photo of an ancient document showing some of the symbols commonly used by alchemists

Alchemical symbols

Understanding of the alchemists is hampered by their predilection for making their writings incomprehensible ( instant knowledge was not to be available to the uninitiated ) and the popular view that their quest was simply to isolate the Philosophers’ Stone and to use it to transform base metals into gold. There was in fact a genuine search for mental and spiritual advance

Using a world-view totally unlike that recognised today, the alchemists’ ideas of ‘spirit’ and ‘matter’ were intermingled – the ability to use ‘spirit’ in their experiments was the difficult part.

alchemical symbol for gold

To transform copper to gold: – copper could be heated with sulphur to reduce it to its ‘basic form’ (a black mass which is in fact copper sulphide) – its ‘metallic form’ being ousted by the treatment. The idea of introducing the ‘form of gold’ to this mass by manipulating and mixing suitable quantities of spirit stymied alchemists for over fifteen centuries.

Whilst this transmutation of metals was the mainstream concern of alchemy, there emerged in the sixteenth century a school that brought the techniques and philosophies of alchemy to bear on the preparation of medicines, the main figures involved being PARACELSUS and JOHANN VAN HELMONT.

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Noticing that burning a candle in an upturned container, the open end of which is submerged in water, causes the water to rise into the container, Philon of Byzantium inferred correctly that some of the air in the container had been used up in the combustion. However, he proposed that this is because this portion of the air had been converted into ‘fire particles’, which were smaller than ‘air particles’.

In 1700 the German physician Georg Ernst Stahl (1660-1734) invoked ‘phlogiston’ to explain what happens when things burn. He suggested that a burning substance was losing an undetectable elementary principle analogous to the ‘sulfur’ of J’BIR IHBIN AYAM, which he re-named ‘phlogiston’. This could explain why a log (rich in phlogiston) could seem to be heavier than its ashes (deficient in phlogiston). The air that is required for burning served to transport the phlogiston away.

The English chemist JOSEPH PRIESTLY (1733-1804), although a supporter of the phlogiston theory, ironically contributed to its downfall. He heated mercury in air to form red mercuric oxide and then applied concentrated heat to the oxide and noticed that it decomposed again to form mercury whilst giving off a strange gas in which things burnt brightly and vigorously. He concluded that this gas must be ‘phlogiston poor’.

Priestly combined this result with the work of the Scottish physician Daniel Rutherford (1749-1819), who had found that keeping a mouse in an enclosed airtight space resulted in its death (by suffocation) and that nothing could be burnt in the enclosed atmosphere; he formed the idea that the trapped air was so rich in phlogiston that it could accept no more. Rutherford called this ‘phlogisticated air’ and so Priestly called his own gas ‘dephlogisticated air’.

In 1774 Priestley visited the French chemist ANTOINE LAVOISIER (1743-1794).
Lavoisier repeated Priestly’s experiments with careful measurements.
Reasoning that air is made up of a combination of two gases – one that will support combustion and life, another that will not; what was important about Lavoisier’s experiments was not the observation – others had reached a similar conclusion – but the interpretation.

Lavoisier called Priestley’s ‘dephlogisticated air’, ‘oxygene’, meaning ‘acidifying principle’, believing at the time that the active principle was present in all acids (it is not). He called the remaining, ‘phlogisticated’, portion of normal air, ‘azote’, meaning ‘without life’

Oxygen is the mirror image of phlogiston. In burning and rusting (the two processes being essentially the same) a substance picks up one of the gases from the air. Oxygen is consumed, there is no expulsion of ‘phlogiston’.

Lavoisier had been left with almost pure nitrogen, which makes up about four fifths of the air we breath. We now know azote as nitrogen. Rutherford’s ‘mephitic air’ was carbon dioxide.


Like phlogiston, caloric was a weightless fluid, rather like elemental fire, a quality that could be transmitted from one substance to another, so that the first warmed the second up.

