Any national banking association may, with the approval of the Comptroller of the Currency, and by a vote of shareholders owning two-thirds of the stock of such associations, increase its capital stock to any sum approved by the said comptroller, but no increase in capital shall be valid until the whole amount of such increase is paid in and notice thereof, duly acknowledged before a notary public by the president, vice president, or cashier of said association, has been transmitted to the Comptroller of the Currency and his certificate obtained specifying the amount of such increase in capital stock and his approval thereof, and that it has been duly paid in as part of the capital of such association: Provided, however, That a national banking association may, with the approval of the Comptroller of the Currency, and by the vote of shareholders owning two-thirds of the stock of such association, increase its capital stock by the declaration of a stock dividend, provided that the surplus of said association, after the approval of the increase, shall be at least equal to 20 per centum of the capital stock as increased. Such increase shall not be effective until a certificate certifying to such declaration of dividend, signed by the president, vice president, or cashier of said association and duly acknowledged before a notary public, shall have been forwarded to the Comptroller of the Currency and his certificate obtained specifying the amount of such increase of capital stock by stock dividend, and his approval thereof.