is the result of imperfect burning. Most of the substances from which we get much smoke would form nothing but gases if they were properly burned; gases which we could not see and which would very soon disappear. But man has not yet invented a furnace that will burn a fuel completely, without any smoke, though some factories have installed devices that almost do the trick. So we burn the coal or wood or oil or other substance only partly, and small specks of it, unburned, are carried up the chimney. The solid substance in smoke is simply fuel in the form of small specks.
Smoke makes black fogs in many cities, and cuts off a great quantity of daylight, besides making everything dirty, destroying plants and trees, and filling our lungs with dirt that we can never get rid of.