THE COLLOIDAL STATE 101 



physico-chemical properties of the salt solutions themselves. These are, rate of 

 diffusion and compressibility, which increase with the favouring action, while 

 surface tension, internal friction, electrical conductivity, diminution of solubility 

 of other solutes, maximum density, effect on catalysis of esters, inversion of 

 cane-sugar by acids, and dissociation of weak acids are properties which decrease 

 along with increase of favouring action. 



At first sight it would seem natural to connect these various phenomena with hydration 

 of the respective crystalloids in solution. A part of the solvent is held in this way in the 

 region of the solute, so that any process in which water is concerned would pursue a different 

 course in presence of crystalloids than in their absence, and, in general, the change would be 

 of the same kind as that caused by increase in concentration. There seems, however, to be 

 some additional factor, because there are some crystalloids whose solutions produce wore 

 swelling than pure water does. The suggestion is made by Samec (p. 157) that adsorption of 

 the crystalloid takes place on the surface of the gel elements and that the adsorbed, highly 

 hydrated, substance brings water into more intimate contact with the colloid. The fact that 

 any particular ion has precisely the same effect on a protein and on starch, as pointed out by 

 Samec (1911, p. 154), shows that the formation of chemical compounds does not play any 

 important part. Further information as to the behaviour of gelatine in various solutions in 

 water, may be found in the paper by Ehrenberg (1913). 



As to the nature of the process itself, Posnyak (1912, p. 154) calls attention 

 to three possibilities : 



1. Condensation of water on the surface of the elementary particles of the 

 gel, leading to filling up of the capillary spaces between them, while the 

 particles themselves remain unchanged in size. 



2. Simple solution of the liquid in the substance of the particles, which 

 thereby change their size, density, etc. 



3. Both processes take place. This is regarded by Posnyak as the most 

 probable one theoretically, although his experiments on the influence of 

 pressure on the liquid content of a gel speak more in favour of the first. He 

 finds, in fact, that the content in solid (c) of such gels as india-rubber and 

 gelatine is related to the pressure (P) by the formula : 



P = Ac* 



where A and k are constants. A varies considerably from gel to gel and from 

 liquid to liquid, while k has always the same value ( = 3). This latter fact is 

 difficult to explain on the basis of a solution of the liquid in the colloid 

 substance and consequent change in its properties. According to Zsigmondy 

 (1913) the lowering of vapour pressure in the imbibition of water by silica 

 gels is due to the formation of a concave meniscus, not to formation of 

 hydrates. Imbibition is the filling of hollow spaces in this case, not the 

 taking up of water into actual substance. 



The similarity of Posnyak's equation to the simple form of the expression for 

 adsorption is obvious. It would seem also to be more advantageous for rapid 

 changes in the distribution of water, such as are required in physiological activities, 

 that the water should be on the surface rather than inside the substance of the 

 colloid. As Posnyak suggests, it is probable that the relative share taken by the 

 two kinds of process differs according to the amount of water available. 



Some experiments which I have recently had occasion to make favour this suggestion. 

 Gelatine is sometimes used to remove water from alcohol that is nearly absolute, say 90 to 95 

 per cent. Of course, it is useless for this purpose unless thoroughly dried first. I found 

 that it does remove water from 90 per cent, alcohol, so that this becomes stronger, but, 

 to my surprise, no increase in volume of the gelatine was to be detected, although the 

 amount of water removed from the alcohol was sufficient to be detected easily. The 

 gelatine also appeared to be just as hard and horny as when put in. The only explanation 

 seems to be that, in order to determine the volume after immersion in alcohol, the 

 pieces were allowed to dry for about a minute in air ; the liquid alcohol evaporated 

 from the surface in this process, and apparently the water concentrated on the surface 

 passed off with the alcohol, a phenomenon that could not have taken place in so short 

 a time if water had penetrated into the substance of the gelatine. 



The facts above described will be found in later pages to have a bearing on the action 

 of enzymes. 



