296 THE COLLOIDAL STATE 



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 C 2 H 3 O 2 > Cl > NO 3 > 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,* in studying the 

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

 clusion that the precipitating power of a salt was an additive 

 property which depended on the constituent ions (see also p. 



318). 



Rations, as a rule, act as precipitants for albumen, while 

 anions tend to keep it in solution. 



The precipitating power of the kations increases in the 

 following order: Mg, NH 4 -, K, Na, Li, while the inhibiting 

 or solvent action of the anion increases in the following 

 order: -C 2 H 3 O 2 , -Cl, -NO,, -Br, -I, -CNS. 



According as the precipitating power of the kation or the 

 inhibiting power of the anion predominates the resulting salt 

 will either precipitate or not precipitate albumen. 



The observations are given below in tabular form. As 



* Pauli : " Beitr. z. chem. Phys. and Path.,'' 1902, 3, 225 ; 1903, 5, 30. 

 fSee p. 319. 



