«The Precipitating Power of Electrolytes and the Valence of their Ions.»—The complete precipitation of colloids, which carry electric charges, depends on the concentrations of the colloid and the electrolyte; in this connection the important observation has been made that the ‹precipitating power› of electrolytes ‹increases decidedly with the valence of the precipitating ions› (H. Schulze's rule[281]). Bivalent ions are far more efficient than univalent; trivalent, in turn, still more effective than bivalent. Thus, colloidal As_{2}S_{3}, carrying a negative charge, is precipitated by the positive ions of added electrolytes. The addition of a few drops of a molar solution of ammonium nitrate (the precipitating ion is NH_{4}^{+}) to 20 c.c. of the colloidal suspension[282] produces a slight precipitate; complete precipitation requires 3.5 to 3.6 c.c. of the ammonium nitrate solution.[283] Only 0.06 c.c. of an equivalent solution[284] of magnesium nitrate (the precipitating ion is Mg^{2+}) is required, and as little as 0.015 c.c. of an equivalent solution of aluminium nitrate[285] (the precipitating ion is Al^{3+}) has the same effect (‹exp.›). An increase in valence of the ‹negative› ion, which is not the precipitating ion in this case, does not affect the result appreciably: 3.5 c.c. of a solution[286] of ammonium sulphate, (NH_{4})_{2}SO_{4}, equivalent to the solution of NH_{4}NO_{3}, is also required for the complete precipitation of the colloidal As_{2}S_{3} (‹exp.›), although the one contains the univalent ion, NO_{3}^{−}, the other the bivalent ion, SO_{4}^{2−}. Conversely, a ‹positively› charged colloid, like ferric hydroxide, may be precipitated by much smaller quantities[287] of bivalent negative ions than of univalent ions, etc. [p136] «Nonprecipitation of Nonelectrified Colloids by Electrolytes.»—Colloids which do not carry any electric charges of moment (see p. 132) are also not precipitated by dilute solutions of electrolytes. Heat, the addition of concentrated salt solutions in great excess (whose effect is probably a dehydrating one), or of other solvents (‹e.g.› alcohol), are the agents most commonly used to effect coagulation in such cases. «Protective Action of Colloids on Other Colloids.»—Colloids, particularly such as are not sensitive to precipitation by electrolytes, increase in a remarkable degree the stability of the colloidal condition of substances, that carry electrical charges and, ordinarily, would be very sensitive to precipitation. Thus, small quantities of albumen are used to render colloidal silver preparations more stable. Tannic acid, gelatine and albumen and related compounds are frequently used as such protective agents. It is supposed that they form protecting films around the colloidal particles.