Joint and several liability of partners binds each partner for firm tax debts; retiring partners remain liable without timely notice. Section 90 makes the firm and each partner jointly and severally liable for tax, interest or penalty payable by the firm; a retiring partner remains liable for amounts attributable to the period up to retirement unless the partner or firm gives written notice of retirement to the Commissioner, and if no notice is received within one month the retiring partner's liability continues until intimation is received.