The Chemical History of a Candle · Lecture I, part 3 · Part 1: Lecture I: A candle and its flame
3. Wax vapor, the shape of a flame, and snapdragon
Goal: Your child can explain that a candle burns as a vapor, not as a solid, and describe the parts and the shape of a candle flame.
You'll need
- A plain candle (a short, thick one is best) and a few tea lights
- A large plate or metal baking tray to stand them on
- Matches or a long lighter (for the adult only)
- A glass of water nearby
- A sheet of white card or a white wall
- A bright flashlight (or a sunny window)
- Science notebook and colored pencils
What's in the book
The end of Lecture I: the vapor seen when a candle is blown out, the shape and brightness of the flame, Hooker's drawing, the shadow of a flame, a flame blown downward, and the tongues of flame in the Christmas game of snapdragon.
The lesson, step by step
Hook question
SayDoes the wax itself burn, or does something happen to it first?
Keep the answer in mind while you read.
Read together
Read pages 31 to 42, to the end of the first lecture.
If you blow a candle out cleverly, you will see the vapour rise from it.
The smoke trail that relights the candle (adult only)
Faraday held a lighted taper in the vapor of a candle he had just blown out:
if I hold a lighted taper two or three inches from the wick, you will observe a train of fire going through the air till it reaches the candle.
The adult does this one. Let the candle burn for a few minutes. Blow it out gently, then at once hold a lit match or long lighter in the rising white smoke, a couple of inches above the wick. The flame jumps down the smoke and relights the wick. Your child watches from a short distance.
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.
SayThe flame traveled down the vapor. So it is wax vapor that burns, not solid wax.
Draw the flame
Light the candle again and let it settle in still air. Look closely (not too close) and draw it in color. Faraday describes it as a bright oblong, brighter at the top, with the wick in the middle and darker parts near the bottom. Can you also see a little blue near the base?
What diamond can shine like flame?
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.
The shadow of a flame
Faraday used sunlight, then a bright electric lamp, to throw the shadow of a candle flame on a screen. Try it with a sunny window or a bright flashlight in a dim room. Keep the candle well away from the card or wall, at least a foot. Look for the wavy shadow of hot air streaming up above the flame.
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.
Snapdragon
Watch only: don't try this at home.
Am I right in supposing that anybody here has played at snapdragon?
Snapdragon was a Victorian Christmas game: raisins in a dish of burning brandy, snatched out by daring players. Faraday lit one to show the many "tongues" of flame that form when air flows in unevenly. It is a real fire hazard, so it is for reading about only.
Indeed, I might say, you have here a multitude of independent candles.
Say it back and check
Narration prompts: What happened when Faraday held a light in the smoke? What does the shadow show? Why does a big flame break into tongues?
Check questions:
1. Why do flames usually point upward?
2. Why must Faraday be "quick and ready" when he relights the vapor?
3. Is a big flickering flame really the shape you see at any one moment?
Afterwards
Notice it this week: watch a candle or a gas-stove flame with an adult. Which parts are blue, which yellow? Which way does the flame lean when someone walks past?
Tip
Faraday ends by laughing at himself for spending so long on snapdragon. It is a good place to stop: the lecture ends in the middle of the fun, just as it did for his audience.
Answers to the book's problems
1. The heat makes a current of air rise, and that rising current draws the flame upward. 2. If the vapor cools it condenses into liquid or solid wax, or the stream gets disturbed, and then it will not carry the flame. 3. No. A large flame is made of many shapes following each other so fast that the eye sees them all at once.
Worked out for this site from the scan. If a number in your copy differs, trust the book.