David Hendricks Bergey

David Hendricks Bergey was born on the Mennonite meetinghouse farm in Shippack township, Montgomery County, Pennsylvania on December 27, 1860. As was the custom for boys on the farm he attended school during the winter and worked the farm during the summer. After turning 18 he attended private and normal schools and taught two winters in rural schools before he decided to study medicine. He started his medical training in the office of Dr. Samuel Wolfe of Shippack, PA.

He went to the University of Pennsylvania and graduated with a B.A. and M.D. simultaneously in 1884 at a time when the discoveries of Luis Pasteur and Robert Koch was causing much discussion in American bacteriology circles. Dr. Bergey went to work in the laboratory of Dr. Henry Formad, who had made two visits to Dr. Koch's laboratory. It was here that Dr. Bergey was introduced to bacteriology.

For nearly ten years Dr. Bergey practiced medicine in North Whales, PA, before returning to the University of Pennsylvania in 1893 first as a student and then as a Scott Fellow in Hygiene in the newly built laboratory of Hygiene. In 1895 he was appointed assistant in chemistry, in 1903 he made assistant professor and in 1926 full professor of hygiene and bacteriology.

Dr. Bergey was responsible for numerous publications during his lifetime but he is best remembered for the manual of bacterial classification that is named after him. The first edition of the manual was published in 1923 by the Society of American Bacteriologists (now the American Society of Microbiologists). Dr. Bergey served as the chairman of the editorial board for the manual. Dr. Bergey had begun preparing the manual soon after he had been the president of the society in 1915 in order to replace the old system of bacterial classification outlines to fit newer knowledge. The manual, which still bears his name, has been constantly revised and is still used today as a standard reference of bacterial classification.

Dr. Bergey died on September 5, 1937. The Bergey award and Bergey medal, awarded for contributions to bacterial taxonomy, given out annually by the Bergey manual trust, are named after him.

References:

Breed, Robert; "David Hendricks Bergey"; Journal of bacteriology(1938)vol.35:p.I2-345

"History of Bergey's Manual" at cme.msu.edu

David Hendricks Bergey, Wikipedia entry

Bergey's Manual Trust Website

Thomas Graham

Thomas Graham was born in Glasgow on December 21, 1805 the the eldest of seven children of a merchant father. After attending preparatory school and high school he started classes at the university of Glasgow in 1819, where he studied under Thomas Thompson. He remained there for seven years taking an M.A. in 1826. His father wanted him to go into the Scottish church, but Thomas showed an aptitude for mathematics and science and against his father's wishes he commenced on a career in science.

Graham was a lecturer in chemistry at the Mechanics Institution in Glasgow and then he was appointed professor of chemistry at Andersonian University in Glasgow. It was at this point that he was able to devote more time to experimentation and the seven years he spent at Andersonian were busy. In 1837 he was appointed professor of Chemistry at London University (now University College, London) where he occupied the chair until 1855 when he succeeded Sir John Herschel as Master of the Mint and remained in that positon until he died.

Graham is best remembered for his discovery that under the same temperature and pressure the rate of effusion of a gas is inversely proportional to the square root of its atomic mass. A demonstration of this can be found here. Basically this law means that the smaller the atomic mass of the gas the faster it will diffuse. Graham was awarded the Keith prize in 1834 by the Royal Society of Edinburgh for this discovery.

Graham is also remembered for his invention of dialysis. Between 1861 and 1864 Graham, while he was studying the ability of dissolved substances to pass through a membrane, noticed that substances that crystallized well like salt passed well through the membrane and substances that did not crystallize like gelatin did not. He distinguished these two classes of substances as crystalloids and colloids. This discovery led to the dialysis that is done on kidney patients today.

Graham is also remembered for is characterization of phosphates in solution. For all of these discoveries Graham was awarded the Copley medal of the Royal Society in 1862.

Graham died on September 16th, 1869.

References:

Williamson, A.W.; Obituary in Nature, Volume 1, (1869) p. 20-22

Obituary in the Proceedings of the Royal Society; Volume 18 (1870) p. xvii-xxvi

Obituary in the Lancet; Volume 2 (1869) p. 456-457

Plimer, Robert Henry Aders; Practical Organic and Biochemistry Chemistry; Logmans, Green and Company; 1920

Mary L. Caldwell

Born on December 18, 1890, Mary Letitia Caldwell was the daughter of Presbyterian missionaries working in Colombia. She attended high school in the United States and then attended the Western College for Women in Oxford, Ohio, graduating in 1913. She taught there for five years following her graduation. She then went to Columbia University, where she studied under Henry S. Sherman and obtaining a M.A. and a Ph.D. in organic chemistry. She stayed at Columbia first as an instructor and then a full professor in 1948. She was the only woman to be a senior chemistry faculty member at the time.

