Ammonium, the hypothetical radical of ammonia, supposed to be metallic. What is called an amalgam of mercury and ammonium was first obtained by Berzelius and Pontin from the aqueous solution of ammonia. Davy produced it with sal ammoniac; and it has since been obtained by simply dropping an amalgam of sodium and mercury into a strong solution of sal ammoniac. At a temperature of 32° F. it is a firm crystalline mass; at 70° to 80° it is a soft solid. It is about three times the density of water. Gay-Lussac and Thenard consider it a mere combination of mercury and ammonia; but Berzelius regards it as a real amalgam of mercury with a metal composed of one volume of nitrogen and four volumes of hydrogen. Since the discovery of other compound radicals that are capable of neutralizing acids, the question of the metallic character of ammonium has lost its significance, and few chemists are now disposed to insist upon calling it a metal.