The 100 Most Influential Scientists of All Time (15 page)

While other chemists were also looking for conservation principles capable of explaining chemical reactions, Lavoisier was particularly intent on collecting and weighing all the substances involved in the reactions he studied. His success in the many elaborate experiments he conducted was in large part due to his independent wealth, which enabled him to have expensive apparatus built to his design, and to his ability to recruit and direct talented research associates. Today the conservation of mass is still sometimes taught as “Lavoisier's law,” which is indicative of his success in making this principle a foundation of modern chemistry.

After being elected a junior member of the Academy of Sciences, Lavoisier began searching for a field of research
in which he could distinguish himself. Chemists had long recognized that burning, like breathing, required air, and they also knew that iron rusts only upon exposure to air. Noting that burning gives off light and heat, that warmblooded animals breathe, and that ores are turned into metals in a furnace, they concluded that fire was the key causal element behind these chemical reactions. The Enlightenment German chemist Georg Ernst Stahl provided a well-regarded explanation of these phenomena. Stahl hypothesized that a common “fiery substance” he called phlogiston was released during combustion, respiration, and calcination, and that it was absorbed when these processes were reversed. Although plausible, this theory raised a number of problems for those who wished to explain chemical reactions in terms of substances that could be isolated and measured.

In the early stages of his research Lavoisier regarded the phlogiston theory as a useful hypothesis, but he sought ways either to solidify its firm experimental foundation or to replace it with an experimentally sound theory of combustion. In the end his theory of oxygenation replaced the phlogiston hypothesis, but it took Lavoisier many years and considerable help from others to reach this goal.

The oxygen theory of combustion resulted from a demanding and sustained campaign to construct an experimentally grounded chemical theory of combustion, respiration, and calcination. Lavoisier's research in the early 1770s focused upon weight gains and losses in calcination. It was known that when metals slowly changed into powders (calxes), as was observed in the rusting of iron, the calx actually weighed more than the original metal, whereas when the calx was “reduced” to a metal, a loss of weight occurred.
The phlogiston theory did not account for these weight changes, for fire itself could not be isolated and weighed. Lavoisier hypothesized that it was probably the fixation and release of air, rather than fire, that caused the observed gains and losses in weight. This idea set the course of his research for the next decade.

Along the way, he encountered related phenomena that had to be explained. Mineral acids, for instance, were made by roasting a mineral such as sulfur in fire and then mixing the resultant calx with water. Lavoisier had initially conjectured that the sulfur combined with air in the fire and that the air was the cause of acidity. However, it was not at all obvious just what kind of air made sulfur acidic. The problem was further complicated by the concurrent discovery of new kinds of airs within the atmosphere. British pneumatic chemists made most of these discoveries, with Joseph Priestley leading the effort.

And it was Priestley, despite his unrelenting adherence to the phlogiston theory, who ultimately helped Lavoisier unravel the mystery of oxygen. Priestley isolated oxygen in August 1774 after recognizing several properties that distinguished it from atmospheric air. In Paris at the same time, Lavoisier and his colleagues were experimenting with a set of reactions identical to those that Priestley was studying, but they failed to notice the novel properties of the air they collected. Priestley visited Paris later that year and at a dinner held in his honour at the Academy of Sciences informed his French colleagues about the properties of this new air. Lavoisier, who was familiar with Priestley's research and held him in high regard, hurried back to his laboratory, repeated the experiment, and found that it produced precisely the kind of air he needed to complete his theory. He called the gas that was produced oxygen, the generator of acids. Isolating oxygen allowed him to explain both the quantitative and
qualitative changes that occurred in combustion, respiration, and calcination.

In the canonical history of chemistry Lavoisier is celebrated as the leader of the 18th-century chemical revolution and consequently one of the founders of modern chemistry. Lavoisier was fortunate in having made his contributions to the chemical revolution before the disruptions of political revolution. By 1785 his new theory of combustion was gaining support, and the campaign to reconstruct chemistry according to its precepts began. One tactic to enhance the wide acceptance of his new theory was to propose a related method of naming chemical substances.

In 1787 Lavoisier and three prominent colleagues published a new nomenclature of chemistry, and it was soon widely accepted, thanks largely to Lavoisier's eminence and the cultural authority of Paris and the Academy of Sciences. Its fundamentals remain the method of chemical nomenclature in use today. Two years later Lavoisier published a programmatic
Traité élémentaire de chimie (Elementary Treatise on Chemistry
) that described the precise methods chemists should employ when investigating, organizing, and explaining their subjects. It was a worthy culmination of a determined and largely successful program to reinvent chemistry as a modern science.

(b. March 23, 1749, Beaumount-en-Auge, Normandy, France—d. March 5, 1827, Paris)

P
ierre-Simon, marquis de Laplace was a French mathematician, astronomer, and physicist and is best known for his investigations into the stability of the solar
system. Laplace successfully accounted for all the observed deviations of the planets from their theoretical orbits by applying Sir Isaac Newton's theory of gravitation to the solar system, and he developed a conceptual view of evolutionary change in the structure of the solar system. He also demonstrated the usefulness of probability for interpreting scientific data.

Laplace was the son of a peasant farmer. Little is known of his early life except that he quickly showed his mathematical ability at the military academy at Beaumont. In 1766 Laplace entered the University of Caen, but he left for Paris the next year, apparently without taking a degree. He arrived with a letter of recommendation to the mathematician Jean d'Alembert, who helped him secure a professorship at the École Militaire, where he taught from 1769 to 1776.

