Before any hearing, trial or investigation shall be held by the secretary of labor, such notice as the secretary shall deem necessary shall be given to all parties interested by registered U.S. mail addressed to the parties to the post office of the usual place of residence or business of the interested parties when same is known, or by the publication of notice in some newspaper of general circulation in the county in which the industry or employment, or the principal office of such utility or common carrier is located, and the notice shall fix the time and place of the investigation or hearing. The costs of publication shall be paid by the secretary out of any funds available therefor. Such notice shall contain the substance of the matter to be investigated, and shall notify all persons interested in the matter to be present at the time and place named to give such testimony or to take such action as they may deem proper.