Prepd by the action of hydrogen selenide on anhydr magnesium chloride at a red heat and by the action of selenium vapor carried in a current of nitrogen on powdered magnesium: Fonzes-Diacon, Contribution a l'Etude des Séléniures Métalliques (Montpellier, 1901); by dropping selenium into molten magnesium: Liddell, Chem. Metall. Eng. 25, 102, 263, 453 (1921).