CHAPTER III 

 SURFACE ACTION 



IT has been shown in Chapter I. how living cells are made up of a highly complex 

 system of constituents, not mixing together liquids, solids, and sometimes gases. 

 Some of the solid substances, the "hydrophile" colloids, contain water in such 

 proportion that many of their properties approximate to those of liquids. 



Investigation has made it plain that where these different "phases," as \\v 

 have been taught to call them by Willard Gibbs, come into contact with each 

 other at their interfaces, the properties are not the same as in the main mass. 



SURFACE TENSION 



One of the most obvious phenomena of this kind is that shown by the surface 

 of contact of liquids with gases, solids, or other liquids immiscible with them. 

 This surface behaves as if stretehed. 



FlO. 30. RlNO OF IRON WIRE, ENCLOSING A SOAP FILM. 



In A there is a loop of flue silk floating in the film. 



In B the portion of the film inside the loop has been broken by touching it with a pointed bit 

 of filter paper. The result is that the tension of the film between the ring and the loop 

 causes this Aim to contract as much as possible, thus drawing the loop into a circle, the 

 figure of maximum area. 



(Van der Mensbrugghe. ) 



One of the simplest ways to demonstrate this is due to van der Meusln-ugghe (1866, 

 p. 312). A loop of fine silk is taken and tied to a wire ring. If the whole be dipped into 

 soap solution, so as to produce a film, the loop floats in the film ; the silk thread forming its 

 boundary is quite loose, and can be readily moved into any shape by means of a fine needle 

 wetted with the soap solution (see Fig. 30). The film inside the loop is now broken by 

 touching it with a bit of filter paper cut to a fine point. The loop is immediately drawn to 

 a circular form by the tension of the film surrounding it, and can be felt to resist attempts 

 to change its shape by the needle. The soap, solution should be prepared by the method of 

 Boys (1912, p. 170) from pure sodium oleate, with the addition of about 25 per cent, of glycerol. 



The best way of showing that the form taken by a liquid when free is that with the least 

 surface, namely the sphere, is by the use of ortho-toluidine, as described by Darling (1911). 

 This liquid has the same density as water at 22, but, since it has a higher coefficient of 

 expansion, it is less dense above 22 and more dense below that temperature. If a leaker half 

 lull df water at 22 is taken, and a solution of scxlium chloride of Moot 0'3 per cent, is run in 