It was believed that all substances contained caloric and that when a kettle was being heated over a fire, the fuel gave up its caloric to the flame, which passed it into the metal, which passed it on to the water. Similarly, two pieces of wood rubbed together would give heat because abrasion was releasing caloric trapped within.

What is being transmitted is heat energy. It was the crucial distinction between the physical and the chemical nature of substances that confused the Ancients and led to their minimal elemental schemes.




Typographic resetting of Gutenberg's 42-line bible of 1452-55, using modern Fraktur and decorative initial in METAFONT by Yannis Haralambous. (Beginning of St. John's Gospel) from a LaTex advertising flyer.

1450 – Mainz, Germany

‘Movable type’


Hand-held block printing – a laborious process of carving whole pages of fixed text out of wooden slabs and reproducing copies using dies – had been used for many decades before the German inventor appeared. What Gutenberg mastered was the idea of placing individual metal letters – (his family background was in minting and metalworking, an ideal foundation for his training as an engraver and goldsmith. His skills enabled him to craft the first individual metal letter moulds) – into temporary mounts, which could then be dismantled or ‘moved’ once a page of text had been completed and reused to produce other pages.

In comparison to engraving and the single use of wooden blocks, the theoretically infinite number of sides which could be made out of a set of metal characters, together with the speed at which a template could be created, revolutionised printing and the spread of the printed word.

engraving of Johannes Gutenberg

Gutenberg printing press. Johannes Gutenberg (c. 13951468) invented the printing press sometime in the mid-fifteenth century. The moveable printing blocks it employed made it far simpler to operate than the complicated machinery of the Far East

Some sources credit the Chinese with inventing moveable type printing, using characters made of wood. What is notable is the quality of Gutenberg’s metal casts and press – they are almost as important as the idea of moveable type itself.

By the end of the fifteenth century tens of thousands of books and pamphlets were already in existence, giving academics the opportunity to share scientific knowledge widely and cheaply.

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1609-19 – Germany

‘1600 – Kepler works in Prague with TYCHO BRAHE the imperial mathematician, under the patronage of Rudolph II
1601 – On Brahe’s death, Kepler inherits his position (and crucially, his astronomical notes)’

portrait of KEPLER ©


  • First Law: The planets move in elliptical orbits with the Sun at one focus

  • Second Law: The straight line joining the Sun and any planet sweeps out equal areas in equal periods of time

  • Third Law: The squares of orbital periods of the planets are proportional to the cube of their mean distances from the Sun

Modern measurements of the planets show that they do not precisely follow these laws; however, their development is considered a major landmark in science.

Kepler’s ardent faith in the Copernican system – ‘The Sun not only stands at the centre of the universe, but is its moving spirit’, he asserted – brought him the disfavour of religious leaders. With his realisation that the planets do not rotate in perfect circles but in fact orbit in an ellipse, he provided the mathematical explanation for planetary motion, which had eluded Copernicus and Ptolemy.

The first two laws were published in 1609 ( Astronomia Nova – New Astronomy ) and the third in 1619 ( Harmonicses Mundi – Harmonics of the World ). Their publication put an end to PTOLEMY’s cycles & epicycles. His work provided the observational and arithmetical proof to support COPERNICUS‘ theories.

His second law states that an imaginary line between the Sun and the planets sweeps out an equal area in equal periods of time.

Stating that the planets ‘sweep’ or cover equal areas in equal amounts of time regardless of which location of their orbit they are in means that, as the Sun is only one of two centres of rotation in a planet’s orbit, a planet is nearer to the Sun at some times than at others. Thus the planet must speed up when it is nearer the Sun and slow down when it is further away.

His third law finds that the period (the time for one complete orbit – a year for the Earth, for instance) of a planet squared is the same as the distance from the planet to the Sun cubed (in astronomical units). This allows distances of planets to be worked out from observing their cycles alone.

Kepler was a versatile genius who, besides discovering these three laws, compiled tables of star positions ( Tabulae Rudolphinae – 1627 ) and developed the astronomical telescope.

