4. PUTTING NUMBERS ON

GEOLOGIC AGES

Niels Bohr’s first meeting with his postdoctoral advisor, Sir J. J. Thomson: “This is wrong.”

PUTTING NUMBERS ON GEOLOGIC AGES

Without having numbers for geologic ages we had no idea how long something had lasted and no idea of rates of change.

Baron Cuvier’s idea of a history punctuated by catastrophes implied that conditions had remained the same for long periods of time, and were then changed by very rapid, catastrophic events.

Charles Lyell’s ‘uniformitarianism’ implied that change had occurred at rates that were imperceptibly slow relative to human lifetimes.

Geology desperately needed a numerical foundation to understand Earth’s history. The knowledge would come in stages, from wholly unexpected sources. First we would learn about the immensity of geologic time, and much later we would figure out how to determine the rates of geologic processes.

The rate at which something happens is critically important in determiningwhat happens.

Consider the difference in what happens if you drive your car into a concrete barrier at 1 mile per hour versus 10 miles per hour or 100 miles per hour. In the first instance the driver might experience a slight discomfort and no damage would occur to the car. In the second instance you might be a little shaken up, and the repair bill for the car would be a few hundred dollars. In the third instance,, you would very likely be killed, so you would no longer need to worry about the damage to your demolished car.

What is happening to our planet right now is change that is going at a rate between 200 and 400 times faster than anything in the geologic past, except for that accident 65 million years ago when an asteroid struck on the northern coast of Yucatan.

PROCESS RATES ARE IMPORTANT

1863 - PHYSICS COMES TO THE RESCUE - EARTH IS NOT MORE THAN 100 MILLION

YEARS OLD

During the 19th century, there were two important questions that needed answers:

1. What caused the ice age?

2. How old is the Earth?

Both were answered by Lord Kelvin

“Lord Kelvin” was William Thomson (1824 – 1907) named by Queen Victoria as the 1st Baron Kelvin.

He was an Irish mathematical physicist and engineer, educated at Cambridge University

He became professor at Glasgow University. He is widely known for developing the scale of absolute temperature measurement named in his honor.

He was given the title of Baron Kelvin in honor of his achievements. The title refers to the River Kelvin, which flows past his university in Glasgow.

Who was Lord Kelvin?

Lord Kelvin’s idea was based on his studies of heat conduction

The Earth had been cooling since its formation some 20 to 40 million years ago.

Prior to the Cenozoic the heat flow from the interior of the Earth was greater than the heat received from the sun.

Therefore there were no climate zones on Earth before the Cenozoic!

The ice age was simply a reflection of the cooling of the Earth

Perhaps the deglaciation reflected an increase in solar luminosity or the general accumulation of heat in the Universe.

Many geologists, and even Charles Darwin were highly skeptical of Kelvin’s 100 million year age for the Earth. They felt intuitively it was not long enough to accommodate everything that had happened or the evolution of life.

But in 1899 when physicist John Joly of Dublin determined the global rate of delivery of salt to the ocean by rivers and concluded from its present salt content that the ocean was about 90 million years old, te matter seemed settled.

The general geological community came to accept the idea that Earth was at most about 100 million years old. After all, the simple fact is that humans have no concept of time longer than a few generations, and imagining how long it might take to erode away a mountain range was purely a guess.

The Earth is not simply cooling down as Lord Kelvin had assumed in his calculation of the age of our planet. It has its own internal heat sources in the slow growth of its core and in the radioactive decay of uranium, thorium and potassium. They produce heat at a rate of about 3.8x1013 watts globally.

Most of these are concentrated in the granitic rocks of the continents. Plate tectonics involves large scale convection of Earth’s mantle, with warm material upwelling along the Mid-Ocean Ridge system.

Directly on the ridge system heat flow may exceed 300 mW/m2 and in regions of old ocean crust it may drop below 40 mW/m2. The highest geothermal heat flux on Earth is at Yellowstone, where it may reach 5.5 W/m2.

The global average is 87 mW/m2, 0.26% (2.6 thousandths!) of the average energy received from the Sun.

Even in Yellowstone heat from the Earth’s interior has no impact on climate.

HEAT FLOWS IN mW (milliwatts)

Yellowstone – 5,000

THE PROBLEM WITH JOLY’S IDEA – THE SALT DELIVERED TO THE SEA BY RIVERS IS RECYCLED

IN REALITY, OCEAN SALINITY HAS ALWAYS BEEN HIGHER IN THE PAST THAN IT IS TODAY

Sal

inity

Age in MY

UNDERSTANDING 19TH CENTURY CHEMISTRY

An unlikely-seeming center for the advancement of science at the time was the soot-covered city of Manchester, England. Mark Twain commented: ‘I would like to live in Manchester, England. The transition between Manchester and death would be unnoticeable.’ The reason for the depressing conditions in Manchester was that it was at the forefront of the industrial revolution set in motion by James Watt’s invention of the steam engine in 1775.

