The century-long debate
In the first years of the 20th century, the physical reality of atoms was not settled science. Many influential thinkers, including physicist Ernst Mach, viewed atoms as a convenient fiction, a useful concept for calculations but not a real physical entity. The core of their argument was simple: no one had ever directly observed one. For them, belief in unseen atoms was philosophical rather than scientific. This debate simmered until a 1905 paper by Albert Einstein offered a testable prediction. He theorized that the random, zigzagging dance of microscopic particles suspended in a fluid, known as Brownian motion, was direct evidence of molecular action. Einstein calculated that the particles were being constantly jostled by billions of invisible water molecules. His equations provided a quantitative link between the observable motion of the particles and the properties of the unseeable molecules, including their size and number.
At the Sorbonne, physicist Jean Perrin saw an opportunity. He realized that if he could precisely measure the motion of suspended particles and if the results matched Einstein's predictions, he could provide definitive, experimental proof of the existence of atoms and molecules. This would transform the atom from a concept into a physical reality.
Grains of gold and a microscope
Perrin began his meticulous experiments around 1908 in his laboratory for physical chemistry at the University of Paris. His first challenge was to find suitable microscopic particles. They needed to be spherical and of a uniform size to match the assumptions in Einstein's mathematics. He chose gamboge, a yellow resin harvested from Garcinia trees in Southeast Asia. He dissolved the raw resin in alcohol and then precipitated microscopic spheres in water. To get uniformly sized grains, he used a centrifuge in a painstaking process of fractional separation.
He then placed a dilute emulsion of these gamboge grains in a drop of water on a microscope slide. Using an ultramicroscope, he could observe the tiny yellow spheres jiggling erratically. At fixed intervals, perhaps every 30 or 60 seconds, he would mark the position of a single grain. The resulting plot was a classic random walk, showing the particle's chaotic path. By measuring the average displacement of many particles over time, Perrin could work backward through Einstein's equations.
His results were successful. From the observable jitter of the gamboge grains, he calculated a value for Avogadro's number—the number of molecules in a mole of a substance. His figures, such as 6.5 x 10²³ and 6.9 x 10²³, were remarkably consistent with estimates derived from entirely different methods like the kinetic theory of gases. He published his findings in 1909, and the scientific community took notice. The agreement between theory and experiment was too strong to ignore. For his work that settled the atomic debate, Perrin was awarded the Nobel Prize in Physics in 1926.