Kepler also studied the anatomy of the human eye and founded the science of geometrical optics ( ‘Dioptrics’ – 1611 ), proposing the ray theory of light after ALHAZEN’s discussion in Opticae Thesaurus ; he described the eye in the same terms – as a pinhole camera, with light entering through the pupil and forming an image of the outside world on the retina at the back of the eye.

His credible solution to predicting planetary motion would act as the stimulus for questions that would lead to ISAAC NEWTON‘s theory of gravity.

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1640 – Italy

‘Together with VINCENZO VIVIANI (1622-1703) realised that the weight of air pushing on a reservoir of mercury can force the liquid to rise into a tube that contains no air; that is, a vacuüm’

In 1650 OTTO VON GUERICKE (1602-1686) invented an air pump and showed that if you remove the air from the centre of two hemispheres that are resting together, the pressure of the outside air is sufficient to prevent a team of horses from pulling them apart.

1657 – Formed the Accademia del Cimento with eight other Florentines to build their own apparatus and conduct experiments to advance the pursuit of knowledge. Disbanded after ten years as a condition of its patron Leopoldo de Medici’s appointment as cardinal, its dissolution followed Galileo’s trial by the Catholic Church and marked the decline of free scientific research in Italy.

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1684 – Germany

‘A new method for maxima and minima, as well as tangents … and a curious type of calculation’

Newton invented calculus (fluxions) as early as 1665, but did not publish his major work until 1687. The controversy continued for years, but it is now thought that each developed calculus independently.
Terminology and notation of calculus as we know it today is due to Leibniz. He also introduced many other mathematical symbols: the decimal point, the equals sign, the colon (:) for division and ratio, and the dot for multiplication.

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1715 – Netherlands

‘The kelvin scale is more suitable for scientific purposes and the celsius scale is neater, based on decimals. The advantage of using the fahrenheit scale is that it is designed with everyday use in mind, rarely needing negative degrees’

Even as late as the start of the eighteenth century, scientists had no reliable means of accurately measuring temperature and a uniform scale by which to describe the limited measurements they could make.

Fahrenheit thermometer


GALILEO had used the knowledge that air expands when heated and contracts when cooled to build a primitive instrument. Using a cylindrical tube placed in water, he noted that when the air in the device was hotter, it pushed the level of the water downwards, just as it rose when the air-cooled. He realised that readings from the device were unreliable because the volume and therefore the behaviour of the air also fluctuated according to atmospheric pressure. Gradually scientists began using more stable substances to improve the accuracy of the reading, with alcohol being introduced as a possible substitute late in the seventeenth century.

Fahrenheit knew that the boiling points of different liquids varied according to fluctuations in atmospheric pressure; the lower the pressure, the lower the boiling point. A producer of meteorological instruments, he first achieved progress in 1709 with an improved alcohol thermometer. Building on the work of GUILLAUME AMONTONS (1663-1705) who investigated the properties of mercury, Fahrenheit took the measurement of temperature into another domain. He produced his first mercury thermometer, particularly useful in its application over a wide range of temperatures, in 1714.

In 1715 he complemented his breakthroughs in instrument making with the development of the fahrenheit temperature scale. Taking 0degrees to be the lowest temperature he could produce (from a blend of ice and salt), he used the freezing point of water and the temperature of the human body as his other key markers in its formulation.

In his initial calculations, he placed water’s freezing point at 30degrees F and the body’s temperature at 90degrees F. Later revisions changed this to 32degrees for the freezing point of water and 96degrees for the body temperature of humans. The boiling point of water worked out to be 212degrees F, giving a hundred and eighty incremental steps between freezing and boiling.

picture of the head and face of DANIEL FAHRENHEIT ©


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1745 – Holland/Germany

‘Electricity produced by electrostatic machines can be stored in a jar’

The Leyden Jar

diagram of the use of the 'LEYDEN JAR'

In modern terms the Leyden jar is a capacitor or condenser.
In 1734 Stephen Gray (c.1666-1736), an English experimenter, discovered that electric charge could be conducted over distance. He also classified various substances into conductors and insulators of electricity. He suggested that metals were the best conductors and thus introduced the use of electric wire.

