Place fresh, cold egg whites and cold water in the bowl of a heavy-duty mixer. Set aside until they are 60 degrees, or slightly below room temperature.
Pour flour, 1/2 cup granulated sugar and salt into a triple sifter or sieve. Sift onto a sheet of waxed paper; set aside. Adjust rack in lower third of oven; preheat oven to 325 degrees. Set nearby a dry, ungreased 10-inch tube pan. Such a pan allows the batter to cling to the sides and rise to its full height. Place a long-necked bottle or large metal funnel nearby for inverting the baked cake.
Whip egg whites, preferably with a whisk attachment until frothy. Add cream of tartar and whip on medium speed until soft peaks form. Continue whipping and gradually add the 1 cup granulated sugar until whites have thickened and form soft droopy white peaks. Add the vanilla and lemon zest in the final moments of whipping.
Sprinkle one-quarter of the flour mixture over the whites, and with a rubber spatula, fold into the whites. Repeat this process with the remaining flour mixture, folding in only one-quarter of the flour mixture at a time. Gently pour the batter into an ungreased tube pan. Bake for 45 to 50 minutes, or until the top is light golden, the cake feels spongy and springs back when touched, and a wooden toothpick inserted in the middle comes out free of cake. Invert cake onto the neck of a bottle or funnel. Cool completely before removing from pan.
To remove cake from the pan, slip just the tip of a small metal spatula between the cake and pan. Slowly trace the perimeter to release any cake sticking to the pan. Tilt the cake pan on its side, and gently tap the bottom against the counter to loosen the cake. Rotate the cake pan, tapping as you turn it a few more times, until it appears free. Cover the cake with a rack or cardboard round, and at the same time that you invert the cake onto the work surface, tap it firmly onto the surface. Lift the pan from the cake.