The 100 Most Influential Scientists of All Time (18 page)

Ampère also offered a physical understanding of the electromagnetic relationship, theorizing the existence of an “electrodynamic molecule” (the forerunner of the idea of the electron) that served as the constituent element of electricity and magnetism. Using this physical understanding of electromagnetic motion, Ampère developed a physical account of electromagnetic phenomena that was both empirically demonstrable and mathematically predictive. In 1827 Ampère published his magnum opus,
Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l'experience
(
Memoir on the Mathematical Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience
), the work that coined the name of his new science, electrodynamics, and became known ever
after as its founding treatise. In recognition of his contribution to the making of modern electrical science, an international convention signed in 1881 established the ampere as a standard unit of electrical measurement, along with the coulomb, volt, ohm, and watt, which are named, respectively, after Ampère's contemporaries Coulomb, Alessandro Volta of Italy, Georg Ohm of Germany, and James Watt of Scotland.

The 1827 publication of Ampère's synoptic
Mémoire
brought to a close his feverish work over the previous seven years on the new science of electrodynamics. The text also marked the end of his original scientific work. His health began to fail, and he died while performing a university inspection, decades before his new science was canonized as the foundation stone for the modern science of electromagnetism.

(b. Aug. 9, 1776, Turin, in the Kingdom of Sardinia and Piedmont—d. July 9, 1856, Turin, Italy)

I
talian mathematical physicist Amedeo Avogadro showed in what became known as Avogadro's law that, under controlled conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules.

The son of Filippo Avogadro, conte di Quaregna e Cerreto, a distinguished lawyer and senator in the Piedmont region of northern Italy, Avogadro graduated in jurisprudence in 1792 but did not practice law until after receiving his doctorate in ecclesiastical law four years later. In 1801 he became secretary to the prefecture of Eridano.

Beginning in 1800 Avogadro privately pursued studies in mathematics and physics, and he focused his early research on electricity. In 1804 he became a corresponding member of the Academy of Sciences of Turin, and in 1806 he was appointed to the position of demonstrator at the academy's college. Three years later he became professor of natural philosophy at the Royal College of Vercelli, a post he held until 1820 when he accepted the first chair of mathematical physics at the University of Turin. Due to civil disturbances in the Piedmont, the university was closed and Avogadro lost his chair in July 1822. The chair was reestablished in 1832 and offered to the French mathematical physicist Augustin-Louis Cauchy. A year later Cauchy left for Prague, and on Nov. 28, 1834, Avogadro was reappointed.

Avogadro is chiefly remembered for his molecular hypothesis, first stated in 1811, in which he claimed that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. He used this hypothesis further to explain the French chemist Joseph-Louis Gay-Lussac's law of combining volumes of gases (1808) by assuming that the fundamental units of elementary gases may actually divide during chemical reactions. It also allowed for the calculation of the molecular weights of gases relative to some chosen standard. Avogadro and his contemporaries typically used the density of hydrogen gas as the standard for comparison. Thus, the following relationship was shown to exist:

To distinguish between atoms and molecules of different kinds, Avogadro adopted terms including
molécule intégrante
(the molecule of a compound),
molécule constituante
(the molecule of an element), and
molécule élémentaire
(atom). Although his gaseous elementary molecules were predominantly diatomic, he also recognized the existence of monatomic, triatomic, and tetratomic elementary molecules.

In 1811 he provided the correct molecular formula for water, nitric and nitrous oxides, ammonia, carbon monoxide, and hydrogen chloride. Three years later he described the formulas for carbon dioxide, carbon disulfide, sulfur dioxide, and hydrogen sulfide. He also applied his hypothesis to metals and assigned atomic weights to 17 metallic elements based upon analyses of particular compounds that they formed. However, his references to
gaz métalliques
may have actually delayed chemists' acceptance of his ideas. In 1821 he offered the correct formula for alcohol (C
₂
H
₆
O) and for ether (C
₄
H
₁₀
O).

Priority over who actually introduced the molecular hypothesis of gases was disputed throughout much of the 19th century. Avogadro's claim rested primarily upon his repeated statements and applications. Others attributed this hypothesis to the French natural philosopher André-Marie Ampère, who published a similar idea in 1814. Many factors account for the fact that Avogadro's hypothesis was generally ignored until after his death. First, the distinction between atoms and molecules was not generally understood. Furthermore, as similar atoms were thought to repel one another, the existence of polyatomic elementary molecules seemed unlikely.

Avogadro also mathematically represented his findings in ways more familiar to physicists than to chemists. Consider, for example, his proposed relationship between
the specific heat of a compound gas and its chemical constituents:

(Here
c, c
₁
,
c
₂
, etc., represent the specific heats at constant volume of the compound gas and its constituents;
p
₁
,
p
₂
, etc., represent the numbers of molecules of each component in the reaction). Based upon experimental evidence, Avogadro determined that the specific heat of a gas at constant volume was proportional to the square root of its attractive power for heat. In 1824 he calculated the “true affinity for heat” of a gas by dividing the square of its specific heat by its density. The results ranged from 0.8595 for oxygen to 10.2672 for hydrogen, and the numerical order of the affinities coincided with the electrochemical series, which listed the elements in the order of their chemical reactivities. Mathematically dividing an element's affinity for heat by that of his selected standard, oxygen, resulted in what he termed the element's “affinity number.” Between 1843 and his retirement in 1850, Avogadro wrote four memoirs on atomic volumes and designated affinity numbers for the elements using atomic volumes according to a method “independent of all chemical considerations”—a claim that held little appeal for chemists.

