COLLOIDS 65 



produce cations. The sum of the cations and anions therefore becomes 

 a minimum, the concentration of the two being equal, and this point 

 the protein, being electrically neutral, has its solubility reduced to a 

 minimum. 



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 well to 

 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- 



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.) 



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. 

 The size of drops of liquid falling from an orifice is dependent on sur- 

 face tension; the larger the drops, the greater the surface tension. If 

 the number of drops obtained by allowing a liquid to drop from a stand- 

 ard orifice in a given time is counted, we have a measure of the surface 

 tension. Account must of course also be taken of the specific gravity 

 of the liquid. The instrument used for this purpose is called a 

 stalagmometer (Fig. 18). Another method depends on the fact that 

 the height to which a i\uid rises in a capillary tube is dependent on 



