Drink a Toast to James Prescott Joule!

by Nicholas Mee on December 6, 2020

Manchester was a boom town in the late eighteenth century. The industrial revolution was well under way and cotton mills were springing up around the city drawing an influx of workers from surrounding villages and towns. William Joule was one of the newcomers enticed by the industrial gold rush.

William Joule came from the village of Youlgrave in Derbyshire where his father was an innkeeper. The letter ‘J’ is a late-comer to the English alphabet, only gradually becoming distinguished from the letter ‘I’ during the eighteenth century, and records survive where the family name has the alternative spelling Youl. So it seems the family name Joule arose from the name of their home village Youlgrave. 

Youlgrave in Derbyshire.

The Joule family business was brewing. By 1780 William’s brother Francis had established a brewery in the town of Stone in Staffordshire, and some time around 1788 William set up a brewery in Salford near Manchester. William’s business prospered and by 1798 he had installed a steam engine. It wasn’t long before William Joule & Son became the biggest brewery in the Manchester area. 

An Atomic Education

The brewery was passed on to William’s son Benjamin who as a wealthy Manchester businessman obtained the best possible education for his own son James Prescott Joule. For two years James was tutored by the great John Dalton (1766-1844), famous for explaining chemistry in terms of atoms.

When James Prescott Joule inherited a share in the family business he began experimenting with ways to make the brewing process more efficient. In particular he was interested in whether it might be profitable to replace the now ageing steam engine with a newly invented electric motor powered by a zinc battery. These investigations revealed that each pound of coal burnt in a steam engine could generate five times as much work as a pound of zinc consumed in an electric battery. And, as coal was far cheaper than zinc, Joule concluded it was much more economical to continue using the steam engine.

The Conservation of Energy

James Prescott Joule (1818-1889).

Joule soon became interested in the fundamental principles of physics rather than the practical considerations of brewing beer. Throughout his career he exchanged ideas with his renowned contemporary John Thomson, better known as Lord Kelvin. Together they worked to understand the connections between energy, work, heat and temperature, a subject known as thermodynamics. Joule’s investigations led to one of the most important principles in physics—the conservation of energy.

This principle remains at the heart of physics. It means that although energy may be converted from one form into another, if all types of energy are taken into account, including heat, then the total amount of energy never changes. In a power station, for instance, coal or gas is burned to heat water and produce steam that drives a turbine and generates electricity, which is fed across the grid to power our computers and other appliances at home. At each step in this sequence energy is transformed from one form to another. But, even though the details of each process might be very complicated, we can be sure that the total amount of energy always remains the same if we take into account the heat that is dissipated along the way.

The Unit of Energy

James Prescott Joule’s gravestone.

Joule studied the relationship between energy and temperature, which we now know is a measure of the energy in the random motions of an object’s molecular constituents. As these internal motions increase the object feels hotter. Conversely, a decrease in the internal motions makes the object feel colder. In his laboratory, Joule measured the amount of energy in the form of physical work that is required to raise the temperature of a body of water by one degree. He would refine these measurements throughout his life.

Joule died in 1889 and is buried in the town of Sale just outside Manchester. The number 772.55 is engraved at the top of his gravestone, shown here. This number is the crux of Joule’s investigations in the laboratory. It is the amount of work in foot pounds required to raise the temperature of one pound of water by one degree Fahrenheit. Fortunately, physicists no longer deal in such arcane units as foot pounds. Today the standard unit of energy is known as the joule in honour of James Prescott Joule and his work. (Incidentally, one foot pound is equivalent to 1.35582 joules.)

Joule’s brewery in Staffordshire, originally established by James Prescott’s great uncle Francis, was resurrected in 2010. So once again it is possible to drink a pint of Joule’s ale and raise a toast James Prescott Joule.

