161 – Rome, Italy

Bust of GALEN


‘A body of work consisting 129 volumes. Some of the deductions were wrong’

Born in Pergamum (now Bergama in Turkey) in the reign of the Emperor Hadrian (76-138AD)
Studied in Corinth and Alexandria
157 – became surgeon to the Pergamum gladiators
161 – became physician to the emperors Marcus Aurielius and Commodus

Famous for the sheer volume of medical thought which he presented. He summarized his observations in books such as ‘On The Usefulness of Parts of The Body’. His works on medical science became accepted as the only authority on the subject for the following 1400 years. One explanation is that Galen not only incorporated the results of his own findings in his texts, but also compiled the best of all other medical knowledge that had gone before him into a single collection, such as that of Hippocrates.
In particular, Galen adopted Hippocrates’ ‘four humors’ approach to the body. This resulted from a desire to see in bodily conditions the attributes of the four Aristotelian elements. Thus earth was reflected in the body as black bile or melancholy; air as yellow bile or choler; fire as blood and water as phlegm.

After the move to Rome in 161 Galen became physician to emperors Marcus Aurelius, Lucius Verus, Commodus and Septimus Severus. This position allowed him the freedom to undertake dissection in the quest for improved knowledge.
Galen was not permitted to scrutinise human cadavers, so he dissected animals and Barbary apes. His most important conclusions concerned the central operation of the human body. Sadly they were only influential in that they limited the search for accurate information for the next millennia and a half.

Many people visited the shrine of Asklepios, the god of healing in Galen’s hometown, to seek cures for ailments and Galen observed first-hand the symptoms and treatment of diseases. After spells in Smyrna (now Izmir), Corinth and Alexandria studying philosophy and medicine and incorporating work on the dissection of animals, he returned to Pergamum in 157, where he took a position as physician to gladiators, giving him further first-hand experience in practical anatomical medicine. He realized that there were two types of blood flow from wounds. In one the blood was bright red and came spurting out, and in the other it was dark blue and flowed out in a steady stream. These observations convinced him these were two different types of blood. He also believed there was a third form of blood that flowed along nerves.

Galen believed that blood was formulated in the liver, the source of ‘natural spirit’. In turn this organ was nourished by the contents of the stomach that was transported to it. Veins from the liver carried blood to the extremes of the body where it was turned into flesh and used up, thus requiring more food on a daily basis to be converted into blood. Some of this blood passed through the heart’s right ventricle, then seeped through to the left ventricle and mixed with air from the lungs, providing ‘vital spirit’ which then passed into the body through the arteries and regulated the body’s heat. A portion of this blood was transported to the brain where it blended with ‘animal spirit’, which was passed through the body by the nerves. This created movement and the senses. The combination of these three spirits managed the body and contributed to the make-up of the soul. It was not until 1628 that WILLIAM HARVEY‘s system of blood circulation conclusively proved the idea of a single, integrated system.

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AL-BIRUNI (973-1050)

The Persian scholar al-Biruni lived around the same time as ibn-Sina. He pioneered the idea that light travels faster than sound, promoted the idea that the Earth rotates on its axis and measured the density of 18 precious stones and metals.

portrait of al-biruni

He classified gems according to the properties: colour; powder colour; dispersion (whether white light splits up into the colours of the rainbow when it goes through the gem); hardness; crystal shape; density.
He used crystal shape to help him decide whether a gemstone was quartz or diamond.

He noted that flowers have 3,4,5 or 8 petals, but never 7 or 9.

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IBN SINA (AVICENNA) (980-1037)

‘al Qann fi al-Tibb’ (The Canon of Medicine), also ‘ The Book of the Remedy

Avicenna lived under the Sammarid caliphs in Bukhara. He identified different forms of energy – heat, light and mechanical – and the idea of a force.

drawing of Ibn Sina ©


Before GALEN, scientists describing nature followed the old Greek traditions of giving the definitions and following them up with the body of logical development. The investigator was then obliged merely to define the various types of ‘nature’ to be found. With Galen this procedure was changed.

Instead of hunting for these natures and defining more and more of them, reproducing ARISTOTLE’s ideas, AVICENNA, a Persian physician, planned inductive and deductive experimental approaches to determine the conditions producing observable results.

His tome surveyed the entire field of medical knowledge from ancient times up to the most up to date Muslim techniques. Avicenna was the first to note that tuberculosis is contagious; that diseases can spread through soil and water and that a person’s emotions can affect their state of physical health. He was the first to describe meningitis and realize that nerves transmit pain. The book also contained a description of 760 drugs. Its comprehensive and systematic approach meant that once it was translated into Latin in the twelfth century it became the standard medical textbook in Europe for the next 600 years.

Arabic Canon of Medicine by Avicenna 1632. Many physicians in the Islamic world were outstanding medical teachers and practitioners. Avicenna (980-1037 CE) was born near Bokhara in Central Asia. Known as the 'Prince of Physicians', his Canon of Medicine (medical encyclopedia) remained the standard text in both the East and West until the 16th century and still forms the basis of Unani theory and practice today. Divided into five books, this opening shows the start of the third book depicting diseases of the brain.