A better understanding of the practical aspects of physics and chemistry was essential to the developing industries.

At that time the only universities in England were Oxford and Cambridge, and study there was restricted to wealthy Anglicans.

The Manchester city fathers saw the need for local opportunities for higher education and the Manchester Academy was founded in 1786.

It was one of several dissenting academies that provided religious nonconformists with higher education, including engineering and science.

Another was University College, in London, which you will hear more about later

Through the 19th century several other institutions were founded in Manchester to further the goal of technological development.

In the late 18th century chemistry was a muddle. No distinction made between ‘atoms’ and ‘molecules.’ In 1803, John Dalton, then teacher of mathematics and natural philosophy at the Manchester Academy’s “New College,” made a presentation to the Manchester Literary and Philosophical Society (better known as the ‘Lit and Phil’) outlining a new way of looking at chemistry in terms of atoms. It was published in 1808 in his book ‘A New System of Chemical Philosophy.’ His ‘atomic theory’ dominated chemical thinking during the 19th century and sounds very familiar today.

There were five main points in Dalton's theory:

1)elements are made of tiny particles called atoms;

2) all atoms of a given element are identical;

3) the atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their atomic weights;

4) atoms of one element can combine with atoms of other elements to form chemical compounds — a given compound always has the same relative numbers of types of atoms;

5) atoms cannot be created, divided into smaller particles, nor destroyed. In the chemical process – a chemical reaction simply changes the way atoms are grouped together.

There is a problem, however, in the rules as stated above. The meaning of the expression ‘atomic weight’ has changed over time. Dalton’s ‘atomic weights’ were established relative to the lightest element, hydrogen, which was denoted as ‘1'. Unfortunately, figuring out an element’s ‘atomic weight’ was a tricky business, and Dalton and subsequent researchers made mistakes.

What Dalton didn’t know was that the hydrogen gas he was weighing is H2 — two atoms of hydrogen are combined. Furthermore, he thought that the simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio, so that his formula for water, for instance, was HO rather than H2O.

His weight for water was 8, so by his reckoning the atomic weight of oxygen was 7. In effect, what he was calling ‘atomic weight’ was what today would be ‘atomic number.’ Today we say that the atomic number of hydrogen is 1 and that of oxygen is 8.

But we now know that their respective atomic weights are 1.0079 and 15.999. These differences from whole numbers reflect the fact that there are more than one kind of atom of each element, isotopes, but they were not discovered until a century later.

Nevertheless, Dalton's atomic theory provided a logical explanationof observations, and opened new fields of experimentation.

It was also in 1808 that Joseph-Louis Gay-Lussac, who had just been appointed Professor of Physics at the Sorbonne in Paris, published his “Law of Combining Volumes of Gases.” He had discovered that when different gases reacted, they did so in small whole number ratios.

In particular, it took two volumes of hydrogen to react with one volume of oxygen to form H2O.

The same year Dalton’s book was published it was shown that his rule that compounds were usually in a 1 to 1 ratio was shown to be incorrect.

In 1811, Amadeo Avogadro, at the time a high school teacher in the town of Vercelli in the Piemonte west of Milan, formulated his famous hypothesis, that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Avogadro’s hypothesis stunned the scientific world. It was a highly controversial idea.

In 1860 there was a Congress of Chemists held in Karlsruhe, Germany, to try to straighten out problems of nomenclature, methods of writing chemical formulae, etc. On the last day of the conference reprints of an 1858 set of notes from a course Professor Stanislao Cannizzaro had taught in Genoa were circulated. In it Cannizzaro showed how Avogadro’s hypothesis, and the ideas that flowed from it, were correct. Members of the Congress were urged to adopt Avogadro’s idea as a working hypothesis, and if serious problems were found, another Conference would be convened in five years to study the matter.

The second conference was never called. Avogadro’s revolutionary idea had been vindicated. Unfortunately he had died in 1856.

In 1815, William Prout, a physician living in London who was also an active chemical investigator, suggested that the atomic weight of every element is a multiple of that of hydrogen, and that the atoms of the other elements are essentially groupings of particles resembling hydrogen atoms.