Mary Caldwell's research centered on enzymes that use starch as a substrate, particularly amylases. She was the first person to purify porcine pancreatic amylase, an enzyme that is used both in industry and research. She also established that amylase is a protein. Amylase is an enzyme that breaks down starches into individual carbohydrate units.

Although Mary Caldwell suffered from a progressive muscular disorder, she never changed her office on the 9th floor of Chandler Hall. She retired in 1959 and was awarded the Garvan Medal by the American Chemical Society in 1960.


References:

Mary Letitia Caldwell, Journal of Chemical Education online

Ogilvie, M.; The Biographical Dictionary of Women in Science: Pioneering Lives From Ancient Times to the Mid-20th Century; p.220-1; Routledge; 2000

Barbosa, Patty; "Mary Letitia Caldwell"; in The Data Bank of Scientists at csupamona.edu

Theodor Schwann

Theodor Schwann was born on December 7, 1810 in Neuss, near Dusseldorf, in Rhenish Prussia, which at the time was a providence of the French Empire. Theodor was the fourth of thirteen children of a goldsmith who had set up a successful printing business. From his father Theodor inherited a proclivity for working with his hands which suited him well in his scientific career. He spent the play hours of his childhood building miniature physical instruments from primitive materials.

Theodor attended a Jesuit college in Cologne and went to the University of Bonn, where after initially studying theology, his natural inclination for science led him to study medicine, studying under Johannes Muller. Muller, recognizing Schwann's abilities, made him an associate and they researched the motor and sensory roots of spinal neurons and blood coagulation. Schwann migrated to Warzburg, and then to Berlin to finish his doctorate and again work with Muller.

In Berlin Schwann became an aid at the Anatomical Museum of which Muller was the director. It was during this time that Schwann laid the basis for the study of nervous and muscle tissue that others would elaborate on. Schwann was one of the first to deal with living tissue on only a chemical and physical basis, ignoring the aid of "vital force". Schwann also discovered that alcoholic fermentation and the fermentation that causes putrefaction were carried out by microbes, a discovery that was ignored and even ridiculed at the time. Adept with the microscope, Schwann was the first to find the thin layer of cells on the inside of blood vessels, which would later be called the endothelium and confirmed the observation of Robert Remark of the cellular sheath around nerve cells, the cells of which would later be named after Schwann.

All of this work would have been enough for Schwann to be considered a great scientist, but Schwann is most famous for his elaboration of the cell theory of biology. In 1839 Schwann published "Microscopical Researches into the Accordance in the Growth and Structure of Animals and Plants", in which he continued the work that Matthias Schleiden had started with plants and identified the cell as the basic unit of living tissue in animals. The discovery was prompted when Schwann was having lunch with Schleiden. Schleiden was describing the nuclei he had found in plant cells and Schwann recognized that he had seen similar structures during his microscopic examinations of animal tissues. The two went to the anatomy theater (this was at Louvin where in 1839 Schwann had been appointed professor of anatomy) where Schwann showed Schleiden animal cell nuclei. From this point on Schwann dedicated his research to studying animal cells.

After all of his early success Schwann did little active research. In 1848 he was called to the University of Liege, where he remained till his death. In 1875 he published an indignant pamphlet denouncing the Catholic clergy for claiming that he testified in favor of the miraculous nature of the appearance of stigmata on Louise Lateau and he died in 1882 at the age of 72.


References:

"Theodor Schwann"; Proceedings of the American Academy of Arts and Sciences; Vol. 17 (1882) p. 460-1

"Heroes of Medicine: Theodor Schwann"; The Practitioner; Vol. 59 (1897) p.498-501

Kettenmann, Helmut; Ransom, Bruce R.; Neuroglia; Oxford University Press, USA; 2004

Leon, Fredericq; "Sketch of Theodor Schwann"; The Popular Science Monthly; Vol. 37 (1897) p.257-264

Otis, Laura; Muller's Lab; Oxford University Press, USA; 2007

Martin H. Klaproth

Martin Heinrich Klaproth was born on December 1, 1743. The second son of a citizen of Wernigerode, who lost all his wealth in a tragic fire in 1751, Martin was forced to sing in the church choir in order to subsidize his studies. Originally he intended to enter the clergy like his older brother, but when faced with hard treatment by his instructors he resolved to study to be an apothecary. He then spent five years as an apprentice and worked for two years in the public laboratory in Quedlinburgh. However it was not until 1766, when he worked at the public laboratory in Hanover that he was able to have access to current scientific texts. This awakened his interest in science. In 1782 he became the pharmaceutical assessor at a medical school in Berlin and in 1810 he was appointed professor of chemistry at the newly founded University of Berlin.

