That depends upon the kind of fuel that is burned. Smoke from a fire of dry hard wood, from anthracite coal or from a gas flame is chiefly a column of hot air. Often it cannot be seen. There is very little solid matter in it. The thick, black smoke from locomotives, factories and house chimneys is made by burning soft coal in a wasteful way. If you will turn back and read the little story of how gas is made, you will understand the changes that take place when coal is heated. In making gas, the coal is not burned but is

roasted in an airless retort or oven. The heat sets the coal gas free. This is allowed to escape into a tank where it is stored for future use. When the oven is opened there is found, not ashes but coke, the carbon of the coal. Nothing has been consumed. The gases and the carbon have simply been separated. Both the coke and the gas can be burned and with very little smoke.

In all coal fires this separation of gases and carbon takes place at low, roasting temperatures—too low, indeed, to heat our houses or to make steam in boilers. Put a shovelful of soft coal into your furnace and watch the thick, yellow gas hover over the black mass before it bursts into flame. Open a draft below the fire box. The oxygen in the air helps to make a hotter fire. Hot air rises. You can see these gases and coal dust rush up the smoke pipe, on the column of hot air. They are carried away before they have time to burn. Of course, then, you understand that smoke means waste of fuel. Under boiler and house furnace fires, from two to five tons of coal are burned to produce the heat that is in one ton.

For more than a hundred years it has been understood that we could save a lot of money if we burned our smoke, and at the same time make this world a sweeter, cleaner, healthier place to live in. Coal gas is poisonous to breathe, and carbon dust is bad for the throat, nose and lungs. Both are bad for plants, for grass, flowers and shade trees. Countless smoke-consuming devices have been patented since James Watt took out a patent in 1785. These devices have aimed at two things : Skillful feeding of fuel and management of drafts to make less smoke, or trapping the smoke and feeding it to the fire again. Some men who have studied the smoke problem, think that all soft coal should be turned into gas and coke at the mine, and then be burned separately, the gas sent to cities through big pipes. This would be possible for factories and houses, but not for locomotives and steam vessels. Can you tell why? There are several fortunes in this field of invention for bright boys who will be men by and by. Any boy has a good beginner's text book on the smoke problem, in the furnace at home.