The surplus provided for in section 26.1-20-02 constitutes a guaranty fund, which must be invested in securities as provided by section 26.1-05-19, and be duly deposited with the commissioner, and the commissioner’s certification of that deposit must be procured, as provided by law. This deposit must be maintained unimpaired and the principal of the fund may be applied only to the payment of losses and expenses by reason of its guaranty and insurance contracts, but the corporation has the right to collect the income from the deposit and to substitute other like securities of equal amount and value from time to time.