According to the phlogiston theory, propounded in the 17th century, every combustible substance consisted of a hypothetical principle of fire known as phlogiston,
which was liberated through burning, and a residue. The word phlogiston was first used early in the 18th century by the German chemist Georg Ernst Stahl. Stahl
declared that the rusting of iron was also a form of burning in which phlogiston was freed and the metal reduced to an ash or calx. The theory was superseded
between 1770 and 1790 when the French chemist Antoine Lavoisier showed that burning and rusting both involved oxygen and concluded that both ash and rust
were compounds of oxygen. Lavoisier’s oxidization theory has been accepted by scientists from about 1800 to the present day.
The theory of phlogiston was predominantly German in origin, with much early work done in Mainz, though it was widely believed through much of the eighteenth
century — two of the most prominent followers of the theory, Johann Joachim Becher and Georg Ernst Stahl (who first used the name phlogiston in 1700), were
Swedish. Phlogiston was not only widespread but deep-seated, and gave way to the atomic theory only slowly.
Phlogiston theorists identified three essences which comprise all matter: sulfur or terra pinguis, the essence of inflammability; mercury or terra mercurialis, the
essence of fluidity; and salt or terra lapida, the essence of fixity and inertness. In this respect phlogiston theory is similar to the ancient alchemical notions of earth,
air, fire, and water. The terra pinguis was renamed phlogiston. In this view, metals were made of a “calx” (or residue) combined with phlogiston, the fiery principle,
which was liberated during combustion, leaving only the calx. Air, according to the theory, was merely the receptacle for phlogiston; all combustible or calcinable
substances, in fact, were not elements but compounds containing phlogiston. Rusting iron, for instance, was believed to be losing its phlogiston and thereby returning
to its elemental state.
Phlogiston theory was widely supported throughout the eighteenth century, although it came under increasing attack as empirical research pointed up its difficulties.
When it was determined that some metals actually gained mass when burnt, partisans explained it by giving phlogiston a negative mass. Even Priestley believed in the
theory until his death, convinced that his discovery of oxygen was “dephlogisticated air.” It was up to Lavoisier to realize the significance of his discovery.
Lavoisier made a symbolic break with phlogiston theory by burning all textbooks that supported the theory, just as Paracelsus had destroyed his copies of the works
of the medieval medical authorities. His theory of oxidation soon replaced phlogiston theory, and remains a part of modern chemistry.
Although he exaggerated its importance, Lavoisier was the first to understand the significance of
Priestley’s work on oxygen, and is considered by some to have discovered the element. He
disproved phlogiston theory by demonstrating that oxygen is required for combustion, rusting, and
respiration. He combined his chemical abilities with an interest in zoology to produce pioneering
work on anatomy and physiology.
phlogiston theory , hypothesis regarding combustion. The theory,
advanced by J. J. Becher late in the 17th cent. and extended and
popularized by G. E. Stahl, postulates that in all flammable materials there is
present phlogiston, a substance without color, odor, taste, or weight that is
given off in burning. emailprotected substances are those that contain
phlogiston and, on being burned, are emailprotected The ash of the
burned material is held to be the true material. The theory received strong
and wide support throughout a large part of the 18th cent. until it was
refuted by the work of A. L. Lavoisier, who revealed the true nature of
combustion. Joseph Priestley, however, defended the theory throughout his
lifetime. Henry Cavendish remained doubtful, but most other chemists of the
period, including C. L. Berthollet, rejected it.
The failure to understand combustion was an insurmountable obstacle to real progress in
chemistry. Any theory of chemical change must be able to explain combustion and phlogiston
was the first real attempt to do so.
The fact that wood turns to ashes and metals become soft powders when heated and can be
changed back into metals in the presence of charcoal is hard to reconcile without imagining the
addition or subtraction of some substance.
Phlogiston was that substance.
The theory was simple, and although having serious contradictions, was better than no theory at
all. Besides, despite the quantitative work of Galileo and Newton, the importance of quantitative
measurements had not yet been impressed upon the chemists.
