6. Why a flame is bright, and what goes up

The lesson, step by step

1. Hook question

   SayA gas-stove flame is blue and gives little light. A candle flame is yellow and bright. What makes the difference?

2. Read together

   Read pages 55 to 67, to the end of Lecture II.

   You would hardly think that all those substances which fly about London, in the form of soots and blacks, are the very beauty and life of the flame

3. The cold spoon test

   Faraday gave this one as homework:

   If you go home and take a spoon that has been in the cold air, and hold it over a candle — not so as to soot it — you will find that it becomes dim, just as that jar is dim.

   Wearing the oven mitt, the adult holds the cold spoon a few inches above the flame for a few seconds. Look at the back of the spoon: a mist of tiny water drops. Then lower the spoon briefly into the tip of the flame: now it collects black soot. Let the spoon cool before anyone touches it.

   Safety: an adult lights the flame and stays right there; tie back long hair and roll up sleeves; set the candle on a plate on a fireproof surface, away from paper and curtains; keep a glass of water nearby.

   let me tell you that it is water which causes the dimness

4. Why the flame shines

   The soot on the spoon is carbon. In the flame, the heat breaks up the wax vapor and sets tiny bits of carbon free. They glow brightly while they are hot, and then they burn away.

   it is to this presence of solid particles in the candle-flame that it owes its brilliancy.

   When air is mixed with the gas before it burns, as in a gas stove, the carbon burns before it can glow, and the flame is pale blue.

5. What Faraday showed

   Watch only: don't try this at home.

   To prove that solid things glow brightly in a flame, Faraday heated platinum wire, then a piece of lime in a hydrogen and oxygen flame:

   This is the glorious lime-light, which rivals the voltaic-light, and which is almost equal to sunlight.

   He also burned phosphorus, set off a mixture with a drop of sulfuric acid, and burned zinc in a red-hot crucible (he said children could do it at home; they should not). Finally he filled a paper fire-balloon with the hot gases from a flame and let it rise. These all involve dangerous chemicals or open fire. If you want to see it, search online for a video of a chemistry teacher doing this demonstration and watch it together.

6. Say it back and check

   Narration prompts: What was on the cold spoon? What makes a candle flame bright? What made the balloon rise?

   Check questions:

   1. What is the black smoke from the bright part of the flame?

   2. Why does a gas flame turn blue when air is mixed into it first?

   3. What causes the dimness on a cold jar or spoon held over a candle?

7. What we know now

   Lime-light really was used to light theaters in the 1800s; that is where the saying "in the limelight" comes from. Faraday's explanation of a bright flame still stands: glowing specks of soot give a candle its yellow light.