One summer afternoon, I arrived at my parents’ house in suburban southeast London armed with some glassware that I’d ordered online and a pack of six Mr. Kipling Bramley apple pies. I was there to do what is probably the silliest experiment I’ve ever attempted.
As a child, my dad was a keen amateur chemist and used to spend happy afternoons in the mid-1960s creating smells and explosions in the shed at the bottom of his parents’ garden. Those were the days when anyone (including teenage boys in possession of an advanced knowledge of chemistry and a healthy disregard for their own safety) could buy a terrifying array of noxious substances from their local chemical supplier. This, it turned out, included all the ingredients of gunpowder. He still recalls with some relish how one of his more dramatic experiments was brought to an abrupt end when his own father, a former artilleryman not unaccustomed to the sound of gunfire, stormed to the bottom of the garden shouting, “That’s enough, that one rattled the windows!” Simpler times. My dad still has some of his old chemistry equipment, including a Bunsen burner that I wanted to get my hands on, and I’d decided that my small London flat was probably not the ideal location for the experiment I had in mind.
The thought behind the experiment was this: if you were presented with an apple pie and had no knowledge of pies, apples, or their composition, what might you do to try to figure out what it was made from? On the workbench in the garage I scraped a small sample of the pie into a test tube, taking care to get a good mix of the crumbly pastry and the soft apple filling, and then sealed it with a cork with a small hole drilled through the middle. After connecting the tube to a second flask floating in a tub of cold water via a long L-shaped glass pipe, we fired up the Bunsen burner, popped it under the test tube, and stood back.
The pie began to bubble and caramelize, and soon the expanding gas within the test tube threatened to force our sample up into the connecting pipe. Reducing the heat slightly we watched the pie slowly start to blacken, and to my delight tendrils of mist started to flow along the pipe and pour into the waiting flask, which before long was overflowing with a ghostly white vapor. Now this was a real chemistry experiment!
Wondering what this white mist might be, I gave it a whiff, a tried and tested method of chemical analysis from before the days of health and safety. Humphry Davy, a pioneering chemist of the Romantic age, famously investigated the medical effects of various gases by inhaling them, which in 1799 led him to discover the pleasurable effects of nitrous oxide, what we now know as laughing gas, which he would inhale in large quantities while locked in a dark room with his poet friends, or sometimes young women of his acquaintance. Mind you, it wasn’t a risk-free strategy. He came close to killing himself during an experiment with carbon monoxide, and on being dragged into the open air remarked faintly, “I do not think I shall die.”
Alas, my apple pie vapor didn’t produce any psychoactive effects, just an extremely unpleasant burnt smell that seemed to hang around for hours afterward. Peering through the mist to the bottom of the flask I found that some parts of the vapor had condensed on contact with the cool water bath, forming a yellowish liquid covered by a dark brown oily film.
After about ten minutes of intense heating, no more vapor seemed to be coming off the charred remains of the apple pie and so we concluded that our experiment was complete. In my keenness to inspect the contents of the test tube, I briefly forgot that when you heat glass with a Bunsen flame for ten minutes it gets really quite hot and badly burned my index finger. There’s a good reason why the most dangerous bit of equipment I am generally allowed near is a desktop computer.
After a much longer wait, I gingerly returned to the test tube and tipped its contents onto the bench. The apple pie had been reduced to a jet-black, rocklike substance whose surface was slightly shiny in places. So what can we conclude about the composition of apple pie from this admittedly rather silly experiment? Well, we’ve ended up with three different substances: a black solid, a yellow liquid, and a white gas, which by now had infused my skin, hair, and clothes with a nauseating burnt smell. I admit that the precise chemical composition of these three apple pie components was not entirely clear to me at the time, though I was pretty sure the black stuff was charcoal and that the yellowish liquid was probably mostly water. To get further toward a list of fundamental apple pie ingredients we are going to need to do some more advanced chemical analysis.
I shouldn’t admit this as a physicist, but chemistry was my favorite subject at school. Physics labs were sterile, joyless places where we were expected to find excitement wiring up a circuit or glumly timing the swing of a pendulum. But the chemistry lab was a place of magic, where you could play with flame and acid, set fire to magnesium ribbon that burned so bright it dazzled, or bubble colored potions through delicate glassware. The safety glasses, the bottles of sodium hydroxide with threatening orange warning labels, and white lab coats stained with the unidentified, perhaps toxic, remains of experiments past, all helped to lend the chemistry lab a frisson of danger. And marshaling all this was our enigmatic teacher, Mr. Turner, who arrived at school in a sports car and was rumored to have made his fortune by inventing the spray-on condom.
In fact, it was a fascination with chemistry that set me on a path toward eventually becoming a particle physicist. Chemistry, like particle physics, concerns itself with matter, the stuff of the world, and how different basic ingredients react, break apart, or change their properties according to certain rules. The reason I didn’t stick with chemistry in the end is because I wanted to know where those rules came from. Had I been born in the eighteenth or nineteenth century, I would most likely have stuck with it. Back then, if you wanted to understand the fundamental building blocks of matter, then chemistry, not physics, was the subject for you.
