“It is hereby expressly provided and declared, that whensoever the said Charles Gilmour, or his heirs above mentioned, succeeding to, and possessing my estate , shall also succeed to the estate now belonging to the said Sir Alexander Gilmour, then and from thenceforth, the right of my estate in their favours shall cease, and be extinct, void, and null, and the same shall fall and pertain to the next heir of entail appointed to succeed.”