Wash and core one tart apple, cut it into slices; put it in a saucepan with a half cupful of cold water, bring to a boil, add two tablespoonfuls of sugar, a teaspoonful of lemon juice and a teaspoonful of gelatin that has been soaked for a half hour in two tablespoonfuls of cold water. When the gelatin is dissolved, press the whole through a sieve, and pour while hot into the beaten white of one egg. Turn into an individual mold and stand aside to harden. Serve with plain or whipped cream, or a soft custard made from the yolk of the egg and a half cupful of milk.