Arabic Canon of Medicine by Avicenna 1632

Avicenna thought of light as being made up of a stream of particles, produced in the Sun and in flames on Earth, which travel in straight lines and bounce off objects that they strike.

A pinhole in a curtain in a darkened room causes an inverted image to be projected, upside-down, onto a wall opposite the curtained window. The key point is that light travels in straight lines. A straight line from the top of a tree some distance away, in a garden that the window of the camera obscura faces onto – passing through the hole in the curtain – will carry on down to a point near the ground on the wall opposite. A straight line from the base of the tree will go upwards through the hole to strike the wall opposite near the ceiling. Straight lines from every other point on the tree will go through the hole to strike the wall in correspondingly determined spots, the result is an upside-down image of the tree (and of everything else in the garden).

He realized that refraction is a result of light traveling at different speeds in water and in air.

He used several logical arguments to support his contention that sight is not a result of some inner light reaching outward from the eye to probe the world around it, but is solely a result of light entering the eye from the world outside – realizing that ‘after-images’ caused by a bright light will persist when the eyes are closed and reasoning that this can only be the result of something from outside affecting the eyes. By effectively reversing the extro-missive theory of Euclid, he formulated the idea of a cone emanating from outside the eye entering and thus forming an image inside the eye – he thus introduced the modern idea of the ray of light.

The idea which was to have the most profound effect on the scientific development of an understanding of the behaviour of light was the thought of the way images are formed on a sunny day by the ‘camera obscura’.

AL HAZEN (c.965-1039)

Born in Basra and working in Egypt under al-Hakim, Abu Ali al-Hassan ibn al-Haytham was one of the three greatest scientists of Islam (along with al-Biruni and ibn-Sina). He explained how vision works in terms of geometric optics and had a huge influence on Western science. He is regarded as one of the earliest advocates of the scientific method.

The mathematical technique of ‘casting out of nines’, used to verify squares and cubes, is attributed to al-Hazen.

Al-Hazen dissented with the J’bir Ayam hypothesis of transmutation, thus providing two different strands for Alchemy in Europe from the Islāmic world.

diagram explaining the working of the eye

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1502 – Florence, Italy

‘In the Renaissance science was reinvented’

Image of the VITRUVIAN MAN


Leonardo is celebrated as the Renaissance artist who created the masterpieces ‘The Last Supper’ (1495-97) and ‘The Mona Lisa’ (1503-06). Much of his time was spent in scientific enquiry, although most of his work remained unpublished and largely forgotten centuries after his death. The genius of his designs so far outstripped contemporary technology that they were rendered literally inconceivable.

The range of his studies included astronomy, geography, palaeontology, geology, botany, zoölogy, hydrodynamics, optics, aerodynamics and anatomy. In the latter field he undertook a number of human dissections, largely on stolen corpses, to make detailed sketches of the body. He also dissected bears, cows, frogs, monkeys and birds to compare their anatomy with that of humans.

It is perhaps in his study of muscles where Leonardo’s blend of artistry and scientific analysis is best seen. In order to display the layers of the body, he developed the drawing technique of cross-sections and illustrated three-dimensional arrays of muscles and organs from different perspectives.

Leonardo’s superlative skill in illustration and his obsession with accuracy made his anatomical drawings the finest the world had ever seen. One of Leonardo’s special interests was the eye and he was fascinated by how the eye and brain worked together. He was probably the first anatomist to see how the optic nerve leaves the back of the eye and connects to the brain. He was probably the first, too, to realise how nerves link the brain to muscles. There had been no such idea in GALEN’s anatomy.

Possibly the most important contribution Leonardo made to science was the method of his enquiry, introducing a rational, systematic approach to the study of nature after a thousand years of superstition. He would begin by setting himself straightforward scientific queries such as ‘how does a bird fly?’ He would observe his subject in its natural environment, make notes on its behaviour, then repeat the observation over and over to ensure accuracy, before making sketches and ultimately drawing conclusions. In many instances he would directly apply the results of his enquiries into nature to designs for inventions for human use.

Self portrait of LEONARDO DA VINCI


He wrote ‘Things of the mind left untested by the senses are useless’. This methodical approach to science marks a significant stepping-stone from the DARK AGES to the modern era.

1469 Leonardo apprenticed to the studio of Andrea Verrocchio in Florence

1482 -1499 Leonardo’s work for Ludovico Siorza, the Duke of Milan, included designs for weaponry such as catapults and missiles.
Pictor et iggeniarius ducalis ( painter and engineer of the Duke )’.
Work on architecture, military and hydraulic engineering, flying machines and anatomy.

1502 Returns to Florence to work for Pope Alexander VI’s son, Cesare Borgia, as his military engineer and architect.

1503 Begins to paint the ‘Mona Lisa’.

1505-07 Wrote about the flight of birds and filled his notebooks with ideas for flying machines, including a helicopter and a parachute. In drawing machines he was keen to show how individual components worked.

