«Precipitation of Colloids by Electrolytes and by Colloids.»—Substances in the colloidal condition, ‹which carry an electrical charge›, are readily precipitated by the addition of electrolytes to the colloidal suspensions (see the behavior of arsenious sulphide, p. 126). Negatively charged colloids are precipitated by the action of positive ions, positively charged colloids by the action of negative ions (Hardy's rule[271]). The precipitated colloid carries [p134] with it a part of the precipitating ion[272] (‹adsorption›) and the weights of ions, carried down by a given quantity of a given colloid, are proportional to the equivalent weights of the ions.[273] The precipitation thus appears to be intimately associated with the neutralization of the charge on the colloid. In accordance with this conclusion, it has also been found that a colloid may be precipitated by a colloid carrying the opposite charge.[274] Thus, colloidal arsenious sulphide, carrying a negative charge, and colloidal ferric hydroxide, carrying a positive charge, mutually precipitate each other (‹exp.›[275]). The purple precipitate (‹purple of Cassius›), which is formed when stannous chloride is added to gold chloride (p. 126), contains ‹colloidal gold›,[276] which, in suspension, is charged with negative electricity, and ‹colloidal stannic acid›,[276] which in acid solution, presumably, carries a positive charge[277]: these two colloids mutually precipitate each other in the presence of hydrochloric acid.[278] When the precipitate is treated with an alkaline liquid (ammonium hydroxide solution), the charge on the stannic acid becomes negative, both colloids acquire the same charge and the precipitate dissolves, to form the beautifully tinted suspensions of colloidal gold (p. 126). In this condition the colloids (gold and stannic acid) are sensitive to precipitating electrolytes, and the solution is more sensitive to a mixture of magnesium nitrate and ammonium nitrate than to ammonium nitrate alone (‹exp.›), as is to be expected from a ‹negative› suspension (see below). The characteristic difference in behavior between ‹ortho-›, ‹pyro-› and ‹metaphosphoric acids› toward albumen, which is used as a characteristic analytical test to distinguish metaphosphoric acid from the other two acids,[279] appears to be due to similar relations[280]: metaphosphoric acid, which precipitates (coagulates) albumen, is colloidal, ortho- and pyrophosphoric acids are not. The [p135] coagulation seems to be the result of the union of the ‹negative colloid› metaphosphoric acid (or a complex negative colloidal ion thereof) with the ‹positive colloid› albumen (or a positive colloidal ion thereof).