Christmas with Faraday: The Chemical History of a Candle

?There is not a law under which any part of this universe is governed which does not come into play and is touched upon in these phenomena. There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle.?
? Michael Faraday, The Chemical History of a Candle

Who would you cite as your favorite physicist? The field has a long, rich history filled with colorful characters and undisputed genius, so it would be a most difficult choice. But Michael Faraday would certainly be near the top of my list of serious contenders for the title. [NOTE: See a related post on Faraday's lecture The Forces of Matter over at Skulls in the Stars.]

Faraday was a 19th century British scientist, the son of a blacksmith, who started out as a? bookbinder?s apprentice and took advantage of that position to read voraciously. His favorite subjects were the natural sciences.

Serendipitously, as his apprenticeship was ending, a friend gave him a ticket to a lecture on electrochemistry by the eminent scientist Humphrey Davey, at the Royal Institution ? not a venue where the young humble-born Faraday would normally be welcomed.

Faraday was entranced, and after the lecture he asked Davy for a job. There wasn?t a position available, Davy gently told the young man, but shortly thereafter he sacked his assistant for brawling and hired Faraday in his stead.

It has famously been said that Michael Faraday was Davy?s greatest discovery; considering that Davy discovered the elements barium, strontium, sodium, potassium, calcium and magnesium, that is no mean compliment. Faraday went on to make conduct a series of seminal experiments in electromagnetism, among other contributions.

He also quickly gained recognition as an excellent public speaker. People in early Victorian England were highly interested in the latest scientific discoveries of the day. (They were also just as prone to superstition, though, and Faraday was a staunch opponent to things like table-turning, seances, and mesmerism.) Fellow naturalist William Crookes described Faraday?s lectures thusly: ?All is a sparking stream of eloquence and experimental illustration.?

One of his favorite demonstrations is now a simple experiment repeated by schoolchildren everywhere. You can see magnetic field lines ? what Faraday called lines of force ? by sprinkling iron filings onto a sheet of paper held over a bar magnet. The filings align themselves within the magnetic field, so we can ?see? the patten normally invisible to us.

In particular, Faraday gave a series of famous Christmas lectures each year at the Royal Institution ? a tradition that continues today. One of the earliest, on the chemistry and physics of flames, became a popular book: The Chemical History of a Candle.

These lectures were a gift that Faraday gave year after year to those who showed up to receive it: the gift of wonder at the natural world that continues to surprise us, even today, with its mysterious workings.

Faraday opened with a discussion of how candles were made, from naturally occurring candles like the paraffin and bits of candlewood found in Irish bogs ? ?a hard, strong, excellent wood? ?to manmade dipped tallow candles, beeswax candles, and something called a? sperm candle, ?which comes from the purified oil of the spermaceti whale.? He even displayed a candle salvaged from the wreck of the Royal George, which sunk at Spithead on the 29th of August, 1782; yet the candle still burned brightly when lit.

As Faraday described the process:

?The fat or tallow is first boiled with quick-lime, and made into a soap, and then the soap is decomposed by sulphuric acid, which takes away the lime, and leaves the fat rearranged as stearic acid, while a quantity of glycerin is produced at the same time. Glycerin?absolutely a sugar, or a substance similar to sugar?comes out of the tallow in this chemical change. The oil is then pressed out of it; and you see here this series of pressed cakes, showing how beautifully the impurities are carried out by the oily part as the pressure goes on increasing, and at last you have left that substance, which is melted, and cast into candles as here represented.?

But the bulk of Faraday?s lecture focused on the science relating to the actual flame of a burning candle. First, Faraday demonstrated a simple experiment, placing a candle inside a lampglass to block out any breezes and achieve ?a quiet flame.? He showed how important a well-made candle could be, demonstrating that ?a beautiful cup is formed? as a result of a ?regular ascending current of air playing upon all sides, which keeps the exterior of the candle cool?:

As the air comes to the candle, it moves upward by the force of the current which the heat of the candle produces, and it so cools all the sides of the wax, tallow, or fuel as to keep the edge much cooler than the part within; the part within melts by the flame that runs down the wick as far as it can go before it is extinguished, but the part on the outside does not melt. If I made a current in one direction, my cup would be lop-sided, and the fluid would consequently run over; for the same force of gravity which holds worlds together holds this fluid in a horizontal position, and if the cup be not horizontal, of course the fluid will run away in guttering. You see, therefore, that the cup is formed by this beautifully.

