The history of

experimental research An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a ...

is long and varied. Indeed, the definition of an experiment itself has changed in responses to changing norms and practices within particular fields of study. This article documents the history and development of experimental research from its origins in Galileo's study of gravity into the diversely applied method in use today.

Ibn al-Haytham

The Arab physicist

Ibn al-Haytham Ḥasan Ibn al-Haytham, Latinized as Alhazen (; full name ; ), was a medieval mathematician, astronomer, and physicist of the Islamic Golden Age from present-day Iraq.For the description of his main fields, see e.g. ("He is one of the prin ...

(Alhazen) used experimentation to obtain the results in his ''

Book of Optics The ''Book of Optics'' ( ar, كتاب المناظر, Kitāb al-Manāẓir; la, De Aspectibus or ''Perspectiva''; it, Deli Aspecti) is a seven-volume treatise on optics and other fields of study composed by the medieval Arab scholar Ibn al- ...

'' (1021). He combined

observation Observation is the active acquisition of information from a primary source. In living beings, observation employs the senses. In science, observation can also involve the perception and recording of data via the use of scientific instruments. The ...

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into Causality, cause-and-effect by demonstrating what outcome oc ...

s and

rational Rationality is the quality of being guided by or based on reasons. In this regard, a person acts rationally if they have a good reason for what they do or a belief is rational if it is based on strong evidence. This quality can apply to an abili ...

arguments to support his intromission theory of

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain un ...

, in which

rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

are emitted from objects rather than from the eyes. He used similar arguments to show that the ancient emission theory of vision supported by

Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...

and

Euclid Euclid (; grc-gre, Wikt:Εὐκλείδης, Εὐκλείδης; BC) was an ancient Greek mathematician active as a geometer and logician. Considered the "father of geometry", he is chiefly known for the ''Euclid's Elements, Elements'' trea ...

(in which the eyes emit the rays of light used for seeing), and the ancient

intromission theory Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum reflec ...

supported by

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...

(where objects emit physical particles to the eyes), were both wrong. Experimental evidence supported most of the propositions in his ''Book of Optics'' and grounded his theories of vision, light and colour, as well as his research in catoptrics and dioptrics. His legacy was elaborated through the 'reforming' of his ''Optics'' by

Kamal al-Din al-Farisi Kamal may refer to: *Kamal (name), a male given name and surname with multiple origins *Kamal (navigation), a navigational instrument for measuring latitude *Kamal, Jhapa, a rural municipality in Nepal *Alfa Romeo Kamal, an SUV by Alfa Romeo *Oper ...

(d. c. 1320) in the latter's ''Kitab Tanqih al-Manazir'' (''The Revision of''

bn al-Haytham's BN, Bn or bn may refer to: Businesses and organizations * RTV BN, a Bosnian Serb TV network * Bangladesh Navy * Barisan Nasional (also known as "National Front"), a political coalition in Malaysia * Barnes & Noble, an American specialty retailer ...

''Optics''). Alhazen viewed his scientific studies as a search for

truth Truth is the property of being in accord with fact or reality.Merriam-Webster's Online Dictionarytruth 2005 In everyday language, truth is typically ascribed to things that aim to represent reality or otherwise correspond to it, such as beliefs ...

: "Truth is sought for its own sake. And those who are engaged upon the quest for anything for its own sake are not interested in other things. Finding the truth is difficult, and the road to it is rough. ... Alhazen's work included the conjecture that "Light travels through transparent bodies in straight lines only", which he was able to corroborate only after years of effort. He stated, "

his His or HIS may refer to: Computing * Hightech Information System, a Hong Kong graphics card company * Honeywell Information Systems * Hybrid intelligent system * Microsoft Host Integration Server Education * Hangzhou International School, in ...

is clearly observed in the lights which enter into dark rooms through holes. ... the entering light will be clearly observable in the dust which fills the air."Alhazen, translated into English from German by M. Schwarz, from "Abhandlung über das Licht", J. Baarmann (ed. 1882) ''

Zeitschrift der Deutschen Morgenländischen Gesellschaft The ''Zeitschrift der Deutschen Morgenländischen Gesellschaft'' (English: "Journal of the German Oriental Society") is a peer-reviewed academic journal covering Oriental studies, published by Harrassowitz Verlag on behalf of the Deutsche Morgenl� ...

