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

Original Article ↗ Hacker News Discussion ↗

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.