It shall be the duty of the said commissioners , to open books in the state treasurer's department, keep separate and distinct accounts of all moneys received and disbursed under this act, specifying the source of revenue and when paid, and from time to time, as the funds paid in may justify, apply the same to the payment of the accruing interest of the public debt and the principal, at or before the semi-annual payments thereof may become due.
72 P.S. § 3646