However, as an increasingly precise list of atomic weights was being developed it became apparent that it was not the simple set of whole numbers Prout predicted, but there were a lot of ractional values. His insight was iscarded for a century.

Nevertheless, the elements could be ranked by increasing atomic weight, and given a sequential ‘atomic number’ according to their position on the list.

Another critically important concept was introduced by another English chemist, Edward Frankland, in London. In 1852 he published the idea that the atoms of each element can only combine with a limited number of atoms of other elements. He termed this property ‘valency’ and in the US we call it ‘valence.’

Nobody had any idea what caused it; it was simply a property thatatoms of each element possessed. However, by the middle of the 19th century the two principles of modern understanding of chemistry, the uniqueness of atoms of each element and the limited number of atoms of other elements with which they could combine, were established and rapid progress was the result.

In 1863, John Newlands, an analytical chamist, gave a talk to the ChemicalSociety pointing out that there seemed to be a periodicity in the characteristics of chemical elements. In 1865, he named this the ‘Law of Octaves.’ Just as on a piano keyboard, every 8th element had properties that were much the same. The properties he was referring to were the ways the atoms of one element could combine with those of another, their valence.

The idea was not very well received; Newlands was not part of the ‘chemical establishment’ of the time.

Julius Lothar Meyer and Dimitri Mendeleyev both studied with the famous chemist Robert Bunsen in Heidelberg, although they did not overlap.

In the late 1860's Meyer was on the faculty of the University in Karlsruhe, and Mendeleyev, who had been born in a village in Siberia, had risen to become Professor of Chemistry at the State University in St. Petersburg.

They both had the same idea about the periodic properties of the elements and both published diagrams showing this phenomenon. Meyer submitted his for publication in 1868, but it dd not appear in print until 1870.

Mendellev submitted his in 1869nad it was pulished that same year.

Priority os publication counts.

Mendeleev’s) Periodic Table. – German version, also 1869.

Mendeleev’s 0rignal (1869) Periodic Table.

Lothar Meier’s 1868 diagram of the elements hydrogen through strontium plotted in terms of atomic volume versus atomic weight and showing the periodic trends. Atomic volume is the volume of a solid or liquid phase of a mole of the element. A mole is the weight in grams corresponding to the atomic weight.307

A MODERN VERSION OF THEPERIODIC TABLE

AT THE END OF THE 19th CENTURY PHYSICISTS BELIEVED THAT VITTUALLY EVERYTHING WORTH KNOWING WAS KNOWN – ALL THAT REMAINED TO DO

WAS TO REFINE THE PHYSICAL CONSTANTS

Lord Kelvin had solved many of the great problems. He had found that:

1) The Sun drew its energy from gravitational contraction.

2) The Earth was about 100 million years old.

3) The ice age reflected the gradual cooling of the Earth.

4) The heat flow from the interior of the planet had dominated the heat flux from the Sun until the beginning of the Tertiary. Prior to the Tertiary there were no climate zones on Earth, and the polar regions had been warm.

In 1895 everything began to unravel.

HENRI BECQUEREL AND THE CURIES

In 1895 Antoine Henri Becquerel had been promoted to Professor of Physics at the Muséum d’histoire Naturelle in Paris. Becquerel was especially interested in two phenomena, fluorescence, in which a material glows when excited by an energy source, and phosphorescence, in which the matter continues to glow for some time after the energy source is removed.

In 1895 Becquerel heard about the discovery of Wilhelm Röntgen at the University of Würzburg in Germany of strange invisible rays that couldpenetrate aluminum, cardboard, and even human flesh and cause fluorescent paint on cardboard clear across the room to light up. Röntgen had recorded an image of his wife’s hand, showing the bones and her wedding ring, on a photographic plate. Röntgen called them ‘x-rays’ because ‘x’ was the mathematical expression for an unknown.

At the end of the 19th century there were no electric lights, and if you were a scientist experimenting with electricity you had to generate it yourself, usually by hand.

Similarly, at that time a cathode ray tube was simply a tube, from which you could evacuate most of the air, with wires at either end connected to the two poles of your electrical generator or a battery.

A visible ray passed down the length of the tube from the negatively-charged cathode to the positive anode. You could put different materials in the tube and see what happened as the electricity passed from one end to the other.

Many minerals and shells would fluoresce brightly. Even the glass at the anode end of the tube would fluoresce.

Röntgen’s strange rays came from a fluorescent spot on the end of a cathode ray tube. (Until recently, all our television sets and computer monitors were cathode ray tubes).

