after stating the case as above reported, delivered the opinion of the court.

It does_not as distinctly appear from the bill itself as from the statement, that the first' action at law, referred to, was *252 brought and is pending in the Circuit Court of the United States for the Southern District of. New York. It may, however, perhaps be fairly inferred from the allegations of the bill that such is the fact, and as it has been so assumed in the argument of the cause, no question is made upon the sufficiency of the bill in that respect. The only ground here urged in support of the decree and of the demurrer to the bill is, that the complainants, upon the case made in the bill, have a complete and adequate defence at law, and that, consequently, they do not bring themselves within the jurisdiction of a court of equity.

If the decedent, Robert Berney, at the time of his death was domiciled in France, and not m Alabama, the letters testamentary issued to his brother, James Berney, as executor in Alabama, were void, and the authority given by James Berney to St. James by the power of attorney was also invalid, and the payment made by the appellants to St. James of the proceeds of the sales of the bonds which belonged to the estate does not bind the rightful executor or protect the complainants. The ground of the bill, therefore, is, that, upon these facts, an action at law may be successfully maintained by the appellee as executrix of Robert Berney against the complainants for the value of the bonds. The question is, whether the other facts set up in the bill furnish a complete and adequate defence to such an action at law, or whether they establish- a right in equity to relief. The rule as laid down by this court in Boyce's Executors v. Grundy, 3 Pet. 210, 215, is, that “ it is not enough that there is a remédy-at law. It must be plain and adequate; or, in other words, as practical and efficient to the ends of justice and its prompt administration as the remedy in equity.” And, as appears by that case, the principle is as applicable in cases where a complainant resorts to a court of equity to enforce a defence to an action at law, as where he seeks by a bill in equity other relief. This is illustrated by the case.of Grand Chute v. Winegar, 15 Wall. 373, 377. That was a case of a bill in equity by a municipal corporation to procure the cancellation of bonds on which an action at law had been brought, alleged to be void in the hands of the *253 holder. The court said: “ A judgment against "Winegar in the suit brought by him would be as conclusive upon the invalidity of the bonds, would as effectually prevent all future vexatious litigation, would expose the fraud, and prevent future deception as perfectly and thoroughly, as would a judgment in the equity suit. Under such circumstances, there is no authority for bringing this suit in equity.”

The ground of relief alleged in the present bill is, that by her acts and conduct the appellee has estopped herself, as against the complainants, from asserting any fact which annuls the executorship of James Berney under the Alabama probate, and the authority of St. James as his attorney in fact. Estoppels of this character, as distinguished from estoppels by record or by deed, are called equitable estoppels. It is not meant thereby that they are' cognizable only in courts of equity, for they are commonly enforced in actions at law, as was fully shown in Dickerson v. Colgrove, 100 U. S. 578. But it does not follow, because equitable estoppels may originate legal, as distinguished from equitable rights, that it may not be necessary in particular cases to resort to a court of equity in order to make them available. All that can properly be said is, that in order to justify a resort to a court of equity, it is necessary to show some ground of equity other than the estoppel itself, whereby the party entitled to the benefit of it is prevented from making it available in a court of law. In other words, the case shown must be one where the forms of the law are used to defeat that which, in equity, constitutes the right. Such a case is one for equitable interposition.

A close analogy is found in the doctrine of equitable set-off. The rule regulating the right of set-off is the same both at law and in equity, and yet there are many cases where set-offs not permissible at law may be enforced in equity. As was said by Mr. Justice Story in Greene v. Darling, 5 Mason, 201, 209: “ Now, the general rule in equity is, like that at law, that there can be no set-off of joint debts against separate debts, unless some new equity justify it: Such an equity may arise under circumstances of fraud; or where the party seeking relief is only a surety for a debt really separate; or where *254 there are a series of transactions in which joint credit is given with reference to the separate debt.” And at page 212: “Since the statutes of set-off of mutual debts and credits, courts of equity have generally followed the course adopted in the construction of the statutes by courts of law; and have applied the doctrine to equitable debts; they have rarely, if ever, broken in upon the decisions at law, unless some other equity intervened which justified them in granting relief beyond the rules of law, such as has been already alluded to.” In Downer v. Dana, 17 Vt. 518, 523, Judge Redfield said: “ Although' a court of equity will not, any more than a court of law, allow a set-off of joint debts against separate debts, yet there are many exceptions. One important exception is where the debts are in reality mutual,'although not so in form, as where one of the joints debtors is a mere surety.” In Smith v. Felton, 43 N. Y. 419, the court said: “ Equity will look through the form of. the transaction, and adjust the equities of the parties with a'view to its substance, rather than its form, so long as no superior equities of third persons will be affected by such adjustment.” In such cases, equity looks to the beneficial ownership of the debt. Kerr on Injunctions, 64, chap. 4, § 5.

The principle of these cases applies, we think, to the present. The ground of equity jurisdiction asserted in the bill is that the estoppel relied on would be good at law as against Louise Berney in her individual right, but not against her in her representative capacity as executrix of the estate of her deceased husband under the New York letters testamentary; but that it is good against her in equity in that capacity to the extent of her own individual interest, and the interest of any distributees of the estate equally bound thereby, in the fund which she is seeking as executrix at law to recover. She sues at law as executrix for the purpose of recovering a sum in dispute for the general benefit of the estate to be applied’ tó the payment of creditors, legatees, and other distributees. 'Under the law of France as widow, and under the will as beneficiary, she is individually entitled to some as yet undetermined portion of the assets of the estate, after the pay *255 ment of creditors, if there are any unpaid. Others named as defendants, similarly bound by the transactions relied upon as an estoppel, are also beneficially interested in the distribution of the estate in some yet unascertained proportions. There may be others entitled to some portion of the estate on distribution, in respect to whom the defence relied upon does not apply. As between them and the appellee and other beneficiaries, it may be necessary to have an account of what they have received, and of what they are still to receive, and an adjustment upon equitable grounds, based on the right of the appellants to enforce the recognition of their payment to St. James as an agent whose authority the appellee and some of the other distributees cannot'in equity be allowed to question. In the action at law, the appellee represents the whole estate, and every one interested in its collection and distribution. It may very well happen, therefore, that in the action at law the right to prove the facts on which the estoppel rests may be questioned and denied on the ground that the plaintiff in the action at law is not bound as executrix for what she did and assented to in her character as widow and legatee.

On this ground, therefore, and because it appears to be altogether uncertain whether the appellants can avail themselves in the action brought against them at law of the defence asserted in this bill, and admitted by the demurrer to be true, we think the demurrer should have been overruled, and the defendant required to answer. For error in this particular,

The decree of the Circuit Court is reversed, and the canse remanded, with directions to take fu/rther proceedings therein as equity and justice may reqvi/re. It is accordingly so ordered.