Atoms & Light

Part 5 - Atoms

Leucippus and Democritus ( about 460 - 370 BCE) were the first Greek philosophers to suggest that everything is composed of indestructible, indivisible elements called atoms. 

In the 1700s, French chemist Charles François du Fay concluded that electricity consisted of two electrical fluids and soon afterwards, Benjamin Franklin proposed that electricity was a single electrical fluid showing an excess of positive or negative. 

John Dalton (1766 – 1844) was an English chemist, estimated the atomic weights, starting with the hydrogen atom taken as one, according to the mass ratios in which they combined. However, Dalton did not realize that some atoms exist normally in molecules, for example pure free oxygen exists only as a molecule of two atoms (O2). He also mistakenly believed that the simplest compound between any two elements was always one atom of each (so he thought water was HO, not H2O). This cause him to make errors. In 1806, he used better data and concluded that the atomic weight of oxygen must actually be 7 rather than 5.5 while other scientists had already concluded that the oxygen atom must weigh 16 times more than hydrogen assuming the modern water formula H2O.

An Italian scientist, Amedeo Avogadro (1776 – 1856), corrected the flaws in Dalton's theory in 1811 by proposing that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (that is, the mass of a gas's molecule did not affect the volume that it occupied).  Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For example, as two litres of hydrogen reacted with just one litre of oxygen to produce two litres of water vapour, it meant that a single oxygen molecule split in two in order to form two molecules of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms.

In 1827, the British botanist Robert Brown observed through his microscope that pollen grains floating in water constantly jiggled about for no apparent reason. This Brownian Motion, explained by Albert Einstein in 1905, was caused by the water molecules continuously knocking the grains about with random collisions.

James Prescott Joule, an English physicist and brewer, discovered the relationship of heat and mechanical work and worked with Lord Kelvin to develop an absolute thermodynamic temperature scale, later known as the Kelvin scale. The Systeme Internationale (SI) unit of energy, the Joule, is named after him.

The joule (symbol: J) is a unit equal to the energy transferred to an object when a force of one newton acts on that object in the direction of the force's motion through a distance of one metre (1 newton metre or N⋅m). It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

In 1841, Joule considered replacing a steam engine at his brewery with a battery powered electric motor and found that burning a pound of coal for a steam engine was cheaper than an expensive pound of zinc consumed in an electric battery. In 1843, Joule challenged Antoine Lavoisier's caloric theory (that heat was a weightless fluid called caloric) by demonstrating that the heat generated in an electrical conductor was not transferred from another part of the equipment.

Joule experimented with his electric motor and estimated the mechanical equivalent of heat as 4.1868 joules/calory (J/cal) but when he proposed this discovery at a meeting of the British Association for the Advancement of Science, few of the audience believed him.

Undaunted, Joule tried various ways to demonstrate the conversion of work into heat and found that it took 4.1868 joules per calorie (J/cal) of work to raise the temperature of one gram of water by one degree Celsius. He was convinced when he discovered that both mechanical and electrical methods produced similar results.

Joule then measured the heat generated, compared to the work done by compressing a gas and obtained a mechanical equivalent of 798 ft·lbf/Btu (foot-pound force per British Thermal Unit or 4.29 J/cal) but the Royal Society rejected his 1844 paper, so he published his findings in the Philosophical Magazine in 1845.  He devised other methods to prove his discovery and published a refined measurement of 772.692 ft·lbf/Btu (4.159 J/cal), closer to twentieth century estimates.

Joule's work led to the first law of thermodynamics and the law of conservation of energy (energy can be transformed from one form to another, but can be neither created nor destroyed).

In 1814, Joseph von Fraunhofer invented the first spectroscope, using a prism, diffraction slit and telescope, to analyze light from the sun and noticed that the spectrum of frequencies produce distinctive lines.

In 1859, Robert Bunsen and Gustav Kirchhoff used a spectroscope to analyze light emitted by a heated material sample. They identified the characteristic spectral lines of lithium, potassium and sodium and proved that each element had a unique spectra. In the process, they detected spectral emission lines of an undiscovered chemical element, which was later found to be caesium. A year later Bunsen discovered rubidium.

The spectroscope was subsequently used to identify the precise amount of alloying elements and impurities in steel, aluminum and other metals.

In 1887, a German physicist, Heinrich Rudolf Hertz, conclusively proved the existence of electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit of frequency, cycle per second, was named the "Hertz" in his honour. 

In the 1840's Richard Laming introduced the idea that an atom was composed of a core of matter surrounded by subatomic particles that had unit electric charges. 

In 1874, Irish physicist George Johnstone Stoney estimate the value of this elementary charge using Faraday's laws of electrolysis and coined the term electron in 1881.

In 1859, while studying electrical conductivity in rarefied gases, German physicist Julius Plücker, was intrigue by a phosphorescent light, on the wall of his glass tube near the cathode, that could be moved by a magnetic field. His student, Johann Wilhelm Hittorf, discovered that the light could be blocked and deduced that the phosphorescence was caused by rays emitted from the cathode and by the rays striking the tube walls. In 1876, another German physicist, Eugen Goldstein, demonstrated the rays were emitted perpendicular to the cathode surface and coined the term cathode rays.

https://www.youtube.com/watch?v=O9Goyscbazk Cathode ray tube

During 1874, the English chemist Sir William Crookes developed the first cathode ray tube with a high vacuum inside and demonstrated that the cathode rays carried momentum as they could turn a small paddle wheel. He also found that a magnetic field deflected the rays, behaving as though the rays were negatively charged. In 1879, he proposed that this could be explained if cathode rays were negatively charged gaseous molecules.

Arthur Schuster applied an electric potential between metal plates mounted parallel to the cathode rays and found that they deflected the rays toward the positively charged plate, confirming that the rays were negatively charged. By measuring the amount of deflection for a given level of current, in 1890 Schuster estimated the charge-to-mass ratio of the ray components. In 1892 Hendrik Lorentz suggested that the mass of these particles (electrons) could be a consequence of their electric charge.