Three Scientific Revolutions: How They Transformed Our Conceptions of Reality (16 page)

Then the Swedish chemist Carl Wilhelm Scheele, in a book translated as
Chemical Treatise on Air and Fire
published in 1777, reported his discovery that air consisted of two components: one highly flammable that he called “Fire air” and the other inflammable designated “Foul air,” the first later renamed “oxygen” and the second “hydrogen.” Though he detected a flammable substance in the air he did not investigate it. Like Scheele, Joseph Priestley in different experiments noticed that when substances are burned in air and the residue and the air are carefully weighed the residue usually
gained
weight while the volume of air
decreased
, contrary to the phlogiston theory that claimed the burning material
gives off
phlogiston and thus should weigh
less
, while the air
gaining
the phlogiston should weigh
more
.

Priestley's description of the transformation of calces (the residue of a burnt mineral) into metals is just one of many examples.

For seeing the metal to be actually revived, and that in considerable quantity, at the same time that the air was diminished, I could not doubt, but that the calx was actually imbibing something from the air; and from its affects in making the calx into metal, it could be no other than that to which chemists had unanimously given the name of
phlogiston
.
⁴⁹

Thus Priestley is credited with discovering that it was “something from the air,” a “new air,” that caused the combustion, but as his final word “
phlogiston
” indicates, he was so committed to the phlogiston theory that he “concluded that the new gas must contain little or no phlogiston, and hence he called it
dephlogisticated air
” (pp. 126–27), which meant air that is free from phlogiston or the element of inflammability.

And so the honor of explaining the significance of the discovery is attributed to Lavoisier. After many failures to explain the process of combustion, it was at a dinner meeting in Paris with Priestley in 1774 that the solution occurred to him. Priestley “told Lavoisier at dinner of his discovery of dephlogisticated air, saying he ‘had gotten it from
precip
[of
mercurius calcinatus
]
per se
and also
red lead
'; whereupon, he says, ‘all the company . . . expressed great surprise'” (pp. 126–27). What caused the surprise was that the so-called dephlogisticated air produced by heating mercury oxide had properties the opposite of carbon dioxide produced by heating charcoal: it supported burning and respiration and did not combine with lime and alkalis. Repeating the experiment Lavoisier obtained a gas purer than ordinary air which convinced him that while measuring the components of chemical reactions is crucial to chemistry, so is choosing the right experiments.

He read two papers describing his experiments on the oxide of mercury titled “On the Nature of the Principle which Combines with Metals during Calcination and Increases their Weight” before the French Academy of Sciences, the first on Easter 1775 and the second on August 8, 1778. Having initially decided that the gas produced in Priestley's experiment though purer than common air was still a form of common air, when he learned of Priestley's later experiment showing that when reacting with nitrous oxide it was more soluble in water than common air, he concluded that while it was a constituent of common air it was not identical to it! He thus considered it a gas that was
absorbed
in the conversion of metals to calces or oxides when burnt in air and
emitted
when the oxides themselves were heated. He named the new gas “oxygene.” According to chemist J. R. Partington:

In 1782 Lavoisier says Condorcet had proposed the name “vital air” for pure air, but in a memoir received in 1777 . . . and published in 1781, entitled “General considerations on the nature of acids and on the principles composing them”, Lavoisier called the base of pure air the “acidifying principle” or “oxigine principle” (
principe oxigine),
which he latter changed to “oxygene”[. . .] . (pp. 131–32)

The publication of Lavoisier's
Traité de Chimie
(Treatise on Chemistry) in 1789 established the superiority of the explanation involving oxygen over that of phlogiston. This not only overthrew the phlogiston theory, it brought about a revolution in chemistry. It no longer was assumed that common substances such as air and water were irreducible, but indicated they were compounded of more basic elements that opened up a whole new world of research. Although Priestley never gave up the phlogiston theory himself, in his last book he graciously acknowledged Lavoisier's contribution.

There have been few, if any, revolutions in science so great, so sudden, and so general, as the prevalence of what is now usually termed the
new system of chemistry
, or that of the
Anti
phlogistons. . . . Though there had been some who occasionally expressed doubts of the existence of such a principle as that of
phlogiston
, nothing had been advanced that could have laid the foundation of
another system
before the labors of Mr. Lavoisier and his friends. . . .
⁵⁰
(italics in original)

Yet as consequential as the chemical revolution has been, there was an impending revolution even more effective in transforming the conception of physical reality, namely, the reconstruction of the atomic theory. The success in chemistry of the experimental identification of oxygen and explanation of its function in combustion convinced scientists of the possibility of discovering the inner corpuscular elements of all ordinary substances, such as air, water, acids, metals, etc., along with the properties that could explain their combinations and reactions.

Thus while the intellectual and technological levels at the time of Leucippus and Democritus were insufficient to promote advances in the atomic theory, that was no longer true. Newton's belief stated earlier “that God in the Beginning form'd Matter in solid, massy, hard, impenetrable, movable Particles, of such Sizes and Figures . . . as most conduced to the End for which he form'd them,” though a misrepresentation of their origin and properties, had finally been vindicated.

In 1787 Lavoisier, Claude Louis Berthollet, Guyton de Morveau, and Antoine François de Fourcroy published a book entitled
Méthode de Nomenclature Chimique
(The Method of Chemical Nomenclature) that presented the first modern list of elements based on recent experimental discoveries. Then in 1799 Joseph Louis Proust introduced “the law of constant proportions” stating that any sample of a compound or molecular substance, such as salt, always contains its constituents, sodium and chlorine, in fixed ratios by weight reinforcing the belief in the constancy of the reagents and the regularity of the reactions. Yet despite the considerable experimental evidence that substances were composed of more basic elements in ratios determined by their weights, there still was no
scientific
explanation as to why or how. And since Newton's belief that they were caused by God was no longer adequate the search began for an explanation, another indication of the transformation in the conception of the external world and how to investigate it and how to understand it.