'' Vol 36 as referenced on p.136 by Shmuel Sambursky (1974) ''Physical thought from the Presocratics to the Quantum Physicists'' He also demonstrated the conjecture by placing a straight stick or a taut thread next to the light beam. Ibn al-Haytham employed

scientific skepticism Scientific skepticism or rational skepticism (also spelled scepticism), sometimes referred to as skeptical inquiry, is a position in which one questions the veracity of claims lacking empirical evidence. In practice, the term most commonly refe ...

, emphasizing the role of

empiricism In philosophy, empiricism is an epistemological theory that holds that knowledge or justification comes only or primarily from sensory experience. It is one of several views within epistemology, along with rationalism and skepticism. Empir ...

and explaining the role of

induction Induction, Inducible or Inductive may refer to: Biology and medicine * Labor induction (birth/pregnancy) * Induction chemotherapy, in medicine * Induced stem cells, stem cells derived from somatic, reproductive, pluripotent or other cell t ...

syllogism A syllogism ( grc-gre, συλλογισμός, ''syllogismos'', 'conclusion, inference') is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be true. ...

. He went so far as to criticize

for his lack of contribution to the method of induction, which Ibn al-Haytham regarded as being not only superior to syllogism but the basic requirement for true scientific research. Something like

Occam's razor Occam's razor, Ockham's razor, or Ocham's razor ( la, novacula Occami), also known as the principle of parsimony or the law of parsimony ( la, lex parsimoniae), is the problem-solving principle that "entities should not be multiplied beyond neces ...

is also present in the ''Book of Optics''. For example, after demonstrating that light is generated by luminous objects and emitted or reflected into the eyes, he states that therefore "the extramission of isualrays is superfluous and useless." He may also have been the first scientist to adopt a form of

positivism Positivism is an empiricist philosophical theory that holds that all genuine knowledge is either true by definition or positive—meaning ''a posteriori'' facts derived by reason and logic from sensory experience.John J. Macionis, Linda M. G ...

in his approach. He wrote that "we do not go beyond experience, and we cannot be content to use pure concepts in investigating natural phenomena", and that the understanding of these cannot be acquired without mathematics. After assuming that light is a material substance, he does not further discuss its nature but confines his investigations to the diffusion and propagation of light. The only properties of light he takes into account are those treatable by geometry and verifiable by experiment.:

Roger Bacon

Roger Bacon Roger Bacon (; la, Rogerus or ', also '' Rogerus''; ), also known by the scholastic accolade ''Doctor Mirabilis'', was a medieval English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empiri ...

's assertions in the ' that "theories supplied by reason should be verified by sensory data, aided by instruments, and corroborated by trustworthy witnesses" were (and still are) considered "one of the first important formulations of the

scientific method The scientific method is an empirical method for acquiring knowledge that has characterized the development of science since at least the 17th century (with notable practitioners in previous centuries; see the article history of scientific m ...

on record".

Galileo Galilei

Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

as a scientist performed quantitative experiments addressing many topics. Using several different methods, Galileo was able to accurately measure time. Previously, most scientists had used distance to describe falling bodies, applying

geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...

, which had been used and trusted since

. Galileo himself used geometrical methods to express his results. Galileo's successes were aided by the development of a new mathematics as well as cleverly designed experiments and equipment. At that time, another kind of mathematics was being developed—

algebra Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics. Elementary a ...

. Algebra allowed arithmetical calculations to become as sophisticated as geometric ones. Algebra also allowed the discoveries of scientists such as Galileo—as well as later scientists such as

Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...

James Clerk Maxwell James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and ligh ...

and

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

—to be summarized by mathematical

equations In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...

. These equations described physical relationships in a precise, self-consistent manner. One prominent example is the "ball and ramp experiment." In this experiment Galileo used an inclined plane and several steel balls of different weights. With this design, Galileo was able to slow down the falling motion and record, with reasonable accuracy, the times at which a steel ball passed certain markings on a beam. Galileo disproved Aristotle's assertion that weight affects the speed of an object's fall. According to Aristotle's Theory of Falling Bodies, the heavier steel ball would reach the ground before the lighter steel ball. Galileo's hypothesis was that the two balls would reach the ground at the same time. Other than Galileo, not many people of his day were able to accurately measure short time periods, such as the fall time of an object. Galileo accurately measured these short periods of time by creating a pulsilogon. This was a machine created to measure time using a

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the ...