In 1734 Musschenbroek, a professor from Leyden in Holland discovered that electricity could be stored in a jar of water.
During the same year, von Kleist, a German scientist also discovered the same principle independently.
In later versions of what became known as the Leyden jar, water was replaced by copper foil inside and outside the jar.
The Leyden jar became a novelty and in village faires magicians used ‘electricity in a bottle’ to amaze and entertain villagers.

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1823 – Germany

‘Why is the sky dark at night?’

This question puzzled astronomers for centuries and no, the answer is not because the Sun is on the other side of the planet.

Olbers pointed out that if there were an infinite number of stars evenly distributed in space, the night sky should be uniformly bright. He believed that the darkness of the night sky was due to the adsorption of light by interstellar space.

This is wrong.

Heinrich-Wilhelm-Matthias-Olbers ©


Olbers’ question remained a paradox until 1929 when it was discovered that the galaxies are moving away from us and the universe is expanding. The distant galaxies are moving away so fast that the intensity of light we receive from them is diminished.

diagram explaining reduced light intensity as the observer travels further from the source

What is light intensity?

In addition, this light is shifted towards the red end of the spectrum. These two effects significantly reduce the light we receive from distant galaxies, leaving only the nearby stars, which we see as points of light in a darkened sky.

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1823 – Germany

‘The spectroscope’

A significant improvement on the apparatus used by Newton. Sunlight, instead of passing through a pinhole before striking a prism, is passed through a long thin slit in a metal plate. This creates a long ribbon-like spectrum, which may be scanned from end to end with a microscope.

image of the visible portion of the electromagnetic spectrum showing a series of dark fraunhofer lines

Cutting across the ribbon of rainbow colours are thin black lines. The lines are present even when a diffraction grating is used instead of a prism, proving that the lines are not produced by the material of a prism, but are inherent in sunlight.

An equivalent way of describing colours is as light waves of different sizes.
The wavelength of light is fantastically small, on average about a thousandth of a millimeter, with the wavelength of red light being about twice as long as that of blue light.

Fraunhofer’s black lines correspond to missing wavelengths of light.

By 1823 Fraunhofer had measured the positions of 574 spectral lines, labeling the most prominent ones with the letters of the alphabet. The lines labeled with the letters ‘H’ and ‘K’ correspond to light at a wavelength of 0.3968 thousandths of a millimeter and 0.3933 thousandths of a millimeter, respectively. The lines are present in the spectrum of light from stars, usually in different combinations.

Fraunhofer died early at the age of 39 and it was left to the German GUSTAV KIRCHOFF to make the breakthrough that explained their significance.

Astronomers today know the wavelengths of more than 25,000 ‘Fraunhofer lines’.

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CARL GAUSS (1777-1855)

1832 – Germany

‘The electrical flux through a closed surface is proportional to the sum of the electric charges within the surface’

 Portrait of GAUSS ©


An electric field may be pictured by drawing lines of force. The field is stronger where these lines crowd together, weaker where they are far apart. Electrical flux is a measure of the number of electric field lines passing through an area.

Gauss’ law describes the relationship between electric charge and electric field. It is an elegant restatement of COULOMB‘s law.


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1842 – Germany

‘Heat is a form of energy and energy is conserved’

In equation form ΔE = H − W where ΔE is the change in the internal energy of a system, H is heat energy received by the system and W is work done by the system.

The first law of thermodynamics is simply a restatement of the law of the conservation of energy: energy is neither created nor destroyed, but may be changed from one form to another.

Mayer and HELMHOLTZ, independent of JOULE and each other, came to similar conclusions at around the same time.

