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'tis, we musicians know, the C Major of this life. Paintings and museum reconstructions depicting the activities of medieval alchemists can easily give the wrong impression of the chemical knowledge of that period. Inanimate objects such as crystals are seen sharing shelves with the preserved bodies of small creatures, a common feature of the early laboratory. This could be taken to suggest that the scientists of those days had grasped the unity of chemistry: that in spite of great differences in their external appearance, Nature makes no distinction between chemical combinations in living and dead substances. Nothing could be farther from the truth. The universally held belief was that the matter contained in living organisms possessed an essential extra ingredient, a vital force, the mysterious origin of which was attributable to divine powers. This attitude was epitomized in the succinct classification to be found in the book Cours de Chyme, published by Nicholas Lemery in 1685. His division is still to be found in the standard first question of a popular parlour game: ‘animal, vegetable or mineral?’. By the end of the eighteenth century, the vital force theory was in rapid decline. Antoine Lavoisier, analyzing typical organic compounds, found them to contain inorganic substances such as hydrogen, oxygen, nitrogen, carbon, sulphur and phosphorus. The basic principles that govern chemical change, such as conservation of total mass, were being established during this period, and in 1844 Jöns Berzelius showed that they apply to all matter irrespective of whether it is inorganic or organic. [ABSTRACT FROM AUTHOR]

The force of nature could no farther go; to make a third she join'd the former two. Most substances are mixtures of two or more elements, and chemical compounds are found to have definite compositions. They are also found to consist of molecules, which are the smallest particles that can exist in the free state and still retain the characteristics of the substance. Thus the chemical formula for water is always H2O, indicating that its molecules contain two hydrogen atoms and one oxygen. (The subscript 1 is not written explicitly; H2O implies H2O1.) The formula H2O2, in which the relative proportions of hydrogen and oxygen are different from those in water, belongs to a different chemical compound, the common bleaching agent hydrogen peroxide. Similarly, sodium chloride, also known as common salt, has the formula NaCl, and not Na2Cl or NaCl3. This shows that the forces that hold atoms together in groups must have certain characteristics which determine the rules of combination. The occurrence of chemical reactions reveals that the forces can be overcome, allowing the atoms to move apart and establish bonds with other atoms. A simple example of such an event is the reaction Zn + 2HCl → ZnCl2 + H2 in which the addition of hydrochloric acid to zinc produces zinc chloride and hydrogen, which is liberated as a gas. This demonstrates that the binding forces between zinc and chlorine atoms are stronger than those which hold hydrogen to chlorine. [ABSTRACT FROM AUTHOR]