. The pendulum was synchronized to the human

pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...

. He used this to measure the time at which the weighted balls passed marks that he had made on the inclined plane. His measurements found that balls of different weights reached the bottom of the

inclined plane An inclined plane, also known as a ramp, is a flat supporting surface tilted at an angle from the vertical direction, with one end higher than the other, used as an aid for raising or lowering a load. The inclined plane is one of the six clas ...

at the same time and that the distance traveled was proportional to the square of the elapsed time. Later scientists summarized Galileo's results as The Equation of Falling Bodies. These results supported Galileo's hypothesis that objects of different weights, when measured at the same point in their fall, are falling at the same speed because they experience the same gravitational acceleration.

Antoine Lavoisier

The experiments of

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794), When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air and th ...

(1743–1794), a French chemist regarded as the founder of modern chemistry, were among the first to be truly quantitative.

Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

, the quantity of

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...

is the same at the end as at the beginning of every chemical reaction. In one experiment, he burned phosphorus and sulfur in air to see whether the results further supported his previous conclusion (

Law of Conservation of Mass In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the system's mass can ...

). In this experiment, however, he determined that the products weighed more than the original phosphorus and sulfur. He decided to do the experiment again. This time he measured the mass of the air surrounding the experiment as well. He discovered that the mass gained in the product was lost from the air. These experiments provided further support for his

. One of Lavoisier's experiments connected the worlds of

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellula ...

and

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...

. Lavoisier's hypothesis was that combustion and respiration were one and the same, and combustion occurs with every instance of respiration. Working with

Pierre-Simon Laplace Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy. He summarized ...

, Lavoisier designed an ice

calorimeter A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimete ...

apparatus for measuring the amount of heat given off during combustion or respiration. This machine consisted of three concentric compartments. The center compartment held the source of heat, in this case the

guinea pig The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent belonging to the genus ''Cavia'' in the family Caviidae. Breeders tend to use the word ''cavy'' to describe the ani ...

or piece of burning

charcoal Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, cal ...

. The middle compartment held a specific amount of ice for the heat source to melt. The outside compartment contained packed snow for insulation. Lavoisier then measured the quantity of carbon dioxide and the quantity of heat produced by confining a live guinea pig in this apparatus. Lavoisier also measured the heat and carbon dioxide produced when burning a piece of charcoal in the calorimeter. Using this data, he concluded that respiration was in fact a slow combustion process. He also discovered through precise measurements that these processes produced carbon dioxide and heat with the same constant of proportionality. He found that for 224 grains of "fixed air" (CO₂) produced, . of ice was melted in the calorimeter. Converting grains to grams and using the energy required to melt . of ice, one can compute that for each gram of CO₂ produced, about 2.02 kcal of energy was produced by the combustion of carbon or by respiration in Lavoisier's calorimeter experiments. This compares well with the modern published

heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...

for carbon of 2.13 kcal/g. This continuous slow combustion, which Lavoisier and Laplace supposed took place in the

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...

, enabled the living animal to maintain its body temperature above that of its surroundings, thus accounting for the puzzling phenomenon of animal heat. Lavoisier concluded, "La respiration est donc une combustion," That is, respiratory gas exchange is combustion, like that of burning a candle. Lavoisier was the first to conclude by experiment that the

applied to chemical change. His hypothesis was that the mass of the reactants would be the same as the mass of the products in a

. He experimented on vinous

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

, determining the amounts of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

, and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

. Weighing a quantity of sugar, he added

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

and

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

in measured amounts, allowing the mixture to ferment. Lavoisier then measured the mass of the carbonic acid gas and water that were given off during fermentation and weighed the residual liquor, the components of which were then separated and analyzed to determine their elementary composition. In this way he controlled a couple of potential confounding factors. He was able to capture the carbonic acid gas and water vapor that were given off during fermentation so that his final measurements would be as accurate as possible. Lavoisier concluded that the total mass of the reactants was equal to the mass of the final product and residue. Moreover, he showed that the total mass of each constituent element before and after the chemical change remained the same. Similarly, he demonstrated via experimentation that the mass of products of combustion is equal to the mass of the reacting ingredients.

Louis Pasteur

The French

biologist A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...

Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, microbial fermentation and pasteurization, the latter of which was named afte ...

