Some of the Science

Atomic and Nuclear Physics

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Far too much history and not enough science

The Greek Concept of atoms

The original concept of atoms can be traced back to the Greek "pre-Socratic" philosophers. They tried to explain what they could see and experience without recourse to the gods that had been seen as the driving force until then. Alhough this can be seen as the birth of science some of their ideas seem bizarre today. Important contributions were made by Leucippus (fifth century BCE) and his student Democritus (c. 460 - c. 370 BCE). However, nothing of their original work has survived. We only know about them from the reports of others.

Leucippus, it seems, suggested that the universe consisted of two elements, the solid and the empty. While things seem to change the constituents of being persist unchanged and are merely rearranged into forms with different appearances (Aristotle later helpfully explained that this was analogous to how the letters of the alphabet can produce a wide range of patterns, a question of matter and form).

Democritus developed these ideas further. In a thought experiment he wondered what would happen if you kept cutting something in half well beyond the ability of any blade. He proposed that, eventually you would reach something that was impossible to cut even in principle. Further these, indestructible entities would move about in an infinite void. Matter, he suggested, consisted of small, indivisible units he termed “atomos” (“un-cutables” in Greek) and the materials that we experience is these atoms in different combinations.

Chemistry

Chemistry is a very old science. We can still see the remnants of colour in cave paintings showing that cavemen were practical chemists, able to make effective dyes.

The refining of metals is another early example of applied chemistry and one that defined the progress made by a ancient society.

Alchemy, with the goal to turn base metals into gold, to cure all diseases and achieve immortality, lead to an increase in learning and the introduction of careful, reproducible methods. It was active from antiquity to the early nineteenth century (if it ever went away).

Modern chemistry, with the conscious use of scientific method, developed with the likes of Robert Boyle (1661), Antoine Lavoisier (1787) and John Dalton (1808).

Boyle gave us a modern definition of a chemical element as a substance that cannot be decomposed by chemical methods.

Lavoisier gave us the law of conservation of mass. Dalton gave us the modern concept of the atom (1808) (Ref)

Mendeleev and periodicity

The modern theory of atoms can be traced back to work in the 17th to 19th centuries with the discovery of many of the natural elements. See the year of discovery of elements (here).

Note: A chemical element is a substance that cannot be broken down by chemical means. We now understand that these are composed of atoms all of which have the same number of protons. (The number of protons in an atom is referred to as its "atomic number" (denoted by the letter Z)).

A number of scientists including Antoine Lavoisier (1789), Johann Dobereiner (1829) (triads of similar properties) and John Newlands (1865) (The Law of Octaves) investigated the relationship between atomic weight and chemical properties and found hints of periodicity but the data they were working on was not very good.

Dmitri Mendeleev produced a periodic table of chemical elements similar to that we currently use in 1869. This was delivered in a paper to the Russian Chemical Society. He observed that "By ordering the elements according to increasing atomic weight in vertical rows so that the horizontal rows contain analogous elements, still ordered by increasing atomic weight" you attain an arrangement from which a few general conclusions can be derived. These conclusions showed the periodicity of chemical properties with atomic mass that we now associate with the electron structure of the atom. You can see a translation of this important paper (here). This was the first iteration of the periodic law. Mendeleev’s great insight was to leave gaps in his table to better match properties. These gaps were later filled by newly discovered elements. The figure below is a version of his table from 1871 compared to a modern version.

The atom, and particularly the nucleus, are nothing like the world we experience; distances are very small, mass and energy densities are very high, time scales are very short, forces can be extremely strong. The classical physics of Newton et al fails under these conditions and quantum effects dominate. This makes explanation and understanding more difficult and means that many features of nuclear physics will be described rather than truly explained.

The discovery of X-rays

Nature notes that “The development of the first good vacuum tubes in the mid nineteenth century led to two fundamental discoveries. The first started with the production of cathode rays and culminated in the discovery (in 1897) of the electron. The second was Wilhelm Rontgen's discovery in 1895 of a new type of penetrating radiation which he termed X-rays "for the sake of brevity"”

A facsimile of Roentgen’s paper reporting the discovery of X-rays, "A new kind of rays. By Dr. Wilhelm Konrad Roentgen, Professor at the K. University Of Würzburg. 2nd Edition (1896) (in German) can be found here. A transcript of the first paper in English can be found in the Nature physics portal or in facsimile here.