1508 Studies anatomy in Milan.

1509 Draws maps and geological surveys of Lombardy and Lake Isea.

1516 Journeys to France on invitation of Francis I.

1519 April 23 – Dies in Clos-Luce, near Amboise, France.

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1543 – Padua, Italy

‘In spite of his premature death, Vesalius left behind a revolutionary legacy to anatomy students’

Portrait of Vesalius &copy:


By his reasoned, critical approach to GALEN he broke the reverence ascribed to the former master and created a model for independent, rational investigation in the development of medical science.


In 1543, Vesalius published ‘De Humani Corporis Fabrica Libri Septem‘ (The Seven Books on the Structure of the Human Body). Book One reveals Vesalius’s understanding of the importance of the skeleton. Book Two is about muscles; Book Three – Veins and Arteries; Book Four – The Nervous System. Book Five concerns the Main Body Organs; Book Six – The Heart & Lungs; Book Seven – The Brain.


At 42cm tall and 28cm wide with over 700 densely packed pages, it was impressive in size alone. It contained 200 illustrations, was the first definitive work on human anatomy actually based on the results of methodical dissections of humans and was the most accurate work on the subject ever written. Galen himself had never dissected a human body as this had been forbidden by Roman religious laws.

Anatomical Study Illustration from De Humani Corporis Fabrica 1543Anatomical Study Illustration from De Humani Corporis Fabrica 1543

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1674 – Netherlands

Portrait of Leeuwenhoek

Leeuwenhoek was probably inspired to take up microscopy after seeing a copy of HOOKE’s Micrographia, though as a draper he was likely to have already been using lenses to examine cloth.
Unlike Hooke, Leeuwenhoek did not use a two lens compound microscope, but a single high quality lens, which could be described simply as a magnifying glass rather than a microscope. Leeuwenhoek is known to have made over 500 of these single–lens microscopes. They are simple devices just a few inches long, with the lens mounted in a tiny hole in a brass plate. The specimen is mounted on a point that sticks up in front of the lens. Two screws move the specimen for focusing. All else that is needed is careful lighting and a very steady, sharp eye.

After an introduction to Henry Oldenburg of the Royal Society in London from Dutch physician and anatomist Regnier de Graaf (discoverer of the egg-making follicles in the human ovary which now bear his name), Leeuwenhoek was encouraged to write to the Society’s journal ‘Philosophical Transactions’.

Leeuwenhoek’s letters were translated into Latin and English from the Dutch and he reported seeing tiny creatures in lake-water.

‘ I found floating therein divers earthly particles, and some green streaks, spirally wound serpentwise, and orderly arranged after the manner of copper or tin worms which distillers use to cool their liquors as they distil over. The whole circumference of each of these streaks was about the thickness of a hair of one’s head ’

Leeuwenhoek’s descriptions of ‘animalcules’ in water from different sources – rainwater, pond water, well water, sea water and so on – were verified by independent witnesses, including the vicar of Delft. Hooke too confirmed his findings with his own observations performed in front of expert witnesses, including Sir Christopher Wren.
Leeuwenhoek came close to understanding that bacteria were germs that cause disease but it took another century before LOUIS PASTEUR made that step.

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1735 – Sweden

‘A system for naming organisms by assigning them scientific names consisting of two parts’

Portrait of Linnaeus ©


Each species is given a two-word Latin name – The genus name that comes first and begins with a capital letter, and the species name, which begins with a lower case letter. The genus name is often abbreviated, and the names are always written in italics or underlined. The Linnaean system has six classification categories – in descending order, kingdoms, phyla, classes, orders, genera and species. Only two are used for naming organisms.

German botanist Rudolph Camerarius (1665-1721) had shown that no seed would grow without first being pollinated. In 1729, Linnaeus wrote in a paper about ‘the betrothal of plants, in which … the perfect analogy with animals is concluded’. He insisted that it is the stamens where pollen is made (the ‘bridegrooms’) and the pistils where seeds are made (‘the brides’) that are the sexual organs, and not the petals as had been considered previously.

As botanists and zoologists looked at nature, or ‘Creation’, there was no way of classifying the animal kingdom depicted in bestiaries of the time but alphabetically; or of distinguishing the real from the mythical.

Linnaeus developed a system of classification. Starting with the plant kingdom, Linnaeus grouped plants according to their sexual organs – the parts of the plant involved in reproduction. Each plant species was given a two-part Latin name. The first part always refers to the name of the group it belongs to, and the second part is the species name.

Linnaeus divided all flowering plants into twenty-three classes according to the length and number of their stamens – the male organs – then subdivided these into orders according to the number of pistils – female organs – they possessed. A twenty-fourth class, the Cryptogamia, included the mosses and other non-flowering plants.

illustration of flower reproductive structures ©

Many people were offended by the sexual overtones in Linnaeus’s scheme. One class he named Diandria, meaning ‘two husbands in one marriage’, while he said ‘the calyx might be regarded as the labia majora; one could regard the corolla as the labia minora’. For almost a century, botany was not seen as a decent thing for young ladies to be interested in.

