The capital stock of a corporation shall be paid in such amounts and at such times as the directors may require. The directors may, from time to time, assess upon each share of stock not fully paid up, such sum of money as the necessities of the business may, in the judgment of the board of directors, require, not exceeding in the whole the balance remaining unpaid on said stock, up to the par value thereof, or in the case of stock without par value, not exceeding the price for which such stock was issued by the corporation, and such sum so assessed shall be paid to the treasurer at such time and by such installments or calls as the directors shall direct. The directors shall notify the time and place of such payments by written notice mailed at least 30 days before the time for such payment, to each stockholder at his last known post-office address.