Re: [Phys-L] Resources about "the atomic hypothesis"...

[John Denker <jsd@av8n.com>]

On 01/24/2013 10:27 AM, Jim Diamond wrote:
> Dalton figured out the law of multiple proportions in the beginning of the
> 19th Century. That is pretty substantial support for the atomic hypothesis
> and doesn't require more than a few calculations of percent mass
> composition.

Actually it requires one more ingredient, namely artfully selecting
what data you are going to look at.

In my line of sight right now, there is *nothing* that upholds the
"law" of multiple proportions.  Wood, cotton, paper, rubber, plastic,
brass, steel, feldspar, brick, porcelain, glass, paint, leather, skin, 
hair, air, et cetera.

  I suppose I could go looking for water, salt, baking soda, highly 
  refined sugar, and copper wire ... but those seem to be the exceptions 
  not the rule.

We agree that the "law" of multiple proportions was important in the 
history of science.  OTOH one must be careful not to overstate the
validity of this "law".

As a partially-related point:  The OP asked on behalf of lay persons.
Alas, even given one or two substances that uphold the "law" it is 
hard for lay persons to do experiments that yield convincing data
on this topic.

Electrolysis of H2O is far and away the best experiment I can think
of on this topic.  In this case you require yet another idea, namely
Avogadro's law (applied to the products) -- before you can interpret 
the results as evidence  of the structure of the reactant.

Also note that setting up the electrolysis experiment beyond the 
skills of most lay persons.  Perhaps better instructions would help;
the instructions I've seen on the web do not even remotely resemble 
the way I would do it.  In any case, understanding it once it's been 
set up is a lot easier than setting it up.

My father set this up for me when I was about 5 years old.  I didn't
realize it at the time, but I probably learned a super-important
lesson from watching him start with a suboptimal method and then
gradually make improvements.