The Ladies' Book of Etiquette, and Manual of Politeness

If you are obliged to leave the company at an earlier hour than the other guests, say so to your hostess in a low tone, when you have an opportunity, and then stay a short time in the room, and slip out unperceived. By a formal leave-taking, you may lead others to suppose the hour later than it is in reality, and thus deprive your hostess of other guests, who, but for your example, would have remained longer. French leave is preferable to a formal leave-taking upon such occasions. If you remain until the usual hour for breaking up, go to your hostess before you leave the room, express the pleasure you have enjoyed, and bid her farewell. Within the next week, you should call upon your hostess, if it is the first party you have attended at her house. If she is an intimate friend, the call should be made within a fortnight.