Liability of firms.

Notwithstanding any contract to the contrary, where any firm is liable to pay tax under this Act, the firm and each of the partners of the firm shall be jointly and severally liable for such payment: Provided that where any such partner retires from the firm he shall also be liable to pay the tax, interest and the penalty, if any, remaining unpaid at the time of his retirement and any tax due upto the date of retirement though un-assessed on that date.