Avogadro's minimal contact with prominent scientists and his habit of citing his own results increased his isolation. Although he argued in 1845 that his molecular hypothesis for determining atomic weights was widely accepted, considerable confusion still existed over the concept of atomic weights at that time. Avogadro's hypothesis began to gain
broad appeal among chemists only after his compatriot and fellow scientist Stanislao Cannizzaro demonstrated its value in 1858, two years after Avogadro's death. Many of Avogadro's pioneering ideas and methods anticipated later developments in physical chemistry. His hypothesis is now regarded as a law, and the value known as Avogadro's number (6.02214179 × 10
²³
), the number of molecules in a gram molecule, or mole, of any substance, has become a fundamental constant of physical science.

(b. Dec. 6, 1778, Saint-Léonard-de-Noblat, France—d. May 9, 1850, Paris)

F
rench chemist and physicist Joseph-Louis Gay-Lussac pioneered investigations into the behaviour of gases, established new techniques for analysis, and made notable advances in applied chemistry.

In 1801 Gay-Lussac became involved in experiments on capillarity in order to study short-range forces. Gay-Lussac's first publication (1802), however, was on the thermal expansion of gases. To ensure more accurate experimental results, he used dry gases and pure mercury. He concluded from his experiments that all gases expand equally over the temperature range 0–100 °C (32–212 °F). This law, usually (and mistakenly) attributed to French physicist J.-A.-C. Charles as “Charles's law,” was the first of several regularities in the behaviour of matter that Gay-Lussac established.

Of the laws Gay-Lussac discovered, he remains best known for his law of the combining volumes of gases (1808). He had previously (1805) established that hydrogen and oxygen combine by volume in the ratio 2:1 to form
water. Later experiments with boron trifluoride and ammonia produced spectacularly dense fumes and led him to investigate similar reactions, such as that between hydrogen chloride and ammonia, which combine in equal volumes to form ammonium chloride. Further study enabled him to generalize about the behaviour of all gases. Gay-Lussac's approach to the study of matter was consistently volumetric rather than gravimetric, in contrast to that of his English contemporary John Dalton.

Another example of Gay-Lussac's fondness for volumetric ratios appeared in an 1810 investigation into the composition of vegetable substances performed with his friend Louis-Jacques Thenard. Together they identified a class of substances (later called carbohydrates) including sugar and starch that contained hydrogen and oxygen in the ratio of 2:1. They announced their results in the form of three laws, according to the proportion of hydrogen and oxygen contained in the substances.

As a young man, Gay-Lussac participated in dangerous exploits for scientific purposes. In 1804 he ascended in a hydrogen balloon with Jean-Baptiste Biot in order to investigate the Earth's magnetic field at high altitudes and to study the composition of the atmosphere. They reached an altitude of 4,000 metres (about 13,000 feet). In a following solo flight, Gay-Lussac reached 7,016 metres (more than 23,000 feet), thereby setting a record for the highest balloon flight that remained unbroken for a half-century. In 1805–06, amid the Napoleonic wars, Gay-Lussac embarked upon a European tour with the Prussian explorer Alexander von Humboldt.

In 1807 Gay-Lussac published an important study of the heating and cooling produced by the compression and
expansion of gases. This was later to have significance for the law of conservation of energy.

When Gay-Lussac and his colleague Louis-Jacques Thenard heard of the English chemist Humphry Davy's isolation of the newly discovered reactive metals sodium and potassium by electrolysis in 1807, they worked to produce even larger quantities of the metals by chemical means and tested their reactivity in various experiments. Notably they isolated the new element boron. They also studied the effect of light on reactions between hydrogen and chlorine, though it was Davy who demonstrated that the latter gas was an element.

Rivalry between Gay-Lussac and Davy reached a climax over the iodine experiments Davy carried out during an extraordinary visit to Paris in November 1813, at a time when France was at war with Britain. Both chemists claimed priority over discovering iodine's elemental nature. Although Davy is typically given credit for this discovery, most of his work was hurried and incomplete. Gay-Lussac presented a much more complete study of iodine in a long memoir presented to the National Institute on Aug. 1, 1814, and subsequently published in the
Annales de chimie
. In 1815 Gay-Lussac experimentally demonstrated that prussic acid was simply hydrocyanic acid, a compound of carbon, hydrogen, and nitrogen, and he also isolated the compound cyanogen [(CN)
₂
or C
₂
N
₂
]. His analyses of prussic acid and hydriodic acid (HI) necessitated a modification of Antoine-Laurent Lavoisier's theory that oxygen was present in all acids.