Where one person has promised to do something which he knew, or, with reasonable diligence, might have known, and which the promisee did not know to be impossible or unlawful, such promisor must make compensation to such promisee for any loss which such promisee sustains through the non-performance of the promise.Illustrations (a) A agrees with B to discover treasure by magic. The agreement is void. (b) A and B contract to marry each other. Before the time fixed for the marriage, A goes mad. The contract becomes void. (c) A contracts to marry B, being already married to C, and being forbidden by the law to which he is subject to practise polygamy. A must make compensation to B for the loss caused to her by the non-performance of his promise. (d) A contracts to take in cargo for B at a foreign port. A's Government afterwards declares war against the country in which the port is situated. The contract becomes void when war is declared. (e) A contracts to act at a theatre for six months in consideration of a sum, paid in advance by B. On several occasions A is too ill to act. The contract to act on those occasions becomes void.