The 100 Most Influential Scientists of All Time (13 page)

(b. Oct. 10, 1731, Nice, France—d. Feb. 24, 1810, London, Eng.)

H
enry Cavendish was a natural philosopher and is considered to be the greatest experimental and theoretical English chemist and physicist of his age. Cavendish was distinguished for great accuracy and precision in researches into the composition of atmospheric air, the properties of different gases, the synthesis of water, the law governing electrical attraction and repulsion, a mechanical theory of heat, and calculations of the density (and hence the weight) of the Earth. His experiment to
weigh the Earth has come to be known as the Cavendish experiment.

Cavendish was a shy man who was uncomfortable in society and avoided it when he could. About the time of his father's death, Cavendish began to work closely with Charles Blagden, an association that helped Blagden enter fully into London's scientific society. In return, Blagden helped to keep the world at a distance from Cavendish. Cavendish published no books and few papers, but he achieved much. Several areas of research, including mechanics, optics, and magnetism, feature extensively in his manuscripts, but they scarcely feature in his published work.

His first publication (1766) was a combination of three short chemistry papers on “factitious airs,” or gases produced in the laboratory. He produced “inflammable air” (hydrogen) by dissolving metals in acids and “fixed air” (carbon dioxide) by dissolving alkalis in acids, and he collected these and other gases in bottles inverted over water or mercury. He then measured their solubility in water and their specific gravity and noted their combustibility. Cavendish was awarded the Royal Society's Copley Medal for this paper. Gas chemistry was of increasing importance in the latter half of the 18th century and became crucial for Frenchman Antoine-Laurent Lavoisier's reform of chemistry, generally known as the chemical revolution.

In 1783 Cavendish published a paper on eudiometry (the measurement of the goodness of gases for breathing). He described a new eudiometer of his own invention, with which he achieved the best results to date, using what in other hands had been the inexact method of measuring gases by weighing them. He next published a paper on the production of water by burning inflammable air
(that is, hydrogen) in dephlogisticated air (now known to be oxygen), the latter a constituent of atmospheric air. Cavendish concluded that dephlogisticated air was dephlogisticated water and that hydrogen was either pure phlogiston or phlogisticated water. He reported these findings to Joseph Priestley, an English clergyman and scientist, no later than March 1783, but did not publish them until the following year.

The Scottish inventor James Watt published a paper on the composition of water in 1783; Cavendish had performed the experiments first but published second. Controversy about priority ensued. In 1785 Cavendish carried out an investigation of the composition of common (i.e., atmospheric) air, obtaining, as usual, impressively accurate results. He observed that, when he had determined the amounts of phlogisticated air (nitrogen) and dephlogisticated air (oxygen), there remained a volume of gas amounting to 1/120 of the original volume of common air.

In the 1890s, two British physicists, William Ramsay and Lord Rayleigh, realized that their newly discovered inert gas, argon, was responsible for Cavendish's problematic residue; he had not made an error. What he had done was perform rigorous quantitative experiments, using standardized instruments and methods, aimed at reproducible results; taken the mean of the result of several experiments; and identified and allowed for sources of error.

Cavendish, as noted before, used the language of the old phlogiston theory in chemistry. In 1787 he became one of the earliest outside France to convert to the new antiphlogistic theory of Lavoisier, though he remained skeptical about the nomenclature of the new theory. He also objected to Lavoisier's identification of heat as having a material or elementary basis. Working within the framework of Newtonian mechanism, Cavendish had tackled the problem of the nature of heat in the 1760s, explaining
heat as the result of the motion of matter. In 1783 he published a paper on the temperature at which mercury freezes and in that paper made use of the idea of latent heat, although he did not use the term because he believed that it implied acceptance of a material theory of heat. He made his objections explicit in his 1784 paper on air. He went on to develop a general theory of heat, and the manuscript of that theory has been persuasively dated to the late 1780s. His theory was at once mathematical and mechanical; it contained the principle of the conservation of heat (later understood as an instance of conservation of energy) and even contained the concept (although not the label) of the mechanical equivalent of heat.

Cavendish also worked out a comprehensive theory of electricity. Like his theory of heat, this theory was mathematical in form and was based on precise quantitative experiments. In 1771 he published an early version of his theory, based on an expansive electrical fluid that exerted pressure. He demonstrated that if the intensity of electric force was inversely proportional to distance, then the electric fluid in excess of that needed for electrical neutrality would lie on the outer surface of an electrified sphere; and he confirmed this experimentally. Cavendish continued to work on electricity after this initial paper, but he published no more on the subject.

Beginning in the mid-1780s Cavendish carried out most of his experiments at his house in London. The most famous of those experiments, published in 1798, was to determine the density of the Earth. His apparatus for weighing the world was a modification of the Englishman John Michell's torsion balance. The balance had two small lead balls suspended from the arm of a torsion balance and two much
larger stationary lead balls. Cavendish calculated the attraction between the balls from the period of oscillation of the torsion balance, and then he used this value to calculate the density of the Earth. What was extraordinary about Cavendish's experiment was its elimination of every source of error and every factor that could disturb the experiment and its precision in measuring an astonishingly small attraction, a mere 1/50,000,000 of the weight of the lead balls. The result that Cavendish obtained for the density of the Earth is within 1 percent of the currently accepted figure.

The combination of painstaking care, precise experimentation, thoughtfully modified apparatus, and fundamental theory carries Cavendish's unmistakable signature. It is fitting that the University of Cambridge's great physics laboratory is named the Cavendish Laboratory.

(b. March 13, 1733, Birstall Fieldhead, near Leeds, Yorkshire [now West Yorkshire], Eng.—d. Feb. 6, 1804, Northumberland, Pa., U.S.)

