Dutch Sweet Potato Puff

Peel and boil 3 sweet potatoes in salted water until tender; then mash
well with 3 beaten yolks of eggs, 1 cup of milk, 3 tablespoonfuls of
butter, 2 tablespoonfuls of sugar, a pinch of nutmeg and lemon-juice.
Beat the whites with a pinch of salt to a stiff froth; add to the
potatoes and put in a well buttered baking-dish and bake. Serve hot.