Further Information

There is more about the re-established Joule’s brewery here: https://www.joulesbrewery.co.uk/our-story


Rutherford’s Protons and Prout’s Protyles

by Nicholas Mee on November 27, 2020

John Dalton (1766-1844) grew up in the English Lake District and spent most of his working life in Manchester. In 1808 he published A New System of Chemical Philosophy where he argued that each chemical element is composed of a distinct type of atom. Dalton listed the elements in a table (shown on the right), assigning a chemical symbol to each and giving its atomic mass as a multiple of that of hydrogen. His figures were, however, rather inaccurate.

Dalton believed, quite correctly, that compounds consist of combinations of atoms and that these combinations are rearranged in chemical reactions. He illustrated his ideas with small wooden balls of varying sizes, similar to the much later ball and stick models used to represent molecules.

Prout’s Hypothesis

William Prout (1785-1850)

William Prout (1785-1850) was a contemporary of Dalton’s. He was a medical doctor and accomplished chemist who studied organic materials and was the first to classify foodstuffs as fats, carbohydrates and proteins.

Prout published two papers in 1815 and 1816 where he observed that the atomic mass of every element so far measured was a multiple of the atomic mass of hydrogen. He suggested that this might be because there was just one fundamental unit or protyle, as he called it, from which all the elements were formed. This idea became known as Prout’s hypothesis. It implied that an atom of hydrogen was formed of a single protyle, whereas each atom of the heavier elements was composed of several protyles that were somehow bound together. This was a great idea. Unfortunately, the experimental basis on which it was constructed seemed to be flawed.

Jöns Jacob Berzelius


The Swedish chemist Jöns Jacob Berzelius (1779–1848) was one of the founders of modern chemistry and made wide-ranging contributions throughout the subject. One of his achievements was to replace the old alchemical symbols with the much more flexible modern chemical notation. He also conclusively demonstrated that elements combine in fixed proportions thereby lending support to Dalton’s atomic hypothesis. But Berzelius’s measurements were far more accurate than Dalton’s. And when he published a table of atomic masses in his Textbook of Chemistry in 1826 he showed that they could not be strict integer multiples of the atomic mass of hydrogen. Berzelius’s results discredited Prout’s hypothesis and it fell out of favour with chemists. 

Lord Rayleigh

But Prout’s hypothesis was never completely forgotten. The simplicity of the idea appealed especially to physicists. In 1904 when Lord Rayleigh accepted the Nobel Prize for Physics he recalled that Prout’s hypothesis had inspired his research.

The subject of the densities of gases has engaged a large part of my attention for over 20 years. In 1882 in an address to the British Association I suggested that the time had come for a re-determination of these densities, being interested in the question of Prout’s law. At that time the best results were those of Regnault, according to whom the density of oxygen was 15.96 times that of hydrogen. The deviation of this number from the integer 16 seemed not to be outside the limits of experimental error.

Rayleigh attempted to resolve the issue by investigating the atomic mass of nitrogen, and this led to the discovery of the noble gas argon and Rayleigh’s Nobel Prize, as discussed here: William Ramsay’s Noble Quest.

Modern values for the atomic masses of many lighter elements (shown in the table above) are indeed remarkably close to multiples of hydrogen’s. Hydrogen is quoted as 1.008, with the atomic masses of helium, carbon, nitrogen and oxygen quoted as 4.003, 12.011, 14.007 and 15.999 respectively. These masses may not be exact multiples of hydrogen’s, but as Rayleigh pointed out, they do seem suspiciously close. There are exceptions, however, most notably chlorine with an atomic mass of 35.453. So was Prout correct or not?

Canal Ray Conundrum

Joseph John Thomson (1856-1940), or J.J. Thomson as he is usually known, succeeded Rayleigh as Director of the Cavendish Laboratories in Cambridge in 1884. During the 1890s Thomson investigated the nature of cathode rays and is credited with the discovery of the first sub-atomic particle—the electron. By 1912, Thomson had turned his attention to streams of positively charged ions, then known as canal rays. Thomson and his research assistant Francis Aston developed a way to measure their mass by firing beams of ions through a magnetic field and recording their trajectories on a photographic plate. The mass of each ion was indicated by the degree to which its beam was deflected by the magnetic field. To Thomson and Aston’s surprise singly ionised neon (Ne) atoms produced two separate tracks, as labelled in the image above. They initially assumed that neon must be composed of two distinct gases neon and meta-neon whose atoms had the same properties apart from their mass. In a sense this is true.

