Line a 23 cm loaf pan with foil or plastic wrap, being sure that enough wrap is used to cover and overhang all 4 sides of the pan and top. Sprinkle gelatin over lemon juice in a saucepan and heat until the gelatin is dissolved. Place in a medium bowl set in a larger bowl of ice. Whip the cream, add the lemon juice, and stir with a rubber spatula until blended. Let stand over the ice for 10 to 15 minutes, or until the mixture is slightly thickened. Remove the bowl from the ice. Beat the egg whites with sugar to form soft peaks. Fold quarter of the egg whites into the lemon-cream mixture, then fold the mixture carefully into the remaining egg whites. Spoon into the lined loaf pan. Cover the top with plastic or foil and freeze. To serve, invert the lemon mousse terrine onto a flat serving platter. Slice off the ends. Cut into slices 0.25to 1 cm thick. Serve with Black Currant Sauce. For Black Currant Sauce: Thaw the currants. Place in a blender with the lemon juice, sugar, and water and blend until smooth.