The phlogiston theory was really quite simple.
Metals and combustible substances contained an imponderable substance known as phlogiston
which was released into the air along with caloric. Air had a limited capacity to absorb
Since phlogiston was an imponderable substance, its properties were incapable of being
detected by senses and could be contradictory.
Sometimes it had weight, sometimes it had negative weight, and sometimes it had no weight at
Phlogiston theory explained many facts about combustion
1.combustibles lose weight when burning because they lose phlogiston
2.a flame goes out in an enclosed space because air becomes saturated with phlogiston
3.charcoal leaves little residue upon burning because it is nearly pure phlogiston
4.a mouse dies in an airtight space because the air becomes saturated with phlogiston
5.some metal calxes turn to metals when heated with charcoal because phlogiston from
charcoal is restored to the calx
A serious problem was that the calx formed when a metal such as magnesium burns weighs more
than the metal from which it formed, just as a rusty nail weighs more than the nail.
The supporters of the phlogiston theory answered this by postulating that metallic phlogiston has
negative weight while other combustibles contain phlogiston with positive weight.
Adding a special postulate such as this signaled a theory in trouble and led to the ultimate
demise of the theory.
Pronounced As: flojiston , hypothesis regarding
combustion. The theory, advanced by J. J. Becher late
in the 17th cent. and extended and popularized by G. E.
Stahl, postulates that in all flammable materials there
is present phlogiston, a substance without color, odor,
taste, or weight that is given off in burning.
“Phlogisticated substances are those that contain
phlogiston and, on being burned, are
“dephlogisticated. The ash of the burned material is
held to be the true material. The theory received strong
and wide support throughout a large part of the 18th
cent. until it was refuted by the work of A. L. Lavoisier,
who revealed the true nature of combustion. Joseph
Priestley, however, defended the theory throughout his
lifetime. Henry Cavendish remained doubtful, but most
other chemists of the period, including C. L. Berthollet,
Hippocrates of Cos (460-ca. 370 BC)
Greek physician who founded a medical school on Cos. This school produced more than 50
books, as well a system of medical methodology and ethics which is still practiced today. Upon
being granted their M. D. degrees, new doctors still swear a so-called Hippocratic oath. In On
Ancient Medicine, Hippocrates stated that medicine is not philosophy, and therefore must be
practiced on a case-by-case basis rather than from first principles. In The Sacred Disease, he
stated that epilepsy (and disease in general) do not have divine causes. He advocated clinical
observations, diagnosis, and prognosis, and argued that specific diseases come from specific
causes. Hippocrates’s methodology relied on physical examination of the patient and proceeded in
what was, for the most part, a highly rational deductive framework of understanding through
observation. (An exception was the belief that disease was caused by “isonomia”, an imbalance in
the four humors originally suggested by Empedocles and consisting of yellow bile, blood, phlegm,
and black bile.) The Hippocratic corpus of knowledge was widely distributed, highly influential,
and marked the rise of rationality in both medicine and the physical sciences.
Galen of Pergamum (ca. 130-ca. 200)
Greek physician considered second only to Hippocrates of Cos in his importance to the
development of medicine, Galen performed extensive dissections and vivisections on animals.
Although human dissections had fallen into disrepute, he also performed and stressed to his
students the importance of human dissections. He recommended that students practice dissection
as often as possible. He studied the muscles, spinal cord, heart, urinary system, and proved that
the arteries are full of blood. He believed that blood originated in the liver, and sloshed back and
forth through the body, passing through the heart, where it was mixed with air, by pores in the
septum. Galen also introduced the spirit system, consisting of natural spirit or “pneuma” (air he
thought was found in the veins), vital spirit (blood mixed with air he believed to found in the
arteries), and animal spirit (which he believed to be found in the nervous system). In On the
Natural Facilities, Galen minutely described his experimentation on a living dog to investigate the
bladder and flow of urine. It was Galen who first introduced the notion of experimentation to
Galen believed everything in nature has a purpose, and that nature uses a single object for more
than one purpose whenever possible. He maintained that “the best doctor is also a philosopher,” and so advocated that medical students be well-versed in philosophy, logic, physics, and ethics. Galen and his work On the Natural Faculties remained the authority
on medicine until Vesalius in the sixteenth century, even though many of his views about human anatomy were false since he had performed his dissections on pigs, Barbary apes, and dogs. Galen mistakenly maintained, for instance, that humans have a five-lobed liver (which dogs do) and that the heart had only two chambers (it has four).
