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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

#161

Post by DonaldFisk »

stevenwoo wrote: post #31
hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

Did you get your physics education in high school or university? I only had to take one physics class in the USA at college for my major, quantum electrodynamics for electrical engineering but my professor wrote the textbook and I recall he went over each experiment starting from the fundamentals of our understanding of the basics of the atom, Newton's understanding of light at the time, double slit experiment, to Ma…

> to deriving then proving experimentally proving general relativity and decomposing GR into Newtonian physics/other laws of electromagnetism

Do you mean Special Relativity, which covers classical mechanics and electromagnetism? General Relativity covers gravitation and cosmology without electromagnetism (though Kaluza and later Klein devised a theory unifying gravity and electromagnetism by adding an fifth dimension to General Relativity, which can then be decomposed into 4-dimensional GR and Maxwell's equations).

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

#162

Post by zokier »

zokier wrote: post #152
tech_ken wrote: post #29

Earlier quoted context omitted.

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_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…

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?

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!

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

#163

Post by knodi123 »

stevenwoo wrote: post #31
hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

Did you get your physics education in high school or university? I only had to take one physics class in the USA at college for my major, quantum electrodynamics for electrical engineering but my professor wrote the textbook and I recall he went over each experiment starting from the fundamentals of our understanding of the basics of the atom, Newton's understanding of light at the time, double slit experiment, to Ma…

you only had to take one physics class, and it was quantum electrodynamics??? That sounds to me as if someone said "The only math class I've taken was differential equations."

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

#164

Post by carlob »

coldcache wrote: post #46

Related: Agnes Pockels’ experiments [0] [0]: https://en.wikipedia.org/wiki/Agnes_Pockels

Ten years earlier and she didn't publish it right away. That really goes to show how much more difficult it was for a woman to become a scientist back then.

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

#165

Post by hn_throwaway_99 »

danielam wrote: post #128
hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

Yes, often what is taught is taught in a manner that seems mysterious in origin, as if it were revealed, certain and final, and developing a sensibility like that concerning scientific matters is not good. You could argue that the viability of science as such rests on certain articles of faith, but the particular findings of science themselves are a matter of demonstration, interpretation of demonstration and argumen…

Yes! Not to get too political, but I saw a lot of this during the Covid debates, e.g. "Trust the science!" Noooo! Science is not just something you're supposed to "trust", but something that's supposed to be supported by evidence.

Yes, I definitely understand that most people don't have the training and background to understand complex scientific topics, and in some ways we do have to trust the scientific community if we're not a part of it. And I get frustrated by the common calls of "Do your own research!", which often means "Look at these YouTube grifters with absolutely no training who are just spouting stuff with no research of their own." But even the underlying problem with that is that most people aren't trained to evaluate the quality of data and motivations of people making it, and that is what scientific education should be about. For example, I may have to "trust" the scientific community when it comes to data about infectiousness of COVID because I'm not an epidemiologist, but how that data is translated into rules and regulations is a policy call, and that policy call is not necessarily one where the epidemiologists are the experts. I shouldn't be told to "trust the science" as though I should just accept policy recommendations even if I do accept the underlying data about transmissibility.

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

#166

Post by fii »

PhasmaFelis wrote: post #19
FpUser wrote: post #8

Earlier quoted context omitted.

[flagged]

> Cortesy of ChatGPT: Can we not, please? At the very least not unless you personally have the knowledge to confirm that ChatGPT didn't make any mistakes, as it did here.

Agreed, although it does make me wonder whether the number of mistakes chatgpt comments make would truly be greater (or at the least more harmful) than the outstanding number of folk who are confidently wrong in a way only obvious to domain experts. It’s easier to be skeptical of a bot.

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

#167

Post by [deleted] »

hn_throwaway_99 wrote: post #2

I love articles like this. I feel like too often in science education (at least my science education) that laws and theories are presented as just something that you need to memorize, when in my opinion the stories of how things were originally discovered and figured out is eminently more fascinating and inspiring. Like I remember having to learn all of these biochemical pathways, but I left school with nary a clue a…

[deleted]

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

#168

Post by wizzwizz4 »

zokier wrote: post #147
NeoTar wrote: post #130

Earlier quoted context omitted.

Well, also knowledge of the distances between the objects involved (at least the Earth and Jupiter), which in turn depended on a series of further investigations. Which is not to denigrate the achievement, but if I were to drop you on an alien world with only a telescope and an accurate time keeper, you're not going to be able to recreate it.

> if I were to drop you on an alien world with only a telescope and an accurate time keeper, you're not going to be able to recreate it. But you can relatively easily derive the distances too with timekeeper and telescope, by applying Keplers laws?

One can, in principle. Most people lack the talent. (It's a skill that almost anyone could learn, but that doesn't make it a skill everyone has.)

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

#169

Post by gnramires »

rkagerer wrote: post #89

This is fascinating, but wasn't it still a bit of a conjecture to assume the oil would spread to a minimum thickness of one molecule? Did he have any doubts, like that surface tension might keep it thicker? Or other clues hinting it was indeed a monolayer?

Perhaps he was observing the layer making sure it had integrity? Oil layers famously have an optical effect (iridescence from interference of reflections). This effect would transition as the layer goes from >= 1 molecule thick to <= 1 molecule thick (on average). It's likely possible to pinpoint this transition experimentally and then using the oil volume obtain the molecular layer thickness.

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

#170

Post by empyrrhicist »

db48x wrote: post #135
wholinator2 wrote: post #132

Earlier quoted context omitted.

No need to nitpick, the original comment states perfectly accurately that he was measuring the fundamental charge _of the electron_, from which the constant e is derived. We've been using e to mean _electron_ charge for a very long time. Where do you get the idea that constant has anything to do with quarks?

https://i.kym-cdn.com/photos/images/original/000/992/402/c35... But seriously, it really is time to fix all the chemistry textbooks so that the charge of the electron is +3 instead of −1.

Why, to make all that quark chemistry a bit easier?
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