The Chemical History of a Candle

Overview

The Chemical History of a Candle is a hundred and twenty pages and it is a transcript. Faraday gave the Royal Institution’s Christmas Lectures for young people nineteen times between 1827 and 1860, and this course, delivered in 1860 and taken down in shorthand by William Crookes, is the one that was published and has never gone out of print.

The Christmas Lectures were his own invention, more or less, and they still run. The audience was children and their parents, in a theatre with a demonstration bench, and the whole thing was performed rather than read: Faraday was by all accounts a mesmerising lecturer who practised his demonstrations obsessively and hired an elocution teacher early in his career.

The book’s structure is its argument. Faraday takes one candle and refuses to leave it, and by working outward from that single object he arrives at the composition of the atmosphere and the chemistry of breathing. It is an object lesson in how much is available in anything if you look hard enough, and he says so explicitly in the first minutes.

Read it knowing who was speaking. A blacksmith’s son who had been taken out of school at thirteen and bound to a bookbinder, Faraday learned his science from the volumes he was sewing and never acquired any mathematics. He also discovered electromagnetic induction, the laws of electrolysis and the magnetic rotation of light, and he chose to spend six evenings a year explaining a candle to children.

The argument

Lecture I: a candle, and where the flame gets its fuel. He begins with the materials: tallow, wax, spermaceti, and a piece of Irish bog butter buried for centuries and still usable. Then the physical problem, which is more interesting than it sounds. Solid wax at the bottom, flame at the top, and nothing obviously connecting them.

The answer is the cup that forms at the top of a burning candle, made because the draught cools the rim, and the wick, which draws melted fuel up by capillary action. He demonstrates capillarity with salt and coloured water, shows that a candle in a draught burns badly because the cup breaks and the wax runs, and points out that the whole thing is a self-regulating system that nobody designed.

Lecture II: the flame itself. A flame is not uniform. There is a dark region near the wick where vapour has not yet burned, a bright luminous zone, and an outer envelope where combustion completes. Faraday proves the dark centre is unburnt fuel by running a glass tube into it and drawing off the vapour, which he then lights at the far end of the tube. He shows the luminosity comes from solid carbon particles heated white, by passing the flame through wire gauze and by producing smoke from the same particles when the air is insufficient.

Lecture III: the products, and water. Hold a cold spoon in the flame and it fogs. Faraday collects the water produced by a burning candle, and then asks where it came from, which leads to the composition of water itself. He decomposes it with a battery into hydrogen and oxygen, burns hydrogen in air to make water again, and weighs the products, establishing by demonstration that water is a compound and not an element.

Lecture IV: hydrogen, and the anatomy of the candle’s fuel. Properties of hydrogen, its lightness, its flame, and the demonstration of the ratio in which it combines with oxygen.

Lecture V: oxygen and the atmosphere. Air is not simple. Faraday separates oxygen and nitrogen, shows what each does to a flame and to a mouse, and works out the proportion. He then turns to carbon dioxide, produced by the candle, and tests it: heavier than air, so that it can be poured from one vessel into another and will extinguish a taper at the bottom of a jar; turns limewater milky; will not support combustion.

Lecture VI: respiration. The lecture the whole course has been building toward. Faraday breathes through limewater and it goes milky, exactly as the candle’s product did. He calculates how much carbon an adult exhales in a day and compares it to a candle’s consumption. The conclusion is that breathing is slow combustion, that the animal and the candle are doing the same chemistry, and that the carbon leaving both enters the air and is taken up by plants.

He then closes with the passage everyone quotes, wishing that the children in front of him may in their generation be fit to compare to a candle, and shine as lights to those about them.

How to read it

Read it with the experiments in mind and, where you can, do a few. The candle, the spoon, the limewater and the jar are all household-scale, and modern editions and several online versions have photographs or video of the demonstrations.

Notice what Faraday never does. He does not assert anything and ask you to accept it, he does not appeal to authority, and he does not skip a step because it would be tedious. When he needs a fact he produces it on the bench.

Read the asides. He stops to tell the children that they should not be satisfied with an answer they have not seen demonstrated, that a failed experiment is worth more than a successful one if you find out why, and that he is speaking to them as a juvenile himself.

Read it after the Experimental Researches, or instead of them. The Researches are the work that made him one of the great experimental scientists; this is the same mind doing something much harder, which is making the work intelligible to someone who knows nothing.

And read it as a piece of writing about method. Everything in the philosophy of science about observation, hypothesis and test is being enacted here rather than described, on an object that costs a penny.

Questions it raises

• Why a candle? Faraday says there is no better door into natural philosophy. What makes an ordinary object a better starting point than an interesting one?
• Every claim is demonstrated in front of the audience. Is that the same as proving it, and what is the audience actually being taught to trust?
• The course ends by showing that a child and a candle are running the same chemistry. What is the effect of arriving at that by six evenings of inference rather than being told it?
• Faraday had no mathematics. What does that suggest about what is essential to doing science and what is essential to communicating it?
• He insists that a failed experiment carries a lesson. Why is that so rarely how science is presented to beginners?
• These lectures were for children and are read by adults. What does it say about scientific writing that the best example of it was aimed at twelve-year-olds?