Linnaeus’s scheme was simple and practical and in 1745 he published an encyclopedia of Swedish plants, when he began considering the names of species. Realizing he had to get the names in place before someone else gave plants other names, he gave a binomial label to every known plant species and in 1753 published all 5,900 in his Species Plantarium.

Believing his work on the plant kingdom complete, he turned his attention to the animal kingdom. In his earlier Systema Naturae of 1735, he had used the classification ‘Quadrupeds’ (four-legged creatures) but replaced this with Mammals, using the presence of mammary glands for suckling young as a more crucial distinguishing characteristic. The first or prime group in the Mammals was the primates, which included Homo sapiens (wise man). His catalogue of animals was included in the tenth edition of Systema Naturae, listed with binomial names.

By the time Linnaeus died it was the norm for expeditions around the world to take a botanist with them, hence CHARLES DARWIN’s famous voyage on the Beagle.

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1859 – England

‘All present day species have evolved from simpler forms of life through a process of natural selection’

Portrait of Charles Darwin ©

Organisms have changed over time and the ones living today are different from the ones that lived in the past. Furthermore, many organisms that once lived are now extinct.

The orthodox view was that of the Creationists. According to the Book of Genesis in the Bible, ‘God created every living creature that moves….’. Against this background, thinkers such as French naturalist Jean-Baptist Lamarck developed a picture of how species evolved from single-celled organisms.

Darwin’s breakthrough was to work out what evolution is and how it happens. His insight was to focus on individuals, not species and to show how individuals evolve by natural selection. The mechanism explained how all species evolved to become well suited to their environment. Later commentators have characterized this idea as ‘survival of the fittest,’ but this was never a phrase that Darwin himself used.

Darwin was influenced by CHARLES LYELL’s newly published book ‘Principles of Geology’, showing how landscapes had evolved gradually through long cycles of erosion and upheaval and by ‘An Essay on the Principle of Population’ written in 1798 by THOMAS MALTHUS.

The publication of Darwin’s book ‘On the Origin of Species by Means of Natural Selection’ in 1859 generated social and political debate that continues to this day. Darwin did not discuss the evolution of humans in this book.
In ‘The Descent of Man’, published in 1871, he presented his explanation of how his theory of evolution applied to the idea that humans evolved from apes. In modern form the theory contains the following ideas:

  • members of a species vary in form and behaviour and some of this variation has an inherited basis

  • every species produces far more offspring than the environment can support

  • some individuals are better adapted for survival in a given environment than others

this means that there are variations within each population gene pool and individuals with most favourable variations stand a better chance of survival – the survival of the fittest.

  • the favourable characteristics show up among more individuals of the next generation

there is thus a ‘natural selection’ for those individuals whose variations make them better adapted for survival and reproduction.

  • the natural selection of strains of organisms favours the evolution of new species, through better adaptation to their environment, as a consequence of genetic change or mutation.

Knowledge of DNA has enriched the theory of evolution. The modern view is still based on the Darwinian foundation; evolution through natural selection is opportunistic and it takes place steadily.

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GREGOR MENDEL (1822- 84)

1865 – Austria

  • ‘Law of Segregation: In sexually reproducing organisms, two units of heredity control each trait. Only one of such units can be represented in a single sexually reproductive cell’

  • ‘Law of Independent Assortment: Each of a pair of contrasted traits may be combined with either of another pair’

These laws laid the foundation for the science of genetics.

The biologist Lamarck (1744-1829) had proposed a theory of inheritance of acquired characteristics and had suggested that inherited characteristics are influenced by environment. Mendel planted an atypical variety of an oriental plant next to a typical variety – the offspring retained the essential traits of their parents, which meant that the characteristics that were inherited were not influenced by the environment. This simple test led Mendel to embark on the path that would lead to the discovery of the laws of heredity.

Mendel’s aim was to discover ” a generally applicable law of the formation and development of hybrids “. He addressed this by studying the effect of cross-breeding on seven pairs of contrasting characteristics of Pisum sativum, a strain of pea.
His work on peas indicated that features of the plant; seed shape, seed colour, pod shape, pod colour, flower colour, flower position and stem length; were passed on from one generation to the next by some physical element. He realised that each characteristic of a plant was inherited independently, and that the ratios of plants exhibiting each trait could be statistically predicted.

photograph of GREGOR MENDEL ©


A common assumption in Mendel’s time was that when two alternative features were combined, an average of these features would occur. For example, a tall plant and a short one would result in medium height offspring. For seven years Mendel kept an exact record of the inherited characteristics of 28,000 pea plants, taking great pains to avoid accidental cross-fertilization; then he applied mathematics to the results. These quantitative data allowed him to see statistical patterns and ratios that had eluded his predecessors.

