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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

#61

Post by namuol »

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…

So very true. The greatest science teachers understand the power that comes with the stories of scientific discovery.

Carl Sagan’s Cosmos and some of Richard Feynman’s best lectures come to mind as some of the most memorable examples, but I’m certain all the best teachers out there know to incorporate the historical and human aspects to bring the essential perspective and natural mnemonic anchors to otherwise “dry” subjects.

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

#62

Post by bryanlarsen »

ummonk wrote: post #37
bryanlarsen wrote: post #24

Earlier quoted context omitted.

He certainly knew oil was hydrophobic. I don't think the hydrophilic nature was necessary for the logic. If it was, I'm sure he knew that soap oils are both hydrophobic and hydrophilic and had ways of figuring out that soap oils consist of a single type of molecule and aren't a mixture.

He used olive oil though, not a soap oil.

Olive oil is a soap oil.

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

#63

Post by bagels »

layer8 wrote: post #45
munchler wrote: post #7

Earlier quoted context omitted.

Agreed. The experiment actually gives an upper limit on the size of a molecule in one particular dimension. Still a very useful result.

It isn’t necessarily an upper bound. The molecules might spread out more distant than their size.

Wouldn't that provide an upper bound then? If the real size is equal to or less than the calculated size?

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

#64

Post by nmstoker »

pechay wrote: post #58

We did this same experiment in school, with a tiny pinprick of oil, estimating the volume of the drop as a sphere, and a small water tank, and then estimated the area of oil slick as a circle.

Yes, we did it in physics at school too, when we were 13 or 14 I think.

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

#65

Post by dilawar »

I went to a talk by a very old physicist. At the end of his talk, he said, recalling from memory, all of the great experiments of the past were done by nothing. If an experiment costs more than $100, I am out.

His setup has mud in a jar and bacteria in it which you can see with a simple microscope or handheld lens.

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

#66

Post by tantalor »

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.

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.

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

#67

Post by taberiand »

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.

Perhaps at the time it was sufficient to define "molecule of oil" as "the height of the amount when spread maximally across the surface of water", and it just so happens that height is only 1 actual molecule

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

#69

Post by ziofill »

>"not more than a Tea Spoonful," according to his diary — Franklin poured it onto the agitated water. The oil spread rapidly across the surface, covering "perhaps half an Acre" of the pond and rendering its waters "as smooth as a Looking Glass."

What??

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

#70

Post by gus_massa »

layer8 wrote: post #45
munchler wrote: post #7

Earlier quoted context omitted.

Agreed. The experiment actually gives an upper limit on the size of a molecule in one particular dimension. Still a very useful result.

It isn’t necessarily an upper bound. The molecules might spread out more distant than their size.

In a very unlucky world, they can form a 2D net, with molecules instead of strings and a lot of tiny holes.

If this seams impossible, remember that when water freeze into ice, it expands to a 3D "net" with empty holes.

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