56 PRINCIPLES OF GENERAL PHYSIOLOGY 



One of the simplest ways to see this fact is to blow a bubble with a solution of saponin, 

 say 1 per cent., as a soap-bubble would be blown. If air be then sucked back out of the 

 bubble, or it be allowed to contract spontaneously, collapse is even and regular in the case 

 of the soap-bubble, so that it remains spherical. In the case of saponin, on the contrary, the 

 film has ceased to be elastic, and can only collapse by falling into folds. It is sometimes 

 possible to see little rods of the solid in the film. A similar phenomenon is found to take 

 place with egg albumin, and is said to be the reason why cooks find that a froth beaten up 

 for meringues, if allowed to stand, cannot be made again into a froth ; the albumin, in fact, 

 has gone out of solution by surface coagulation. This coagulation in surface films is also, 

 no doubt, the cause of the inactivation of enzymes when shaken with air, as found hy 

 Schmidt-Nielsen (1909 and 1910). 



When surface tension is measured, as in the experiments of several workers, 

 by means of vibrating drops or surface waves, the surface tension plays the 

 part of elasticity in the ordinary form of wave motion in air, so that, when 

 this surface tension changes, the rate of vibration changes also. When pure 

 liquids are investigated by this dynamic method, in which' the surface is being 

 continually renewed, the same values are obtained as by static methods, such as 

 rise in a capillary tube or drop method, where time is allowed for the surface to 

 attain a state of equilibrium. With solutions of substances which lower surface 

 tension, on the other hand, and are therefore concentrated in the surface layer, 

 it depends upon the rate at which this adsorption takes place whether the 

 two kinds of method give identical values. Conversely, if the values are not 

 identical, it is clear that the adsorption has not had sufficient time for completion 

 before a new surface is formed in the dynamic method. To take a well marked 

 instance : A '025 per cent, solution of sodium oleate has a static surface tension 

 of 26 dynes, but a dynamic one of 79 dynes, practically the same as water, 

 so that no adsorption has taken place in the time allowed before a new surface 

 is formed. The fact is of interest in that it shows that the actual process of 

 adsorption is not instantaneous, although it is extremely rapid. 



The Gibbs principle implies, as will be obvious, that if a substance raises 

 surface energy, its concentration at an interface will be lowered, giving rise to 

 negative adsorption. Such a case has been described by Lagergren (1898). When 

 sodium chloride solution is shaken with charcoal, its concentration is raised, owing 

 to its being displaced from the interface and sent into the main body of the 

 solution. This effect seems to depend on change in solubility with pressure, since 

 the water film on the surface of the adsorbing powder is, probably, in a highly 

 compressed state owing to molecular forces (Nernst, 1911, p. 124). 



Although the principle of Carnot and Clausius shows that adsorption must 

 take place if free energy is lowered thereby, we have, as yet, made no reference to 

 the forces which produce this surface action. Titoff points out (1910, p. 674) that 

 the quantity adsorbed in the case of gases increases with the well-known quantity 

 a of the Van der Waals equation 



The meaning of this equation will be discussed later (page 149), but it may be 

 stated here that a expresses the mutual attraction of the molecules. Therefore, as 

 Arrhenius puts it (1912, p. 40) : "The forces which produce adsorption are of the 

 same order and of the same nature as those which cause the mutual attraction of 

 molecules." This view is confirmed by the fact, shown by Freundlich (1909, 

 p. 154), that the extent to which a series of different substances is adsorbed by 

 charcoal follows the same order, although different in absolute amounts, when 

 adsorbed by wool, silk, cotton, and so on. 



Four special cases of adsorption are of interest to the physiologist, on account 

 of the part they play in the phenomena with which he has to deal. These may be 

 given here as illustrating the nature of the process. Other cases will appear in the 

 course of this book. 



I. The Adsorption of Gases by Solids. This is familiar to all chemists in the 

 use of charcoal. It is characteristic of adsorption to be diminished by rise of 

 temperature, and here it is of importance to remember that this statement refers 

 only to the condition of equilibrium, and that the rate of adsorption is increased by 