From his analysis he found that certain characteristics of plants are due to factors passed intact from generation to generation.
Mendel observed that individual plants of the first generation of hybrids (crossbred plants) usually showed the traits of only one parent. The crossing of yellow seeded plants with green seeded ones gave rise to yellow seeds; the crossing of tall stemmed ones with short-stemmed varieties gave rise to tall-stemmed plants.

The factors determining a trait are passed on to the offspring during reproduction.

Mendel worked out that the factors for each trait are grouped together in pairs and that the offspring receives one part of a pair from each parent.

Contrary to the popular belief of the time, these factors do not merge. Any individual pea always exhibits one trait or the other, never a mixture of the two possible expressions of the trait; only one trait from each pair of factors donated by the parents would be expressed in the offspring, although there are four possible combinations of factors.
This is now described as Mendel’s law of segregation.
An offspring inherits from its parents either one trait or the other, but not both.

He decided that some factors were ‘dominant’ and some were ‘recessive’ and was able to conclude that certain expressed traits, such as yellow seeds or tall stems, were the dominant ones and that other traits, such as shortness of stem and green seeds, were recessive. It appeared that the dominant factors consumed or destroyed the recessive factors – but this could not be the case, as the second generation of hybrids exhibited both the dominant and recessive traits of their ‘grandparents’. Across a series of generations of descendants, plants did not average out to a medium, but instead inherited the original features (for example, either tallness or shortness) in consistent proportions, a ratio of 3:1, according to the dominant factor.
The 3:1 ratio would apply because the dominant factor would feature whenever it was present.

He also noted that the different pairs of factors making up the characteristics of the pea plant ( such as the pair causing flower colour, the pair causing seed shape and so on ), when crossed, occurred in all possible mathematical combinations. This convinced him that the elements regulating the different features acted independently of each other, so the inheritance of one particular colour of flower was not influenced, for example, by the inheritance of pea shape.
This is now described as Mendel’s law of independent assortment.

He first articulated his results in 1865 and in 1866, which was shortly after Darwin’s ‘Origin of Species’ appeared, published them in an article ‘Versuche über Pflanzen-Hybriden’ (Experiments with plant hybrids).

No one before him had attempted to use mathematics and statistics as a means of understanding and predicting biological processes and during his lifetime and for some time after, his results were largely ignored.

Around the time of Mendel’s death, scientists using ever improving optics to study the minute architecture of cells coined the term ‘chromosome’ to describe the long, stringy bodies in the cell nucleus.

The seven traits studied in peas
Type of seed surface smooth wrinkled
Colour of seed albumen yellow green
Colour of seed coat grey white
Form of ripe pod inflated constricted
Colour of unripe pod green yellow
Position of flowers on stem axial terminal
Length of stem tall short

‘There is doubt as to the probity of this Jesuit scholar, some claiming that his data was falsified whilst others argue that it is accurate’
Pilgrim, I. (1984) The Too-Good-to-be-True Paradox and Gregor Mendel. Journal of Heredity,#75, pp 501-2. Cited in Brake,M.L. & Hook, N. Different Engines – How science drives fiction and fiction drives science

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LOUIS PASTEUR (1822- 95)

1865 – France

‘Many human diseases have their origin in micro-organisms’

1862 – ‘Memoire sur les corpuscles organises qui existent dans l’atmosphere’ (Note on Organized Corpuscles that exist in the Atmosphere) – Puts an end to centuries of debate on the theory of spontaneous generation.

Although a chemist, Pasteur is best remembered for his contributions to medicine. His name is used to describe the process of ‘pasteurisation’.
Pasteur proved that living microorganisms cause fermentation. Previously scientists had assumed that fermentation was a chemical process.
Pasteur showed that the alcohol in fermentation was made by the yeast microbe. He also realised that when fermentation went wrong it was due to other germs.

Portrait of Louis Pasteur in his laboratory


In 1863 he showed that brief, moderate heating of wine and beer kills germs, thereby sterilizing the foodstuffs and ending the fermentation process. The process now known as pasteurisation is still used in the food industry.

His investigations led him to believe that microorganisms could also cause disease in humans. Pasteur realized the dangers of infection, but the English surgeon JOSEPH LISTER (1827-1912) is credited with developing and systematizing the notion of antiseptic surgery so that operations could be made safer if an ‘antiseptic’ procedure was introduced to destroy microbes and curb the infections that followed wounds or surgery.

In 1876, Pasteur confirmed the findings of ROBERT KOCH’s discovery of the anthrax bacillus. After EDWARD JENNER’s breakthrough in the development of vaccination against smallpox, little had been done to take advantage of the potential of this treatment against other disease.

In 1882 Pasteur successfully applied his discovery of vaccination by attenuated culture of microorganisms to anthrax and in 1885 to the treatment of rabies in humans.

On 14 November 1888 the Pasteur Institute opened in Paris.