In 1896, Becquerel was trying to expand on Röntgen’s experiments in order to understand the relationship between fluorescence and x-rays. He wanted to see if the rays produced by fluorescence included not only visible light, but x-rays. For these experiments he used a mineral known as Zippeite, a uranium salt.

Zippeite is brilliantly luminescent, glowing bright yellow when excited and has a very short (1/100 sec) persistence of luminescence once the source of excitation is removed.

The Zippeite came from the mines at Joachimsthal (Jáchymov in Czech) in Bohemia. Those mines had operated for centuries, producing a lot of silver for coins that were called ‘Jochimsthaler’ or simply ‘Thaler’ — this is the origin of the word ‘Dollar’.

Becquerel knew that Zippeite, when excited, was extremely fluorescent, and to excite it he planned to use sunlight. You would not be able to see the fluorescence in the bright sunlight, but if the fluorescence also included x-rays they would expose a photographic plate shielded from the light by being wrapped in black paper.

The Zippeite was clamped onto the wrapped photographic plate and setout to be exposed to the sun. Unfortunately, it was late February and the weather was bad. The paper-wrapped plate with its uranium had been exposed only to diffuse daylight. Becquerel already knew that diffuse light would produce at best only a weak fluorescence. He put the photographic plate with its attached uranium salt away in a dark drawer.

A few days later, on March 1, 1896, wanting to have some result to present at the weekly meeting of the Academie Francaise, and probably thinking that perhaps he could show that even weak fluorescence might have produced x-rays, he developed the plate.

.

To his great surprise there was a strong image of the uranium salt on the plate. It was evident that the image had been produced while the plate was in storage in the dark. He presented his result to the Academie the next day.It was considered an interesting result, but the question was raised whether the ‘Becquerel rays’ were the same as ‘x-rays’?

March 1, 1896 is often cited as the discovery of radioactivity, but at the time the real nature of the ‘Becquerel rays’ was not yet known.

Henri Becquerel was 44 years old when he made his discovery. He conducted a number of other experiments in the following weeks and found that the image on the photographic plate was just as intense after many days. It became evident that it was not the result of fluorescence or phosphorescence, which must decay away with time, but from some sort of radiation emanating from the uranium salts that did not require external stimulation. This was totally new to science.

Two of hhis co-workers, Marie and Pierre Curie took up the study of these strange rays. Among other things they found that the air around the minerals was more electrically conductive than normal. The Curies also found that the ore of uranium, pitchblende, produced even stronger radiation. They thought that this must be coming from an unknown chemical element that was a stronger emitter than uranium. Chemical analyses of the rocks had recorded every element present in proportions greater than 1%, and they guessed that the new chemical must be present in very small amounts. By 1902, using sophisticated chemical techniques, they succeeded in isolating tiny amounts of the salts of not one, but two new elements from a couple of tons of uranium ore from Joachimsthal. The first they named ‘Polonium,’ after Poland where Marie had been born 35 years earlier, and the second ‘Radium’ which was so intensely radioactive as to produce an eerie green glow in a darkened room. Even more strange, they noticed that the cup containing it was warm

They coined the term ‘radio-active’ to describe the atoms which produced the radiation. The amount of radium they had extracted was about 1/10 gram (= 0.0035 ounce).! In 1903 Pierre Curie measured the heat emitted by the radium; it was not just a little, but enough to melt its own weight of ice in about an hour.

Sir George Darwin (Charles’ son, who had become an astronomer) and John Joly seized on this discovery and concluded that the heat generated by radioactivity was partly responsible for the heat flow from the interior of the Earth. The Earth had its own interior heat source from the decay of radioactive elements. One of Lord Kelvin’s basic premises was gone.

The Curie’s laboratory.It was in a simple wooden shed behind the École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris, in the rue Lhomond, a few blocks from the Museum d’histoire Naturelle.

In 1903 Henri Becquerel and the Curies were awarded separate Nobel Prizes in Physics for their discoveries. Marie’s was the first Nobel Prize awarded to a woman.

In 1906, at the age of 47, Pierre was run over by a carriage in the streets of Paris and killed.

In 1911 Marie Curie was awarded the Nobel Prize in Chemistry for isolation of these rare elements, becoming the first person toreceive two Nobel Prizes.

To put things in context, it was in 1896, the year after the discovery of radioactivity, that Swedish scientist Svante Arrhenius published a paper entitled ‘On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.’

NONCONFORMISTS AND THE BRITISH UNIVERSITIES OPEN TO ALL – MANCHESTER AND LONDON

The Manchester Academy (1786) became home to the Mechanics Institute (1825) – founded under the leadership of John Dalton.