Erasistratus of Chios (ca. 304-ca. 250 BC)
Greek anatomist who continued the systematic investigation of anatomy begun by Herophilus in Alexandria. He described the cerebrum
and cerebellum, studied nerves (which he believed to be hollow) and the valves of the heart. He distinguished between veins and arteries, believing the latter to be full of air. He proposed mechanical explanations for many bodily processes, such as digestion. He believed in a tripartite system of humors consisting of nervous spirit (carried by nerves), animal spirit (carried by the arteries), and blood
(carried by the veins). After the work of Erasistratus, the use of dissection and study of anatomy declined.
Flemish anatomist who founded the sixteenth century heritage of careful observation characterized
by “refinement of observation.” Vesalius changed the organization of the medical school
classroom, bringing the students close to the operating table. He demonstrated that, in many
instances, Galen and Mondino de’ Luzzi were incorrect (the heart, for instance, has four
chambers). He conducted his own dissections, and worked from the outside in so as not to
damage the cadaver while cutting into it. Vesalius also wrote the first anatomically accurate
medical textbook, De Humani Corporis Fabrica (1543), which was complete with precise
illustrations. Vesalius’s careful observation, emphasis on the active participation of medical
students in dissection lectures, and anatomically accurate textbooks revolutionized the practice of
medicine. Through Vesalius’s efforts, medicine was now on the road to its modern
implementation, although major modifications and leaps of understanding were, of course,
necessary to make its practice actually safe for the patient.
Harvey, William (1578-1657)
English physician who, by observing the action of the heart in small animals and fishes, proved that
heart receives and expels blood during each cycle. Experimentally, he also found valves in the
veins, and correctly identified them as restricting the flow of blood in one direction. He developed
the first complete theory of the circulation of blood, believing that it was pushed throughout the
body by the heart’s contractions. He published his observations and interpretations in Exercitatio
Anatomica de Motu Cordis et Sanguinis in Animalibus (1628), often abbreviated De Motu
Harvey also noted, as earlier anatomists, that fetal circulation short circuits the lungs. He
demonstrated that this is because the lungs were collapsed and inactive. Harvey could not explain,
however, how blood passed from the arterial to the venous system. The discovery of the
connective capillaries would have to await the development of the microscope and the work of Malpighi. He was heavily influenced bythe mechanical philosophy in his investigations of the flow of blood through the body. In fact, he used a mechanical analogy withhydraulics. He could not, however, explain why the heart beats. Furthermore, Harvey used quantitative methods to measure the
capacity of the ventricles.
Harvey was the first doctor to use quantitative and observation methods simultaneously in his medical investigations. In Exercitationesde Generatione Animalium (On the Generation of Animals, 1651), he was extremely skeptical of spontaneous generation and proposed that all animals originally came from an egg. His experiments with chick embryos were the first to suggest the theory of
epigenesis, which views organic development as the production in a cumulative manner of increasingly complex structures from aninitially homogeneous material.
Lavoisier, Antoine (1743-1794)
French chemist who, through a conscious revolution, became the father of modern chemistry. As
a student, he stated “I am young and avid for glory.” He was educated in a radical tradition, a
friend of Condillac and read Maquois’s dictionary. He won a prize on lighting the streets of Paris,
and designed a new method for preparing saltpeter. He also married a young, beautiful
13-year-old girl named Marie-Anne, who translated from English for him and illustrated his
books. Lavoisier demonstrated with careful measurements that transmutation of water to earth
was not possible, but that the sediment observed from boiling water came from the container. He
burnt phosphorus and sulfur in air, and proved that the products weighed more than he original.
