Section 8. The words “child”, “grandchild”, “issue”, “heir” or “heir-at-law”, or their respective equivalents, in a grant, trust settlement, entail, devise or bequest, shall include one who is adopted to the same extent as if born to the adopting parent or parents in lawful wedlock, whether the adoption was decreed before or after the date of execution or the effective date of any such grant, trust settlement, entail, devise or bequest, unless the contrary plainly appears by the terms of the instrument.