The precipitation of sols by electrolytes naturally attracted the attention of the early investigators, and certain general relations were established. As these are to be found in any standard textbook on colloids, it is unnecessary to restate them here at length. They may be summarised as follows:— (i) A sol is precipitated by the ion of opposite electric charge, i.e. negative sols by kations, positive sols by anions; (ii) the precipitating power increases with the valency of the ion, the ratio for univalent, bivalent, trivalent being I: X:X2, the so-called valency rule; (iii) the other ion of the electrolyte tends to stabilisation of the sol; (iv) excess of the precipitating ion reverses the electric charge of the sol, which is then only precipitated by the ion of opposite electric charge; this occurs more readily with polyvalent ions.