Nevertheless, the weight gained was lost from the air. Thus he established the Law of
Conservation of Mass.
Repeating the experiments of Priestley, he demonstrated that air is composed of two parts, one of
which combines with metals to form calxes. However, he tried to take credit for Priestley’s
discovery. This tendency to use the results of others without acknowledgment then draw
conclusions was characteristic of Lavoisier. In Considrations Gnrales sur la Nature des
Acides (1778), he demonstrated that the “air” responsible for combustion was also the source of
acidity. The next year, he named this portion oxygen (Greek for acid-former), and the other azote (Greek for no life). He also
discovered that the inflammable air of Cavendish which he termed hydrogen (Greek for water-former), combined with oxygen to
produce a dew, as Priestley had reported, which appeared to be water.
In Reflexions sur le Phlogistique (1783), Lavoisier showed the phlogiston theory to be inconsistent. In Methods of Chemical
Nomenclature (1787), he invented the system of chemical nomenclature still largely in use today, including names such as sulfuric acid,
sulfates, and sulfites. His Trait lmentaire de Chimie (Elementary Treatise of Chemistry, 1789) was the first modern chemical
textbook, and presented a unified view of new theories of chemistry, contained a clear statement of the Law of Conservation of Mass,
and denied the existence of phlogiston. In addition, it contained a list of elements, or substances that could not be broken down further,
which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc, and sulfur. His list, however, also included light, and caloric,
which he believed to be material substances. In the work, Lavoisier underscored the observational basis of his chemistry, stating “I
have tried…to arrive at the truth by linking up facts; to suppress as much as possible the use of reasoning, which is often an unreliable
instrument which deceives us, in order to follow as much as possible the torch of observation and of experiment.” Nevertheless, he
believed that the real existence of atoms was philosophically impossible. Lavoisier demonstrated that organisms disassemble and
reconstitute atmospheric air in the same manner as a burning body.
With Laplace, he used a calorimeter to estimate the heat evolved per unit of carbon dioxide produced. They found the same ratio for a
flame and animals, indicating that animals produced energy by a type of combustion. Lavoisier believed in the radical theory, believing
that radicals, which function as a single group in a chemical reaction, would combine with oxygen in reactions. He believed all acids
contained oxygen. He also discovered that diamond is a crystalline form of carbon. Lavoisier made many fundamental contributions to
the science of chemistry. The revolution in chemistry which he brought about was a result of a conscious effort to fit all experiments into
the framework of a single theory. He established the consistent use of chemical balance, used oxygen to overthrow the phlogiston
theory, and developed a new system of chemical nomenclature. He was beheaded during the French revolution.
avoisier, Antoine Laurent
Pronounced As: Ntwn lorN lvwzya , 1743-94,
French chemist and physicist, a founder of modern
chemistry. He studied under eminent men of his day,
won early recognition, and was admitted to the
Academy of Sciences in 1768. Much of his work was
the result of extending and coordinating the research of
others; his concepts were largely evolved through his
superior ability to organize and interpret and were
substantiated by his own experiments. He was one of
the first to introduce effective quantitative methods in
the study of chemical reactions. He explained
combustion and thereby discredited the phlogiston
theory. He also described clearly the role of oxygen in
the respiration of both animals and plants. His
classification of substances is the basis of the modern
distinction between chemical elements and
compounds and of the system of chemical
nomenclature. He also conducted experiments to
establish the composition of water and of many
organic compounds. Lavoisier worked as well to
improve economic and social conditions in France,
holding various government posts. He was appointed
director of the gunpowder commission (1775),
member of the committee on agriculture (1785),
director of the Academy of Sciences (1785), member of
the commission on weights and measures (1790),
and commissioner of the treasury (1791). As one of the
farmers general, however, charged with the collection
of taxes, he was guillotined during the Reign of Terror.