The paper reports that “A discharge from a large induction coil is passed through a Hittorf's vacuum tube, or through a well-exhausted Crookes' or Lenard's tube. The tube is surrounded by a fairly close-fitting shield of black paper; it is then possible to see, in a completely darkened room, that paper covered on one side with barium platino-cyanide lights up with brilliant fluorescence when brought into the neighbourhood of the tube, whether the painted side or the other be turned towards the tube. The fluorescence is still visible at two metres distance. It is easy to show that the origin of the fluorescence lies within the vacuum tube.”. The paper goes on to say that there is clearly "some agent" being produced that can pass through black paper that visible light cannot. He then goes on to describe a series of experiments looking at the shielding power of various things he had to hand (glass plates, lead glass, Ebonite, water, hydrogen, thin layer of metal and so on).

The tubes mentioned consist of glass vessel out of which the air is pumped and inside which is a pair of electrodes separated by some distance and attached to an electrical supply. We now understand that when a high voltage is applied across the electrodes electrons are forced off the negatively charged electrode (the cathode) and flow towards the positively charged electrode (the anode). These electrons are “cathode rays” and their properties were of much interest in the late nineteenth century and still made great undergraduate experiments when I was at university.

What Roentgen discovered was that another type of radiation was also generated and that this could excite a fluorescent material despite the tube being covered in black paper in such a way as to absorb any visible light from the tube, “paper covered on one side with barium platinocyanide lights up with brilliant fluorescence when brought into the neighbourhood of the tube, whether the painted side or the other be turned towards the tube. It is easy to show that the origin of the fluorescence lies within the vacuum tube”.

He then reports a number of experiments. The first being a rather informal investigation of the penetrating power of the radiations, reporting that they pass through a 1000 page book, two packs of cards, tinfoil, wood, aluminium. He states that “A piece of sheet aluminium, 15 mm. thick, still allowed the X-rays (as I will call the rays, for the sake of brevity) to pass, but greatly reduced the fluorescence” and thus the radiations were given the name that we still use for them.

He concluded from his experiments that “density of the bodies is the property whose variation mainly affects their permeability. At least no other property seems so marked in this connection. But that the density alone does not determine the transparency”. This is backed up by more careful experiments tabulating the thicknesses of different metals to achieve the same level of attenuation. These used photographic plates rather than a fluorescent screen to improve the reproducibility and traceability of the results.

He tried to measure the diffraction of X-rays by a prism and then by powder and determined that X-rays were far less diffracted than visible light.

He determined that X-rays were not visibly deflected by a magnet but that the effect of a magnet bending the cathode rays affected where the X-rays were generated and concluded that “the X-rays are not identical with the cathode rays, but are produced from the cathode rays at the glass surface of the tube.”

He also reports the production of “shadow pictures”, including the famous print of an x-ray of his wife’s hand clearly showing the bone structure and her ring. This the excited medical and X-ray departments soon appeared in hospitals to exploit with phenomenon.

He ends the paper with a speculation about the nature of X-rays, pointing out that the evidence suggests that they are not cathode rays or ultra-violet light but do have some properties similar to light. “Should not the new rays be ascribed to longitudinal waves in the ether ? I must confess that I have in the course of this research made myself more and more familiar with this thought, and venture to put the opinion forward, while I am quite conscious that the hypothesis advanced still requires a more solid foundation”.

Roentgen’s work was preceded by a number of other papers. The edition of Nature in which it appeared referred to earlier works by Hertz and by Lenard. It also reports the repetition of a number of the experiments reported (A.A.C.Swinton).

Going even further back, a paper to the Royal Society of London in 1785 by William Morgan reporting a green coloured glow when a small amount of air was admitted to an evacuated glass tube through which a passed an electrical discharge. This may have been the first report of an X-ray generator (ref here).