Beat egg yolks and 1/4 teaspoon salt in a 3-1/2 quart mixing bowl until very thick and light colored. Add cold water and beat one minutes. Then hot water next and beat 5 minutes or until mixture fills bowl 3/4 full. Gradually beat in sugar; stir in vanilla. Beat untilfluffy. Blend in flour. Beat egg whites until frothy; add cream of tartar and remaining salt and continue beating until stiff but not dry. Fold whites into egg-yolk mixture. Pour into an ungreased 10-inch tube pan and bake in moderate oven (425 degrees) for one hour. Invert pan on cake rack and let cool fo 20 minutes; cut cake away from pan with sharp knife and turn out on rack to cool. ÿ