His works include Trait lmentaire de chimie (1789)
and the posthumously published Mmoires de chimie
Lavoisier, Antoine Laurent , 1743B94, French chemist and physicist, a
founder of modern chemistry. He studied under eminent men of his day, won
early recognition, and was admitted to the Academy of Sciences in 1768.
Much of his work was the result of extending and coordinating the research
of others; his concepts were largely evolved through his superior ability to
organize and interpret and were substantiated by his own experiments. He
was one of the first to introduce effective quantitative methods in the
study of chemical reactions. He explained combustion and thereby
discredited the phlogiston theory. He also described clearly the role of
oxygen in the respiration of both animals and plants. His classification of
substances is the basis of the modern distinction between chemical
elements and compounds and of the system of chemical nomenclature. He
also conducted experiments to establish the composition of water and of
many organic compounds. Lavoisier worked as well to improve economic and
social conditions in France, holding various government posts. He was
appointed director of the gunpowder commission (1775), member of the
committee on agriculture (1785), director of the Academy of Sciences
(1785), member of the commission on weights and measures (1790), and
commissioner of the treasury (1791). As one of the farmers general,
however, charged with the collection of taxes, he was guillotined during the
Reign of Terror. His works include Trait lmentaire de chimie (1789) and
the posthumously published Mmoires de chimie (1805).
Introduction to the Scientific Method
The scientific method is the process by which scientists, collectively and over time, endeavor to construct an accurate (that is,
reliable, consistent and non-arbitrary) representation of the world.
Recognizing that personal and cultural beliefs influence both our perceptions and our interpretations of natural phenomena, we aim
through the use of standard procedures and criteria to minimize those influences when developing a theory. As a famous scientist
once said, “Smart people (like smart lawyers) can come up with very good explanations for mistaken points of view.” In summary,
the scientific method attempts to minimize the influence of bias or prejudice in the experimenter when testing an hypothesis or a
I. The scientific method has four steps
1. Observation and description of a phenomenon or group of phenomena.
2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or
a mathematical relation.
3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations.
4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments.
If the experiments bear out the hypothesis it may come to be regarded as a theory or law of nature (more on the concepts of
hypothesis, model, theory and law below). If the experiments do not bear out the hypothesis, it must be rejected or modified. What
is key in the description of the scientific method just given is the predictive power (the ability to get more out of the theory than you
put in; see Barrow, 1991) of the hypothesis or theory, as tested by experiment. It is often said in science that theories can never be
proved, only disproved. There is always the possibility that a new observation or a new experiment will conflict with a long-standing
II. Testing hypotheses
As just stated, experimental tests may lead either to the confirmation of the hypothesis, or to the ruling out of the hypothesis. The
scientific method requires that an hypothesis be ruled out or modified if its predictions are clearly and repeatedly incompatible with
experimental tests. Further, no matter how elegant a theory is, its predictions must agree with experimental results if we are to
believe that it is a valid description of nature. In physics, as in every experimental science, “experiment is supreme” and experimental
verification of hypothetical predictions is absolutely necessary. Experiments may test the theory directly (for example, the
observation of a new particle) or may test for consequences derived from the theory using mathematics and logic (the rate of a
radioactive decay process requiring the existence of the new particle). Note that the necessity of experiment also implies that a
theory must be testable. Theories which cannot be tested, because, for instance, they have no observable ramifications (such as, a
particle whose characteristics make it unobservable), do not qualify as scientific theories.
If the predictions of a long-standing theory are found to be in disagreement with new experimental results, the theory may be
discarded as a description of reality, but it may continue to be applicable within a limited range of measurable parameters. For
example, the laws of classical mechanics (Newton’s Laws) are valid only when the velocities of interest are much smaller than the
speed of light (that is, in algebraic form, when v/c