At his time, Klaproth was the leading chemist in Germany. He was one the first non-French adherents to the antiphlogistic theories of Lavosier and in the course of his research Klaproth was the first to describe the elements uranium, zirconium and cerium, although he did not obtain the elements in pure metallic form. He also confirmed the existence of titanium as an element. His exact experimentation and use of quantitative methods did much to develop analytical chemistry and mineralogy.

Klaproth is best remembered for his discovery of uranium, which he named after the newly discovered planet Uranus, rather than after himself which was the custom at the time. In 1789 he was examining waste product from St. Joachimstahl and noticed that the stuff associated with lead. When he heated it in solution a yellow crystal was produced, which was unlike anything he had ever seen before. He added wax and a little oil to produce a heavy grayish residue which he identified as a new element. This was uranium.

Klaproth has a crater on the moon named after him.

For his work in analytical chemistry and his discovery of uranium Martin Klaproth is the Dead Scientist of the week for the week of November 29-December 5, 2009


References:

Martin Klaproth Wikipedia Entry

Zoellner, Tom; Uranium: War, Energy and the Rock That Shaped the World; Viking Adult; 2009

Fischer, E. G.; Memoir of the Life of Martin Henry Klaproth; Edinburgh Philisophical Journal; (1821) Vol. 5, Part 2, Issue 10, p.319-334

Julius Robert von Mayer

Julius Robert von Mayer, born on November 25, 1814, was one of the pioneers of thermodynamics for enunciating one of the first formulations of the First Law: "Energy can not be created or destroyed".

Mayer grew up in Heilbronn, in southern Rhineland, the son of a pharmacist he studied medicine at the University of Tubingen. After attaining his doctorate, he signed on as ship's physician on Dutch three-masted sailing ship on a trip to Jakarta.

This trip spurred his interest in the physical sciences. His observation that wind swept waves were warmer than calm seas led him to wonder about the physical phenomenon of warmth. Also while in the tropics he observed that the blood of his patients was redder than it would be in northern climes. While operating on a patient he saw the redder blood and feared he had severed an artery. Only after being reassured by local physicians did he realize that this was normal. In the tropics not as much oxygen is required to warm the body and thus the blood traveling back to the heart is more oxygenated and redder than venous blood in cooler climes where more oxygen is required to maintain temperature. This led Mayer to believe that oxidation was the primary source of energy in living things.

Upon returning to Germany Mayer wrote a paper outlining his observations, however the paper contained no experimental results and was rejected. Unable to get clarification on why his paper was rejected Mayer (relying on the experimental work of others) wrote another paper in which he proposed that mechanical energy and heat energy were equivalent, specifically that the dropping of a weight from 365meters was equivalent to warming an equivalent mass of water from 0 to 1 degree Celsius. The current accepted value for this standard is 418.4meters.

At the time Mayer's work was largely ignored and credit for the discovery of the equivalence of mechanical and heat energy was initially given to James Joule (for whom the unit of energy is named). In 1848 when Mayer tried to assert his precedence, Joule admitted Mayer's precedence but claimed credit for the experimental demonstration of the theory. This controversy, plus the loss of two of his children, caused Mayer to attempt suicide in 1850. Mayer's work led Herman von Helmholtz to formulate the general principal of conservation of energy in 1848.

For his work in describing the principal of the conservation of energy Julius Robert von Mayer is the Dead Scientist of the Week for the week of November 22-28, 2009.

References:

Julius Robert von Mayer Wikipedia Entry

Mechanical equivalent of heat Wikipedia Entry

Cobb, Kathy; Goldwhite, Harold; Creations of Fire: Chemistry's Lively History from Alchemy to the Atomic Age; Basic Books, 2002

James B. Sumner

James B. Sumner, born on November 19, 1887 in Canton, MA, was the first person to isolate an enzyme. While he was growing up he was an avid hunter and lost his right arm below his elbow in a hunting accident. Having been left handed, this forced him to adapt to using his right hand to do things and this effort allowed him to become an expert at tennis, skiing, billiards and clay-pigeon shooting.

