WHAT SMOKE IS. Smoke consists mostly of little specks of unburned carbon. That is why it is gray or black. When you have black smoke, you may always be sure that some of the carbon particles are not combining properly with oxygen. Yellow flames are usually smoky; that is, they usually are full of unburned bits of carbon that float off above the flame. But by letting enough air in with the flame, it is possible to make all the little pieces of carbon burn (combine with the oxygen of the air) before they leave the heat of the burning hydrogen. That is why kerosene lamps do not smoke when the chimney is on. The chimney keeps all the hot gases together, and this causes a draft of fresh air to blow up the chimney to push the hot gases on up. The fresh air blowing up past the flame gives plenty of oxygen to combine with the carbon. The drum part of an oil heater acts in the same way; when the drum is open, the heater smokes badly; when it is closed up, enough air goes past the flame to burn up all the carbon. But if you turn either lamp or heater too high, it will smoke anyway; you cannot get enough air through to combine with all the carbon. The hottest flames are the blue flames. That is because in a blue flame all the carbon is burning up along with the hydrogen of the fuel--both are combining with the oxygen of the air as rapidly as possible. A gas or gasoline stove is so arranged that air is fed into the burner with the gas. You will see this in the following experiment: EXPERIMENT 96. Light the Bunsen burner in the laboratory. Open wide the little valve in the bottom. Now put your finger and thumb over the hole in the bottom of the burner. What happens to the flame? Turn the valve so that it will close the hole in the same way. Now hold a white saucer over the flame, being careful not to get it hot enough to break. What is the black stuff on the bottom of the saucer? Examine the gas plate (small gas stove) in the laboratory. Light it, and see if you can find the place where the air is fed in with the gas. Close this place and see what happens. Open it wider and see what happens. If the air opening is too large, the flame "blows"; there is too much cold air coming in with the gas, and your flame is not as hot as it would be if it did not "blow." Also, the stove is liable to "back-fire" (catch fire at the air opening) when the air opening is too wide. [Illustration: FIG. 168. The Bunsen burner smokes when the air holes are closed.] _APPLICATION 69._ An oil lamp tipped over and the burning oil spread over the floor. Near by were a pail of water, a pan of ashes, a rug, and a seltzer siphon. Which of these might have been used to advantage in putting out the fire? _APPLICATION 70._ My finger was burned. I wanted the flesh around it to heal and new skin cells to live and grow rapidly around the burn.