Estates of Future Enjoyment — Remainders, Created by Act of Par ties — Reversions Created by Operation of Law — Rules Governing the Creation of Remainders — Vested Remainders — Rule in Shel ley's Case — Contingent Remainders — Uncertainty of the Person Of the Event — Contingent Uses — Executory Dm-ises these Difi'er from Contingent Remainders — Merger.