As an undergrad at Harvard College he specialized in chemistry. He then went on to study Biochemistry at Harvard Medical School from which he obtained his Ph.D. in 1914. Offered an assistant professorship at Cornell Medical School he went there where he was made a full professor in 1929.

His research at Cornell centered around analytical methods, but despite hard work he was unable to obtain any interesting results. He then decided to try to isolate an enzyme, a feat which until then many of his colleagues thought was impossible. In 1921, when he work was still in the early stages he received a fellowship to travel to Belgium. He wanted to go there to work with Jean Effront, who had written several books on enzymes, however Erront thought the idea of isolating an enzyme was ridiculous, so the trip fell through.

In 1926 he finally succeeded in isolating the enzyme urease. Many biochemists disbelieved or ignored his results, but in 1929 it brought him a full professorship. In 1929 John Northrop of the Rockefeller Institute isolated pepsin and subsequently other enzymes it became clear that Sumner had developed a general method for isolating enzymes. Other biochemists gradually were forced to admit Sumner's claims were correct. For his work demonstrating that an enzyme could be isolated Sumner, along with Northrop, were awarded the Nobel Prize for Chemistry in 1946.

Sumner's work demonstrated that enzymes were proteins, which at the time was controversial. Enzymes function as catalysts in biological systems. They allow chemical reactions, particularly those of metabolic pathways, to occur much faster than they would under normal conditions. Enzymes speed chemical reactions by lowering the activation energy required for a reaction to occur. Enzymes are also specific, in that they will only catalyze a specific chemical reaction. For example Urease (the enzyme isolated by Sumner) catalyzes the conversion of urea into carbon dioxide and ammonia and it occurs in bacteria, yeast and some higher plants. An article with more information about enzymes can be found here.

For his research in isolating enzymes, James Sumner is the Dead Scientist of the Week for the Week of November 15-21, 2009.

References:

James Sumner Wikipedia Entry

James Sumner Nobel Biography

Enzymes Wikipedia Entry

Urease Wikipedia Entry

Andres M. Del Rio

Born on November 10th in 1764 Andres Manuel Del Rio was the Spanish-Mexican mineralogist who discovered the element vanadium.

After his studies in Spain, Germany, and France (where he studied under Antoine Lavoisier) Del Rio was appointed to the College of Mines in New Spain (Mexico) as the chair of chemistry and mineralogy. It was there he taught the first course of mineralogy offered in New Spain.

In 1801, while examining samples from a mine in Zimpan, he arrived at the conclusion that he had found a new metallic element. He prepared various compounds of the new element which he named pancromium. Later, upon observing that when the compounds were heated, he decided on the name eritronium (Eritros is red in Greek). A year later he sent the samples to Alexander von Humbolt, who sent them to Hippolyte Victor Collet-Descotils who analyzed the samples and found only chromium, causing Del Rio to claim that his discovery had been in error.

Later, in 1830, Swedish chemist Nils Gabriel Sefstrom rediscovered the element, which he named Vanadium after the Scandinavian goddess of love and beauty Vanadis. In the same year German chemist Friedrich Wholer analyzed Del Rio's samples and found that eritronium and vanadium were the same.

Vanadium is an example of a transition metal. Transition metals are the elements found in the middle part of the periodic table (see a periodic table here where the transition metals are in green). These elements are very hard in their elemental from with high melting and boiling points. In this part of the periodic table the electrons in the loosely held d-orbitals are being filled in which make transition metals malleable and able to conduct electricity.

For his discovery of the element Vanadium, Andres M. Del Rio is the Dead Scientist of the week for the week of November 8-14, 2009.

References:

Andres Manuel Del Rio Wikipedia Article

Lise Meitner

Born on November 7 1878 Lise Meitner played an important part in the discovery of atomic fission. She was the second woman to obtain a doctoral degree in physics from the University of Vienna.

At the time many male scientists did not approve of women in science and her career was hampered (including her being omitted from the Nobel Prize in 1944 for her work in discovering the nuclear chain reaction of atomic fission according to some historians).

Because of her Jewish ancestry (she had been baptised a Protestant in 1908) she was forced to flee Germany in 1938 (after the German annexation of Austria). In Holland she was unable to get a position so she went to Stockholm, Sweden where she got a position in Manne Siegbahn's laboratory.

