No county in this state shall be authorized to levy a greater rate of taxation in any one year on the value of the taxable property therein than one-half of one per centum; provided, that to pay debts existing on the sixth day of December, eighteen hundred and seventy-five, an additional rate of one-fourth of one per centum may be levied and collected which shall be appropriated exclusively to the payment of such debts and the interest thereon; provided, further, that to pay any debt or liability now existing against any county, incurred for the erection, construction, or maintenance of the necessary public buildings or bridges or that may hereafter be created for the erection of necessary public buildings, bridges, or roads, any county may levy and collect such special taxes, not to exceed one-fourth of one per centum, as may have been or may hereafter be authorized by law, which taxes so levied and collected shall be applied exclusively to the purposes for which the same were so levied and collected.