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

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

#191

Post by hollerith »

qsdf38100 wrote: post #190
ForOldHack wrote: post #180

Earlier quoted context omitted.

The speed of light, because it is so fast, is the most precise physical constant known : 299 792 458 m / s. Less than 7/1000ths away from 300,000,000 m / s. I am not going to sweat this in the least. So light travels only 0.3 m / nano second, or 11.802 inches. https://www.youtube.com/watch?v=9eyFDBPk4Yw

Speed of light has recently been redefined, and is now _exactly_ 299792458 m/s. It’s no longer a measurement, it’s a definition.

That's a little misleading: either the meter or the second has been redefined to make c = 299792458 m/s.

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

#192

Post by zokier »

lIl-IIIl wrote: post #109
nielsbot wrote: post #99

Earlier quoted context omitted.

My question exactly.. I hope someone can chime in :) EDIT: Thinking a bit more... I suppose it's a reasonable assumption that the molecules (mostly) wouldn't stack on top of each other. They all want to get lower and perhaps the resistance to the oil spreading out is much lower proportionately that the gravitational force encouraging the oil to flatten

But if I spill some oil on my plate it doesn't look a molecule thin to me. Why is it different with water?

I believe this is at least part of the explanation. Although there might have been some further developments in the 100 years since this was published

    Oleic acid on water forms a film one
    molecule deep, in which the hydrocarbon chains stand vertically on the water surface
    with the COOH groups in contact with the water.

    Acetic acid is readily soluble in water because the COOH group has a strong
    secondary valence by which it combines with water. Oleic acid is not soluble because
    the affinity of the hydrocarbon chains for water is less than their affinity for each other.
    When oleic acid is placed on water the acid spreads upon the water because by so doing
    the COOH can dissolve in the water without separating the hydrocarbon chains from
    each other.

    When the surface on which the acid spreads is sufficiently large the double bond
    in the hydrocarbon chain is also drawn down on to the water surface, so that the area
    occupied is much greater than in the case of the saturated fatty acids.

    Oils which do not contain active groups, as for example pure paraffin oil, do not
    spread upon the surface of water
https://zenodo.org/records/1429064

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

#193

Post by [deleted] »

bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. 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.

[deleted]

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

#194

Post by cb321 »

bqmjjx0kac wrote: post #4

> Assuming that the oil formed a single layer of molecules — a monolayer — then the thickness of the oil film is the same thing as the length of one oil molecule. 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.

The molecular scale was well estimated (at least for the gaseous phase of matter) by 1865 (25 years earlier not 5 as the incorrect HN title would suggest) https://physics.stackexchange.com/questions/13757/how-was-av... (and guesstimated by just "following your nose" in Gandalf-of-Lord-Of-The-Rings-ese in 1646!)

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.

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

#195

Post by ghkbrew »

hollerith wrote: post #191
qsdf38100 wrote: post #190

Earlier quoted context omitted.

Speed of light has recently been redefined, and is now _exactly_ 299792458 m/s. It’s no longer a measurement, it’s a definition.

That's a little misleading: either the meter or the second has been redefined to make c = 299792458 m/s.

The second is (and has been) defined independent of length for a while. It's the time it takes for a certain number oscillations of a caesium atom.

The meter was redefined as the distance light travels in a specific time. So you could say that either the meter or the speed of light was redefined to make the speed of light a round constant, but not the second.

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

#196

Post by tantalor »

kccqzy wrote: post #176
tantalor wrote: post #66

Earlier quoted context omitted.

Scientists frequently have to make assumptions in order to make progress. 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.

This is why I guess I was never really interested in scientific experiments personally and decided to study mathematics. These assumptions don't seem justified to me. At least in mathematics you always state these assumptions or hypotheses very clearly, or make them into axioms.

I don't know if anybody challenged Darwin on that point. It's an interesting question.

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.

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

#197

Post by ummonk »

bryanlarsen wrote: post #156
ummonk wrote: post #150

Earlier quoted context omitted.

No the idea that Rayleigh knew the film was one molecule thick, let alone why it was one molecule thick, is clearly made up. The paper makes it clear that he was reporting the thickness, and thought this might provide information on molecule size.

LOL, the first line of the ChatGPT response: `Lord Rayleigh didn’t know a priori that surface tension would cause the oil film to be exactly one molecule thick, `

And then it goes on to provide various explanations as to why he may have known it a posteriori.

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

#198

Post by [deleted] »

ghkbrew wrote: post #195
hollerith wrote: post #191

Earlier quoted context omitted.

That's a little misleading: either the meter or the second has been redefined to make c = 299792458 m/s.

The second is (and has been) defined independent of length for a while. It's the time it takes for a certain number oscillations of a caesium atom. The meter was redefined as the distance light travels in a specific time. So you could say that either the meter or the speed of light was redefined to make the speed of light a round constant, but not the second.

[deleted]

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

#199

Post by RheingoldRiver »

jonny_eh wrote: post #97
RheingoldRiver wrote: post #83

Earlier quoted context omitted.

I highly recommend this book! https://www.goodreads.com/book/show/25238350-the-hunt-for-vu...

I'd recommend this one instead: https://en.wikipedia.org/wiki/A_Short_History_of_Nearly_Ever...

Why "instead" ? they seem completely different from each other

Hunt for Vulcan is a fun history lesson with some interesting insights about human nature. It's history of science, not science. It took about 3 hours to read and I had a lot of fun with it.

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

#200

Post by lern_too_spel »

tsimionescu wrote: post #121
lern_too_spel wrote: post #96

Earlier quoted context omitted.

How we derived the laws and theories is science. (Some of the other commenters are mixing this together with biographies of the scientists, which is not science but is sometimes interesting in its own right.) The laws and theories in isolation are just trivia, and any class that teaches just those cannot truthfully be called a science class. Demand a better education.

Both have their value, both the process and the results. And given the immensity of scientific knowledge, you can only learn so much of it in a K-12 education, or even in college. I don't think it's a priori wrong to teach students our current understanding of the world, without going into the details of how we came up with it. I also don't think it's wrong to add those details, but the more details you add, the less…

The results are useful in engineering, but the fact that the results are useful does not mean that a class that teaches just the results is a science class.

You don't have to include the boring bits of who paid for the research or the day to day lives of the researchers. That isn't science, and it doesn't help the student understand science. A mere description of the fact that Tycho Brahe kept meticulous records of the positions of the planets in the sky and a walkthrough of the math that shows this data matches elliptical orbits and the math that shows that Newton's laws lead to those elliptical orbits is sufficient. The fact that there is a discrepancy for Mercury motivates further developments.

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