Yesterday I was thinking about how they originally demonstrated that atoms exist. The traditional story is, of course, that Einstein showed in his 1905 paper that Brownian motion was explicable in terms of atoms, but it occurred to me that I remembered another experiment from when I was at school that also demonstrated the existence of atoms, and I wondered why noone used it as an argument before 1905.

The experiment in question was the one where you mix 50ml of one chemical 50ml of another (I can't remember which chemicals they were, and I imagine there are many that you could pick from and see the same effect) and find that the mixture only occupies 97ml. This can be explained by imagining that the molecules of one of the chemicals are smaller than those of the other, so could occupy some of the gaps between the bigger molecules. Just as if you mixed half a cup of coffee beans with half a cup of sugar, some of the sugar would fill in the gaps between the beans and so the mixture would not occupy the full cup.

Does anyone know if this effect had been noticed before the Einstein paper? Were there any explanations put forward that didn't require atoms?

Thanks for any answers.

Both the 97% and Brownium movement are evidence, not proof of atoms. Einstine was highly respected so his evidence became mainstream. Lots more evidence, and few problems with atoms have occured since so we consider them proved beyond reasonable doubt. If it works, it likely is correct. Neil

See this from Kyushu University in Japan, and continue to the next several pages.

http://www2.kutl.kyushu-u.ac.jp/semi...overAtom_E.htm

Alcohol and water exhibit this effect enough that alcohol content in beverage is listed explicitely by weight or by volume as they are noticeably different.

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Isn't that just because water and alcohol have different density?

neilzero. In 1905, when Einstein published his 5 papers, he was an unknown scientist, and a very good patent clerk. The respect came slowly at first, and not without some trepidation on the part of millions. The clincher was the eclipse event, ~ a decade later ...the sun bent starlight.
There was certainly the Law of Definite Composition for pure compounds, and the Law of Multiple Proportions, which yielded easily to an atomic model, and kinetic theory in gas laws which inferred colliding particles...all of which pointed to discrete structures in nature. So he wasn't out on a limb, tooting atoms. The guy who did that first was English schoolmaster, John Dalton...(many people thought he was nuts ).

Contemporary electron microscopy clearly shows atomic structures, but that technology was far removed from the turn of the century. Advances in instrumentation often lead to advances in theory, and lots of surprises through the years. pete

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Not just electron microscopes, but a much earlier development as well. I believe X-ray crystallography could also something of atomic structure, though this was first developed about 7 years after Einsteins paper.

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You're right, I had a brain fart.
The effect that the combined volume is smaller than the sum of the volumes is still easily observed.

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And the "driving on the freeway on a scooter" analogy still holds true because the pilots are sitting in 7 to 30 ton aircraft o' doom and you are running around them in your very own Meatbody, Mark I. Beep, beep.
Trying to make sense of computers, The Error Log.

Yes, that's interesting-- I hadn't heard that evidence in favor of atoms.

I'm not sure if I'm nitpicking or I just don't understand.

These proofs seem to be about molecules, not atoms.

What's the obvious shift in proof from molecules to atoms?


(There's a song on the local radio playlist with the line "I can split the atom of a molecule". That really bugs me. Maybe it shouldn't.)

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You're right, and I wouldn't call it a nitpick, but perhaps it's not the core of the debate. The main issue is continuous matter vs. discrete pieces, so if the molecules represented discrete pieces of internally continuous matter, your objection would be founded-- but arguers for continuous matter would find no reason to have molecules in the first place. If matter was continuous, why would it need molecules? Even the existence of atoms that are hard to "split" does not prove matter isn't continuous at some deeper level, it just shows that it comes in discrete bits at a level underneath what seems continuous.

I guess it depends on what you mean by "proof".

John Dalton and other early 19th century chemists showed that substances (elements) combine in set, whole number ratios, and explained that using the concept of atoms.

Brownian motion was first described in 1827 by Robert Brown and as early as 1877 was explained by the motion of water molecules. Einstein was the first to describe it mathematically.

Personally, I think Thomson's work with cathode rays (electrons) in 1897, Rutherford's gold foil experiments in 1909, and the x-ray diffraction work already mentioned, really set the stage for the modern concept of atoms, more than Einstein's work.

This wikipedia article has a good history of the whole thing.

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