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1843 – Germany

‘The number of visible sunspots varies in a regular cycle that averages about 11 years’

image of the Sun from space

GALILEO was the first to study sunspots. Schwabe made careful records of sunspots almost daily for 17 years before announcing his theory. He continued his observations for another 25 years.

Wherever magnetic fields emerge from the Sun, they suppress the flow of surrounding hot gases, creating relatively cool regions that appear as dark patches in the Sun’s shallow outer layer, the photosphere.

Sunspots vary in size from 1000 to 40,000 kilometres across and may last from a few days to many months.

Near a solar minimum there are only a few sunspots. During a solar maximum, solar flares can produce dramatic changes in the emission of ultraviolet rays and X-rays from the Sun.

Hot plasma of several thousand degrees rises upwards from within the Sun, then cools down and sinks back into the depths. Where the strong magnetic fields hold the plasma, dark sunspots emerge.

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1845 – Germany

‘First law (Junction law): At any junction point in an electrical circuit, the sum of all currents entering the junction must equal the sum of all currents leaving the junction’

‘Second law (Loop law): For any closed loop in an electrical circuit, the sum of the voltages must add up to zero’

In equation form the first law is I = I1 + I2 + I3 + I4 +… where I is the total current and I1, I2, I3 etc. are the separate currents.

Second law is V = V1 + V2 + V3 + … where V is the total voltage and V1, V2, V3 etc. are the separate voltages.

These laws are an extension of OHM‘s law and are used for calculating current and voltage in a network of circuits. Kirchhoff formulated these laws when he was a student at the University of Konisburg.

Kirchhoff also showed that objects that are good emitters of heat are also good absorbers. This is Kirchhoff’s law of radiation.

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1850 – Germany

‘Heat does not flow spontaneously from a colder to a hotter body’

’The second law of thermodynamics’. The law says that many processes in nature are irreversible, never going backwards. It defines the direction of time (time cannot go backwards).

In 1857 Clausius wrote a paper entitled ‘The Kind of Motion We Call Heat’, relating average molecular motion to thermal quantities. Two years later, JAMES CLERK MAXWELL took up the problem using a statistical approach.

Clausius tried to understand why mechanical energy is in some sense a ‘higher’ form of energy than heat, and why it isn’t possible to change heat into mechanical energy with 100% efficiency, although the opposite is true.

He managed to link the degree of order and disorder in a system to the reversibility of a process.

ca. 1850s-1888 --- Original caption: Portrait of German mathematical physicist Rudolph Clausius (1822-1888), one of the founders of thermodynamics. Undated photograph. --- Image by © Bettmann/CORBIS ©


In 1865, Clausius used the term entropy as a measure of the disorder or randomness of a system. The more random and disordered a system is, the greater the entropy. The entropy of an irreversible system must increase; therefore, the entropy of the universe is increasing. A force acts to minimize the disequilibrium of energy and to maximize entropy, an object rolling down a hill can come to a stop by friction, but the heat generated through that friction cannot be used to bring the object back to the top.

  • First Law – The energy of the universe is constant

  • Second Law – The entropy of the universe tends to a maximum (overall disorder always increases)

  • The third law of thermodynamics, enunciated by Hermann Nernst (Nernst’s theorem) dictates that it is impossible to cool an object to a temperature of absolute zero ( -273.15 degrees Celsius ). Absolute zero temperature is a state of complete order.

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1860 – Germany

‘Each chemical element, when heated to incandescence, produces its own characteristic lines in the spectrum of light’

For example, sodium produces two bright yellow lines.
Bunsen developed the Bunsen burner in 1855.
In the flame test the Bunsen burner’s non-luminous flame does not interfere with the coloured flame given off by the sample.


Kirchhoff was a professor of physics at Heidelberg. Bunsen and Kirchhoff together developed the first spectroscope, a device used to produce and observe a spectrum. They used their spectroscope to discover two new elements, caesium (1860) and rubidium (1861).

