Brownian motion as a measurement of molecules
The random walk of particles suspended in a liquid, explained by the impacts of the molecules of the solvent. From the observed motion Avogadro's number (how many molecules are in a mole of substance) can be computed.
In 1905 the German Albert Einstein derived from the molecular-kinetic theory of heat that such particles must perform a random walk visible under a microscope, with a mean square displacement over time tied to Avogadro's number. In 1908 the Frenchman Jean Perrin measured both that displacement and the vertical distribution of the particles, and obtained concordant values of Avogadro's number by several independent routes.
Molecular statistics stopped being a convenient model and became measurable physics. Together with the counting of gaseous ions by J. J. Thomson this brought even Ostwald to acknowledge atoms in 1909.
Why this date
The axis year is 1905, the year the theory was first published: Einstein, Annalen der Physik, volume 322, pages 549-560. The upper bound of the label corresponds to Perrin's experiments of 1908, published as Mouvement brownien et realite moleculaire in Annales de chimie et de physique in 1909. Prediction and measurement are dated together because apart they settle nothing: the formula without Avogadro's number remains a hypothesis, and the measurement without the formula has nothing to compute.
What it grew from
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Kinetic theory of gases and statistical mechanics
prerequisite for · Confidence: high
Einstein derived the law of Brownian motion directly from the molecular-kinetic theory of heat, as the very title of his 1905 paper states.
Link sources
- A. Einstein, Annalen der Physik 322 (8), 549-560 (1905)
Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen
- Jean Baptiste Perrin, Biographical, The Nobel Foundation (1926) Verified 20 Sep 2026
His results in this field were able to confirm Einstein's theoretical studies in which it was shown that colloidal particles should obey the gas laws, and hence to calculate Avogadro's number N.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- A. Einstein, Annalen der Physik 322 (8), 549-560 (1905)
Other links
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Energetics: physics without atoms
refuted by · Confidence: medium
The energeticist refusal of atoms was closed by the experiments of the early twentieth century, and in 1909 Ostwald acknowledged the experimental proof of the atomic nature of matter, naming among his grounds the agreement of Brownian movement with the kinetic hypothesis established by Perrin.
Dispute: Brownian motion was not the only cause. In 1909 Ostwald named two grounds and put the counting of gaseous ions by J. J. Thomson first, that is work from an entirely different field, mentioning Perrin second. The encyclopedia's wording is more cautious than a verdict: it says 'usually considered', reporting the received view of Perrin's decisive role. Mach, unlike Ostwald, never accepted the reality of atoms, so the end of the dispute refers to the scientific community as a whole and not to every one of its participants.
Link sources
- W. Ostwald, Grundriss der allgemeinen Chemie, 4th ed., Preface, quoted in Hiebert and Körber, Dictionary of Scientific Biography, vol. 15-16, 464 (1909) Verified 20 Sep 2026
I am now convinced that we have recently become possessed of experimental evidence of the discrete or grained nature of matter, which the atomic hypothesis sought in vain for hundreds and thousands of years. The isolation and counting of gaseous ions, on the one hand, which have crowned with success the long and brilliant researches of J.J. Thomson, and, on the other, agreement of the Brownian movement with the requirements of the kinetic hypothesis, established by many investigators and most conclusively by J. Perrin, justify the most cautious scientist in now speaking of the experimental proof of the atomic nature of matter.
- Klodian Coko (rev. of A. Chalmers), Atomism from the 17th to the 20th Century, Stanford Encyclopedia of Philosophy (2026) Verified 20 Sep 2026
The factor that is usually considered as turning the tables decisively in favour of the atomists is Jean Perrin's experiments on Brownian motion.
- Jean Baptiste Perrin, Biographical, The Nobel Foundation (1926) Verified 20 Sep 2026
In this way the discontinuity of matter was proved by him beyond doubt: an achievement rewarded with the 1926 Nobel Prize.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- W. Ostwald, Grundriss der allgemeinen Chemie, 4th ed., Preface, quoted in Hiebert and Körber, Dictionary of Scientific Biography, vol. 15-16, 464 (1909) Verified 20 Sep 2026
Node sources
- A. Einstein, Annalen der Physik 322 (8), 549-560 (1905)
Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen
- Klodian Coko (rev. of A. Chalmers), Atomism from the 17th to the 20th Century, Stanford Encyclopedia of Philosophy (2026) Verified 20 Sep 2026
The factor that is usually considered as turning the tables decisively in favour of the atomists is Jean Perrin's experiments on Brownian motion.
- Jean Baptiste Perrin, Biographical, The Nobel Foundation (1926) Verified 20 Sep 2026
His results in this field were able to confirm Einstein's theoretical studies in which it was shown that colloidal particles should obey the gas laws, and hence to calculate Avogadro's number N.
- W. Ostwald, Grundriss der allgemeinen Chemie, 4th ed., Preface, quoted in Hiebert and Körber, Dictionary of Scientific Biography, vol. 15-16, 464 (1909) Verified 20 Sep 2026
The isolation and counting of gaseous ions, on the one hand, which have crowned with success the long and brilliant researches of J.J. Thomson, and, on the other, agreement of the Brownian movement with the requirements of the kinetic hypothesis, established by many investigators and most conclusively by J. Perrin, justify the most cautious scientist in now speaking of the experimental proof of the atomic nature of matter.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.