Section 5. If the office of the superintendent of any of the said institutions is vacant, or if the said officer is absent from the institution or is unable to perform his duties, an acting superintendent designated by the commissioner shall have the powers to perform the duties and be subject to the liability of the said office; and the commissioner may required such acting superintendent, before entering on the performance of his official duties, to give a bond to the commonwealth in such sum as the comptroller may prescribe, with sureties approved by the commissioner, conditioned on the faithful performance of such duties, and faithfully to account for all money received by him, as such acting superintendent. After the approval of such bond, the acting superintendent shall, as long as he performs the duties of the office, receive the salary of the superintendent in lieu of his salary prior to his designation as acting superintendent.