will now be pulled into a 

 circular shape by the tension of the film around it (Fig. 17). 



For the measurement of surface tension, various methods are used. 



