Linda Sadowsky, age 13, of St, Catharines, On ., for her question:

What is smoke?

Pure smoke is invisible. It is gas and water vapor given off from a fire. When every scrap of fuel is burned, pure smoke pours forth and mingles with the other invisible gases of the air. But almost every fuel leaves some ashes. This means that the fuel is not completely burned. Almost every fire gives off fragments of ashy material into the air. Some fuels give off clouds of unburned sulphur. Almost all fuels give off fine fragments of black carbon.

These fine ashes rise up on the warm gases escaping from the fire. They mix with the rising water vapor and color it black or dirty grey. The vapor bwomea amoks  as we see it, As the murky smoke mingles with surrounding air, the fragments of carbon and other ash are dropped as soot. In one month, the factories, furnaces and trains of a busy city may dump 80 tons of soot on a square mile area.