Now: the University of Manchester’s Institute of Science and Technology

Owens College, Manchester (1873) was founded in 1851 by John Owens, a cotton merchant, to further education for “men that would not have to submit to any test whatsoever of their religious opinions.“

Now: Victoria University

University College, London, 1826, open to all students regardless of race or religious beliefs.

Jeremy Bentham, one of University College’s founders.

Jeremy died in 1832, six years after the College was founded.

He believed in individual and economic freedom, the separation of church and state, freedom of expression, equal rights for women, the right to divorce, decriminalization of homosexual acts, the abolition of slavery, and animal rights.

He opposed physical punishment (including that of children) and the death penalty.

He believed that mankind should strive for “the greatest good for the greatest number of people.”

He had his body mummified, and dressed in his finest clothes and great top hat. He is seated on a chair in a glass box just to the right of the College’s main entrance. Having worked hard for the establishment of the College and wanted to be able to greet the faculty and students even after his death.

These nonconformist British Universities became centers for laboratoryexperiments, while Cambridge and Oxford specialized in mathematics and‘natural philosophy,’ which we usually refer to as ‘physics.’

Manchester’s Mechanics Institute became a factory of great scientists producing to date 18 Nobel Prize winners in Chemistry and Physics, and 5 inother fields. It also supplied Cambridge University’s prestigious CavendishLaboratory with much of its faculty.

Among Owens College’s early undergraduate students was 14 year old John Joseph (‘J.J.’) Thomson (1856-1904) who went on to get his advanced degrees at Trinity College, Cambridge.

In 1895, Sir William Ramsay of University College in London, a chemist, discovered helium on Earth (it had been detected earlier as an unknown yellow spectral line in sunlight during a solar eclipse in 1868) — it was a gas trapped in a specimen of uranium ore. Helium became an important piece of the radioactivity puzzle.

THE DISCOVERY OF ELECTRONS, ALPHA-RAYS, AND BETA-RAYS

Joseph John (‘J.J.’) Thomson had become Cavendish Professor of Physics at Cambridge in 1884. In 1897 he figured out that the mysterious ‘cathode rays’ were tiny particles with a negative electrical charge being emitted from supposedly solid, indivisible atoms. He called them corpuscules.’

However, in 1891, George Johnston Stoney had suggested the name ‘electron’ for the undamental unit quantity of electricity, the name being derived from the Greek word for amber (ηλεκτρον).

It had long been known that rubbing amber against fur is a very good way to generate ‘static electricity’ which is free electrons. Much to Thomson’s distress,Stoney’s earlier word won out over ‘corpuscles’ and to this day we call them ‘electrons.’

One of Thomson’s students at Cambridge was Ernest Rutherford, a 28 year old New Zealander. In 1898 Rutherford graduated from Cambridge and accepted a position at McGill University in Montreal, Canada.

From his work in Thomson’s laboratory at Cambridge, he concluded “that the uranium radiation is complex and that there are present at least two distinct types of radiation - one thatis very readily absorbed, which will be termed for convenience the alpha-radiation, and the other of more penetrative character which will be termed the beta-radiation.”

(Philosophical Magazine, 1899).

In the meantime, a lot was happening in Paris. A young friend of the Curies, 26 year old André-Louis Debierne, discovered another new element in 1899. He named it ‘Actinium.’

The next year, Pierre Curie reported that radium radiation consists of two distinct types: rays that pass straight through (α-rays), and rays that are bent in a magnetic field (β-rays).

Deflection meant that the β-rays had an electrical charge, and it was soon realized it had the same mass as the electrons (corpuscles) discovered by J.J. Thomson three years earlier.

Another physicist in Paris, Paul Villard,discovered in 1900 that there was a third type of radiation coming from radium. It was similar to x-rays, but able to penetrate much more material. We now know these as gamma rays (γ-rays). It would be a long time before the importance of this discovery would be recognized.

THE DISCOVERY OF RADIOACTIVE DECAY SERIES, EXPONENTIAL DECAY, HALF-LIFE, AND SECULAR EQUILIBRIUM

In 1900 Rutherford’s laboratory at McGill had a strange problem. The count of alpha particles from radioactive thorium varied according to whether the window was open or closed. Rutherford guessed that one of the products might be a gas, ‘Thorium emanation’ (now know as radon). He was able to capture some of it, and found that it had a very short lifetime, after only a few minutes it was gone. But then the glass of the vessel that had held the gas had become radioactive. Some other ‘decay product’ had been formed and was adhering to the glass; it had a lifetime in the order of hours. It was the study of this short-lived gas and its decay product that gave rise to the idea of ‘exponential decay’ and the concept of the ‘half-life’ — the length of time required for half of a radioactive substance to decay.

