THE COLLOIDAL STATE 97 



When we consider the series of salts investigated by Hofmeister (1888), as 

 regards their relative effect on the salting out of albumin, and known as the 

 " Hofmeister series" no obvious reason is apparent for the different behaviour of 

 the salts. A chemical one is excluded by the fact that the same series is found in 

 the action on substances so different in constitution as albumin, gelatine, agar, and 

 starch. As Hatschek (1912, p. 46) points out, the only view which co-ordinates the 

 various phenomena is that they are all manifestations of a change in the 

 distribution of water between the two phases ; the salts of the Hofmeister series 

 do this by their action on the compressibility of water. The solution of emulsoids 

 is usually associated with contraction. These phenomena, in general, belong "to 

 that class called by Freundlich " lyotropic " (1909, pp. 54 and 412), as dependent 

 on changes in the solvent itself. When water is the solvent, we speak of the 

 hydration of the ions of the salt, and the changes in the equilibrium between the 

 various molecular states of fluid water. These changes give rise, in their turn, to 

 alterations in the internal pressure, expressed in changes of compressibility, viscosity, 

 solubility, and so on. For further details as to the Hofmeister series, and the 

 action of salts on emulsoids, the reader is referred to the book of Freundlich (1909, 

 pp. 424 seq.). 



It was known to Faraday (1858, p. 175) that the precipitating action of "salt " 

 on gold solutions could be prevented by the addition of a trace of "jelly." Other 

 emulsoid colloids have this action, although in different degree, and the fact 

 serves as the basis for the "gold number " of Schulz and Zsigmondy (1903) as a 

 characteristic of individual proteins. It seems certain that this protection 

 against the action of electrolytes conferred by an emulsoid on a suspensoid is due 

 to the deposition of a film of the former over the surface of the solid particles, 

 thus practically converting the system into an emulsoid one. Mines (1912, p. 219), 

 by the application of his test with complex trivalent ions, finds that a gold 

 hydrosol protected by an emulsoid behaves in the same insensitive way as the 

 emulsoid itself. Moreover, if the protective colloid be a protein, which has the 

 sign of its charge easily reversed by acid, as gelatine, it will be found that the gold 

 particles, previously insensitive to acid, have become sensitive (ibid., p. 222). It 

 can be shown by electric convection that it is not easy to reverse the sign of the 

 charge on gold particles by acid alone ; when they are coated with gelatine this is 

 easy, although gelatine itself does not affect the sign of the charge. The adsorption 

 of protective colloid by the surface of the gold particles is no doubt due to the 

 lowering of surface tension thereby brought about ; and the gold number varies 

 according to the capacity in this respect. 



The protective action is not necessarily complete, as I noticed in some experiments made 

 with arsenious sulphide and with Congo-red. In these cases, actual precipitation by calcium 

 sulphate was prevented by the addition of albumin, as in the case of gold, but if such 

 mixtures were carefully compared with the original, it was noticed that they were somewhat 

 more turbid. Under the ultra-microscope, the change was very obvious in the case of Congo- 

 red. This dye, in the absence of electrolytes, is not resolvable into particles. After the 

 addition of serum-albumin and calcium sulphate, although no precipitation occurred, as when 

 the salt was added alone, the solution was nevertheless found to be full of very distinct, but 

 not brilliant, particles. This effect is in agreement with the small, but not negligible, 

 effect of salts on emulsoids. 



Walpole (1913, 3) has shown that gelatine in very low concentration (1 in 100,000,000) 

 increases the effect of hyfoochloric acid in the aggregation of hydrosols of gold, mastic or 

 oil. In concentrations of 10~ 6 to 10~ 4 ' 4 of gelatine there are two concentrations of the acid 

 which produce aggregation, whereas, between these two, no effect is produced. In cases of 

 aggregation due to the assistance of a "protective " colloid, reversal is obtained by the addition 

 of alkali, and ultra-microscopic examination shows that the aggregates in the case of oil 

 solutions consist of numbers of the original minute particles, stuck together ; whereas, in the 

 case of aggregation by hydrochloric acid in the presence of gelatine of concentration lower 

 than 10~ 8 , the aggregates are comparatively large drops of oil. There is no change of sign of 

 the electric charge in these cases of aggregation brought about by traces of gelatine. When 

 concentrations of gelatine greater than 10~ 4 - 4 protect from the action of acid, the sign of the 

 charge of the particles is converted from negative to positive by the acid. For further details 

 see Walpole's second paper (1914, 2). 



A marked difference in viscosity, or internal friction, is shown by emulsoids 

 and suspensoids. While that of the latter is not greatly different from that of 

 water, the former, as a rule, have a very considerable viscosity (Freundlich, 1 909, 



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