The person credited with initiating the explanation, John Dalton, like many of his predecessors, was a most unlikely candidate. Born to a Quaker family in the tiny, rustic Village of Eaglesfield, England (1766–1844), his father was a cottage weaver while his mother supplemented their income by selling writing materials. Unable to enter a private school, he attended the village schools in the neighborhood but had acquired a sufficient background that he was able to teach in the village school from the early age of twelve to fourteen. While teaching there he was fortunate to meet a wealthy Quaker named Elihu Robinson who, along with being educated in natural philosophy, especially meteorology, corresponded with Benjamin Franklin.

Having noticed Dalton's mathematical aptitude when he won a dispute in mathematics, Robinson began tutoring him in mathematics with Dalton always appreciating the kindnesses, good advice, and intellectual awakening that Robinson and his cultured wife had contributed to his early development. Then, when he was fifteen, he moved to Kendal to become assistant in a boarding school rising to the position of principal. During his spare time he studied Latin, Greek, French, mathematics, and natural philosophy.

During the twelve years he lived in Kendal he made the acquaintance of a more unusual benefactor, another Quaker by the name of John Gough who, despite being blind and suffering from epilepsy, owing to his wealthy, well-educated, and intellectual family was able to acquire a sound knowledge of the classics, physics, mathematics, botany, and zoology. Though nine years older than Dalton and considerably more advanced in his studies, when he learned of their common interests and Dalton's intellectual aptitudes, Gough became his close friend and academic mentor. His family having an excellent library and an extensive collection of scientific instruments, he shared these with Dalton who, in gratitude, served as his reader and amanuensis. As a result, Dalton became well schooled in “mechanics, [Newton's] fluxions, algebra, geometry, chemistry (including some French chemical writings), astronomy and meteorology . . .”
⁵¹
(brackets added).

Because of this close intellectual relationship, when Dr. Barnes of New College in Manchester wrote to Gough in 1793 (who had become a widely respected mathematician) seeking his suggestions in filling a position of professor of mathematics and natural philosophy at New College, Gough unselfishly recommended Dalton for the position, even though it would mean severing their very close association. When the position of tutor at New College was offered to him, Dalton readily accepted, partially because of his dissatisfaction with his teaching at Kendal and also because he foresaw a more promising future in Manchester, which was confirmed when he later described his life there as “very happy and fulfilling.”

Moving to Manchester he was immediately welcomed by the eminent “Mancunians,” as the patricians of Manchester were called, and elected to the prestigious Manchester Literary and Philosophical Society the following year. As author Elizabeth C. Patterson states:

The association which Dalton began with the Manchester Literary and Philosophical Society in 1794 was to continue until his death in 1844. During this half century the Society would play a central role in his life and he in its. Before it he read one hundred and seventeen papers, of which fifty-two were printed. For forty-four years he served as an officer—first as Secretary, then as Vice-President, and as President. To think of either—the Society or the man—is to think of the other. (pp. 59–60)

It was hearing the lectures and witnessing the experiments of English physician Dr. Thomas Garnett at New College that aroused his interest in molecular chemistry.

His early investigations and publications were centered on meteorology, including the nature of water vapor and the composition of the air and whether its components consisted of a mixture, a chemical compound, or some other structure. Then in a series of four essays he presented his research conclusions regarding gases, meteorology, and chemistry, in the last essay declaring that he had independently discovered Jacques Charles's gas law that all gases at constant temperature will, with the same increase in temperature, expand equally. In tribute, Patterson declares that the “wealth of material in these four essays is extraordinary. Even today they are hailed as ‘epoch-making' and as ‘laying the foundations for modern physical meteorology'” (p. 94).

It apparently was these initial experiments of the solubility of gases in water (similar to those of Robert Boyle) that led to his crucial insight that each element was composed of characteristic atoms that would be possible to distinguish by their atomic weights. The first explicit statement of this is in a paper he read to the Literary and Philosophical Society on October 21, 1803, en­titled “On the Absorption of Gases by Water and Other Liquids.”

The greatest difficulty attending the mechanical hypothesis arises from different gases observing different laws. Why does water not admit its bulk of every kind of gas alike [i.e., why are they not equally soluble in water]? This question I have duly considered, and though I am not yet able to satisfy myself completely, I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases. . . . An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new; I have lately been prosecuting this enquiry with remarkable success.
⁵²
(brackets in the original)

While other chemists were only investigating the
relative
weights in which the components of substances
combine
, such as the densities or weights of hydrogen and oxygen composing water, Dalton was the first to attempt to determine the relative weights of the
components themselves.
Appended to the paper was a “Table of the relative weights of the ultimate particles of the gaseous and other bodies,” while the paper itself presented the basic features of his atomic theory at the time.

After giving a series of lectures in Edinburgh and Glasgow that were highly praised—which must have been very gratifying considering his humble origins—he began writing his great work, a
New System of Chemical Philosophy
, published in 1808. The chapter “On Chemical Synthesis” was particularly significant because it explicitly states his original thesis that every sample of a basic substance such as water contains “
ultimate particles
” that always “
are perfectly alike in weight, figure, etc.
” When one considers the various possibilities as to how the ratios of the constituent particles could be construed, one begins to appreciate the complexity of the problem he faced. The key, he believed, lay in determining the individual atomic weights of the elements composing the substances. However, one can only claim that water is H
₂
O rather than HO
₂
or H
₃
O
₄
if one knows not only their atomic weights, but also in what numerical proportion they combine. As he summarized the challenge: