1890 – Ireland
1904 – Holland

‘A moving object appears to contract’

The contraction is negligible unless the object’s speed is close to the speed of light.

In 1890 Fitzgerald suggested that an object moving through space would shrink slightly in its direction of travel by an amount dependent on its speed.

In 1904 Lorentz independently studied this problem from an atomic point of view and derived a set of equations to explain it. A year later, Einstein derived Lorentz’s equations independently from his special theory of relativity.

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LEON FOUCAULT (1819- 68)

1850 – France

portrait of LEON FOUCAULT (seated) ©

LEON FOUCAULT  (Smithsonian)

‘A Foucault pendulum is a simple pendulum – a long wire with a heavy weight (bob) at the end – except that at the top it is attached to a joint which allows it to swing in any direction’

Foucault’s pendulum proved that the Earth is rotating

Once a Foucault pendulum is set in motion, it seems not to swing back and forth in the same direction but to rotate. In fact, it is the rotation of the Earth beneath the pendulum which gives rise to its apparent rotation.
The angle of rotation per hour, which is constant at any particular location, can be calculated from the formula 15 sin Φ, where Φ is the geographical latitude of the observer. At the North or South Pole, the pendulum would rotate through 360 degrees once a day. At the equator it would not rotate at all.

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1869 – Russia

‘The properties of elements are periodic functions of their atomic weights’

Arrange the atoms in order of their atomic weight (relative atomic mass) and elements are also arranged in order of their properties. This arrangement of the elements is called the periodic table.

In the modern periodic table elements are no longer arranged by their atomic weight but by a more fundamental quantity; ‘atomic number’.

photo portrait of DIMITRY IVANOVICH MENDELEYEV© - link to Britannica online article


The atomic number of an element is the number of protons in the nucleus of one of its atoms; the number of neutrons, which contributes to atomic weight, is ignored. The modern periodic law is that ‘The properties of elements are periodic functions of their atomic numbers’.

In 1860 Dmitri Ivanovich Mendeleev attended a chemistry conference in Karlsruhe where the Italian Stanislao Cannizzaro’s speech announcing his rediscovery of the distinction between atoms and molecules (originally announced in 1811 by AVOGADRO) made a profound impression.

The German chemist Johann Wolfgang Döbereiner (1780-1849) had recognised mathematical patterns in elements that had similar properties. He found that adding the atomic weights of calcium (40) and barium (137) and dividing the total in two left a value close to the weight of strontium (88). Finding this same pattern repeated for lithium, sodium and potassium, and for chlorine, bromine and iodine confirmed the relationship, which he termed the Law of Triads.

In 1862, French scientist Alexandre Beguyer de Chancourtois developed a way of representing the elements by wrapping a helical list around a cylinder.

A repeating pattern in natural phenomena is a strong indication that there exists a simple, compact description.
The periodic table suggests that the distinct atoms of the elements may be described in terms of significantly fewer building blocks than the number of the individual elements. Atoms, then, were made of significantly fewer subatomic building blocks.

In 1869 the 35-year-old Mendeleev published a table of the 61 elements then known. His list of elements – ‘On the Relation of the Properties to the Atomic Weights of Elements’ – occupied a grid where the atomic weight increased as you went down a column (periods) and the elements in any particular row (groups or families) shared similar properties and valencies (metals and gases, for instance).

Mendeleev had to juggle the order of a few elements, assuming their weights to have been incorrectly measured, and predicted that some undiscovered elements would fill the gaps in the table, based on the properties of the elements surrounding the gaps.
The modern periodic table has been turned sideways.

By 1886, with the discoveries of gallium, scandium and germanium with the properties he had foretold, his prediction was fulfilled. By 1925, chemists had successfully identified all the 92 elements they believed to exist in nature. The first artificial element, neptunium, was synthesised in 1940. Many more elements have been made since then.

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‘The Genesis of a Law of Nature’ – Mendeleev


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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1864 Scotland

The Scottish physicist examined Faraday’s ideas concerning the link between electricity and magnetism interpreted in terms of fields of force and saw that they were alternative expressions of the same phenomena. Maxwell took the experimental discoveries of Faraday in the field of electromagnetism and provided his unified mathematical explanation, which outlined the relationship between magnetic and electric fields. He then proved this by producing intersecting magnetic and electric waves from a straightforward oscillating electric current.

‘Four equations that express mathematically the way electric or magnetic fields behave’

In 1831 – following the demonstration by HANS CHRISTIAN OERSTED that passing an electric current through a wire produced a magnetic field around the wire, thereby causing a nearby compass needle to be deflected from north – MICHAEL FARADAY had shown that when a wire moves within the field of a magnet, it causes an electric current to flow along the wire.
This is known as electromagnetic induction.

In 1864 Maxwell published his ‘Dynamical Theory of the Electric Field’, which offered a unifying, mathematical explanation for electromagnetism.

In 1873 he published ‘Treatise on Electricity and Magnetism’.

The equations are complex, but in general terms they describe:

  • a general relationship between electric field and electric charge
  • a general relationship between magnetic field and magnetic poles
  • how a changing magnetic field produces electric current
  • how an electric current or a changing electric field produces a magnetic field

The equations predict the existence of electromagnetic waves, which travel at the speed of light and consist of electric and magnetic fields vibrating in harmony in directions at right angles to each other. The equations also show that light is related to electricity and magnetism.

Maxwell worked out that the speed of these waves would be similar to the speed of light and concluded, as Faraday had hinted, that normal visible light was a form of electromagnetic radiation. He argued that infrared and ultraviolet light were also forms of electromagnetic radiation, and predicted the existence of other types of wave – outside the ranges known at that time – which would be similarly explainable.

Verification came with the discovery of radio waves in 1888 by HEINRICH RUDOLPH HERTZ. Further confirmation of Maxwell’s theory followed with the discovery of X-rays in 1895.

photo portrait of JAMES CLERK MAXWELL ©


Maxwell undertook important work in thermodynamics. Building on the idea proposed by JAMES JOULE, that heat is a consequence of the movement of molecules in a gas, Maxwell suggested that the speed of these particles would vary greatly due to their collisions with other molecules.

In 1855 as an undergraduate at Cambridge, Maxwell had shown that the rings of Saturn could not be either liquid or solid. Their stability meant that they were made up of many small particles interacting with one another.

In 1859 Maxwell applied this statistical reasoning to the general analysis of molecules in a gas. He produced a statistical model based on the probable distribution of molecules at any given moment, now known as the Maxwell-Boltzmann kinetic theory of gases.
He asked what sort of motion you would expect the molecules to have as they moved around inside their container, colliding with one another and the walls. A reasonably sized vessel, under normal pressure and temperature, contains billions and billions of molecules. Maxwell said the speed of any single molecule is always changing because it is colliding all the time with other molecules. Thus the meaningful quantities are molecular average speed and the distribution about the average. Considering a vessel containing several different types of gas, Maxwell realized there is a sharp peak in the plot of the number of molecules versus their speeds. That is, most of the molecules have speeds within a small range of some particular value. The average value of the speed varies from one kind of molecule to another, but the average value of the kinetic energy, one half the molecular mass times the square of the speed, (1/2 mv2), is almost exactly the same for all molecules. Temperature is also the same for all gases in a vessel in thermal equilibrium. Assuming that temperature is a measure of the average kinetic energy of the molecules, then absolute zero is absolute rest for all molecules.

The Joule-Thomson effect, in which a gas under high pressure cools its surroundings by escaping through a nozzle into a lower pressure environment, is caused by the expanding gas doing work and losing energy, thereby lowering its temperature and drawing heat from its immediate neighbourhood. By contrast, during expansion into an adjacent vacuüm, no energy is lost and temperature is unchanged.

The explanation that heat in gas is the movement of molecules dispensed with the idea of the CALORIC fluid theory of heat.

The first law of thermodynamics states that the heat in a container is the sum of all the molecular kinetic energies.
Thermal energy is another way of describing motion energy, a summing of the very small mechanical kinetic energies of a very large number of molecules; energy neither appears nor disappears.
According to BOYLE’s, CHARLES’s and GAY-LUSSAC’s laws, molecules beating against the container walls cause pressure; the higher the temperature, the faster they move and the greater the pressure. This also explains Gay-Lussac’s experiment. Removing the divider separating half a container full of gas from the other, evacuated, half allows the molecules to spread over the whole container, but their average speed does not change. The temperature remains the same because temperature is the average molecular kinetic energy, not the concentration of caloric fluid.

In 1871 Maxwell became the first Professor of Physics at the Cavendish Laboratory. He died at age 48.

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GEORGE BOOLE (1815- 64)

1854 – England

‘Logical operations can be expressed in mathematical symbols rather than words and can be solved in a manner similar to ordinary algebra’

Boole’s reasoning founded a new branch of mathematics. Boolean logic allows two or more results to be combined into a single outcome. This lies at the centre of microelectronics.

picture of mathematician George Boole


Boolean algebra has three main logical operations: NOT, AND, OR.
In NOT, for example, output is always the reverse of input. Thus NOT changes 1 to 0 and 0 to 1.