It was also in 1900 that Rutherford acquired a colleague to help in his research. Frederick Soddy, a fresh 23 year old Oxford graduate who had gotten appointed as ‘demonstrator’ in the Chemistry Department at McGill.

As Rutherford and Soddy worked together, they realized that something very odd was happening. If you had a pure sample of one of the uranium or radium salts it was soon contaminated by other heavy elements.

In 1902, using a strong magnetic field, Rutherford succeeded in deviating the path of the alpha rays. They were bent in the opposite direction as the beta rays, but to a much lesser degree. Pierre Curie’s magnet had simply been too weak to show this effect.

Rutherford realized that the ‘α-radiation’ must be particles with the same mass as the helium discovered in uranium samples by Sir William Ramsay at University College in London in 1895.

A cathode ray tube showing the beam of negatively ionized particles moving from their source at the negative cathode toward the positive anode showing what happens when a magnetic field is introduced. The deviation of the beam when the magnetic field is applied means that the particles have an electric charge. Your older television set is a more elaborate and complex version of this, but the idea that a shifting magnetic field could move the beam in such a way as to make a picture goes back to the 1870's,

(from Frederick Soddy’s book ‘Radio-Activity’ published in 1904)

The uranium was undergoing ‘spontaneous disintegration’ with the loss of the alpha particles transforming uranium into other elements. They had discovered ‘transmutation’ of one element into another.

They soon figured out that uranium (and thorium) were the parents of ‘decay series’ resulting in new atoms with different atomic weights or valences. At the time chemists still followed Dalton’s idea, that an atom of an element could have a single atomic weight.

Rutherford and Soddy were hesitant to assume that each of the radiation decay products was a new element, so they used terms like (U = uranium; Ra = radium) UX, UX2, UXII, etc., and RaD, RaE, RaF, etc. They were discovering what we now know as the uranium decay series. Their observations clearly didn’t fit the accepted rules of the time. In this process they discovered something that was the key to dating ancient rocks as well as young sediments — secular equilibrium:

“The normal or constant radioactivity possessed by thorium is an equilibrium value, where the rate of increase of radioactivity due to the production of fresh active material is balanced by the rate of decay of radioactivity of that already formed.” (Rutherford and Soddy, 1902).

THE DISCOVERY THAT RADIOACTIVE DECAY SERIES MIGHT BE USED TO DETERMINE THE AGE OF ROCKS

In 1905 Rutherford, in a talk at Harvard University, suggested that radioactive decay and the production of helium might be used to determine the age of the Earth. Soon after, he did just that and came up with an age of about 500 million years for a sample of pitchblende.

Rutherford’s Helium ages were suspect because they assumed that the helium remained trapped in the rock, even though helium was known to slowly escape from almost any container.

.On his trip through New England, Rutherford also gave a talk at Yale. In the audience was a fresh young graduate in Chemistry, Bertram Boltwood. He looked into the matter and found that the ratios between uranium and lead in rocks from the same layer were the same, but that there was less uranium and more lead present in older samples. The conclusion was that lead was the end-member of the uranium decay series, and the age of the rock could be determined from the uranium/lead ratio/.

REVISING THE AGE OF EARTHBoltwood figured that if one unit of radium decays to become lead for every 10 billion units of uranium in the parent rock, the age equals 10 billion times the ratio of uranium to lead. He dated 26 mineral samples, finding ages that ranged from 92 to 570 million years, but held back from publication. It was a good thing he did because these ages were based on the then current estimate of the half-life of radium of 2,600 years which was too large by a factor of about 150%. The ages were soon corrected and Precambrian samples with ages over 1 billion years were included in his publication. The decimal point on the age of the Earth had moved over one place, from hundreds of millions to billions of years.

.Lord Kelvin, still active in Glasgow, objected to all this radioactivity nonsense because it violated the principle of ‘conservation of energy,’ one of the basic tenets of physics. In his view, the energy released by radioactive decay simply appeared from nowhere - something absolutely forbidden by physics.

Rutherford countered that the energy must have been put into the uranium by some ‘event.’ That would later become the ‘Big Bang.’

THE DISCOVERY OF STABLE ISOTOPES

In 1913,, J.J. Thomson was investigating the effect of a magnetic field on a stream of positively ionized particles termed ‘canal rays’ or ‘anode rays.’ They boiled off the tip of a heated positive anode rather than being emitted from the negative cathode.