In 1860 Kirchhoff made the discovery that when heated to incandescence, each element produces its own characteristic lines in the spectrum.

This means that each element emits light of a certain wavelength – sodium’s spectrum has two yellow lines (wavelengths about 588 and 589 nanometres). The Sun’s spectrum contains a number of dark lines, some of which correspond to these wavelengths.

The Swedish scientist ANDERS ANGSTROM had, four years earlier, found that a gas always absorbs light at the same wavelength that it emits light. If the gas is hotter than the light source, then more light is emitted by the gas than absorbed, creating a bright line in the spectrum of the light source. If the gas is cooler than the light source the opposite happens; more light is absorbed by the gas than is emitted, creating a dark line.
The dark solar D lines told Kirchhoff that sodium is present in the relatively cool outer atmosphere of the Sun. This could be tested in the laboratory by burning a piece of chalk in a hot oxygen-hydrogen torch. The intensely bright limelight that is produced may be passed through a cooler sodium flame and the light emerging examined through a spectroscope. Crossing the spectrum of the artificial light occur black lines at the same wavelength that a sodium flame emits light. This solved the mystery of the FRAUNHOFER LINES.

Scientists now had a means to determine the presence of elements in stars. By comparing the dark lines in the spectra of light from the stars with the bright lines produced by substances in the laboratory, Kirchhoff had been able to identify the elements that made up a celestial body millions of miles away in space.



In England the astronomer William Huggins recorded the spectra of hundreds of stars and showed the unmistakable fingerprints of familiar elements that are found on the Earth’s surface. The stars are made of exactly the same kind of atoms as the Earth.

In 1868 Norman Lockyer described a spectral line in the yellow region very close to the wavelength of the two ‘D’ spectral lines of sodium. After repeated attempts to discover a substance that produced the same line on Earth, it appeared that the line did not correspond to any hitherto known element. Lockyer gave the element the name ‘helium’, the gas later to be found associated with radioactive decay in ores containing uranium.


Helium had not previously been found on Earth because it is both inert and lighter than air, ironic because after hydrogen, helium is the second most common element in the universe.

In 1904 RUTHERFORD would declare that the presence of helium in the Sun was evidence that sunlight was a product of radioactive processes. The absence of any FRAUNHOFER lines in sunlight that corresponded to radium dealt a blow to this hypothesis. Was there another way of releasing atomic energy than radioactivity?


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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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1885 – Germany

‘Daimler was convinced that steam power was outdated’

early motor vehicle

In 1885 he perfected the first petroleum-injected internal combustion engine and produced the first motorcycle and the first four-wheeled petrol driven car.

The foundation for Daimler’s work had already been laid in the creation of two- and four-stroke gas-fuelled internal combustion engines by early pioneers Joseph Etienne Lenoir (1822-1900), Alphonse Beau de Rochas (1815-93) and Nikolaus August Otto (1832-91).

diamler-benz engine serial number

Although liquid petroleum was well-known, it had been of no use in developing the internal combustion engine because the liquid could not be compressed in the same manner as gas. The four-stroke engine awaited the development of the carburetor, which converted the liquid petroleum into a thin spray, which could be compressed and sparked.

In 1885 Karl Benz (1844-1929) designed and constructed a three-wheel vehicle powered by a 0.75 horsepower engine.

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1888 – Germany

‘Radio waves can be produced by electric sparks. They have the same speed as light and behave as light’

Hertz’s discovery provided the basis of radio broadcasting.

In 1864 MAXWELL‘s equations predicted the existence of electromagnetic waves.
His thinking had shown that electromagnetic waves could be refracted, reflected and polarized in the same way as light. Hertz was able to measure the speed of these waves and to show that the speed is the same as that of light.

