How did he know that the film of oil was one molecule thick?
It feels like a huge assumption to me, but maybe this blog post left something out.
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.
> 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
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
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!
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
"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.
Most fluids do not behave this way in most circumstances, because of surface tension, so it's really not intuitive.
This experiments is one of the few ways you can get an accurate measurement. Many other fluids will either mix or end up as bubbles/blobs many orders of magnitude thicker than a molecule.
Or did he pour it into a huge lake or something?
If this seams impossible, remember that when water freeze into ice, it expands to a 3D "net" with empty holes.
Famous example is Darwin figured out that traits are inheritable by natural selection, and this is the driving force of evolution, without having any concept of the physical nature of DNA, or how genes could change (eg. by DNA mutation) to develop adaptations and thus make an organism more fit.
The simplest explanation is that he deduced such a physical mechanism must exist but the science and technology available at the time could not locate it.
Rayleigh's experiment is just accessible requiring very little training / background to describe. To interpret as a monolayer is honestly probably not so accessible at all, though, and a weakness of the atomsonly.news piece and seemingly not even done by Rayleigh himself. Modern retconning as zokier says elsewhere.
Of course this also fails if the oil formed a disc of X layers of molecules
He didn't. It was an assumption