Parboil a sweetbread, adding to water a bit of bay leaf, a slice of onion, and a blade of mace. Cool, and cut in small cubes; there should be three-fourths cup. Add an equal quantity of cucumber cubes. Beat one-half cup thick cream until stiff; add one-fourth tablespoon granulated gelatine soaked in one-half tablespoon cold water and dissolved in one and one-half tablespoons boiling water, then add one and one-half tablespoons vinegar. Add sweetbread and cucumber season highly with salt and paprika, mould, and chill. Arrange on lettuce leaves, and serve with French Dressing.