Chemistry 101 with Faraday

Lesson 1

Faraday, the bookbinder's apprentice

The lecturer in this course began working life not in a laboratory but in a bookbinder's shop, where the boy who bound books also read them. The story told about Faraday is that his apprenticeship put science books into his hands, that he went to hear Humphry Davy lecture, took careful notes, and sent them to Davy, and that Davy took him on at the Royal Institution in London. Keep that starting point in mind, because the scientist who resulted never forgot what it is like to be a beginner holding a book.

You can hear it in the lectures themselves. Faraday opens The Chemical History of a Candle by telling his audience of boys and girls that he claims the privilege of speaking to juveniles as a juvenile himself, and that he has taken this same subject on a former occasion and would gladly repeat it almost every year. He promises to treat the subject honestly, seriously, and philosophically, and then makes his famous claim for the candle: there is no better, no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle.

Lesson 2

The candle flame: shape and parts

Shield a candle from draughts, as Faraday did with a lamp glass, and the flame settles into a steady shape: a bright oblong, brighter at the top than towards the bottom, with the wick in the middle and darker parts towards the bottom where the ignition is not so perfect. Around it, invisible, rises a current of hot air. Faraday proved that current exists by putting a lit candle in sunlight and catching the shadow of the flame on paper: you can see matter streaming upward that is not part of the flame, drawing the flame out and supplying it with air.

The flame is not burning everywhere at once. Push a glass tube into that dark middle and a white vapour travels down the tube; collected and lit, it burns, because it is the wax of the candle turned to vapour, not yet burned. Push the tube into the top of the flame and what comes away will no longer burn, because it is already burned. Heat lives in a ring, too. Faraday held paper across the middle of a flame and found it charred in two places and barely touched in the centre. The fierce action is where fuel vapour and air meet.

Lesson 3

What burns, and where it goes

Before anything can burn, the fuel has to travel. Heat melts the top of the candle into a little cup, a cup kept neat because the rising current of air cools the outer edges of the wax. Then the melted fuel climbs the wick by capillary attraction, the same pull that makes water creep up a towel or coloured fluid rise through a column of salt in Faraday's demonstration. The fuel arrives in small quantities, right into the centre of the action, and there the heat turns it into vapour.

So what does the burning, and where does the candle go? The vapour does. Faraday showed the two actions are separate, production of vapour and combustion of it, by drawing vapour through a tube and lighting the candle flame again at a distance from the candle. Burned properly, a candle disappears without the least trace of dirt in the candlestick, but nothing vanishes. Its products ride the rising current upward in two kinds: one part that condenses against a cold spoon or plate, and one part that does not condense. Starve the flame of air and the story changes: combustion turns imperfect, and black soot, which is the candle's own carbon escaping unburned, streams away instead.

Lesson 4

Water from fire: hydrogen

Here is Faraday's surprise: the condensable product of a candle is just water, nothing but water. Hold a cold jar, spoon, or poker head over the flame and dew gathers on it, deposited by the combustion itself. He collected water from a candle, from a spirit lamp, and from a gas lamp, and reported that a pint of oil, burned fairly, produces rather more than a pint of water. The water from fire tests exactly like any other water, because water is one individual thing that never changes, only its state does.

Water, though, is a compound, built from two substances. One of them is hydrogen, the element Faraday calls the one that generates water. He makes it in a small bottle, the philosopher's candle, from bits of zinc with sulphuric or muriatic acid, warning his audience to let the air be swept out first because the gas mixed with air is explosive. Hydrogen is astonishingly light: a cubic foot of it weighs a twelfth of an ounce, against almost a thousand ounces for a cubic foot of water, so bubbles blown with it rise to the ceiling. And when hydrogen burns, in a cold glass held over its flame, it produces water only, the same water the candle made.

Lesson 5

Oxygen and the air we breathe

The other substance in water is oxygen, the part the candle takes from the air. Faraday gets it several ways. A battery acting on water through platinum plates splits water into its two gases, giving twice the volume of hydrogen as oxygen. More conveniently, he warms chlorate of potash, which he calls chlorate of potassa, mixed with oxide of manganese, and the salt gives off oxygen freely at less than a red heat. However he makes it, the gas looks like plain air, which is why tests matter.

The tests are dramatic. A taper lowered into oxygen burns beautifully. Even iron wire, with a bit of burning wood to start it, burns in oxygen. Air works more gently because air is mostly not oxygen: Faraday's table gives one hundred parts of air as twenty parts oxygen and eighty parts nitrogen by bulk. Nitrogen will not burn and will not let a taper burn, and that indifference is a mercy. In pure oxygen, even an iron grate would burn more powerfully than its coals. One more of his demonstrations belongs here: a platinum wire thrust into a flame does not change or melt, but glows far brighter than the flame around it, showing how a solid body carries light.

Lesson 6

Carbon dioxide and the breath of life

The candle's second product does not condense, and Faraday hunts it with lime water, made by stirring quicklime into water and filtering it. Shake lime water with plain air and it stays clear. Shake it with the products rising from a candle and it turns milky, because the lime has combined with a substance from the candle to make chalk, carbonate of lime. That substance is carbonic acid, now called carbon dioxide. It is heavier than air, so Faraday can pour it between jars; it does not burn or support combustion, so a taper lowered into it goes out.

Then comes the turn that makes these lectures famous. Breathe out through lime water and it clouds, as candle fumes cloud it, while air drawn in from the room leaves it clear. Respiration, Faraday says, is combustion slowed down: food is fuel, the carbon in it combines with oxygen in the lungs, and carbonic acid and warmth come out. He reckons a man converts as much as seven ounces of carbon into carbonic acid in twenty four hours. And the gas that is poison to us is life to plants, which absorb carbon from the air and grow, so animal and vegetable kingdoms are tied together by one exchange.

Lesson 7

Kitchen experiments to do yourself

Faraday wanted his audience to experiment, carefully, and several of his own observations are safe to repeat in a modern kitchen with an adult supervising from start to finish. Use one plain candle in a stable holder on a heatproof surface, tie back hair and sleeves, keep water nearby, and never leave a flame unattended. Do not taste any chemical, do not make or handle acids, lime water, chlorate, or hydrogen, and keep every scale small: those lecture hall preparations are for reading about, not for home.

1. Flame map: on a still day, study the shielded flame and sketch its bright top, dark centre around the wick, and the invisible rising current you can reveal by watching its shadow on card in strong light.
2. Water from fire: hold a cold, dry glass jar or spoon high above the flame for a few seconds and watch dew condense on it, then wipe it dry and repeat.
3. Air runs out: have your adult place a large jar over the candle and time how long the flame lasts before it fades, showing that fresh air, not air alone, is what burning needs.

Write down what you see before you explain it. That order, observation first and conclusion second, is Faraday's real lesson.

Sources

The Books Behind This Course

This course is taught from classic texts that are free for anyone to read. Every method in the lessons traces back to one of these books.

• The Chemical History of a Candle by Michael Faraday (1861), Project Gutenberg

Graduation

You are a chemist now

You began with a candle and ended with the chemistry of your own breath. Keep asking Faraday's question whenever a result surprises you: what is the cause, and why does it happen? Light a candle at the table this week, watch it with your new eyes, and you will never see a plain flame again. Congratulations, and keep experimenting carefully.

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