E
nglish clergyman, political theorist, and physical scientist Joseph Priestley contributed to advances in liberal political and religious thought and in experimental chemistry. He is best remembered for his contribution to the chemistry of gases.

In 1765 Priestley met the American scientist and statesman Benjamin Franklin, who encouraged him to publish
The History and Present State of Electricity, with Original Experiments
(1767). In this work, Priestley used history to show that scientific progress depended more on the accumulation of “new facts” that anyone could discover than on the theoretical insights of a few men of genius. This
view shaped Priestley's electrical experiments, in which he anticipated the inverse square law of electrical attraction, discovered that charcoal conducts electricity, and noted the relationship between electricity and chemical change.

In 1767 Priestley began intensive experimental investigations into chemistry. Between 1772 and 1790, he published six volumes of
Experiments and Observations on Different Kinds of Air
and more than a dozen articles in the Royal Society's
Philosophical Transactions
describing his experiments on gases, or “airs,” as they were then called. British pneumatic chemists had previously identified three types of gases: air, carbon dioxide (fixed air), and hydrogen (inflammable air). Priestley incorporated an explanation of the chemistry of these gases into the phlogiston theory, according to which combustible substances released phlogiston (an immaterial “principle of inflammability”) during burning.

Priestley discovered 10 new gases: nitric oxide (nitrous air), nitrogen dioxide (red nitrous vapour), nitrous oxide (inflammable nitrous air, later called “laughing gas”), hydrogen chloride (marine acid air), ammonia (alkaline air), sulfur dioxide (vitriolic acid air), silicon tetrafluoride (fluor acid air), nitrogen (phlogisticated air), oxygen (dephlogisticated air, independently codiscovered by Carl Wilhelm Scheele), and a gas later identified as carbon monoxide. Priestley's experimental success resulted predominantly from his ability to design ingenious apparatuses and his skill in their manipulation. He gained particular renown for an improved pneumatic trough in which, by collecting gases over mercury instead of in water, he was able to isolate and examine gases that were soluble in water. For his work on gases, Priestley was awarded the Royal Society's prestigious Copley Medal in 1773.

Upon contemplating the processes of vegetation and the “agitation” of seas and lakes, Priestley envisioned the means by which a benevolent nature restored the “common air” that had been “vitiated and diminished” by such “noxious” processes as combustion and respiration. Apart from strengthening his own spiritual views, these observations informed the photosynthesis experiments performed by his contemporaries, the Dutch physician Jan Ingenhousz and the Swiss clergyman and naturalist Jean Senebier.

When confronted by the multitude of diseases that plagued the growing populations in towns and military installations, Priestley designed an apparatus that produced carbonated water, a mixture that he thought would provide medicinal benefit to sufferers of scurvy and various fevers. Although it ultimately proved ineffective in treating these disorders, the “gasogene” that employed this technique later made possible the soda-water industry. Priestley also designed the “eudiometer,” which was used in the general movement for sanitary reform and urban design to measure the “purity” (oxygen content) of atmospheric air.

Priestley's lasting reputation in science is founded upon the discovery he made on Aug. 1, 1774, when he obtained a colourless gas by heating red mercuric oxide. Finding that a candle would burn and that a mouse would thrive in this gas, he called it “dephlogisticated air,” based upon the belief that ordinary air became saturated with phlogiston once it could no longer support combustion and life. Priestley was not yet sure, however, that he had discovered a “new species of air.” The following October, while in Paris on a journey through Europe, he informed the
French chemist Antoine-Laurent Lavoisier how he obtained the new “air.” This meeting between the two scientists was highly significant for the future of chemistry. Lavoisier immediately repeated Priestley's experiments and, between 1775 and 1780, conducted intensive investigations from which he derived the elementary nature of oxygen, recognized it as the “active” principle in the atmosphere, interpreted its role in combustion and respiration, and gave it its name. Lavoisier's pronouncements of the activity of oxygen revolutionized chemistry.

In 1800 Priestley published a slim pamphlet,
Doctrine of Phlogiston Established, and That of the Composition of Water Refuted
, which he expanded to book length in 1803. The
Doctrine of Phlogiston
provided a detailed account of what he envisioned to be the empirical, theoretical, and methodological shortcomings of the oxygen theory. Priestley called for a patient, humble, experimental approach to God's infinite creation. Chemistry could support piety and liberty only if it avoided speculative theorizing and encouraged the observation of God's benevolent creation. The phlogiston theory was superseded by Lavoisier's oxidation theory of combustion and respiration.

The English press and government decreed that Priestley's support, together with that of his friend, the moral philosopher Richard Price, of the American and French Revolutions was “seditious.” On July 14, 1791, the “Church-and-King mob” destroyed Priestley's house and laboratory. Priestley and his family retreated to the security of Price's congregation at Hackney, near London. Priestley later began teaching at New College, Oxford, and defended his anti-British government views in
Letters to the Right Honourable Edmund Burke
(1791).

In 1794 Priestley fled to the United States, where he discovered a form of government that was “relatively tolerable.” His best-known writing in the United States,
Letters to the Inhabitants of Northumberland
(1799), became part of the Republican response to the Federalists. Priestley died at Northumberland, Pennsylvania, mourned and revered by Thomas Jefferson, the third president of the United States.

(b. Sept. 9, 1737, Bologna, Papal States [Italy]—d. Dec. 4, 1798, Bologna, Cisalpine Republic)

L
uigi Galvani was an Italian physician and physicist who investigated the nature and effects of what he conceived to be electricity in animal tissue. His discoveries led to the invention of the voltaic pile, a kind of battery that makes possible a constant source of current electricity.