(1822-1895), regarded as the "Father of microbiological sciences and immunology", worked during the 19th century. He postulated - and supported with experimental results - the idea that disease-causing agents do not spontaneously appear but are alive and need the right environment to prosper and multiply. Stemming from this discovery, he used experimentation to develop vaccines for

chicken cholera Fowl cholera is also called avian cholera, avian pasteurellosis, avian hemorrhagic septicemia. Abraham b. It is the most common pasteurellosis of poultry. As the causative agent is ''Pasteurella multocida'', it is considered to be a zoonosis. Adu ...

anthrax Anthrax is an infection caused by the bacterium ''Bacillus anthracis''. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day and more than two months after the infection is contracted. The sk ...

and

rabies Rabies is a viral disease that causes encephalitis in humans and other mammals. Early symptoms can include fever and tingling at the site of exposure. These symptoms are followed by one or more of the following symptoms: nausea, vomiting, vi ...

, and developed methods for reducing

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

in some food products by heating them (

pasteurization Pasteurization or pasteurisation is a process of food preservation in which packaged and non-packaged foods (such as milk and fruit juices) are treated with mild heat, usually to less than , to eliminate pathogens and extend shelf life. The ...

). Pasteur's work also led him to advocate (along with the English physician Dr.

Joseph Lister Joseph Lister, 1st Baron Lister, (5 April 182710 February 1912) was a British surgeon, medical scientist, experimental pathologist and a pioneer of antiseptic surgery and preventative medicine. Joseph Lister revolutionised the craft of s ...

) antiseptic surgical techniques. Most scientists of that day believed that microscopic life sprang into existence from

spontaneous generation Spontaneous generation is a superseded scientific theory that held that living creatures could arise from nonliving matter and that such processes were commonplace and regular. It was hypothesized that certain forms, such as fleas, could arise fr ...

in non-living matter. Pasteur's observations of tiny

organisms In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...

under the

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...

caused him to doubt spontaneous generation. He designed an experiment to test his

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

that life could not arise from where there is no life. He took care to control possible confounding factors. For example, he needed to make sure there was no life, even microscopic, in the flasks of broth he used as a test medium. He decided to kill any microscopic organisms already present by boiling the broth until he was confident that any microorganisms present were dead. Pasteur also needed to make sure that no microscopic organisms entered the broth after boiling, yet the broth needed exposure to air to properly test the theory. A colleague suggested a flask with a neck the shape of an "S" turned sideways. Dust (which Pasteur thought contained microorganisms) would be trapped at the bottom of the first curve, but the air would flow freely through. Thus, if bacteria should really be spontaneously generated, then they should be growing in the flask after a few days. If spontaneous generation did not occur, then the contents of the flasks would remain lifeless. The experiment appeared conclusive: not a single microorganism appeared in the broth. Pasteur then allowed the dust containing the microorganisms to mix with the broth. In just a few days the broth became cloudy from millions of organisms growing in it. For two more years he repeated the experiment in various conditions and locales to assure himself that the results were correct. In this way Pasteur supported his hypothesis that spontaneous generation does not occur. Despite the experimental results supporting his hypotheses and his success curing or preventing various diseases, correcting the public misconception of spontaneous generation proved a slow, difficult process. As he worked to solve specific problems, Pasteur sometimes revised his ideas in the light of the results of his experiments, as when faced with the task of finding the cause of disease devastating the French silkworm industry in 1865. After a year of diligent work he correctly identified a culprit organism and gave practical advice for developing a healthy population of moths. However, when he tested his own advice, he found disease still present. It turned out he had been correct but incomplete – there were two organisms at work. It took two more years of experimenting to find the complete solution.Dubos (1986; p.210)

References

*Bell, Madison Smartt (2005) ''Lavoisier in the Year One''.. W.W. Norton & Company, Inc. *Holmes, Frederic Lawrence (1987) ''Lavoisier and the chemistry of life: an exploration of scientific creativity'', Univ. Wisconsin Press. Reprint. . *Dubos, Rene J. (1986) ''Louis Pasteur: Free Lance of Science''. Da Capo Press. *Kupelis, Theo; Kuhn, Karl F. (2007) ''In Quest of the Universe''. Jones and Bartlett Publishers. {{ISBN, 978-0-7637-4387-1. Experiments