Cream sugar and butter. Add egg yolks, one at a time and beat until light and fluffy. Sift together flour and baking powder. In small amounts, add dry ingredients with sugar/butter mixture, alternating with milk. Add vanilla and lemon juice. Beat egg whites until stiff and fold into mixture. Bake in 10 inch angel food cake pan which has been greased and floured. Bake 1 hour and 15 minutes at 350 degrees. Cool in cake pan about 10 minutes. Then place on wire rack top side up.