Preheat the oven to 325 degrees. Grease a large loaf pan. Cream the butter and add the sugar, beating until foamy. Add the egg yolks, beaten light, then the grated lemon rind. Beat for 15 minutes. Fold in the egg whites, stiffly beaten, then add the flour. Put the batter into the prepared loaf pan and bake at 325 degrees for 1 to 1 1/4 hours. This is a large, rich cake which will keep fresh for a long time if wrapped in waxed paper and kept tightly covered.
Source: "The American Woman's Cook Book", Consolidated Book Publishers, Inc., 1939