A firm is dissolved—

(a) by the adjudication of all the partners or of all the partners but one as insolvent, or

(b) by the happening of any event which makes it unlawful for the business of the firm to be carried on or for the partners to carry it on in partnership :

Provided that, where more than one separate adventure or undertaking is carried on by the firm, the illegality of one or more shall not of itself cause the dissolution of the firm in respect of its lawful adventures and undertakings.