Further light was shed on the conundrum the following year when Frederick Soddy showed that a radioactive species referred to as ionium was chemically identical to thorium, but with atoms of a slightly different mass. Soddy soon realised that many radioactive elements have atoms with several distinct masses. He described them as different isotopes of the element. Although all Soddy’s examples were found amongst the heaviest known elements, most of which were radioactive, he believed that even the lighter elements might be mixtures of different isotopes.

The Mass Spectrometer

Francis Aston

In 1918 J.J. Thomson became Master of Trinity College, Cambridge, and the following year Ernest Rutherford took over his position as Director of the Cavendish Laboratories.

Aston had spent the war years working for the newly formed Royal Air Force at Farnborough. On his return to the Cavendish, Rutherford encouraged him to further develop the apparatus he had used as Thomson’s assistant prior to the war. By November 1919 he had constructed the mass spectrograph, a device that would evolve into the mass spectrometer. By now Soddy’s proposal of isotopes was generally accepted and the meta-neon atoms were recognised as a second isotope of neon. 

With the mass spectrograph Aston could determine atomic masses with greater accuracy than ever before. His measurements showed that the atomic masses of the two neon isotopes were almost exactly twenty and twenty-two times the mass of hydrogen. He then found that chlorine had two isotopes with masses thirty-five and thirty-seven times that of hydrogen. This explained why the atomic mass of chlorine was not close to an integer—it was a mixture of two isotopes.

The Whole Number Rule

During the early months of 1920 Aston systemically analysed various elements and discovered that many were composed of more than one isotope. (Over his career he would discover more than 200 isotopes.) Aston found that every isotope had an atomic mass that was almost exactly a whole number multiple of hydrogen’s. He referred to this observation as the Whole Number Rule. It implied that whenever previous measurements had assigned an element a fractional atomic mass it was because the element was a mixture of isotopes. 

Francis Aston with his mass spectrograph.

Aston’s Whole Number Rule resurrected Prout’s hypothesis of a century earlier. But now the picture was looking decidedly clearer. In 1917 Rutherford had discovered one of the constituents of the atomic nucleus, a positively charged particle identical to the nucleus of a hydrogen atom. In 1920, and partly as a tribute to Prout, he named this particle the proton, taking its stem from Prout’s fundamental unit—the protyle. Rutherford speculated that the nucleus might have a second electrically neutral constituent with a similar mass to the proton. He named it the neutron.

Nobel Prizes

In 1922 Soddy and Aston both received a Nobel Prize for Chemistry. Soddy was given the 1921 award and Aston the 1922 award. Soddy’s award was

for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes.

Aston’s award was

for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule.

The Neutron

Rutherford’s team at the Cavendish searched for the neutron throughout the 1920s, but without success. Finally in 1932, James Chadwick saw an opportunity that others had missed and tracked down the elusive particle. This discovery resolved the issue of isotopes. The nucleus of an atom is composed of two types of particle—protons and neutrons. The number of protons determines the chemical identity of the atom and the element to which it belongs, but the number of neutrons can vary and this is why some elements have more than one isotope. For instance, neon’s isotopes are neon-20 whose nucleus contains ten protons and ten neutrons, and neon-22 whose nucleus contains ten protons and twelve neutrons.

Nevertheless, Aston’s Whole Number Rule cannot be exactly true. The nucleus of a hydrogen atom is a single proton, whereas other atoms have neutrons and protons in their nucleus. But the neutron’s mass is 0.1% greater than the proton’s mass. Also, binding energy is released when a nucleus forms, so it has slightly less mass than an equivalent number of free neutrons and protons. Therefore the mass of a nucleus cannot simply be a multiple of the proton’s mass.

Replica of Francis Aston’s third mass spectrometer. Credit: Jeff Dahl/Wikimedia.

