You Burn the Same Fuel a Candle Does
Blow through the straw into the clear limewater, gently, for about half a minute.
It goes milky.
The same white cloud, the same chalk, the same speed. Whatever the candle was sending up its column of hot air, you are sending out of your mouth right now, sitting still, at room temperature, without a flame anywhere near you.
Line the two up
Put a candle's account beside your own and they are the same account.
Four things are moving. Sort each one into where it belongs.
- Oxygen, from the air
- Water
- Carbon dioxide
- Heat
That list is true of a candle. It is also true of you. The only difference worth naming is the fuel: a candle carries its carbon and hydrogen as wax, and you carry yours as food.
A candle takes oxygen from the air and gives back water, carbon dioxide and heat. You take oxygen from the air and give back water, carbon dioxide and heat. A candle's carbon and hydrogen arrive as wax, drawn up a wick. Yours arrive as breakfast, carried round in your blood.
That is not a comparison or a poetic way of putting it. It is the same reaction. Scientists call it combustion in the candle and respiration in you, and the two words describe one piece of chemistry happening in two places.
The differences that are real
Three things genuinely are different, and each of them is interesting.
Speed. A flame is a rush; you are a slow burn. Your body takes the reaction apart into a long chain of small steps, each releasing a little of the energy, so nothing anywhere in you ever gets hot enough to glow. Same start, same finish, spread over hours instead of an instant.
Temperature. A candle needs to be lit — the wax will sit in a drawer for a century without doing anything. You need no such thing. The reaction starts in you the moment air arrives and keeps going at ordinary body temperature, because your cells hold tools that let carbon and oxygen meet without any heat to get them going. Faraday put it exactly right: gunpowder waits, and respiration waits no time at all.
Where the water goes. The candle's water goes straight up as invisible vapour. Yours comes out in your breath — breathe on a cold window and you will see it — and the rest leaves in other ways. You are quietly making water all day, out of the air and your lunch.
The other half of the circle
There is one obvious problem with all of this, and Faraday's audience would have felt it. If every person, every animal and every fire is turning oxygen into carbon dioxide, then the air ought to have been used up long ago.
It has not been, because something is running the reaction backwards.
A tree takes in carbon dioxide and water, and — using sunlight for the energy that burning gave off — puts them back together into the very thing a candle is made of: carbon and hydrogen, joined up, solid. Wood is stored fuel. So is wax. So is the food on your plate. The oxygen the tree does not need is pushed back out into the air, which is where your next breath comes from.
Burn a candle and you are taking that apart again and giving the air back what a tree borrowed. If you want to see the other half of this properly, the article A Tree Is Mostly Built From Air is about exactly where a tree's carbon comes from, and the answer is stranger than it sounds.
You breathe through clear limewater and it turns milky, just as the candle's air did. What does that show?
- Your breath is warm, and warm air clouds limewater
- Your breath contains carbon dioxide, so you are making the same gas a candle makes
- There is soot in your lungs from the air you have been breathing
- Limewater clouds when anything at all is bubbled through it
What the candle was for
Faraday finished these six lectures on Christmas Eve, in front of a hall of children, with a wish rather than a fact. He hoped they might turn out to be worth comparing to a candle: that they would shine as lights to the people around them, and be, as he put it, honourable and useful.
It reads as a nice thing to say at the end of a talk. It is more than that, because of everything that came before it. He had spent six evenings showing that a candle is a small, well-made machine that builds its own cup, pumps its own fuel, boils it, burns it at exactly the rate it can manage, gives out light and heat, and hands the air back everything it borrowed. It wastes nothing and it makes no fuss.
And he had just finished proving that you are doing the same thing.
A candle is worth taking seriously, which is the whole point of spending six parts on one. Anything is, if you look at it hard enough and refuse to be satisfied by the first answer. There is no such thing as a simple object. There are only objects nobody has asked enough questions about yet.
Go and find one.