ROBERT KOCH (1843-1910)

1876 – Germany

‘Koch’s postulates – four conditions that need to be satisfied to be sure that a particular type of bacteria causes disease’

Koch developed methods of staining bacteria that enabled him not only to see them under a microscope, but also to differentiate between the various strains of microorganisms that he found.

Koch proved that specific organisms cause specific diseases and in addition, that pollution could spread disease.
He developed methods for obtaining pure cultures of bacteria and laid down Koch’s Postulates.

photo portrait of ROBERT KOCH who devised 'KOCH's POSTULATES' ©


His colleague RICHARD JULIUS PETRI (1852-1921) designed a shallow flat dish that allowed him to grow microorganisms on a solid flat surface, and thus easily separate colonies of bacteria. Until then scientists had grown bacteria in flasks, or injected them into animals.

Koch’s rules for identifying harmful bacteria

  • – the micro-organism must be identified and seen in all animals that suffer the same disease
  • – it must be cultured through several generations
  • – these later generations of bacteria must be capable of causing the disease
  • – the same agent must be found in a newly infected animal as was found in the original victim

Using this set of criteria he identified the organisms responsible for more than twenty diseases, including tuberculosis, salmonella, cholera, pneumonia and meningitis.

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1895 – Germany

picture of the diploma awarded to Roentgen by the NOBEL foundation. link to:link to:
Photograph of ROENTGEN ©


‘X-Rays are high energy radiation given off when fast-moving electrons lose energy very rapidly’

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IVAN PAVLOV (1849 – 1936)

1903 – Russia

‘A conditioned reflex is a learnt response to an environmental stimulus’

The process of learning to connect a stimulus to a reflex is called conditioning.

An innate or built-in reflex is something we do automatically without thinking (such as moving our hand away from a flame).

photograph of ivan pavlov and his staff c1925 pavlov-whose-views-formed-the-foundation-of-behaviorism-believed-that-learning-consisted-of-a-series-of-conditioned-responses

Ivan Pavlov and his staff

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1914 – Manchester, England

‘Moseley’s law – the principle outlining the link between the X-ray frequency of an element and its atomic number’

ca. 1910s --- Physicist Henry Gwyn Jeffreys MOSELEY --- Library Image by © Bettmann/CORBIS


Working with ERNEST RUTHERFORD’s team in Manchester trying to better understand radiation, particularly of radium, Moseley became interested in X-rays and learning new techniques to measure their frequencies.
A technique had been devised using crystals to diffract the emitted radiation, which had a wavelength specific to the element being experimented upon.

In 1913, Moseley recorded the frequencies of the X-ray spectra of over thirty metallic elements and deduced that the frequencies of the radiation emitted were related to the squares of certain incremental whole numbers. These integers were indicative of the atomic number of the element, and its position in the periodic table. This number was the same as the positive charge of the nucleus of the atom (and by implication also the number of electrons with corresponding negative charge).

By uniting the charge in the nucleus with an atomic number, a vital link had been found between the physical atomic make up of an element and its chemical properties, as indicated by where it sits in the periodic table.
This meant that the properties of an element could now be considered in terms of atomic number rather than atomic weight, as had previously been the case – certain inconsistencies in the MENDELEEV version of the periodic table could be ironed out. In addition, the atomic numbers and weights of several missing elements could be predicted and other properties deduced from their expected position in the table.

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1923 – Toronto, Canada

‘Discovery of insulin’

Early research had shown that there was almost certainly a link between the pancreas and diabetes, but at the time it was not understood what it was.

We now know a hormone from the pancreas controls the flow of sugar into the blood stream. Diabetics lack this function and are gradually killed by uncontrolled glucose input into the body’s systems.

Banting believed that the islets of Langerhans might be the most likely site for the production of this hormone and began a series of tests using laboratory animals.
After successfully treating dogs – showing signs of diabetes after the pancreas had been removed – with a solution prepared from an extract from the islets of Langerhans, Banting’s team (Best, MacLeod and Collip) purified their extract and named it insulin.

Human trials successfully took place in 1923 and dying patients were restored to health. The same year, industrial production of insulin from pigs’ pancreas began.

In the Second World War Banting undertook dangerous research into poisonous gas and was killed in an air crash while flying from Canada to the United Kingdom.

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1929 – England

‘First identification of an antibiotic – the discovery of penicillin’

The chance discovery of a mould in 1928 led to the development of a non-toxic drug, which is used to combat the bacteria that infect wounds.

Whilst Paul Erlich (1854-1915) worked in Germany to produce a ‘magic-bullet’, a compound or dye that could stick to bacteria and damage them, Alexander Fleming’s chance discovery of the antibacterial properties of the mould Penicillium notatum led him to conclude there was a chemical produced by the mould that would attack the bacterial agents of disease.

Whilst searching for a naturally occurring bacteria-killer, Fleming’s experiments were concentrated on the body’s own sources, tears, saliva and nasal mucus.

The chance discovery of the anti-bacterial properties of Penicillium notatum was not developed commercially until World War Two over a decade later.

