Difficulties Getting the Kinetic Theory Moving
Oddly, the first published calculation of the average speed of a molecule in the kinetic theory of
gases appeared in the
, of all places, in 1836. Why there?
calculation was by John Herapath—he owned the magazine.
He was definitely not part of the
scientific establishment: a previous paper of
his on the kinetic theory had been rejected by the
But the calculation of molecular speed was in fact correct.
John James Waterston, submitted an excellent paper on the kinetic theory to the Royal Society in
1846, to have it rejected as “nonsense”.
This was evidently still the age of the caloric theory, at
least in the Royal Society.
In 1848, Joule (who had worked with Herapath, and was also
something of an outsider) presented a paper at a meeting of British Association where he
announced that of the speed of hydrogen molecules at 60F was about 1 mile per second, close to
Again, though, this did not excite wide interest…
Finally, in 1857, a pillar of the scientific establishment—Clausius—wrote a paper on the kinetic
theory, repeating once more the calculation of
average molecular speed (around 460 meters per
second for room temperature oxygen molecules). He mentioned the earlier work by Joule, and
some more recent similar calculations by Krönig. Suddenly people sat up and took notice!
highly respected German professor was willing to entertain the possibility that the air molecules
in front of our faces were mostly traveling faster than the speed of sound, perhaps there was
something to it…
How Fast Are Smelly Molecules?
But there were obvious objections to this vision of fast molecules zipping by.
As a Dutch
meteorologist, C. H. D. Buys-Ballot, wrote: [if the molecules are traveling so fast] how does it
then happen that tobacco-smoke, in rooms, remains so long extended in immoveable layers?”
(Nostalgia trip for smokers!)
He also wondered why, if someone opens a bottle of something
really smelly, like ammonia, you don’t smell it across the room in a split second, if the molecules
are moving so fast.
And, why do gases take ages to intermingle?
These were very good questions, and forced Clausius to think about the theory a bit more deeply.
Buys-Ballot had a point: at that speed, the smelly NH
’s really would fill a whole room in
So what was stopping them?
The speed of the molecules follows
measuring the pressure and density—you don’t need to know the size of molecules. If the kinetic
theory is right at all, this speed has to be correct. Assuming, then, the speed
more or less
correct, the molecules are evidently not going in straight lines for long.
They must be bouncing