All interest which shall be received by the State Treasurer upon monies paid into the state depository, to the credit of the Commissioner of Highways, by any county, to be expended for the construction of bridges or highways, shall be credited to the fund which shall earn the same; and all interest heretofore earned by any such deposits or funds, and by the State Treasurer credited to the general revenue fund, shall be transferred from said fund and credited to the fund which earned said interest; Provided, there is any unexpended balance of such fund to which to credit said interest.
Laws 1921, c. 49, p. 69, § 1.