In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

Original Article ↗ Hacker News Discussion ↗

Lord Rayleigh’s classic oil-on-water experiment, inspired by Benjamin Franklin’s observations of “oiling the seas,” is used as a springboard to explore how scientists first estimated molecular dimensions long before molecules could be seen. Commenters probe the key assumptions behind treating the oil film as a one-molecule-thick layer, compare Rayleigh’s method to other landmark measurements like Millikan’s oil-drop experiment and early estimates of Avogadro’s number, and note where later analysis suggests the popular story glosses over uncertainties. The exchange broadens into a critique of science education that presents laws as facts to memorize rather than as the outcome of clever, often simple experiments, with many recommendations for historical and pedagogical resources that better convey how scientific knowledge is built.

Value of historical narratives in science

  • Many commenters say school science over-emphasized memorizing laws and pathways, under-emphasized how discoveries were made.
  • Historical experiments are seen as both more engaging and more faithful to how science actually works.
  • Physics education is often praised for including history and foundational experiments, while chemistry and biology are criticized for focusing on facts and details.
  • Several recommend history-of-science style books, lectures, and videos as superior pedagogy.

Rayleigh’s oil-film experiment and its assumptions

  • Core question: how did he know the film was one molecule thick?
  • Multiple commenters stress he did not “know”; it’s a hypothesis that oil spreads to a monolayer on water, giving at best an upper bound on molecular length.
  • Others argue surface-tension behavior and repeatability (same thickness across many drops and areas) make the monolayer assumption reasonable, though not certain.
  • Some note complications: oil could form regions with 1–2 layers, packing density may change at the air–water interface, and volume conservation is not obviously guaranteed.
  • A later paper by Rayleigh is cited where he links the first drop in surface tension to a one-molecule-thick layer, and his numerical estimate is closer to half the “celebrated” value in the blog, suggesting some present-day retconning and numerological coincidence.

Experimental details and replications

  • Commenters discuss how area was measured: fixed-size bowls, weight/volume of oil needed to calm the surface, or visual methods using powders or surfactant films.
  • Several people report reproducing the experiment in high school or university; results were instructive but often noisy or off by orders of magnitude.
  • Related work by other surface-tension experimenters is mentioned as historically connected.

Broader themes: experiment design and philosophy of science

  • Thread branches into other classic experiments (oil-drop charge measurement, interferometer tests of relativity, early speed-of-light estimates) as examples of ingenuity from limited tools.
  • Many emphasize that experiments are hard, assumptions are unavoidable, and results are models, not final truth.
  • There’s debate on how much science education should prioritize methods and history versus present-day results, and on public calls to “trust the science” without understanding evidence.

Corrections and context

  • Commenters note the article’s date is off; Rayleigh’s key paper is from 1890, not 1870.
  • Earlier work had already estimated molecular scales; Rayleigh’s contribution is framed as a particularly accessible, elegant method rather than the very first determination.