398 THE COLLOIDAL STATE 



The case of zinc is peculiar, inasmuch as very dilute 

 solutions of zinc salts produce irreversible precipitation of 

 egg albumen, whereas strong solutions may either not pro- 

 duce a precipitate, or else cause one already formed to 

 dissolve.* 



The anion also plays an important part in influencing 

 the precipitating power of a given salt. By arranging the 

 various salts of sodium in the order of decreasing precipita- 

 ting power, the so-called Lyotropic series is obtained as 

 follows : — 



Citrate > tartrate > sulphate > acetate > chloride > nit- 

 rate > chlorate > bromide > iodide > sulphocyanide. 



Here, again, there is no relation between precipitating 

 power and electric charge of the ion, and the fact that citric 

 acid comes first in the list has nothing to do with its being 

 tribasic. 



The precipitating effect of a salt appears rather to be con- 

 nected with its water-binding power, and it may be assumed 

 that the presence of a citrate, tartrate, or sulphate of an alkali 

 metal leaves less water available to the colloid. 



This assumption would also explain the fact that a gela- 

 tine gel containing such salts has a higher melting-point than 

 one containing a sulphocyanide which leaves the gelatine so 

 much water that it is reluctant to set. 



On the other hand, these salts are also known to affect the 

 compressibility of water, and their action on emulsoids may 

 possibly be connected with this fact. 



The precipitating power of the anions when combined with 

 one of the metals of the alkaline earths is exactly the reverse 

 of that observed when the same anions were combined with 

 the alkali metals. Thus the precipitating power of the anions 

 increases in the order C2H302> CI > N03> Br > I >CNS, 

 whereas when combined with the alkali metals the inhibiting 

 power increases in this same order. 



In conformity with the above facts, Pauli,t in studying the 

 precipitation of albumen by various salts, came to the con- 



* Pauli : " Beitr. z. chem. Phys. and Path.," 1905, 6, 233, 259. 

 t Ibid., 1902. 3, 225 ; 1903, 5, 30. 



