Any funds accumulated as provided in section 31-1007, Idaho Code, may, so far as practicable without jeopardy thereto, be invested by the county treasurer, under the direction of the board of county commissioners; and provided, further, that any portion of such fund not so invested, shall be deposited in accordance with the requirements of the public depository law. All interest earned by such fund shall accrue and be added to the principal thereof and become subject to investment as such.