On Sat, Feb 9, 2013 at 12:29 AM, Allan Wechsler <acwacw@gmail.com> wrote:
One of the shows in Philip Morrison's great series, "The Ring of Truth", has an experiment that at least suggests the order of magnitude of the size of molecules. He rows out into a pond, and dumps a small vial of light oil onto the surface. [...]
Yep, I did that in science class at age 14 ("9th grade" in USA). You put talcum powder on the surface of the water and the oil-drop pushes the particles of powder aside, and forms a lipid monolayer. One can then calculate the thickness of the monolayer, and thus how many "cubes" would be needed to make it. We cheated a bit, the teacher told us the approximate length:width ratio of the molecules. We didn't bother dividing the answer by the number of atoms in the molecule. Surprisingly though, this was not one of the methods used to estimate the Avogadro number 100 years ago. In 1914 Perrin [1] summarized the work up to that point; the methods used were: Viscosity of gases (Maxwell/Clausius); Brownian motion (Einstein/Perrin; 4 methods); Critical opalescence (Smoluchowski); Black-body spectrum (Planck); charged spheres (Millikan); Radioactivity (e.g. Rutherford, Curie/Debierne; 4 methods). The results were all between 6.0e23 and 7.5e23, so they were off by 10%, but still pretty impressive. [1] Jean Perrin, Atoms (translated from the original French by D. Ll. Hammick) D. Van Nostrand (New York), 1916. Page 206. -- Robert Munafo -- mrob.com Follow me at: gplus.to/mrob - fb.com/mrob27 - twitter.com/mrob_27 - mrob27.wordpress.com - youtube.com/user/mrob143 - rilybot.blogspot.com