Working in Sweden she corresponded with her cousin Otto Hahn who she had worked with before she fled Germany and clandestinely traveled to Copenhagen in November 1938 and after corresponded with Hahn to design a set of experiments that Hahn would later carry out that would provide evidence of nuclear fission of uranium.

Nuclear fission is the process by which the nuclei of large atoms break down into smaller nuclei. Large nuclei (with high number of protons) will break down into smaller nuclei because the large amount of electrostatic repulsion (positive protons repelling each other) overcomes the strong nuclear force which holds the nucleus together. Meitner (along with her nephew Otto Frisch) discovered why no elements larger than uranium naturally occur; the electrostatic repulsion of so many protons is so great it overcomes the strong nuclear force which holds the nuclei together. She was able to explain the large amount of heat energy produced by a nuclear chain reaction and realized that this process could be used to produce atomic weapons.

A nuclear chain reaction occurs when a one nuclear reaction causes one or more nuclear reactions to occur. For example when a uranium nucleus breaks down it releases neutrons which bombard other uranium nuclei and cause them to break down. In turn when these nuclei break down they release neutrons that cause more uranium nuclei to break down. This self perpetuating process produces thousands of times more energy than any chemical reaction. The rate at which this process occurs can be controlled by absorbing some of the neutrons and thus slowing the reaction. In nuclear power plants this is done by inserting control rods which absorb neutrons. In nuclear weapons the process is allowed to continue without moderation.

For her work discovering the nuclear fission and the nuclear chain reaction Lise Meitner is the Dead Scientist of the Week for the week of November 1-7, 2009

References:

Lise Meitner Wikipedia entry

Splitting of the Atom at sjmv.org

Lise Meitner: A Battle for Ultimate Truth at sdsc.edu

Nuclear Chain Reaction Wikipedia Entry

Marcellin Berthelot

Marcellin Berthelot was born in Paris, France on October 27, 1827. He is regarded as one of the greatest chemists of all time and was responsible for the Thomsen-Berthelot principle of thermochemistry and for disproving the theory of vitalisim by synthesizing many organic compounds from inorganic substances.

The fundamental conception that underlay all of Berthelot's work was that all chemical phenomena depend on the action of physical forces which can be determined and measured. When he began his career it was assumed that all organic chemistry depended upon vital forces to produce organic compounds. Berthelot opposed this idea and in order to disprove it he synthesized many organic molecules (including hydrocarbons, and natural fats and sugars) from inorganic starting materials, thus proving that the synthesis of these molecules did not depend of vital forces and that organic chemicals obeyed the same principals as inorganic compounds.

His other major contribution is the Thomsen-Berthelot principle of thermochemistry. Berthelot and Dutch chemist Julius Thomsen both independently came up with slightly different formulations for this principle which states that all chemical changes are accompanied by the production of heat and the processes which occur will be the ones in which the most heat is produced. This postulate led to the thermal theory of affinity which postulated that the true measure of chemical affinity was the amount of heat that was produced by a particular reaction. This was later disproved by Herman von Helmholtz, who discovered that the true affinity was not the amount of heat produced by a reaction, but the maximum amount of work or free energy produced when the reaction was carried out reversibly.

For his work in disproving the theory of vitalism and pioneering work in the field of thermochemistry Marcellin Berthelot is the Dead Scientist of the Week for the Week of October 25-31, 2009.

References:

Marcellin Berthelot Wikipedia Entry

Thomsen-Berthelot principle of thermochemistry Wikipedia Entry

Marcellin Berthelot NNDB Entry

James Chadwick

Born on October 20, 1891, James Chadwick won the Nobel Prize in Physics in 1935 for his discovery of the neutron.

In 1932 Chadwick discovered a nuclear particle that did not have any charge (one of his letters announcing the discovery can be found here). These chargeless particles, called neutrons, differ from the previously discovered protons in that they do not have any charge. Because they do not have any charge they can be combined with other nuclei without having to overcome electrostatic repulsion.

Atomic nuclei are composed of two different types of particles, protons and neutrons. They both have about the same mass (neutrons are slightly more massive than protons) but differ in the amount of electrostatic charge they carry. Protons carry a positive charge and neutrons have no charge. Because they are all positively charged protons repel each other and therefore it is difficult to add alpha particles (alpha particles have two protons) to a nucleus. A neutron has no charge and can be used to bombard a nucleus without having to overcome the electrostatic charge.

