As a teenager I once set fire to a field of wheat chaff. I didn't do it maliciously; in fact, the farmer I was working for that summer paid me to do it. As it was, the field was a fire hazard but by choosing a time when the wind was at the right speed and from the proper direction, we could control the course of the fire, eliminating the danger of an accidental fire and returning nutrients to the soil. But none of that mattered to me—I was awestruck by the spectacle of the fire itself. Our fascination with fire is something primal, irresistible, and ancient, passed from one human being to another since the dawn of time. I call that part of human nature—the part that thrills to a fireworks display, the part that slouches before a fireplace, the part that insists on dinner by candle light—I call that part "Lucifer." I have given Lucifer an independent voice in this book, separate and distinct from my own. Lucifer's pronouncements are preceded by the alchemical symbol for fire, an upward-pointing triangle reminiscent of a flame. In addition, you will be haunted by three other spirits, those of Earth, Air, and Water, but for the moment it falls to me, the twenty-first century chemist, to describe the phenomenon of fire.

Figure 1-1. Aristotle's Elements

Lucifer was alive and well in 350 BC. Driven by curiosity, philosophers engaged in a lively debate on the nature of the universe; was it made of infinitely many kinds of substances or just a few? Aristotle summarized the opposing viewpoints:


Anaximenes and Diogenes make air prior to water, and the most primary of the simple bodies, while Hippasus of Metapontium and Heraclitus of Ephesus say this of fire, and Empedocles says it of the four elements, (adding a fourth—earth—to those which have been named); for these, he says, always remain and do not come to be, except that they come to be more or fewer, being aggregated into one and segregated out of one.

Anaxagoras of Clazomenae, who, though older than Empedocles, was later in his philosophical activity, says the principles are infinite in number; for he says almost all the things that are made of parts like themselves, in the manner of water or fire, are generated and destroyed in this way, only by aggregation and segregation, and are not in any other sense generated or destroyed, but remain eternally.[1]

Aristotle divided the world into opposites, noting, for example, that things are either hot or cold, never both. They are either wet or dry, never both. But a thing can be both hot and dry or cold and wet. And so fire was assigned dominion over things "hot and dry." water included all things "cold and wet," earth described anything that was "cold and dry," and air, anything "hot and wet," as illustrated in Figure 1-1.

Figure 1-2. Lavoisier's Elements

If longevity is a sign of success, the I-dea of "element" must be considered a great one, having held sway for more than two millennia. And the division into four makes a certain logical sense, but there was always the temptation to view these elements as "ingredients." The practical questions asked by crafts people and artisans had less to do with logic than with logistics. "How much of which ores will produce a ton of copper?" "What kinds of herbs, and in what proportions, will cure a fever?" "What fertilizers will yield the best crops?" "Which plant will dye cloth a permanent blue color?" The four elements are descriptive, not prescriptive. They describe qualities, not quantities. Any attempt to view them as the literal ingredients of nature is fraught with difficulty. Let us examine each of these elements in turn, beginning with earth.

Wood, being cold and dry, belongs to the earthy domain. Look at it closely and you will see that its composition is not uniform; there are dark areas and light areas. The composition of the wood in the light areas is different from that in the dark areas; it cannot be a single substance because it is clearly heterogeneous. Most of the matter encountered in nature is heterogeneous. A handful of earth can be separated into sand and clay, decayed leaves and insects. The sky is divided into a blue expanse across which distinct white clouds roam. Water contains algae and fish and scum. Look at most matter closely enough and you will see non-uniformities and these are the hallmark of heterogeneous matter. Even blood and milk are heterogeneous when viewed under a microscope; Blood consists of the colorless plasma and the colored cells, while milk consists of the colorless whey and various suspended solids and liquids.

Heterogeneous matter can be separated into its constituents by mechanical means, by sorting, sifting, filtering, or sometimes by just letting it settle. For wood, this would entail grinding the wood to a powder and separating the white bits from the brown. I might do this with tweezers and a magnifying glass, or I might find an easier, more ingenious method (see Chapter 14, for example) for achieving the separation, but in the end, I would have the white pile and the brown pile, each one uniform in appearance and composition. I would have rendered the wood homogeneous. Let us call the white pile cellulose for future reference, and move on from earth to a consideration of air.

