Section 8. That the commissioners of Philadelphia county are hereby authorized and directed to borrow, on the credit of the county, the sum of thirty thousand dollars, at the rate of interest not exceeding six per centum per annum, said loan to be redeemable in twenty years. But no certificate of stock shall be issued for any fractional part of one hundred dollars: Provided, That the money so borrowed shall be appropriated by the county board to pay the present indebtedness of the county prison.