Every person above the age of twenty-one years, who is a citizen of the United States, or who has declared his intention to become such, or any association of persons severally qualified as above, shall, upon application to the register of the proper land office, have the right to enter, by legal subdivisions, any quantity of vacant coal lands of the United States not otherwise appropriated or reserved by competent authority, not exceeding one hundred and sixty acres to such individual person, or three hundred and twenty acres to such association, upon payment to the register of not less than $10 per acre for such lands, where the same shall be situated more than fifteen miles from any completed railroad, and not less than $20 per acre for such lands as shall be within fifteen miles of such road.