The person who probably did more than anyone else to invent modern chemistry was Antoine-Laurent Lavoisier, a brash, ambitious, and fabulously rich young Frenchman who lived and worked in the second half of the eighteenth century. Born in Paris in 1743 into a wealthy family steeped in the legal profession, he used a large inheritance from his father to equip his personal lab at the Paris Arsenal with the most sophisticated chemical apparatus money could buy. Aided by his wife and fellow chemist, Marie-Anne Pierrette Paulze, he brought about a self-declared “revolution” in chemistry by systematically dismantling the old ideas that had been inherited from ancient Greece and inventing the modern concept of the chemical element.
The idea that everything in the material world is made up of a number of basic substances, or elements, has been around for thousands of years. Different element theories can be found in ancient civilizations including Egypt, India, China, and Tibet. The ancient Greeks argued that the material world was made of four elements: earth, water, air, and fire. However, there is a big difference between what the ancient Greeks thought of as an element and the definition of a chemical element that we learn about in high school.
In modern chemistry, an element is a substance like carbon, iron, or gold that can’t be broken down or converted into anything else. On the other hand, the ancient Greeks thought that earth, water, air, and fire could be transformed into one another. On top of the four elements they added the concept of four “qualities”: hotness, coldness, dryness, and moistness. Earth was cold and dry, water was cold and moist, air was hot and moist, and fire was hot and dry. This meant that it was possible to convert one element into another by adding or removing qualities; adding hotness to water (cold and moist) would produce air (hot and moist), for example. This theory of matter raised the prospect of transforming, or “transmuting,” one substance into another—most famously common metals into gold—through the practice of alchemy.
It was the concept of transmutation that Lavoisier attacked first. As with many of his greatest breakthroughs, his approach was based on a simple assumption, namely that mass is always conserved in a chemical reaction. In other words, if you weigh all the ingredients at the start of an experiment, and then all the products at the end, taking care to make sure no sneaky wisps of gas escape, then their masses should be the same. Chemists had been making this assumption for some time, but it was Lavoisier, aided by a set of extremely precise (and expensive) weighing scales, who popularized the idea when he published the results of his own painstaking experiments in 1773. In Mr. Turner’s high school chemistry lessons, the law of the conservation of mass was taught to me as Lavoisier’s principle.
One piece of evidence in transmutation’s favor was the fact that when water was slowly distilled in a glass container, a solid residue was left behind, which seemed to confirm that water could be converted into earth. Lavoisier had his doubts. Weighing the empty glass container before and after the experiment, he found that it had lost some mass, which was almost exactly equal to the mass of the so-called earth. In other words, the idea was nonsense. The solid residue was just made up of bits of the glass container.
By demolishing the idea of the transmutation of water into earth, Lavoisier fired the first shot in a campaign that would totally upend how people thought about the chemical world. Declaring with characteristic swagger his intention to bring about “a revolution of physics and chemistry,” he then set about tearing down the elements themselves. His next move was to take on the most mysterious and powerful of them all: fire.
In the mid-eighteenth century, flammable materials like charcoal were believed to contain a substance known as “phlogiston” that was given off when they were set on fire. A fuel like charcoal contained lots of phlogiston, which was released during burning, with the burning eventually stopping either when all the phlogiston in the charcoal had run out or when the surrounding air had become so full of phlogiston that it couldn’t absorb any more.
One problem with this phlogiston business came with the discovery that metals actually get heavier when they are burned, whereas you’d expect them to get lighter if phlogiston was being released. This was explained away by the Dijon-based lawyer and chemist Louis-Bernard Guyton de Morveau as being due to the fact that phlogiston was incredibly light and when stored in metals somehow “buoyed” them up, a bit like a hot air balloon. When the metal was burned, the buoyancy provided by the phlogiston was lost and so the metal appeared to get heavier.
Lavoisier was less than impressed by Guyton’s idea and argued the complete opposite—instead of burning releasing phlogiston, burning involved air being absorbed. This explained why metals got heavier when burned: they weren’t releasing floaty phlogiston, they were combining with air.
It’s worth taking a moment to appreciate how brilliant an insight this is. If you’re briefly able to forget everything you were taught at school about combustion, then thinking that phlogiston is released by fire actually makes a lot of sense. Fire definitely seems to be a process that releases stuff—light, heat, and smoke at the very least. The idea that burning combines air with the fuel, effectively sucking something out of the air, is really quite counterintuitive. Lavoisier’s ability to follow the experimental evidence and reject what might seem like common sense is what allowed him to leap to such a radically different conclusion.
The question was, what exactly was it in air that was consumed in burning? Unknown to Lavoisier at the time, significant advances in the understanding of air had recently been made across the Channel in Britain. In 1756 the Scottish natural philosopher Joseph Black had discovered a peculiar new type of air that was released when certain salts were heated. Most surprisingly, he found that it was impossible to set things on fire when they were surrounded by this “fixed air”—what we now know as carbon dioxide. A decade later, Henry Cavendish found that when sulfuric acid was poured over iron it gave off another, lighter air that would catch fire with a characteristic pop. But the most prolific discoverer of new airs was the English natural philosopher Joseph Priestley.
Priestley was inspired to begin his own investigations of air when he learned of Cavendish’s discovery of “inflammable air” in 1767. At the time he was working as a Presbyterian minister in Leeds and living next door to a brewery, a bit of a contrast to Lavoisier’s lavishly equipped laboratory in central Paris. However, being next door to a brewery did have its benefits, aside from an ample supply of beer. The fermentation process released large quantities of fixed air, which, among other things, Priestley used to develop a technique for making fizzy drinks, laying the foundations for the future soft-drink industry.
Copyright © 2021 by Harry Cliff. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.