Joint and several liability of partners for a firm's tax obligations, with retiring partners liable until retirement intimation is filed. Where a firm is liable to pay tax, interest, or penalty under the Act, the firm and each partner are jointly and severally liable. A retiring partner remains liable for amounts due up to the date of retirement unless the retiring partner or the firm gives written notice of retirement to the Commissioner; absence of such notice within one month extends the retiring partner's liability until the Commissioner receives the intimation.