The civil service commissioners shall, twice a year or oftener if they deem it necessary, under such rules and regulations as they may prescribe, hold examinations for the purpose of determining the qualifications of applicants for positions on the police force, in the fire department, in the health department in cities of more than ten thousand and less than twenty thousand inhabitants and of more than fifty thousand and less than one hundred thousand inhabitants or in any other special line of service in the city government which may be placed by the city council under civil service regulations. The examination for each line of service shall be practical, fairly testing the fitness of the persons examined to discharge efficiently the duties of the particular line of employment. The commissioners shall, as soon as practicable after each such examination, certify to the council the persons who have satisfactorily passed such examination, stating the order of excellence. But the commissioners shall not certify the name of, and may deny the examination to, any person as to whose honesty and integrity or general moral character they have not reasonably satisfied themselves by affirmative investigation.