4 ms·

Blog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lit

Blog post seems to have elided this point, but it did link the original paper which was quite short: https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_literature/Rayleigh1890.pdf https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite... Rayleigh's experiment was actually trying to solve for the minimum thickness of oil required to stop some camphor shavings from moving around on the water. He never states it explicitly, but I think the assumption is that the minimum thickness required to stop the shavings' movement would be such that the oil volume 'just' covers the surface, ie. is 1 molecule thick everywhere and hence the shavings never touch water. I think he's specifically making a slightly more clever point about surface tension, but that's a little beyond me.

youainti 2y ago
Camphor would release compounds that adjust the surface tension of water. So the oil would break that direct relationship.
- tech_ken 2y ago
  Ahhh that's cool, thanks for clarifying
zokier 2y ago
Reading the paper, there is no mention of sizes of molecules. Did Rayleigh actually make the connection between film thickness and molecular size at some point? Or is that just modern retconning?
- tech_ken 2y ago
  It's really (really) quick but the first line of the second para is: > In view, however, of the great interest which attaches to the determination of molecular magnitudes, the matter seemed well worthy of investigation... So it seems like his main goal was to understand the size of molecules via his film-thickness measurements
- zokier 2y ago
  Replying to myself, I found 1899 paper which is more explicit on the matter, and shows how Rayleigh was not all that certain about the results: The comparison of the present with former results throws an interesting light upon molecular magnitudes. It has been shown (Proc. Roy. Soc. March 1890) that the thickness of the film of olive-oil calculated as if continuous, which corresponds to the camphor-point, is about 2.0 μμ while from the present curves it follows that the point at which the tension begins to fall is about half as much, or 1.0 μμ [...] If we accept this view as substantially true, we conclude that the first drop in tension corresponds to a com- plete layer one molecule thick, and that the diameter of a molecule of oil is about 1.0 μμ XXXVI. Investigations in Capillarity:—The size of drops.—The liberation of gas from supersaturated solutions.—Colliding jets.—The tension of contaminated water-surfaces If we assume that the "about 2.0 μμ" value is just the previously mentioned 1.63 nm value rounded up, then that throws a wrench into the story, in particular this bit from blog post > Rayleigh’s final result was 1.63 nanometers. Olive oil is mainly composed of fat molecules called triacylglycerols, and we now know that they measure about 1.67 nanometers in length, implying that Rayleigh’s “primitive” estimates were off by just 2 percent is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that!
  - tech_ken 2y ago
    > is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that! I definitely thought so too, in the first paper the 1.67 isn't even really his primary guess so definitely some presentist bias in the OP
    - bialpio 2y ago
      From the paper: "The thickness of oil required to take the life out of the camphor movements lies between one and two millionths of a millimetre, and may be estimated with some precision at 1’6 micromillimetre." Looks like a primary guess to me, even if the table lists more data points.
      - tech_ken 2y ago

youainti 2y ago

Camphor would release compounds that adjust the surface tension of water. So the oil would break that direct relationship.

tech_ken 2y ago
Ahhh that's cool, thanks for clarifying

zokier 2y ago

Reading the paper, there is no mention of sizes of molecules. Did Rayleigh actually make the connection between film thickness and molecular size at some point? Or is that just modern retconning?

tech_ken 2y ago
It's really (really) quick but the first line of the second para is: > In view, however, of the great interest which attaches to the determination of molecular magnitudes, the matter seemed well worthy of investigation... So it seems like his main goal was to understand the size of molecules via his film-thickness measurements
zokier 2y ago
Replying to myself, I found 1899 paper which is more explicit on the matter, and shows how Rayleigh was not all that certain about the results: The comparison of the present with former results throws an interesting light upon molecular magnitudes. It has been shown (Proc. Roy. Soc. March 1890) that the thickness of the film of olive-oil calculated as if continuous, which corresponds to the camphor-point, is about 2.0 μμ while from the present curves it follows that the point at which the tension begins to fall is about half as much, or 1.0 μμ [...] If we accept this view as substantially true, we conclude that the first drop in tension corresponds to a com- plete layer one molecule thick, and that the diameter of a molecule of oil is about 1.0 μμ XXXVI. Investigations in Capillarity:—The size of drops.—The liberation of gas from supersaturated solutions.—Colliding jets.—The tension of contaminated water-surfaces If we assume that the "about 2.0 μμ" value is just the previously mentioned 1.63 nm value rounded up, then that throws a wrench into the story, in particular this bit from blog post > Rayleigh’s final result was 1.63 nanometers. Olive oil is mainly composed of fat molecules called triacylglycerols, and we now know that they measure about 1.67 nanometers in length, implying that Rayleigh’s “primitive” estimates were off by just 2 percent is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that!
- tech_ken 2y ago
  > is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that! I definitely thought so too, in the first paper the 1.67 isn't even really his primary guess so definitely some presentist bias in the OP
  - bialpio 2y ago
    From the paper: "The thickness of oil required to take the life out of the camphor movements lies between one and two millionths of a millimetre, and may be estimated with some precision at 1’6 micromillimetre." Looks like a primary guess to me, even if the table lists more data points.
    - tech_ken 2y ago