Boole’s first book ‘Mathematical Analysis of Logic’ was published in 1847 and presented the idea that logic was better handled by mathematics than metaphysics. His masterpiece ‘An Investigation into the Laws of Thought’ which laid the foundations of Boolean algebra was published in 1854.

Unhindered by previously determined systems of logic, Boole argued there was a close analogy between algebraic symbols and symbols that represent logical interactions. He also showed that you could separate symbols of quality from those of operation.

His system of analysis allowed processes to be broken up into a series of individual small steps, each involving some proposition that is either true or false.
At its simplest, take two proposals at a time and link them with an operator. By adding many steps, Boolean algebra can form complex decision trees that produce logical outcomes from a series of previously unrelated inputs.

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1843 – England

‘A given amount of work produces a specific amount of heat’

4.18 joules of work is equivalent to one calorie of heat.

In 1798 COUNT RUMFORD suggested that mechanical work could be converted into heat. This idea was pursued by Joule who conducted thousands of experiments to determine how much heat could be obtained from a given amount of work.

Even in the nineteenth century, scientists did not fully understand the properties of heat. The common belief held that it was some form of transient fluid – retained and released by matter – called CALORIC. Gradually, the idea that it was another form of energy, expressed as the movement of molecules gained ground.
Heat is now regarded as a mode of transfer of energy – the transfer of energy by virtue of a temperature difference. Heat is the name of a process, not that of an entity.

Joule began his experiments by examining the relationship between electric current and resistance in the wire through which it passed, in terms of the amount of heat given off. This led to the formulation of Joule’s ideas in the 1840s, which mathematically determined the link.

Joule is remembered for his description of the conversion of electrical energy into heat; which states that the heat (Q) produced when an electric current (I) flows through a resistance (R) for a time (t) is given by Q=I2Rt

Its importance was that it undermined the concept of ‘caloric’ as it effectively determined that one form of energy was transforming itself into another – electrical energy to heat energy. Joule proved that heat could be produced from many different types of energy, including mechanical energy.

john collier portrait of james prescott joule


Joule's apparatus to show equivalence of work and heat

Joule’s apparatus to show equivalence of work and heat

Joule was the son of a brewer and all his experiments on the mechanical equivalent of heat depended upon his ability to measure extremely slight increases in temperature, using the sensitive thermometers available to him at the brewery. He formulated a value for the work required to produce a unit of heat. Performing an improved version of Count Rumford’s experiment, he used weights on a pulley to turn a paddle wheel immersed in water. The friction between the water and the paddle wheel caused the temperature of the water to rise slightly. The amount of work could be measured from the weights and the distance they fell, the heat produced could be measured by the rise in temperature.

Joule went on to study the role of heat and movement in gases and subsequently with WILLIAM THOMSON, who later became Lord Kelvin, described what became known as the ‘Joule-Thomson effect’ (1852-9). This demonstrated how most gases lose temperature on expansion due to work being done in pulling the molecules apart.

Thomson thought, as CARNOT had, that heat IN equals heat OUT during a steam engine’s cycle. Joule convinced him he was wrong.

The essential correctness of Carnot’s insight is that the work performed in a cycle divided by heat input depends only on the temperature of the source and that of the sink.

Synthesising Joule’s results with Carnot’s ideas, it became clear that a generic steam engine’s efficiency – work output divided by heat input – differed from one (100%) by an amount that could be expressed either as heat OUT at the sink divided by heat IN at the source, or alternatively as temperature of the sink divided by temperature of the source. Carnot’s insight that the efficiency of the engine depends on the temperature difference was correct. Temperature has to be measured using the right scale. The correct one had been hinted at by DALTON and GAY-LUSSAC’s experiments, in which true zero was minus 273degrees Celsius.

A perfect cyclical heat engine with a source at 100degrees Celsius and a sink at 7degrees has an efficiency of 1 – 280/373. The only way for the efficiency to equal 100% – for the machine to be a perfect transformer of heat into mechanical energy – is for the sink to be at absolute zero temperature.

Joule’s work helped in determining the first law of thermodynamics; the principle of the conservation of energy. This was a natural extension of his work on the ability of energy to transform from one type to another.

Joule contended that the natural world has a fixed amount of energy which is never added to nor destroyed, but which just changes form.

The SI unit of work and energy is named the joule (J).

link to James Joule - Manchester Museum of Science & Industry

Manchester Museum of Science & Industry

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