It shall be lawful for the county commissioners of such county, and they are hereby authorized and empowered, to incur or increase the indebtedness of the county to an amount sufficient to pay its share of the cost of any land or lands required and of erecting the building or buildings aforesaid, by issuing coupon bonds in sums of not less than one hundred dollars each, bearing interest at a rate not exceeding five per centum per annum, and the principal thereof reimbursable at a period not exceeding thirty years from the date at which the same are authorized.
53 P.S. § 1340