Section 18. A deed in substance following the form entitled ''Mortgage Deed'' shall when duly executed have the force and effect of a mortgage deed to the use of the mortgagee and his heirs and assigns with mortgage covenants and upon the statutory condition and with the statutory power of sale, as defined in the three following sections, to secure the payment of the money or the performance of any obligation therein specified. The parties may insert in such mortgage any other lawful agreement or condition.