Pish, P. J.
1. Bowman’s agreement, which appears in the foregoing statement of facts, was evidently without consideration when made, as Brown and Taylor, at that time, neither did nor agreed to do anything. It was, therefore, when made, unenforceable for want of mutuality. When, however, Brown and Taylor within a reasonable time, and in the absence of any revocation of *156the agreement by Bowman, by much labor and considerable expense, as the petition alleged, located and developed gold in the land in question, they did the very thing they were expected to do, and thereby furnished the contemplated consideration, which changed the nudum pactum into a valid and binding contract. “ A contract is often such that, until something is done under it, the consideration is imperfect, yet a partial performance, or a complete performance on one side, supplies the defect. If, for example, one promises another, who makes no promise in return, to pay him money when he shall have done a specified thing, if he does it, not only is the contract executed on one side, but also the consideration is perfected and payment can be enforced. And in more general terms, when from any cause the party from whom the consideration moves is not compellable to render it, if he does render it, the contract becomes thereby perfected.” .Bishop on Contracts, § 87. To the same effect, see Parsons on Contracts, * 451; Clark on Contracts, 169 — 170 ; Story on Contracts, § 569 ; 7 Am. & Eng. Ene. L. 115. In Hamm on on Contracts, 683, it is said: “The test of mutuality is to be applied, not as of the time when the promises are made, but as of the time when one or the other is sought to be enforced. A promise may be unenforceable for want of mutuality when made, and yet the promisee may render it valid and binding by supplying a consideration on his part before the promise is withdrawn.” This doctrine is well settled by many adjudged cases cited by the above-named text-writers, and has been fully recognized by this court. Sivell v. Hogan, post, and cit. Counsel for defendant in error cite Peacock v. Deweese, 73 Ga. 570; Grizzle v. Gaddis, 75 Ga. 350; Lindsay v. Warnock, 93 Ga. 619; Morrow v. Southern Express Co., 101 Ga. 810; Perry v. Paschal, 103 Ga. 134; Marietta Paper Co. v. Bussey, 104 Ga. 477; but nothing was decided in any of these cases in conflict with the principle above announced. The contract in Peacock v. Deweese was purely unilateral when made, and it was not even claimed that the promisee did anything afterwards which furnished a consideration. Under the facts, the court held that a specific performance could not be decreed in behalf of the promisee. It was held in Grizzle v. Gaddis that “ A parol contract by which it was agreed that if a man and his family would take possession of certain land and cultivate and improve *157it, they should have it as their home during the lives of himself and wife, paying therefor a reasonable rent, and that if at any time, after paying such rent from time to time, he should become able to purchase, the owner would convey to him a title for such price or sum as it was then worth, was too vague, uncertain, and wanting in mutuality to furnish a foundation for a decree for specific performance, requiring a conveyance from the other party to the contract or one who purchased from him with notice.” There the insufficiency of the contract resulted more from its vagueness and uncertainty than from its lack of mutuality. - See remarks of Mr. Justice Cobb in Perry v. Paschal, 103 Ga. 139.
The facts in Lindsay v. Warnock, were very similar to those in the present case, and the decision there made is really in line with the well-settled rule as stated in the text-books to which we have hereinbefore referred. It was held in that case: “Where by a written contract the owner of a tract of land stipulated to convey to the other contracting party a half interest in all the minerals that the latter might find, open, and develop ‘ to the extent that it will justify the employ of labor, ’ with timber and water for mining purposes, the other party stipulating in the writing to prospect the land within a specified time at his own expense, and the latter having complied with this stipulation and discovered, opened, and developed a mineral of unknown name but of sufficient value to justify the employment of labor in mining the same, equity will, at his instance, compel the former to specifically execute his contract to convey in accordance with its terms. There was no want of mutuality in the terms of the contract as set forth in the writing; and the failure to sign the writing by the party who performed his undertaking is of no consequence, after full performance on his part.” In Morrow v. Southern Express Co., the agreement was entirely without consideration, and Mr. Justice Little, who delivered the opinion, said : “ There are instances in which a promise, though a mere nudum pactum when made, because the promisee is not bound, may become binding on his afterwards furnishing the consideration contemplated.” Nor does the case of Perry v. Paschal help the defendant; for it was there held: “ Where one by written agreement binds himself to convey to another a designated tract of land upon the payment of a sum stated, and by the same agreement likewise undertakes to extend the *158time of payment to a definite time in the future, and the vendee upon his part undertakes to pay a certain sum annually by way of rent for the immediate use of the premises, the vendee has the election at any time to tender the full amount of the purchase-money with interest, and then to demand a conveyance; and a continuing tender being made by him in accordance with the terms of the contract, the right of the vendor to demand rent thenceforth ceases.” In Marietta Paper Co. v. Bussey, the ruling was only as to the allowance of an amendment, and the case is not in point. Counsel for defendant in error further contend'that the agreement was merely a license, a gratuitous privilege granted by Bowman to Brown and Taylor to make a certain use of Bowman’s land, and that as there was no consideration for such license, it was revocable at any time. Granting that the written instrument involved in this case was originally a mere license, revocable at the will of the licensor, still, after the licensees had done the work and gone to the expense contemplated by the agreement, the right of revocation was lost; certainly there could not then be an unconditional revocation. Even a parol license is not revocable after the licensee has executed it and in doing so has incurred expense. Civil Code, § 3069. See City Council of Augusta v. Burum, 93 Ga. 68. After Brown and Taylor had, as alleged in the petition, by much labor and at considerable expense, located and developed gold in the land, Bowman could not revoke the license and free himself from liability for one half of the valué of the gold so located and developed. We conclude, therefore, that the petition set forth a good cause of action for the recovery of one half of the value of the gold which was located and developed in the land «by the plaintiffs; and hence the first, second-, third, and fourth grounds of the demurrer should not have been sustained.
2. The judgment of the court was not. only general, but it expressly sustained the demurrer upon all the grounds taken therein. This judgment must be affirmed, as one of the special grounds of the demurrer was good. That ground was, that the petition did not allege the nature, character, and value of the minerals located and developed. The petition did not allege the value of the gold located and developed. It alleged that the plaintiffs “ developed and located valuable minerals, including gold,” but was silent as do the amount or value of the gold; and the defendant was en*159titled to be informed what the plaintiffs expected to prove upon this subject. It is true that the petition alleged that, by the discovery, location, and development of the valuable minerals, the land was increased in value from one hundred dollars to fifteen hundred dollars, and that therefore the plaintiffs were entitled to recover seven hundred and fifty dollars. But the plaintiffs were not entitled to recover the value of a one-half interest in the land, nor were they entitled to recover anything for the enhanced value of the land. What they were entitled to recover for was a one-half interest in the value of the gold which they had located and developed. It is apparent that there was no sufficient averment as to the value of the gold. In view of the fact that the trial judge did not base his judgment simply upon this special ground, but expressly placed it upon all the grounds taken in the demurrer ; and the probability that, if he had intimated or announced his intention to sustain this special ground, the plaintiffs would have prevented a dismissal of their case, by offering a proper amendment; and the further fact that the plaintiffs, by their petition, seem to present a meritorious case, we affirm the judgment, with direction that plaintiffs be allowed to amend the petition before the judgment of this court is made the judgment of the court below.
Judgment affirmed, with direction.
All the Justices concur.