In large mixing bowl combine sugar, flour, and salt. Stir in butter, lemon peel, and lemon juice. In small bowl combine eggs, milk; add to flour mixture. In a mixer bowl beat egg whites to stiff peaks. Gently fold egg whites into lemon batter. Turn into an ungreased 8x8x2 baking pan. Place in a larger pan on oven rack. Pour hot water into larger pan to a depth of 1 inch. Bake in a 350 oven for 35 to 40 minutes or till top is golden and springs back when touched. Serve warm or chilled in individual dessert dishes.