Robert Boyle (1627-1691), an Irishman who wrote The Sceptical Chymist in 1661, is often considered the father of chemistry. He held that matter was made of atoms or groups of atoms that were constantly moving. He urged chemists to accept only those results that could be demonstrated by experiment.

The great French chemist Antoine-Laurent de Lavoisier (1743-1794) demanded that measurements be made with precision and that scientific terms be clearly defined and carefully used. With the discovery that air contained oxygen, which is necessary to support animal life, he clearly demonstrated that air was not an element, but a mixture of oxygen and nitrogen. By showing that hydrogen, discovered by Henry Cavendish (1731-1810), when mixed with oxygen formed water, Lavoisier showed that water also was not an element, but a compound of two elements. Lavoisier’s care with weighing both the reactants and products of reactions allowed him to make one of the earliest statements that mass is conserved, that is, neither created nor destroyed in chemical reactions. Lavoisier’s wife was a brilliant woman who assisted her husband in many ways, including translating letters from foreign scientists. He was executed during the French Revolution for being a tax collector. In supporting the guilty verdict, the judge said “The Republic needs neither chemists nor scientists.”

Lavoisier met with Joseph Priestley (1733-1804) shortly after Priestley discovered oxygen in 1774. Lavoisier did extensive work on oxygen and gave it its present name. Priestley invented carbonated water and experimented with ammonia and laughing gas (nitrous oxide) but when he opposed much of the work of chemists of his generation he was pushed to the sidelines. After writing theological books that opposed traditional Christianity in England he was forced to flee to America in 1794.

John Dalton (1766-1844) was an English chemist who, because he was a Quaker, could not obtain a position in a state-run university. For a number of years he taught at a college in Manchester for dissenters from the Church of England. His strengths were his rich imagination and clear mental pictures, but especially his astonishing physical intuition.

His first interest was meteorology. He wondered how Earth’s atmosphere, consisting of gases of very different densities, could have the same composition at different altitudes. His meteorological studies led him to the conclusion that atoms were physical entities and their relative weight and number were crucial in chemical combinations. Dalton’s atomic theory of chemistry was published in “A New System of Chemical Philosophy” in 1808 and 1810. Briefly, it has five parts:

• Elements are made of tiny particles called atoms that cannot be divided into smaller particles. Nor can they be created or destroyed or changed into another kind of atom.
• All atoms of an element are identical. So there are as many kinds of atoms as there are elements.
• The atoms of an element are different from those of any other element in that they have different weights.
• Atoms of one element can combine with atoms of another element to form a chemical compound (today called a molecule). A compound always has the same relative numbers and kinds of atoms.
• In a chemical reaction atoms are rearranged among the compounds; they are neither lost nor gained.

The first and last parts give an atomic basis for Lavoisier’s conservation of mass that he had confirmed in many careful experiments. Two of Dalton’s specifications are now known to be false. Atoms can be changed from one kind to another by the process of radioactive decay. Not all atoms of an element have the same weight. Both of these results will be discussed in the chapter “At the Heart of the Atom.”