Chadwick also discovered that atomic number is determined by the number of protons that are found in a nucleus. This is now the definition of atomic number. What element a particular atom is is determined by the number of protons present in its nucleus. For example, 1 proton is hydrogen, two protons is helium, three protons is lithium, etc. A full list of atom:atomic number corespondences can be found on a periodic table.

Chadwick later work with particle accelerators contributed to the making of the atomic fission bomb.

For discovering the neutron James Chadwick is the Dead Scientist of the Week for the week of October 18-24, 2009.

References:

James Chadwick Wikipedia entry

James Chadwick Nobel biography

James Chadwick Answers. com biography

Arthur Harden

Born on October 12, 1885, Arthur Harden shared the 1929 Nobel Prize in chemistry for his work elucidating the glycolytic pathway. This is the pathway by which glucose and other sugars are broken down into smaller molecules and energy is extracted in living cells. Arthur Harden discovered posphorylated intermediates along this pathway and he characterized glucose phosphate and fructose diphosphate, two of the molecules along the pathway.

It has been known since ancient times that fruit juices and other sugar containing liquids, under the right conditions, undergo fermentation. Fermentation is the process by which microorganisms convert sugar into alcohol. Ancient cultures used knowledge of fermentation to produce wine and beer.

The glycolytic pathway is a number of steps in which a six-carbon long sugar molecule is broken down into two three-carbon pyruvate molecules (a really cool discussion of all the steps can be found here). This pathway occurs in nearly all biological organisms, both aerobic (those using oxygen) and anaerobic (those that don't use oxygen). Each sugar molecule that goes through the pathway produces two molecules of pyruvate, two molecules of hydrogenated nicotinamide adenine dinucleotide (NADH+) and two molecules of adenosine triphosphate (ATP). ATP is the primary molecule by which living cells store and transfer chemical energy.

In order to get the glycolytic pathway started two molecules of ATP are used to add phosphate to the six-carbon sugar molecule. The addition of the first phosphate produced glucose phosphate and the addition of the second phosphate produces fructose diphosphate (the two molecules that Arthur Harden characterised). This six-carbon sugar diphosphate is then broken down (through a number of steps) into two three carbon molecules, each of which will add phosphate to two molecules of adenosine diphosphate (ADP) to make two molecules of ATP. In summary two molecules of ATP are used to energize the process which will produce four molecules of ATP, giving a net production of two molecules of ATP for each molecule of sugar that is broken down.

For his work studying the glycolytic pathway Arthur Harden is the Dead Scientist of the Week for the week of October 11-18, 2009.

References:

Arthur Harden, Wikipedia Entry

Glycolysis, Wikipedia Entry

Moran, Laurence, a blog entry on Arthur Harden

Niels Bohr

Born on October 7, 1885, Niels Bohr won the Nobel Prize in physics in 1922 for proposing a structure for the atom and his work in quantum mechanics.

Before Bohr physicists knew that the structure of an atom consisted of a small dense nucleus orbited by electrons. Earnest Rutherford in 1911 published the results of an experiment in which alpha particles emitted by the decay of radium were used to bombard a thin piece of gold foil. The results of the experiment showed that a small amount of the alpha particles were deflected and did not penetrate the gold foil. Rutherford hypothesized that these alpha particles were deflected by the small, hard nuclei of the gold atoms. This led him to propose that atoms were composed of small nuclei surrounded by orbiting electrons that orbited the nuclei in a similar manner to the way planets orbit stars. This model is sometimes called the planetary model (wikipedia entry).

In 1913 Niels Bohr proposed a model for atomic structure where:

1. Electrons orbit nuclei only in certain orbits: orbits set at discrete distances from the nucleus.

2. Electrons can change orbitals, but in doing so they must either absorb (when moving to a higher orbital, further from the nucleus) or emit energy (when moving to a lower orbital, closer to the nucleus).

3. The frequency of the light emitted by an electron changing orbitals is related to the period of the orbital.

The emission of light by electrons falling back into lower orbitals can be seen in neon lights. A gas, sealed in a tube is electrified, causing electrons (normally staying in the lowest energy levels) to move to higher energy levels (higher orbits). When they fall back to their lower, ground state they emit light. This is how neon lights work, and each gas emits a different spectrum giving each gas a distinct color.

For his work determining the structure of atoms, Neils Bohr is our Dead Scientist of the Week for the week of October 4-10, 2009.

References:

Neils Bohr Wikipedia Entry

Bohr Model, Wikipedia Entry

Carpi, Anthony, Vision Learning, Atomic Structure II