Sift the flour and half the sugar together several times. Beat the egg whites with the salt until frothy. Add the cream of tartar and beat until thee egg foam starts to peak. Fold in the other half of the sugar to form a meringue. Then fold in the flour sugar mixture gradually and gently, and when the whole is partly blended add the vanilla. Only a gently motion should be used in mixing, for stirring tends to release the air depended on for leavening. A tube pan is best for baking angel food cake, and a fairly large cake will require about an hour in an oven at a temperature of 325 degrees F. After baking, invert the cake and remove from the pan when almost cold.