The council may provide by ordinance for the appointment of, and may appoint, such number of fire wardens as may be deemed necessary; and for the examination by them, from time to time, of the stoves, furnaces and heating apparatus and devices in all the dwellings, buildings and structures within the city; and in all places where combustible or explosive substances are kept; and to cause all such as are unsafe with respect to fire to be put in a safe condition.