This line of investigation was the beginning of what is now known as ‘mass spectroscopy.’ A mass spectrometer is analogous to the glass prism used to divide light into its different wavelengths, but it uses a magnetic field to spread out a beam of charged particles having different masses.

Thomson concluded that those particles having a smaller mass were deviated more by the magnetic field than those having a greater mass. In an experiment with the then newly discovered gas neon, the photographic plate revealed not one, but two closely spaced sites where the ions had struck. Thomson realized that there were two kinds of neon, which differed in atomic weight. The stream of ions of the heavier form was bent less than that of the lighter form. He had discovered that stable (non-radioactive) element, neon, had two isotopes, one with an atomic weight of 20, the other 22.

RETHINKING THE STRUCTURE OF THE ATOM

John Dalton (1808) had though of atoms as being rather like billiard balls.

In 1815 William Prout had proposed that the atoms of the different elements might be built up of units of hydrogen. But because the atomic weights of elements were almost never integers and always had decimal fractions, his idea was discarded.

In 1904 J.J. Thomson, after hs discovery of the ‘corpuscles’ (electrons) proposed that the atmom consisted of a positively charged body with the electrons embedded in it: the Plum Pudding model.

From Thomson’s experiment it was evident that the atomic weight of neon, 20.18, could be the result of a combination of the two isotopes in a ratio of about 90% 20Ne and 10% 22Ne isotopes. In 1907 Ernest Rutherford moved back to England to head up the Physics Laboratory at the University of Manchester (formerly Owens College). One of the post-doctoral students he inherited from his predecessor was Hans Geiger.Rutherford wanted to learn more about the α-ray particles, particularly how much matter they could penetrate, and how they were scattered as they passed through it.

In those days the way this was done was to sit in front of a glass covered with fluorescent material and count and note the position of the scintillations as the alpha rays struck it. It was a task that drove Ernest Rutherfordwild. He simply didn’t have the patience.

Geiger immediately devised a device that could do this automatically; this became the Geiger Counter we use to detect radiation today. With it he and Rutherford were able to demonstrate that the alpha particles had a double positive charge. Rutherford then co-opted a 20 year old undergraduate, Ernest Marsden, into helping out with what has become famous as the ‘gold foil experiment.’

Using their radium source, they used an electric field to direct a stream of the positively charged alpha particles toward the piece of very thin gold foil. The foil was surrounded by a circular sheet of zinc sulfide (ZnS) which wouldlight up when hit with alpha particles. According to J. J. Thomson’s plumpudding model, the alpha particles should have been deflected by no more than a few degrees as they passed through the gold atoms. However Geiger andMarsden found that a few of the alpha particles were deflected through angleslarger than 90 degrees.

When they presented the results of the experiment to Rutherford he was astonished:

It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge. Rutherford had started to develop our modern view of the atom, as having an incredibly tiny positively-charged nucleus surrounded by a negatively- charged cloud of electrons. It was mostly empty space. It could be thought of as a miniature solar system. But there was a problem with this simple concept. In Rutherford’s atom the electrons should gradually lose energy and spiral in to the nucleus. They don’t.

Niels Bohr was a Dane and received his doctorate from the University ofCopenhagen in 1911 when he was 26 years old. He was awarded a fellowship to spend a year abroad. He chose to go to Cambridge to work with J.J. Thomson. According to legend, their first meeting did not go well. Bohr walked into J.J.’s office with a copy of Thomson’s book, opened it, pointed to an equation and said “This is wrong.” Thomson was not amused.

A few weeks later Bohr moved to Manchester to work with Rutherford. Bohr solved the ‘electrons spiraling in’ problem by introducing quantum physics into atomic theory

The electrons were arranged in shells about the nucleus. The innermost shell can have no more than two electrons, the next no more than 8, the next no more than 18,and the two subsequent shells no more than 32 electrons. Bohr’s model of the hydrogen atom was published in 1913.External energy can ‘excite’ an electron and cause t to jump to a higher shell. Later it will fall back, emitting energy.

Meanwhile, from work carried out between 1911 and 1913, physicists in three different laboratories, Soddy in Glasgow, Kazimierz Fajans in Germany,and A. S. Russell in Oxford, all came to the same conclusion about how the radioactive decay series took place. results within a few months of each other in 1913.

When an alpha particle was emitted, the atomic weight decreased by two and the new atom belonged to the element two spaces to the left (‘lower’) on the periodic table. When beta decay took place the atomic weight stayed the same, but the element moved one place to the right (‘upward’) on the periodic table. This has become known as the ‘displacement rule’ and its formulation was critical to understanding what is going on in radioactive decay.