Granted, air may have dust or fog in it, but let us filter it until it is clean and dry. No matter how closely I look at this sample, it is the same everywhere, i.e. its composition is uniform. There are no light bits and dark bits: it is clearly homogeneous. I may now ask whether or not this air is an element. This question was explored by Antoine Lavoisier late in the eighteenth century.[2] Mercury was boiled in air for 12 days, during which time a red solid formed on the surface of the mercury. At the end of the experiment, 42-43 cubic inches of the original 50 cubic inches of air remained. This gas extinguished candles and suffocated animals immersed into it, and he called it nitrogen. The red solid was collected and, when heated, produced 7-8 cubic inches of gas. Either this was an amazing coincidence, or this was the same 7-8 cubic inches which went missing from the original air. This new gas, in contrast to nitrogen, caused candles to burn more brightly than in normal air, and was breathable by animals. Lavoisier gave it the name oxygen and concluded that air was not an element, but a mixture of nitrogen and oxygen. Today, air is recognized as a solution of 78% nitrogen and 21% oxygen,[3] but these percentages are not fixed. A solution has a uniform but variable composition. Air is still air if it has 18% or 25% oxygen. Its composition may vary from city to suburb, from mountain to valley, or from the first to the twenty-first century.

Whereas a solution is described by its percentage composition, which may vary, a pure substance has a fixed composition. The solution called air can be separated into the substances nitrogen and oxygen. While there are many methods for separating a solution into its substances, we will consider three in detail. Recrystallization will be discussed in Chapter 8, distillation in Chapter 16, and chromatography in Chapter 22.

Earth is heterogeneous; air is a solution; what about water? Filter it so that it is homogeneous; distill it until it is pure. The question remains, "Is it an element, or is it a combination of other materials?" All of our work in defining a pure substance has been leading up to this fundamental distinction. The evidence that water is a compound is also summarized in Elements of Chemistry. First, Lavoisier decomposed water by passing steam over iron. 100 parts (by weight) of water decomposed into 15 parts of hydrogen and 85 parts of oxygen. Furthermore, 15 parts of hydrogen combined with 85 parts of oxygen to produce 100 parts of water. He concluded that pure water is composed of 15% hydrogen and 85% oxygen. These proportions have been refined over the years as our ability to weigh gases has improved; water is precisely 11.190% hydrogen and 88.810% oxygen. Any sufficiently careful experiment will confirm these percentages, and they are the same for water collected and purified anywhere. Water is never 25% hydrogen or 3% hydrogen; its composition is fixed and this is what makes it a substance rather than a solution.

While the composition of water is fixed, it is not robust; after all, Lavoisier had showed that it can be decomposed into hydrogen and oxygen. A substance is classified as a compound when it can be decomposed into two or more other substances. Similarly, cellulose, the white homogeneous solid separated from wood, becomes black when charred. Careful observations reveal that when cellulose is heated in a closed container, it decomposes primarily into the substances, charcoal and water. When one substance decomposes into two, it must be a compound.

With earth, air, and water stripped of their elemental status, one might wonder whether any substance can resist such analysis. While even fire has not survived as an element, one of its products, charcoal, has done so. According to Lavoisier, "As charcoal has not been hitherto decomposed, it must, in the present state of our knowledge, be considered as a simple substance."[4] Lavoisier's notion of a "simple substance," or element, is thus provisional; while a compound is a pure substance which has been decomposed, an element is one which, so far, has resisted all such attempts. Charcoal's composition is robust. Unlike wood, charcoal can be heated in the absence of oxygen without suffering further decomposition. That is not to say that charcoal is inert; charcoal burns in the presence of oxygen to produce a gas with a fixed composition, that is, two substances combine to make one substance. To show that charcoal is a compound we would have to turn one substance into two substances. No process has yet been found for doing so and since the time of Lavoisier, charcoal has been known as the element, carbon, after the French word for coal.