Hertz hypothesised that he could experimentally examine the waves by creating apparatus to detect electromagnetic radiation. He devised an electric circuit with a gap that would cause a spark to leap across when the circuit was closed. If Maxwell’s theory was correct and electromagnetic waves were spreading from these oscillator sparks, appropriately sensitive equipment should pick up the waves generated by the spark.
Hence he constructed the equivalent of an antenna.
His simple receiver consisted of two small balls at the ends of a loop of wire, separated by a small gap. This receiver was placed several yards from the oscillator and the electromagnetic waves would induce a current in the loop that would send sparks across the small gap. This was the first transmission and reception of electromagnetic waves. He called the waves detected by the antenna ‘Hertzian waves’.

We are now familiar with all the types of electromagnetic waves that make up the complete electromagnetic spectrum. They all travel with the speed of light and differ from each other in their frequency. We measure this frequency in hertz.

It was left to the Italian electrical engineer GUGLIELMO MARCONI to refine this equipment into a device that had the potential of transmitting a message and to develop technology for the practical use of Hertzian  waves – when they became commonly known as radio waves.

Further experimentation showed that these waves had the properties that Maxwell had predicted.
As well as being important as a newly discovered phenomenon, Hertz’s discovery helped to prove that Maxwell had been correct when he suggested that light and heat were forms of electromagnetic radiation.

Radio waves are electromagnetic waves. Other main kinds of electromagnetic waves are: gamma rays; X-rays; ultra-violet radiation; visible light; infrared radiation and microwaves.

This radiation was behaving in all the ways that would be expected for waves, the nature of the vibration and the susceptibility to reflection and refraction were the same as those of light and heat waves. Hertz found that they could be focused by concave reflectors.

Experimenting further, Hertz spotted that electrical conductors reflect this electromagnetic radiation and that non-conductors allow most of the waves to pass through.

In honour of Hertz’s achievements, the SI unit of frequency, the hertz (Hz), was named after him.

Hertz’s discoveries came at an early age. The German physicist died at the age of thirty-six.

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1894 – Germany

‘A catalyst can change the rate of a chemical reaction, but is not itself used up in the reaction’

Portrait of Wilhelm Ostwald ©


The effect of a catalyst is known as catalysis.
The action of a catalyst is specific (a particular catalyst is necessary to catalyse a given reaction) and it can increase or decrease the rate of the reaction.

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MAX PLANCK (1856-1947)

1900 – Germany

‘Energy is not a continuous quantity but it is quantised; it flows in discrete packets or quanta. When particles emit energy they do so only in quanta’

According to Quantum theory, the energy (E) of one quantum (photon) is given by E = hf where f is the frequency of radiation and h is Planck’s constant.
Its value is 6.63 x 10-34 joules per second

h is a tiny number, close to zero, but it is has a finite value. This implies energy is released in discrete chunks, a revolutionary notion.

photo portrait of MAX PLANCK ©


By the late 1800s the science of thermodynamics was developing to the point that people were beginning to understand the nature of energy.
The traditional view was that energy was released in a continuous stream and that any amount of energy could be indefinitely divided into smaller and smaller ‘lumps’. Planck’s work on the laws of thermodynamics and black body radiation led him to abandon this classical notion of the dynamic principles of energy and formulate the quantum theory, which assumes that energy changes take place in distinct packages, or quanta, that cannot be subdivided. This successfully accounted for certain phenomena that Newtonian theory could not explain.

The basic laws of thermodynamics recognised that energy could not be created or destroyed, but was always conserved. The second law was drawn from an understanding that heat would not pass from a colder body to a hotter body.
The study of thermodynamics was based on the assumption that matter was ultimately composed of particles. LUDWIG BOLTZMAN had proposed an explanation of thermodynamics, saying the energy contained in a system is the collective result of the movements of many tiny particles rattling around. He believed the second law was only valid in a statistical sense; it only worked if you added up all the bits of energy in all the little particles.
Among his detractors was Max Karl Ernst Ludwig Planck.

Planck began his work on the second law of thermodynamics and the concept of entropy. He investigated how materials transform between solid, liquid and gaseous states. In doing so he found explanations for the laws governing the differing freezing and boiling points of various substances.
He also looked at the conduction of electricity through liquid solutions (electrolysis).