Next, Faraday asked (rhetorically), how is it that a candle burns so steadily, when the it is impossible for a solid fuel to ?flow? up to the wick to feed the flame at the top, as in an oil lamp? The oil in the lamp flows upward thanks to something called capillary action: ?the ability of a substance to draw another substance into it.? (It?s also behind the so-called ?wick effect? explanation for cases of suspected spontaneous human combustion.) Basically, it?s the same thing that causes a sponge (a porous material) to soak up liquids from a surface.

You can witness capillary action for yourself with a simple vertical glass tube open at either end. Place the lower end in a glass of water, you?ll notice that the water rises up to a certain point and then stops. Surface tension basically pulls the liquid column up until the mass of the liquid is large enough so that gravity can overcome the intramolecular forces. You know when a drop of water forms on the spigot of your tap and suspends there until you touch it? Capillary forces hold it there.

And the same is true of candles. To demonstrate this, Faraday showed a ?vessel made of wire gauze filled with water.? It was porous, since water poured into the top would run out at the bottom, and yet the vessel remained filled with water. Faraday compared the wire gauze to a candle?s wick, and explained:

?the wire, being once wetted, remains wet; the meshes are so small that the fluid is attracted so strongly from the one side to the other, as to remain in the vessel, although it is porous. In like manner, the particles of melted tallow ascend the cotton and get to the top: other particles then follow by their mutual attraction for each other, and as they reach the flame they are gradually burned.?

Faraday went on to muse upon the connection between the burning candle and the formation of soot and smoke, as well as air currents and how they influence the shapes of flames. He illustrated this last point with an impromptu version of ?snapdragon?: he took a warmed dish, poured in some brandy (the fuel), then lit it. Then he dropped in some plums (which served as a wick) and pointed out how ?beautiful tongues of flame? were formed.

?You have the air creeping in over the edge of the dish forming these tongues. Why? Because, through the force of the current and the irregularity of the action of the flame, it can not flow in one uniform stream. The air flows in so irregularly that you have what would otherwise be a single image broken up into a variety of forms, and each of these little tongues has an independent existence of its own.

?Indeed, I might say, you have here a multitude of independent candles. You must not imagine, because you see these tongues all at once, that the flame is of this particular shape. A flame of that shape is never so at any one time. Never is a body of flame, like that which you just saw rising from the ball, of the shape it appears to you. It consists of a multitude of different shapes, succeeding each other so fast that the eye is only able to take cognizance of them all at once.?

It?s a wonderful lecture, and worth reading in its entirety. For all our technological advancement, I find it charming that, even today, scientists still find much to puzzle about when it comes to burning candles. ?There are literally thousands of reactions that go on from the moment the fuel vapor is produced and leaves the wick to the time it actually burns and produces Co2 and water,? NASA researcher Howard Ross told Discover in 2001.

This is why I love Faraday so much. No matter how accomplished he became in the world of science, no matter how much he learned through his experiments (which gave us the dynamo, among other things), he never lost the ability to keenly observe even the simplest things around him, noting tiny details and reveling in the intricacy of Nature. He closed his candle lecture by telling his audience,

?Indeed, all I can say to you at the end of these lectures (for we must come to an end at one time or other) is to express a wish that you may, in your generation, be fit to compare to a candle; that, in all your actions, you may justify the beauty of the taper by making your deeds honourable and effectual in the discharge of your duty to your fellow-men.?

On August 25, 1867, the flame of Faraday?s life was snuffed out; his (physical and mental) health had been deteriorating for a good 20 years by then. But his gifts keep on giving, all these years later. And Christmas seems a particularly apt time to honor the man.

Source: http://rss.sciam.com/click.phdo?i=815f28817721d7527decbf55a2b18fb0

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