Table 1-1. Formulae for Some Common Substances

Let us now examine the nature of fire using the combustion of wood in air as an example. Wood is a heterogeneous material composed chiefly of cellulose; air is a solution composed mainly of nitrogen and oxygen, so let us sharpen our discussion of fire by considering only the reaction of cellulose with oxygen. The combustion of cellulose occurs in two stages. When cellulose is heated, it does not burn immediately; it first releases steam and turns from white to black, that is, it chars, becoming charcoal. It is this hot charcoal which burns when it comes into contact with oxygen, producing a new gas, carbon dioxide. The heat released by the combustion causes more cellulose to char, producing more steam and charcoal. Since cellulose, steam (water), charcoal, oxygen, and carbon dioxide are all substances, they can be represented by chemical formulae, as defined in Table 1-1. Reactions involving these substances are represented by the equations shown in Equation 1-1. An equation is said to be balanced if the amount of each element is the same on either side of the equal sign. Equation (a) describes the charring of cellulose and (b) describes the combustion of charcoal. In such equations the attributes (s), (l) and (g) refer to the states solid, liquid and gas.

Equation 1-1. The Combustion of Cellulose

Figure 1-3. Fire as a Process

The equations of Equation 1-1 correspond to the process schematic of Figure 1-3, and vice versa. In such a schematic the cellulose reactant enters from the left and moves into reactor (a), a furnace, where it is charred. The lower circle of the furnace, bearing the alchemical symbol for fire, represents any source of heat. The middle circle, bearing the symbol for earth, represents the transformation of the solid cellulose into solid charcoal. The top circle, bearing the symbol for air, represents the gases produced in the furnace, in this case water vapor. Because water is a waste product in this reactor, it exits to the top of the figure, as if it were going up a chimney. The intended product of the reactor, charcoal, exits to the right. The convention established here is that reactants enter a reactor from the left, useful products exit to the right, and waste products exit to the top or bottom of a schematic.

Reactor (b) is a burner, represented by the alchemical symbol for fire. The reactants, charcoal and oxygen, enter from the left and carbon dioxide goes up the chimney. Taken together, the two reactors of Figure 1-3 give a pictorial representation of the corresponding reactions for the combustion of cellulose. Study them carefully, as the conventions established here will allow us to represent quite complicated chemical processes using simple figures.

WarningMaterial Safety

We live in a litigious society. Consequently lawn mowers must carry warnings that they are not to be used for trimming hedges. Sand destined for the sand-box must carry a hazardous material warning. In short, manufacturers are forced to warn consumers of every conceivable danger, no matter how bizarre, involving their products. Given this atmosphere, I had better tell you that the projects described in this book require a certain amount of common sense to be completed safely. A stupid or careless person will, no doubt, be able to find ingenious ways to hurt himself[5] no matter how many warnings are given. And, incredibly, far from being embarrassed by his[6] stupidity, he may believe that someone else should have protected him from his own stupidity. If you intend to make fire, do I really have to tell you to be careful? Do not make fire near flammable materials. Do not make fire in small, unventilated places. Avoid inhaling smoke. If you are stupid, careless, or unwilling to accept responsibility for your own safety, let me ask you to save us all a lot of trouble by putting this book away and taking up some safer activity, like sitting quietly or walking carefully in slow circles.

NoteResearch and Development

Before proceeding with your work, you must master the following material:



Reference [2] Metaphysica, p. 984a.


Reference [17], pp. 34-35.


The remaining 1% consists of argon, carbon dioxide, and a host of less abundant gases.


Reference [17], p. 208.


In the case of non-characters of indefinite gender, pronouns have been chosen by coin toss. An initial coin toss established the convention that "heads" would result in a female pronoun and vice versa. One toss was allowed for each pronoun and this toss was never second-guessed. Any suspicions of gender bias should therefore be attributed to the coin, not to the author. Such pronouns are listed in the index under He/his and She/her.


When a string of pronouns or adjectives refer to a non-character of indefinite gender, the first one has been chosen by coin toss and subsequent references made to agree with it.