In the mid 1890s Planck turned his attention to the question of how heated substances radiate energy. Physicists were aware that all bodies radiate heat at all frequencies – although maximum radiation is emitted only at a certain frequency, which depends on the temperature of the body. The hotter the body, the higher the frequency for maximum radiation. (Frequency is the rate per second of a wave of any form of radiation).

Planck had been considering formulae for the radiation released by a body at high temperature. Using ideas developed by ROBERT KIRCHOFF, he knew it should be expressible as a combination of wavelength frequency and temperature. For a theoretical ‘black body’, physicists could not predict expressions that were in line with the behaviour of hot bodies at high frequencies and were in agreement with other equations showing their nature at low frequencies. Thus no law could be found which fitted all frequencies and obeyed the laws of classical physics simultaneously.
Plank resolved to find a theoretical formula that would work mathematically, even if it did not reflect known physical laws. His first attempts were partially successful, but did not take into account any notion of particles or quanta of energy, as he was certain of the continuous nature of energy. In an ‘act of despair’ he renounced classical physics and embraced quanta.

The final straw had been a concept developed by John Rayleigh and James Jeans that became known as the ‘ultraviolet catastrophe’ theory. They had developed a formula that predicted values for radiation distribution and worked at low frequencies, but not at high frequencies. It was at odds with Planck’s formula, which worked for high frequencies but broke down at low frequencies. In June 1900 Rayleigh had pointed out that classical mechanics, when applied to the oscillators of a black-body, leads to an energy distribution that increases in proportion to the square of the frequency. This conflicted with all known data.

Planck’s answer was to introduce what he called ‘energy elements’ or quanta and to express the energy emitted as a straightforward multiplication of frequency by a constant, which became known as ‘Planck’s constant’ (6.6256 x 10-34 Jsec-1). This only works with whole number multiples which means for the formula to have any practical use one must accept the radical theory that energy is only released in distinct, non-divisible chunks, known as ‘quanta’, or for a single chunk of energy, a ‘quantum’. This completely contradicts classical physics, which assumed that energy is emitted in a continuous stream. The individual quanta of energy were so small that when emitted at the everyday large levels observed, it appears that energy could seem to be flowing in a continuous stream.
Thus classical physics was cast into doubt and quantum theory was born.

Planck announced his theory on December 14 1900 in his paper ‘On the Theory of the Law of Energy Distribution in the Continuous Spectrum’. Planck said ‘energy is made up of a completely determinate number of finite equal parts, and for this purpose I use the constant of nature h = 6.55 x 10-27(erg sec)’

photo portrait of MAX PLANCK ©

When ALBERT EINSTEIN was able to explain the ‘photoelectric’ effect in 1905, suggesting that light is emitted in quanta called ‘photons’, by applying Planck’s theory – and likewise NIELS BOHR in his explanation of atomic theory in 1913 – the abstract idea was shown to explain physical phenomena.

Planck was awarded the Nobel Prize for Physics in 1918.

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1915 – Germany

‘Continental land masses are constantly in motion. The Earth’s land surface was once one big super-continent. About 250 million years ago it broke up into the continents we know today, which have since drifted to their present positions’

Photograph of ALFRED WEGENER ©


Wegener proposed that the continental land masses are moving over the face of the Earth.
Rock under the ocean is principally Basalt, a denser rock than the Granite that makes up the continents. At the start of the Earth’s history there was just a single landmass, which began to break up 200 million years ago, and the parts are still moving. Mountain ranges have been produced where one moving land mass crashes into another, pushing rocks together and forcing them upwards in folds. The tectonic plates move over the asthenosphere carried by convection currents in the magma below.

Up until and beyond Wegener’s death his ideas had little scientific credence – until in the 1950s the mid-Atlantic ridge was discovered. It was this discovery that led to the concept of the tectonic plates.