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LINUS PAULING (1901- 94)

1931 USA

‘A framework for understanding the electronic and geometric structure of molecules and crystals’

An important aspect of this framework is the concept of hybridisation: in order to create stronger bonds, atoms change the shape of their orbitals (the space around a nucleus in which an electron is most likely to be found) into petal shapes, which allow more effective overlapping of orbitals.

A chemical bond is a strong force of attraction linking atoms in a molecule or crystal. BOHR had already shown that electrons inhabit fixed orbits around the nucleus of the atom. Atoms strive to have a full outer shell (allowed orbit), which gives a stable structure. They may share, give away or receive extra electrons to achieve stability. The way atoms will form bonds with others, and the ease with which they will do it, is determined by the configuration of electrons.

Earlier in the century, Gilbert Lewis (1875-1946) had offered many of the basic explanations for the structural bonding between elements, including the sharing of a pair of electrons between atoms and the tendency of elements to combine with others to fill their electron shells according to rigidly defined orbits (with two electrons in the closest orbit to the nucleus, eight in the second orbit, eight in the third and so on).

Pauling was the first to enunciate an understanding of a physical interpretation of the bonds between molecules from a chemical perspective, and of the nature of crystals.

In a covalent bond, one or more electrons are shared between two atoms. The two atoms are bound together by the shared electrons. This was proposed by Lewis and Irving Langmuir in 1916. Two hydrogen atoms form the hydrogen molecule, H2, by each sharing their single electron.

In an ionic bond, one atom gives away one or more electrons to another atom. So in common salt, sodium chloride, sodium gives away its spare electron to chlorine. As the electron is not shared, the sodium and chlorine atoms are not bound together in a molecule. However, by losing an electron, sodium acquires a positive charge and chlorine, by gaining an electron, acquires a negative charge. The resulting sodium and chlorine ions are held in a crystalline structure.
Until Pauling’s explanation it was thought that they were held in place only by electrical charges, the negative and positive ions being drawn to each other.

Pauling’s work provided a value for the energy involved in the small, weak hydrogen bond.
When a hydrogen atom forms a bond with an atom which strongly attracts its single electron, little negative charge is left on the opposite side of the hydrogen atom. As there are no other electrons orbiting the hydrogen nucleus, the other side of the atom has a noticeable positive charge – from the proton in the nucleus. This attracts nearby atoms with a negative charge. The attraction – the hydrogen bond – is about a tenth of the strength of a covalent bond.
In water, attraction between the hydrogen atoms in one water molecule and the oxygen atoms in other water molecules makes water molecules ‘sticky’. It gives ice a regular crystalline structure it would not have otherwise. It makes water liquid at room temperature, when other compounds with similarly small molecules are gases at room temperature.Water_animation

He devised the electronegativity scale, which ranks elements in order of their electronegativity – a measure of the attraction an atom has for the electrons involved in bonding ( 0.7 for caesium and francium to 4.0 for fluorine ). The electronegativity scale lets us say how covalent or ionic a bond is.

One aspect of the revolution he brought to chemistry was to insist on considering structures in terms of their three-dimensional space. Pauling showed that the shape of a protein is a long chain twisted into a helix or spiral, now known as an alpha-helix. The structure is held in shape by hydrogen bonds.
He also explained the beta-sheet, a pleated sheet arrangement given strength by a line of hydrogen bonds.

1922 – while investigating why atoms in metals arrange themselves into regular patterns, Pauling used X-ray diffraction at CalTech to determine the structure of molybdenum.

When X-rays are directed at a crystal, some are knocked off course by striking atoms, while others pass straight through as if there are no atoms in their path. The result is a diffraction pattern – a pattern of dark and light lines that reveal the positions of the atoms in the crystal.
Pauling used X-ray and electron diffraction, magnetic effects and measurements of the heat of chemical reactions to calculate the distances and angles between atoms forming bonds. In 1928 he published his findings as a set of rules for working out probable crystalline structures from the X-ray diffraction patterns.

Pauling’s application of quantum theory to structural chemistry helped to establish the subject. He took from quantum mechanics the idea of an electron having both wave-like and particle-like properties and applied it to hydrogen bonds. Instead of there being just an electrical attraction between water molecules, Pauling suggested that wave properties of the particles involved in hydrogen bonding and those involved in covalent bonding overlap. This gives the hydrogen bonds some properties of covalent bonds.

1939 – ‘The Nature of the Chemical Bond and the Structure of Molecules’
Pauling suggests that in order to create stronger bonds, atoms change the shapes of their waves into petal shapes; this was the ‘hydridisation of orbitals’.

Pauling developed six key rules to explain and predict chemical structure. Three of them are mathematical rules relating to the way electrons behave within bonds, and three relate to the orientation of the orbitals in which the electrons move and the relative position of the atomic nuclei.



Describing hybridisation, he showed that the labels ‘ionic’ and ‘covalent’ are little more than a convenience to group bonds that really lie on a continuous spectrum from wholly ionic to wholly co-valent.

