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John Dalton (1766-1844)

In 1803, John Dalton, an obsure English school teacher, published an article in which he outlined the first modern atomic theory of matter. Using quantitative data on the reaction of various simple elements and compounds, he deduced the existence of invisible, yet indivisible atoms

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John Dalton (1766-1844)

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  1. In 1803, John Dalton, an obsure English school teacher, published an article in which he outlined the first modern atomic theory of matter. Using quantitative data on the reaction of various simple elements and compounds, he deduced the existence of invisible, yet indivisible atoms Direct physical evidence of atoms was not to appear for more than a century. Many nineteenth century chemists, although they acknowledged the convenience of the atomic approach in explaining reactions, doubted the physical reality of atoms. John Dalton (1766-1844)

  2. In 1827, the Scottish botanist and microscopist Robert Brown observed the “ceaseless motion” of particles within a pollen grain. He conclusively showed that this motion was not a result of convective currents in the liquid (water), nor was it due to the animated motion of living organisms. Robert Brown (1773-1858)

  3. Although Brown’s microscope was primitive by modern standards, he was able to make remarkable observations with it. His microscope has been preserved, and this video (taken of fat droplets in milk) is similar to what he observed in 1827. Brown’s “simple” microscope

  4. In 1905, Einstein published three papers in the same issue of Annalen der Physik. One introduced special relativity; the second, on the photoelectric effect, laid one of the cornerstones of quantum theory. The third, on Brownian motion, conclusively demonstrated the physical reality of atoms for many remaining skeptics. Albert Einstein as a patent clerk, ca. 1905

  5. Einstein showed, by a statistical mechanical argument, that the “random walk” path followed by the visible particle was the result of continual collisions with much smaller (hence invisible) particles—molecules of liquid or gas. This “classical” paper, using well-accepted arguments, provided convincing evidence of the physical reality of atoms and molecules. Through the use of the Scanning Tunneling Electron Microscope (STEM), atoms have been successfully “visualized” only within the past few years. Cesium atoms (red) on a Ga/As surface

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