Frederick Soddy’s 1913 diagram of the decay of uranium.

THE DISCOVERY OF PROTONS AND NEUTRONS

In 1919, Rutherford moved to Cambridge, succeeding J.J. Thomson as head of the Cavendish Laboratory. In June of that year he announced success in artificially disintegrating nitrogen into hydrogen and oxygen by alpha particle bombardment, proving that the hydrogen nucleus is present in other nuclei. This result is considered to be the discovery of the proton.

Rutherford named the ‘hydrogen nucleus’ the proton, either after the neuter singular of the Greek word for "first", or to honor William Prout, who had postulated its existence a century earlier, or both.

There were two nagging questions. If the particles in the nucleus were protons why didn’t the atom have an electrical charge commensurate with its atomic weight? Worse, if protons carry a positive electrical charge, and positive charges repel each other, why doesn’t the nucleus simply fly apart?

CHANGING IDEAS ABOUT THE ATOMIC NUCLEUS

In J.J. Thomson’s 1904 ‘Plum Pudding’ model of the atom, the nucleus is as large as the entire atom.

In Ernest Rutherford’s 1911 model, the atom is mostly empty space and the nucleus is an extremely small object in its center.

In Rutherford;s 1919 model, the nucleus consists of positively charged particles called protons, and maybe something else.

In 1932, James Chadwick, another of Rutherford’s students at Manchester, proposed a new model: the nucleus contains both positively charged protons and new perticles, neutrons with about the same mass as a proton but no electrical charge.This finally provided an explanation for Soddy’s, Fajan’s, and Russell’s (1913) ‘displacement rule’ and explains the nature of isotopes.

ARTHUR HOLMES AND THE AGE OF THE EARTH

Arthur Holmes (1890-1965) was from Gateshead in the north of England. As a child he had heard about Archbishop James Ussher’s determination that the Earth was created in 4004 BC.

Sir Isaac Newton had also tried to figure out historical dates, but didn’t get back as far as the Creation with the details so he put it at a simple round number — 4000 BC.

Arthur was troubled by that last 4 on Ussher’s date.

In his teens Arthur Holmes was an undergraduate at the Royal College of Science (now Imperial College) in London, originally intending to study physics. He took a course in geology, and decided that was what he wanted to do.

Maybe he could figure out the real age of the Earth. As an undergraduate he used the uranium to lead ratio in a mineral of Devonian age from Norway to date it at 370 Ma.

In 1911, when he was 21 years old and had not yet finished his doctorate, he published a book,‘The Age of the Earth,’ in which he discussed the various techniques that had been used before, including not only Kelvin’s hypothesis, but estimates based on erosion rates as well.

At that time Holmes’ preferred estimate of the age of the Earth was 1,600 million years.

Holmes argued that the future of the development of the geologic time scale hinged on the use of radioactive materials. It is difficult to imagine his audacity. He had not even been old enough nor had the credentials to join the Geological Society. However he gave a talk there.

He was astonished to discover he was the only one in the room who knew that there were different isotopes of lead. Most probably didn’t even know what an isotope was.

Holmes set about a lifelong career of refining the geologic timescale. In 1927 he published an updated version of his book, The Age of the Earth, an Introduction to Geological Ideas, which was read by a wider audience. In it he pushed back the age of the Earth to 3 billion years. (Now: 4.54 ± 0.05 billion years)

The ‘Geol Soc’ lecture hall in Burlington House, Piccadilly, London

THE DEVELOPMENT OF A NUMERICAL GEOLOGICAL TIMESCALE

Louis Pirsson & Charles Schuchert’s (Yale) Textbook scale was based on dates published before Holmes’. Joseph Barrell (also at Yale) interpolated between Holmes’ radiometric ages based on the thicknesses of rock of the different Systems. Schuchert & Carl Dunbar and others refined this. Felix Gradstein and James Ogg have published the latest scale, but further refinements are under way.

The first geologic revolution at the end of the 18th and early in the 19th century established that the Earth was ancient.

Catastrophic changes in Earth History were replaced by the gradual slow change implied by ‘Uniformitarianism.’

The second geologic revolution, during the first half of the 2oth century pushed the age of the Earth back from 100 million years to at least 3 billion years.

Unfortunately, the radiometric dates were not close enough together so that the rates of geologic processes could be established.

A third geologic revolution, ‘plate tectonics’ would come in the second half of the 20th century.

None of the major characters mentioned in this lecture was over 30 when they made their great discovery !

SUMMARIZING PROGRESS IN UNDERSTANDING GLOBAL CHANGE