The secret molecular life of soap bubbles (1913)
Most of us would look at a soap film image and marvel at the beautiful rainbow colors; others would investigate the optics underlying them. But it took an exceptional physicist, Jean Baptiste Perrin (1870-1942), to realize that these colors concealed something more: direct evidence that matter consists of discrete atoms and molecules!
Today we take for granted that all material objects in the universe are comprised of discrete “bits” of matter, which we call atoms; however, even up until the early 20th century there were still proponents of the continuum hypothesis, in which all matter is assumed to be infinitely divisible.
Jean Baptiste Perrin was also the author of Les Atomes which presented a series of maybe a dozen ways of determining Avogadro’s Number and which finally end the last few serious objections to atomic theory. (His method, which he included, worked by measuring the half pressure height of a variety of colloidal particles he created.)