Why Does a Lamp Give a Better Light With the Chimney on?
When a lamp is burning without a chimney it generally smokes. That is because the oil which is coming up through the wick is being only partially burned. The carbon, which is about one-half of what the oil contains, is not being burned at all, and goes off into the air in little black specks with the gases which are thrown off. The reason the carbon is not burned when the chimney is off is that there is not sufficient oxygen from the air combining with it, as it is separated from the oil in the partial combustion that is going on. To make the carbon in the oil burn you must mix it with plenty of oxygen at a certain temperature, and this can only be done by forcing sufficient oxygen through the flame to bring the heat of the flame to the point where the carbon will combine with it and burn. When you put the chimney on the lamp you create a draft which forces more oxygen through the flame, brings the heat up to the proper temperature and enables the carbon to combine with it and burn. When you take the chimney off again the heat goes down, when the draft is shut off and the lamp smokes again.
The chimney also protects the flame of the lamp from drafts from the sides and above, and helps to make a brighter light, because a steady light is brighter than a flickering one.
The draft created by the chimney also forces the gases produced by the burning oil up and away from the flame. Some of these gases have a tendency to put out a light or a fire.