What evidence led Wilhelm Ostwald to accept the atomic hypothesis in 1909?

For approximately 15 years, Wilhelm Ostwald rejected the atomic hypothesis in favor of his own philosophical framework known as "energetics," which prioritized energy over matter and rejected theoretical concepts lacking strict empirical foundations.1

However, by 1909, Ostwald acknowledged the validity of atomism due to the mounting experimental evidence that confirmed the existence of atoms as physical entities. A pivotal factor in this shift was the work of French physicist Jean Perrin, who provided experimental confirmation of Albert Einstein's mathematical analysis of Brownian motion.123

By using an ultramicroscope to observe the erratic movement of particles suspended in liquid, Perrin verified that these movements were caused by the bombardment of invisible, randomly moving molecules. This research allowed for the reliable calculation of the size and quantity of atoms and molecules, effectively raising them from useful hypothetical objects to observable, undeniable physical realities.23