1 53 



the top and bottom layers was as 11 to 12, while that of calcium bicarbonate was as 1 to 4. 

 In another set of experiments (p. 557 of Collected Papers), it was found that in foui -ti-cn <l;i\ s 

 the concentration of sodium chloride at the top of a column of 127 mm. was only l/> of that 

 at the bottom, while sugar was only just to be detected at the top in that space of time, the 

 uppermost 50 c.c. of solution contained only. (HJ05 g. of glucose. 



The different diffusion rates of various substances may give rise temporarily to 

 considerable differences of osmotic pressure between two solutions in an osmometer, 

 even when the two solutions are actually of equal osmotic pressure to Ix'u'in 

 with, and separated by a membrane permeable to both solutes. Sodium chloride 

 diffuses more rapidly than magnesium sulphate, so that, if we take isotonic solu- 

 tions on the two sides of a membrane permeable to both solutes, the former 

 salt will diffuse through the membrane faster than the latter, the molar concen- 

 tration and osmotic pressure of the sodium chloride solution will diminish, 

 while that of the magnesium sulphate will increase, and water will pass tu 

 the latter. The difference in concentration is, of course, only temporary, but 

 may give rise to considerable changes in osmotic pressure, and is of importance 

 in the process of absorption from the alimentary canal. 



When we have a solution of an electrolytically dissociated salt in contact 

 with water, if the anion and cation move at different rates, it is clear that 

 there will be a difference of potential between the front and back of the 

 advancing surface of the diffusing column, the faster moving ions giving the 

 sign of their charges to the front layer. Owing to electrostatic forces, the one 

 set of ions cannot outdistance the other set further than their kinetic energy 

 can carry them in opposition to the electrostatic attraction. (For the magnitude 

 of these forces see the calculation by Arrhenius on page 179 below.) This 

 phenomenon is a possible source of potential differences in tissues, and will be 

 discussed later in Chapter XXII. 



OSMOTIC PRESSURE OF COLLOIDS 



The osmotic pressure of a solution is found to be, by whatever method it 

 is measured, in direct relation to the molecular concentration. If a molecule 

 is dissociated in any way, electrolytically or hydrolytically, each fraction acts as 

 an element, equivalent osmotically to a molecule. Similarly, if there is association 

 of molecules, the associated group behaves as a single molecule. The measurement 

 of osmotic pressure is thus the most valuable means of determining the actual 

 molecular concentration of a given solution. 



Have we then any reason to limit the powers of giving an osmotic pressure 

 to associations of a small number of molecules and deny it to those where a 

 larger number are associated, as in colloids ? Or at what particular number 

 does osmotic pressure cease ? Some substances, moreover, as we saw in Chapter TV., 

 owe their colloidal properties to the fact that their single molecules or ions are too 

 large to pass through parchment paper. If colloids have no osmotic pressure, 

 it must be denied also to some molecules, so that we may again ask, at what 

 molecular dimensions does it cease 1 



Any colloidal solution which remains in permanent suspension consists of 

 particles in perpetual Brownian movement, precisely similar to the molecular 

 movement postulated by the kinetic theory. Moreover, as shown by Perrin 

 (page 85 above), each particle possesses the same mean kinetic energy as a molecule. 

 If, then, this kinetic energy is the cause of osmotic pressure, it follows that 

 colloidal particles must manifest it. 



A brief consideration will show, however, that it cannot be expected to be great, at all 

 events as far as the association of molecules constituting a suspensoid colloid are concerned. 

 A true solution in decimolar concentration has an osmotic pressure of 1,702 mm. of mercury 

 at 0, as can be seen from the following calculation. One gram-molecule of a gas, at normal 

 temperature and pressure, occupies a volume of 22 '4 litres ; therefore, to compress it to 

 one litre, the volume of a solute in molar solution requires, by Boyle's law, a pressure 

 of 22 - 4 atmospheres, or 17,024 mm. of mercury. But suppose that the same number of 

 molecules as those in a decimolar solution are aggregated in masses of 500, then the solution, 

 although containing the same amount of total solid, will have only 0'002 times the number 



