The directors of a savings bank may declare dividends on common or preferred stock of so much of the net profits of the corporation as they shall judge expedient; but the corporation shall, before the declaration of a dividend on common stock from the net profits, carry 50% of its net profits of the preceding period for which the dividend is paid to its surplus fund until the same shall amount to 50% of its capital stock, and thereafter shall carry 25% of its net profits of the preceding period for which the dividend is paid to its surplus fund until the same shall amount to 100% of its capital stock.