The mass spectrometer is now a standard piece of laboratory equipment with widespread applications in physics, chemistry, biology and medical research. Carbon isotope separation for radiocarbon dating is just one example where this technology has had an enormous impact.

During World War II separating the isotopes of uranium became a top priority and we are still living with the repercussions today. But that is another story.

Further Information

There is more information about Frederick Soddy and his role in the discovery of radioactive isotopes here: Frederick Soddy and the World Set Free.

The World’s First Nuclear Reactor

November 22, 2020

The investigations by the French authorities in 1972 led to a uranium mine in the Central African country of Gabon, then a French overseas territory. The reason for the missing uranium-235 turned out to be quite incredible. Fermi’s Chicago nuclear reactor had been foreshadowed 1.7 billion years earlier by a natural nuclear reactor in a uranium deposit at Oklo near Franceville. Detailed analysis of the unusual nuclear isotopes found in the deposit has enabled physicists to construct a picture of what happened there long long ago.

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Twenty Thousand Leagues Under the Seas

November 21, 2020

Walt Disney played his part in selling Rickover’s revolutionary submarine to the American public. In 1954 Disney released its steampunk blockbuster 20,000 Leagues Under the Sea based on the Jules Verne masterpiece. It was the most expensive Hollywood film up to that date and starred James Mason as the dark anti-hero Captain Nemo. Although the film is set in the nineteenth century, it suggests that Nautilus is powered by a new and mysterious source of energy discovered by Nemo. The nature of this energy would be all too obvious to the cinema-goers of the 1950s who were very familiar with the nuclear ambitions of the United States.

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The Age of Rock

October 31, 2020

Rutherford suggested that the amount of helium trapped within a rock would reveal how much uranium has decayed since the rock formed. And by comparing the amount that has decayed to the amount that remains the age of the rock could be determined. For instance, if half the uranium has decayed then the rock’s age equals the half-life of uranium, which can be measured in the laboratory. 

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Pregnant Camels Often Sit Down Carefully

October 30, 2020

After several months of arduous work Holmes could assign an age to the rock—370 million years. This is considered to be the first accurate radiometric dating. (It agrees with modern results for the Devonian, according to which this period lasted from 419-359 million years ago.) Not bad for an undergraduate project! Holmes was just twenty years old and would graduate later that year. His project must rank as one of the most remarkable ever completed by an undergraduate.

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Why Does Antimatter Matter?

October 17, 2020

There is a very important modern technology that makes use of antimatter. It is known as PET (Positron Emission Tomography) and it is a non-invasive medical imaging technique.

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Hell Creek Apocalypse

September 27, 2020

Four decades of detective work by geologists, nuclear physicists, palaeontologists and geochronologists has given us an incredibly detailed and compelling picture of what happened one dramatic day just over sixty-six million years ago—the day that sealed the fate of the dinosaurs.

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A Goddess Spurned and the Fate of the Sun

September 25, 2020

So how will the Sun evolve and what is its ultimate fate?
Within the Sun’s core hydrogen nuclei are gradually fusing to form helium nuclei, releasing a steady stream of energy. The flow of thermal energy from the nuclear furnace supports the Sun against its tendency to collapse under gravity. For as long as the nuclear fuel holds out this balance will be maintained. In the case of the Sun this is around ten billion years. Currently, the Sun is nearly half way through its allotted time span.

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An Exceptionally Cold Case

September 19, 2020

Much has been learnt about Ötzi the Iceman. High levels of copper and arsenic were found in his body, which suggests that he may have been involved in the smelting of copper. He was about 45 years old and he suffered from a number of ailments. He had parasitic worms in his gut, gallstones, arthritic joints, hardened arteries and rotting teeth. He also had numerous tattoos produced by rubbing charcoal into lesions in the skin. From the contents of Ötzi’s stomach it seems that his last meal included fatty ibex meat and einkorn grain. The radiocarbon dating of Ötzi’s remains reveal that he lived some time around 5300 years ago.

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