§ 54. Exemption of property from state taxation. The said property shall be and continue forever thereafter exonerated and discharged from all taxes, assessments and other charges, which may be levied or imposed under the authority of this state; but the jurisdiction hereby ceded and the exemption from taxation hereby granted, shall continue in respect to said property so long as the same shall remain the property of the United States, and be used for the purposes aforesaid, and no longer.