The subscribers to the guarantee capital of any mutual life insurance company shall be entitled to receive from such company interest, payable semiannually at such rate, not exceeding six per centum, as may be agreed upon at the time of subscribing, if the net surplus over a requisite reservation for liabilities and contingencies is sufficient to pay the same, and, if less than the sum originally agreed on, it shall be made equal to it when the profits of the company are sufficient.