Sieve together the flour, salt and baking powder. Cream the butter and sugar together. Separate the eggs. Beat the yolks and stir them into the creamed butter mixture, then fold in the flour mixture, and stir in the milk. Add the caraway seeds and the peel. Whisk the egg whites very stiffly, and fold them into the cake mixture. Put into a greased seven-inch tin, and bake on the middle shelf of the oven at 350°F, 180°C, gas mark 4 for one hour. Leave in the tin for five minutes before turning out on a wire cake tray to cool.