In common law, what conditions must be met for a salvage service to qualify for a salvage reward?

(1) There must be a peril (e. g. the threat of the property sinking); 

(2) the salved property must be a recognised subject of salvage; (3) the salvage service must be a voluntary act; and (4) the salvage service must result in success (i. e. some or all of the property must be salved).