152 PRINCIPLES OF GENERAL PHYSIOLOGY 



THE CAUSE OF OSMOTIC PRESSURE 



The basis of the foregoing considerations has been that of the kinetic theory, 

 according to which the osmotic pressure, developed by a solution constrained by a 

 membrane permeable only to the solvent, is due to the impacts of the molecule <-t' 

 the solute against the membrane through which they cannot pass (Nernst, 1911, 

 p. 244). It is well to note that there are other views on the question, such as 

 surface tension, attraction of solute for solvent, and so on, but it would exceed tin- 

 scope of the present book to discuss them. Although van : t Hoff made use of the 

 therinodynamic method in the quantitative mathematical treatment of osin<t it- 

 pressure, he interprets the phenomenon in terms of the kinetic theory as j, r i\m 

 above (see p. 482 of his paper, 1887). For our purposes, the kinetic theory 

 satisfies requirements best. Those who are interested in the question are referred 

 to the monograph by Findlay (1913, pp. 65-76), and to the paper by Callendar (1908). 

 Callendar remarks, " It is probable that all the theories possess some elements of 

 truth, and that they may be to some extent merely different aspects of the same 

 phenomenon." 



HYDRATION OF SOLUTE 



There is one point that requires a few words. Many solutes are hydrated in 

 solution in water. That is, each molecule is associated with a larger or smaller 

 number of water molecules. The result of this is that the number of molecules of 

 water in a given volume is reduced, although that of the solute is not._ 



As far as dilute solutions are concerned, as Nernst (1911, pp. 271 and 469) 

 points out, this fact will have no influence on the osmotic pressure, however 

 measured. The number of molecules of water is so great in proportion to those 

 of the solute that the fixation of a certain number of them will have no measurable 

 effect. On the other hand, calculations of the osmotic pressure of concentrated 

 solutions of cane-sugar, made on the hypothesis that each molecule is associated 

 with five molecules of water, gives values more nearly approximating to those 

 obtained experimentally (see Findlay, 1913, p. 42). 



There is at present much difference of opinion as to the nature of this hydration. For 

 example, it is stated by Callendar (1908, p. 498) that the conclusions of Jones and Bassett (1905) 

 are "diametrically opposed" to his. 



METHODS OF MEASUREMENT 



The direct measurement of osmotic pressure, either by Pfeffer's method of 

 measuring the pressure produced in the osmometer when one side of the membrane 

 is immersed in water at atmospheric pressure, or by that of Berkeley by measuring 

 the pressure necessary to be applied to the solution in order to prevent passage 

 of solvent in either direction, is of considerable experimental difficulty, and only 

 applicable in certain cases, owing to the fact that we know of so few appropriate 

 semi-permeable membranes. In practice, the determination of other properties, 

 which are related in a known way to the osmotic pressure, is usually resorted to. 

 Fig. 48 shows the construction of some of the cells used by Morse. 



Before passing to the indirect methods, a direct method due to Fouard (1911) will, perhaps, 

 sometimes bo found useful. In speaking of the semi -permeable membranes prepared by 

 Traube, that made by the action of tannin on gelatine was referred to. Fouard makes use of 

 this, but instead of measuring the pressure in a manometer, he balances it by the use of 

 solutions of cane-sugar of known osmotic pressure outside. It is clear that the approximate 

 osmotic pressure of the solution inside should be known, in order to save a large number of 

 preliminary trials. A small cylinder of silver gauze is taken, immersed in 6 per cent, collodion 

 in order to form a film, washed with water, and then filled with 1 per cent, gelatine, which is 

 then poured out. After soaking for five to six days in dilute tannin solution, it is ready for 

 use. It should be kept in dilute solutions of the membrane formers, presumably gelatine 

 inside and tannin outside, or vice versa. For use, it is connected with a capillary tube, bent 

 liori/.oiitally BO as to be at the same level as the top of the outer solution. The solution whose 

 osmotic pressure is to be measured is placed inside, so as to form a meniscus in the capillary 

 tube. If the osmotic pressure of the cane-sugar solution is greater than that of the inner 

 solution, water will pass out and the meniscus will move towards the cell and vice versa. By 

 adding either water or sugar, as the case may be, a solution can be found which has the same 



