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The Structure of Matter

The Route to the Atomic Theory

Chemistry is a subject of vital importance to human society. We even measure the progress of civilisation by the chemical technology that our ancestors possessed at various stages in history. Thus, the earliest phase of civilisation is known as the Stone Age, when humans used readily available materials such as stone to form tools. In chemical terms, the stone was used as it was found. The only manipulation was to shape it by physical means into knives, axes, and so on. The discovery of bronze moved civilisation forward significantly and gave birth to the Bronze Age. As an example of this technological advancement, bronze axes could be made with much more acute angles at the cutting edge of the blade than can stone axes, and so fewer strokes were required to cut through a tree trunk. Now chemistry was involved, since ores such as malachite had to undergo a chemical conversion to release the copper metal that they contained. Heating the ore to a high temperature brought about this chemical change. The temperature required to release iron from its ores, such as haematite, is even higher, so it was not until furnace technology had reached the required level that the Iron Age began.

Chemistry is important to all industries to some extent, but to perfumery, it is absolutely central. The odorous substances that produce the sensation of smell, whether of natural or synthetic origin, are chemicals. The receptors in our noses that perceive them are chemicals. Smell begins with the process of chemical recognition of the odorant by the olfactory receptor, and therefore smell is very much a chemical sense. To understand fragrance perception, we must understand chemistry. The products into which perfumes are incorporated are also composed of chemicals and chemical interactions can occur between the perfume and the product. Thus, in order to understand the interaction of perfume with products such as soaps and detergents, we must understand chemistry.

Chemistry is very much a practical science and people were practising it long before theories about the nature of matter and of these chemical processes were developed. Metallurgy, which is one branch of chemistry, started in the Nile Delta in ancient Egypt. Because of the colour of the rich alluvial soil, the Greeks knew this region as 'The Black Country'. Metallurgy was considered to be the art of Egypt, the Black Country, and hence became to be known as the ' Black Art'.

The debate about the nature of matter began in Greece around the fourth century. Democritus (460-370 BCE) and Epicurus (341-270 BCE) argued that matter was made of small indivisible particles that they called atoms. The word atom is derived from the Greek verb t?µe? (tomeo), which means 'I cut', and at?µ?? (atomos), meaning 'uncuttable' or 'indivisible'. On the other hand, Empedocles (c. 450 BCE) and later Aristotle (384-322 BCE) believed that matter was continuous and composed of four basic ingredients or elements: earth, air, fire, and water. In order to distinguish living from inanimate matter, Aristotle invoked a fifth element or quintessence, which he called spirit. The legacy of his erroneous theory still survives in our language today. Adherents of the Aristotelian philosophy believed that by heating plant material, they were removing the spirit (or quintessence) of the plant and so the oil they obtained was called the quintessential (later shortened to essential) oil. Similarly, we refer to other distillates, such as whisky, gin, or brandy, as spirits. With these two philosophical schools came the first theories of how the sense of smell worked. Epicurus believed that odours were made up of atoms that travelled through the air from the source to the nose. Smooth, rounded atoms gave rise to sweet smells and pointed ones to sharp odours. Aristotle believed that odours radiated from the source to the nose, just as heat radiated from the sun to the earth.

In CE 50, Dioscorides produced a book called De Materia Medica in which he listed all the known facts about herbal medicines. The compilation of what was known about the physical universe gained further momentum in CE 866 when Razi began a systematic collection of facts. Around CE 1000 the Arabs invented distillation, which meant that liquids could be produced in a pure state. New solvents for distillation such as alcohol were used in addition to water and therefore allowed for a great increase in the ability to manipulate materials. For instance, the odorous components of plants had previously been capable only of being extracted into fats and oils through the process of enfleurage (see Chapter 4, for details). With distillation, the volatile oils could be extracted directly from the plant material. The availability of alcohol as a solvent meant that the odorous principles could also be extracted from the fatty concretes by dissolving them in ethanol . (Again, more detail will be found in Chapter 4.)

The alchemists of medieval Europe searched for a method to turn base metals into gold. We now know that this is a futile endeavour but, in their work, they built up a fund of experimental evidence about interconversions of substances. In the thirteenth century, Roger Bacon, an English Franciscan friar and scientist, laid the foundations of what we now call 'The Scientific Method'. Scientific method uses five steps in developing theories about the physical universe. These steps are observe, correlate, postulate, test, and revise. Thus, true science begins with the observation of facts. It then seeks to find relationships between them and to devise theories to account for them. The next step is to devise experiments that will test the theories. If the theory passes the test, it remains valid. If not, the theory must be abandoned or revised until a new theory is developed that passes all known practical tests. We must always remember that in science nothing is ever established beyond doubt; every theory, every model is only accepted, while no exceptions are known. The possibility always exists of an inconvenient fact turning up and forcing us to revise our theories again - hence the saying 'The exception proves the rule', the verb prove here being used in the sense of tests.

Armed with Bacon's powerful scientific method, the scientists of the Age of Enlightenment were able to start interpreting the growing body of facts in a more rational way, and, in one sense, the opposing theories of Democritus and Aristotle began to come together to form a more accurate picture of the universe. Democritus had seen each type of matter as being composed of characteristic particles or atoms. Aristotle saw different forms of matter as being composed of combinations of four basic elements. Gradually, a new picture began to emerge in which atoms of a larger number of elements came together in different ways to form other substances.

As an illustration, let us look at some chemical relationships between iron and sulfur. These two substances appear in various guises, and so the suspicion arose that they might be elements, basic building blocks of matter. Heating iron ore produces iron, which can be purified by heating to burn off some of the contaminants present and then pouring the molten iron away from the more refractory minerals around it. Sulfur was collected from the rims of volcanoes, hence its former name of brimstone. If iron powder and sulfur are mixed together, they can easily be separated again with a magnet. However, if they are heated together, they form a new substance that turns out to be identical to the mineral known as pyrites or 'fool's gold'. Burning sulfur in air produces an irritant gas that is referenced in Homer's Odyssey, when Odysseus burnt sulfur in his house to cleanse it from the traces of those who had occupied it during his famous return journey from Troy. If pyrites is burnt in air, we drive off the same acidic gas and obtain iron. So iron and sulfur can be chemically combined to form a new substance. However, they are not lost, and both can be recovered from the combination. Therefore, we can conclude that they are both elements, as opposed to pyrites, which is a compound of iron and sulfur. Of course, another element, oxygen, is involved in the above conversions. However, oxygen is difficult to characterise and it was not identified as an element until much later.

In this way, a number of elements were identified and then laws about the way they combined began to be discovered. The first was the law of definite proportions that was first defined by J.B. Richter in 1792. This law states, 'The ratios of the weights of elements which are present in a given chemical compound are constant'. So, taking our example of pyrites, the ratio of the weights of iron to sulfur in any given sample will be the same. Then, came the law of equivalent proportions, which states: 'The proportions in which two elements separately combine with the same weight of a third element are also the proportions in which the first two elements...