Preheat oven to 350 degrees. Line bottom of a 9 by 5 by 3 loaf pan with waxed paper; grease paper well, also sides of pan. Sift flour, salt and nutmeg together and set aside. Cream butter until light, then add sugar slowly, creaming until fluffy. Beat egg yolks until thick and light, then add to butter mixture, a little at a time, beating well after each addition. Blend flour in gradually, then mix in vanilla and beat just until smooth. Beat egg whites until soft peaks form and fold gently into batter. Spoon into pan and bake 65-70 minutes until cake pulls slightly from sides of pan. Cool upright in pan on a wire rack 30 minutes; carefully turn out, peel off waxed paper, turn cake right-side-up, then cool thoroughly before slicing.