In 1773 he began his major lifework—applying Newtonian gravitation to the entire solar system—by taking up a particularly troublesome problem: why Jupiter's orbit appeared to be continuously shrinking while Saturn's continually expanded. The mutual gravitational interactions within the solar system were so complex that mathematical solution seemed impossible; indeed, Newton had concluded that divine intervention was periodically required to preserve the system in equilibrium. Laplace announced the invariability of planetary mean motions (average angular velocity). This discovery in 1773, the first and most important step in establishing the stability of the solar system, was the most important advance in physical astronomy since Newton. It won him associate membership in the French Academy of Sciences the same year.

Applying quantitative methods to a comparison of living and nonliving systems, Laplace and the chemist Antoine-Laurent Lavoisier in 1780, with the aid of an ice calorimeter that they had invented, showed respiration to be a form of combustion. Returning to his astronomical
investigations with an examination of the entire subject of planetary perturbations—mutual gravitational effects—Laplace in 1786 proved that the eccentricities and inclinations of planetary orbits to each other will always remain small, constant, and self-correcting. The effects of perturbations were therefore conservative and periodic, not cumulative and disruptive.

During 1784–85 Laplace worked on the subject of attraction between spheroids; in this work the potential function of later physics can be recognized for the first time. Laplace explored the problem of the attraction of any spheroid upon a particle situated outside or upon its surface. Through his discovery that the attractive force of a mass upon a particle, regardless of direction, can be obtained directly by differentiating a single function, Laplace laid the mathematical foundation for the scientific study of heat, magnetism, and electricity.

Laplace removed the last apparent anomaly from the theoretical description of the solar system in 1787 with the announcement that lunar acceleration depends on the eccentricity of the Earth's orbit. Although the mean motion of the Moon around the Earth depends mainly on the gravitational attraction between them, it is slightly diminished by the pull of the Sun on the Moon. This solar action depends, however, on changes in the eccentricity of the Earth's orbit resulting from perturbations by the other planets. As a result, the Moon's mean motion is accelerated as long as the Earth's orbit tends to become more circular; but, when the reverse occurs, this motion is retarded. The inequality is therefore not truly cumulative, Laplace concluded, but is of a period running into millions of years. The last threat of instability thus disappeared from the theoretical description of the solar system.

In 1796 Laplace published
Exposition du système du monde (The System of the World
), a semipopular treatment of his
work in celestial mechanics and a model of French prose. The book included his “nebular hypothesis”—attributing the origin of the solar system to cooling and contracting of a gaseous nebula—which strongly influenced future thought on planetary origin. His
Traité de mécanique céleste (Celestial Mechanics
), appearing in five volumes between 1798 and 1827, summarized the results obtained by his mathematical development and application of the law of gravitation. He offered a complete mechanical interpretation of the solar system by devising methods for calculating the motions of the planets and their satellites and their perturbations, including the resolution of tidal problems. The book made him a celebrity.

In 1814 Laplace published a popular work for the general reader,
Essai philosophique sur les probabilités (A Philosophical Essay on Probability
). This work was the introduction to the second edition of his comprehensive and important
Théorie analytique des probabilités (Analytic Theory of Probability
), first published in 1812, in which he described many of the tools he invented for mathematically predicting the probabilities that particular events will occur in nature. He applied his theory not only to the ordinary problems of chance but also to the inquiry into the causes of phenomena, vital statistics, and future events, while emphasizing its importance for physics and astronomy. The book is notable also for including a special case of what became known as the central limit theorem. Laplace proved that the distribution of errors in large data samples from astronomical observations can be approximated by a Gaussian or normal distribution.

Probably because he did not hold strong political views and was not a member of the aristocracy, he escaped imprisonment and execution during the French Revolution. Laplace was president of the Board of Longitude, aided in the organization of the metric system, helped found the
scientific Society of Arcueil, and was created a marquis. He served for six weeks as minister of the interior under Napoleon, who famously reminisced that Laplace “carried the spirit of the infinitesimal into administration.”

(b. May 17, 1749, Berkeley, Gloucestershire, Eng.—d. Jan. 26, 1823, Berkeley)

E
nglish surgeon Edward Jenner is best known as the discoverer of vaccination for smallpox. Jenner lived at a time when the patterns of British medical practice and education were undergoing gradual change. During this time, the division between the trained physicians and the apothecaries or surgeons—who acquired their medical knowledge through apprenticeship rather than through academic work—was becoming less sharp, and hospital work was becoming much more important.

Jenner attended grammar school and at the age of 13 was apprenticed to a nearby surgeon. In the following eight years Jenner acquired a sound knowledge of medical and surgical practice. On completing his apprenticeship at the age of 21, he went to London and became the house pupil of John Hunter, who was on the staff of St. George's Hospital and was one of the most prominent surgeons in London. Even more important, however, he was an anatomist, biologist, and experimentalist of the first rank; not only did he collect biological specimens, but he also concerned himself with problems of physiology and function.

The firm friendship that grew between the two men lasted until Hunter's death in 1793. From no one else could Jenner have received the stimuli that so confirmed his natural bent—a catholic interest in biological phenomena, disciplined powers of observation, sharpening of critical faculties, and a reliance on experimental
investigation. From Hunter, Jenner received the characteristic advice, “Why think [i.e., speculate]—why not try the experiment?”

In addition to his training and experience in biology, Jenner made progress in clinical surgery. After studying in London from 1770 to 1773, he returned to country practice in Berkeley and enjoyed substantial success. He was capable, skillful, and popular. In addition to practicing medicine, he joined two medical groups for the promotion of medical knowledge and wrote occasional medical papers. He played the violin in a musical club, wrote light verse, and, as a naturalist, made many observations, particularly on the nesting habits of the cuckoo and on bird migration. He also collected specimens for Hunter; many of Hunter's letters to Jenner have been preserved, but Jenner's letters to Hunter have unfortunately been lost. After one disappointment in love in 1778, Jenner married in 1788.