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.

Included in the area of distribution of Salmo salar are the western coasts of Europe as far south as the Franco-Spanish border as well as the British Isles and Iceland, and, in addition, the eastern coast of Canada and the United States down to the State of Maine. A very large number of investigations have been made in Great Britain and various European countries, both by marking the fish in order to trace their subsequent growth and movements, and by reading their age and history from the scales. Length calculations from scale measurements have also been made in Scotland, Norway, and Sweden.