Whenever the commissioners of any county of the sixth or seventh class of this Commonwealth have been heretofore, or may hereafter be, incorporated as commissioners or trustees of any real or personal property, the income whereof is to be expended for any public purpose or purposes within the county, it shall be lawful for such commissioners to invest and reinvest any moneys realized from the sale of any of such property, or from the repayment of any loan, in such securities as are by law made legal investments for funds in the hands of fiduciaries.