THOMAS HUNT-MORGAN (1866-1945)

1933 – USA

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

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

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

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

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

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

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

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

In 1933 Hunt Morgan received the Nobel Prize for Physiology.

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ALBERT MICHELSON (1852-1931) EDWARD MORLEY (1838-1923)

1887 – USA

‘The aim of the experiment was to measure the effect of the Earth’s motion on the speed of light’

This celebrated experiment found no evidence of there being an effect.

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ALEXANDER GRAHAM BELL (1847-1922)

1875 – USA

‘The inventor of the telephone, Bell devoted much of his life to working with the deaf’

After emigrating to Canada from Scotland in 1870, Bell met Thomas Watson, who would help Bell’s theoretical ideas become physical reality. Bell believed that if the right apparatus could be devised, sound waves from the mouth could be converted into electric current, which could then be sent down a wire relatively simply and converted into sound at the other end using a suitable device. Bell’s telephone was patented in 1876.

Bell used the money brought in from his invention to found his company AT & T and the Bell Laboratories.

Just as THOMAS EDISON improved the viability of Bell’s telephone, so Bell enhanced Edison’s phonograph.

Bell spent some time educating Helen Keller and was instrumental in founding the journal ‘Science‘.

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THOMAS ALVA EDISON (1847-1931)

1875 – USA

‘We don’t know one millionth of one percent of anything’

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THOMAS ALVA EDISON

‘Genius is one percent inspiration and ninety-nine percent perspiration’
Scorning high-minded theoretical and mathematical methods was the basis of Edison’s trial and error approach to scientific enquiry and the root of his genius.

1877 – Patents the carbon button transmitter, still used in telephones today.
1877 – Invents the phonograph.
1879 – Invents the first commercial incandescent light after more than 6000 attempts at finding the right filament and finally settling on carbonized bamboo fibre.

Edison held 1093 patents either jointly or singularly and was responsible for inventing the Kinetograph and the Kinetoscope (available from 1894) the Dictaphone, the mimeograph, the electronic vote-recording machine and the stock ticker.

His laboratory was established at Menlo Park in 1876, establishing dedicated research and development centres full of inventors, engineers and scientists. In 1882 he set up a commercial heat, light and power company in Lower Manhattan, which became the company General Electric.

Experimenting with light bulbs, in 1883 one of his technicians found that in a vacuüm, electrons flow from a heated element – such as an incandescent lamp filament – to a cooler metal plate.
The electrons can flow only from the hot element to the cool plate, but never the other way. When English physicist JOHN AMBROSE FLEMING heard of this ‘Edison effect’ he used the phenomenon to convert an alternating electric current into a direct current, calling his device a valve. Although the valve has been replaced by diodes, the principle is still used today.

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LINUS PAULING (1901- 94)

1931 USA

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

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

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

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

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

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

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

Pauling’s work provided a value for the energy involved in the small, weak hydrogen bond.
When a hydrogen atom forms a bond with an atom which strongly attracts its single electron, little negative  charge is left on the opposite side of the hydrogen atom. As there are no other electrons orbiting the hydrogen nucleus, the other side of the atom has a noticeable positive charge – from the proton in the nucleus. This attracts nearby atoms with a negative charge. The attraction – the hydrogen bond – is about a tenth of the strength of a covalent bond. 
In water, attraction between the hydrogen atoms in one water molecule and the oxygen atoms in other water molecules makes water molecules ‘sticky’. It gives ice a regular crystalline structure it would not have otherwise. It makes water liquid at room temperature, when other compounds with similarly small molecules are gases at room temperature.Water10_animation

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

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

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

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

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

1939 – ‘The Nature of the Chemical Bond and the Structure of Molecules’
Pauling suggests that in order to create stronger bonds, atoms change the shapes of their waves into petal shapes; this was the ‘hydridisation of orbitals’.
Describing hybridisation, he showed that the labels ‘ionic’ and ‘covalent’ are little more than a convenience to group bonds that really lie on a continuous spectrum from wholly ionic to wholly co-valent.

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

          

   

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

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

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

  

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

diagram of CO2 bond length

CO2 bond length

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

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EDWARD LORENZ (1917-2008)

1963 – USA

‘The behaviour of a dynamic system depends on its small initial conditions’

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EDWARD LORENZ

While working at the Massachusetts Institute of Technology, Lorenz developed a simple computer model to forecast changes in weather at a number of places. In one of his equations he used a rounded number (for example 0.156 127 became 0.156); his model now predicted quite different conditions.
He suggested that even a small initial unpredictable condition such as a flapping butterfly could produce a larger global change in weather.

The butterfly effect is one aspect of chaos theory that describes disorderly systems. The behaviour of a chaotic system is difficult to predict because there are so many variable or unknown factors in the system.

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Chaos in a driven double well system

ROBERT MILLIKAN (1868-1953)

1909 – USA

‘The charge on the electron’

Millikan measured the charge on the electron.

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ROBERT ANDREWS MILLIKAN

His experiment showed that the electron is the fundamental unit of electricity; that is, electricity is the flow of electrons. From the experiment Millikan calculated the basic charge on an electron to be 1.6 × 10-19 coulomb. This charge cannot be subdivided – by convention this charge is called unit negative, -1, charge.

Millikan also determined that the electron has only about 1/1837 the mass of a proton, or 9.1 × 10-31 kilogram.

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