1951 – published his findings one year after WILLIAM LAWRENCE BRAGG’s team at the Cavendish Laboratory.

As carbon has four filled and four unfilled electron shells it can form bonds in many different ways, making possible the myriad organic compounds found in plants and animals. The concept of hybridisation proved useful in explaining the way carbon bonds often fall between recognised states, which opened the door to the realm of organic chemistry.

X-ray diffraction alone is not very useful for determining the structure of complex organic molecules, but it can show the general shape of the molecule. Pauling’s work showed that physical chemistry at the molecular level could be used to solve problems in biology and medicine.

A problem that needed resolving was the distance between particular atoms when they joined together. Carbon has four bonds, for instance, while oxygen can form two. It would seem that in a molecule of carbon dioxide, which is made of one carbon and two oxygen atoms, two of carbon’s bonds will be devoted to each oxygen.

Well-established calculations gave the distance between the carbon and oxygen atoms as 1.22 × 10-10m. Analysis gave the size of the bond as 1.16 Angstroms. The bond is stronger, and hence shorter. Pauling’s quantum .3-2. explanation was that the bonds within carbon dioxide are constantly resonating between two alternatives. In one position, carbon makes three bonds with one of the oxygen molecules and has only one bond with the other, and then the situation is reversed.

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1933 – USA

‘The Mechanism of Mendelian Heredity’ (1915), ‘The Theory of the Gene’ (1926)’

Morgan laid the foundation for understanding MENDEL’s observations and helped to provide the science required to reinforce CHARLES DARWIN’s conclusions.

Starting with Mendel’s laws of segregation and independent assortment, Morgan investigated why there are far fewer chromosomes – the long thread-like structures present in the nucleus of every living cell, which grow and divide during cell splitting, – than there are ‘units of heredity’. Morgan could not see how these few chromosomes could account for all the changes that occur from one generation to the next.

Mendel’s ‘factors of heredity’ had been renamed ‘genes’ in 1909 by the Dane Wilhelm Johannsen.

When the organism forms its reproductive cells (gametes), the genes segregate and pass to different gametes.
Since it had been separately established that chromosomes play an important part in inheritance, then groups of genes had to be present on a single chromosome.
If all the genes were arranged along chromosomes, and all chromosomes were transmitted intact from one generation to the next, then many characteristics would be inherited together. This implicitly invalidates Mendel’s law of independent assortment, which dictated that hereditary traits caused by genes would occur in all possible mathematical combinations in a series of descendants, independent of each other.

Experimental evidence often seemed to back-up the mathematical forecasts for characteristics present in descendants that Mendel had suggested; Morgan felt that the law of independent assortment could not accurately model the process of arriving at the end result.

He began his experiments with the fruit fly, which has just four pairs of chromosomes, in 1908.
He observed a mutant white-eyed male fly, which he extracted for breeding with ordinary red-eyed females. Over subsequent generations of interbred offspring, the white-eyed trait returned in some descendants, all of which turned out to be males. Clearly, certain genetic traits were not occurring independently of each other but were being passed on in groups.
Rather than invalidating Mendel’s law of independent assortment, a simple adjustment was required to unite it with Hunt’s belief in chromosomes to produce his thesis.
He suggested that the law of independent assortment did apply – but only to genes found on different chromosomes. For those on the same chromosome, linked traits would be passed on; usually a sex-related factor with other specific features (such as, the male sex and the white-eyed characteristic).

The results of his work convinced Morgan that genes were arranged on chromosomes in a linear manner and could be mapped. Further testing showed that, as chromosomes actually break apart and re-form during the production of sperm and egg cells, linked traits could occasionally be broken during the exchange of genes (recombination) that occurred between pairs of chromosomes during the process of cell division. He hypothesised that the nearer on the chromosome the genes were located to each other, the less likely the linkages were to be broken. Thus by measuring the occurrence of breakages he could work out the position of the genes along the chromosome.
In 1911 he produced the first chromosome map showing the position of five genes linked to gender characteristics.

In 1933 Hunt Morgan received the Nobel Prize for Physiology.

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1934 – England

Photograph of Dorothy Mary Hodgkin OM, FRS (12 May 1910 – 29 July 1994), née Crowfoot,  a British chemist, credited with the development of protein crystallography. Dorothy Crowfoot was born in Cairo on May 12th, 1910 where her father, John Winter Crowfoot was working in the Egyptian Education Service. ©


‘X-ray diffraction’

X-ray crystallography. X-rays are unique in that their wavelength is about the length of bonds within molecules. When X-rays hit a crystallized molecule, the electrons surrounding each atom cause the beam to bend. Because there are many atoms the result is that when the X-rays exit the crystal and fall onto a photographic plate, they produce a series of light and dark patches. Measuring the intensity and relative position of each patch indicates the relative positions of atoms within the crystal.

photo of Dorothy Hodgkin ©

With her co-worker John Desmond Bernal (1901-1971) Hodgkin produced the first diffraction patterns for proteins.

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