Re: Proof for atoms?

[Jeff Radtke <jr@REFLECTIONIMAGING.COM>]

A most striking qualitative demonstration of molecular size occurs
when I very quietly open and slice a piece of cheese and my
salivating dog arrives moments later from the other end of the
house.

An excellent quantitative explanation of measurements of Avagadro's
number can be found in the Textbook of Physical Chemistry by Samuel
Glasstone, 1940.  Experiments undertaken by Perrin beginning in 1908
involved observations of particles in suspension, and Glasstone's
description includes mathematical details and historical references.
He goes on to write that later measurements using X-ray diffraction
gave more accurate values using X-ray wavelengths precisely
determined by independent means.  X-ray diffraction techniques
can measure inter atomic or intermolecular distances in gasses,
liquids, or solids.

- Jeff Radtke
jr@cloudchambers.com
http://www.cloudchambers.com



Savinainen Antti wrote:
> Hello,
>
> I  had an interesting discussion about atoms with a doubter who asked=
>  if there
> is any proof for existence of atoms. The argument was that of course =
> physicists
> find atoms because the idea of atoms is their very starting point! Th=
> e doubter
> asked experiments which could provide convincing evidence for the exi=
> stence of
> atoms without having initially decided that idea of atoms is used to =
> analyze
> the data.
>
> What would you answer?
>
> In a way I think that the doubter would have been quite right in the =
> 19th
> century. At the time atoms were used as an hypothesis which allowed n=
> ice
> explanations and predictions in thermal physics.
>
> The question is interesting from the point of view of philosophy of s=
> cience.
> Einstein said that it is the theory that determines what can be obser=
> ved (I
> don't have an exact citation here; I read it from an issue of Physics=
>  Today a
> year or two ago). I also recall that Hawking said that there is no ex=
> perimental
> data on cosmology which would be independent on theory (again this is=
>  not exact
> citation). OTOH it is not easy at all to construct theories which wou=
> ld provide
> correct predictions even in previously unkonown phenomena. It is very
> convincing if a theory is capable of doing that.
>
> I think that these "how do you know" questions could be useful in tea=
> ching as
> well. It forces students (and me) to think critically. Late Arnod Aro=
> ns has
> written about this issue in his marvelous "Teaching Introductory Phys=
> ics" book.
>
> Regards,
>
> Antti
>
> Antti Savinainen
> Physics Teacher
> Kuopion Lyseo High School/IB
> Finland
>
> Homepage: <http://personal.inet.fi/tiede/physics/>