"--My will & intention also is the the particular pecuniary legacies give to my children or any of them shall not be demandable to my executors until a sufficient sum be raised for discharging the whole Tho my executors are left at liberty sooner to pay any one or more of the said legacies or part thereof if they judge it necessary or convenient so to do & more especially with respect to the sums d" . . . . .