Bonds issued under the provisions of this act [21-8-15, 21-8-16, 21-8-18 to 21-8-26 N.M. Stat. Ann.], being for the sole purposes specified in Section 1 [21-8-15 N.M. Stat. Ann.] hereof, shall forever be and remain free and exempt from taxation by this state or any subdivision thereof. Such bonds may be deposited as security for public monies by depositaries thereof within the state of New Mexico.