Beat yolks until thick and lemon-colored, add sugar gradually, and continue beating, using Dover egg-beater. Add lemon-juice, rind, and whites of eggs beaten until stiff and dry. When whites are partially mixed with yolks, remove beater, and carefully cut and fold in flour mixed and sifted with salt. Bake one hour in a slow oven, in an angel cake pan or deep narrow pan.
Genuine sponge cake contains no rising properties, but is made light by the quantity of air beaten into both yolks and whites of eggs, and the expansion of that air in baking. It requires a slow oven. All so-called sponge cakes which have the addition of soda and cream of tartar or baking powder require same oven temperature as butter cakes. When failures are made in Sunshine and Angel Cake, they are usually traced to baking in too slow an oven, and removing from oven before thoroughly cooked.