This was a favorite of Professor Edward Mason, Newport Rogers Professor Emeritus, Department of Chemistry, Brown University.
By the way, Avagadro did not do this experiment, nor
did Ben Franklin determine Avagadro's number.
"At length being at Clapham, where there is on the common a large
pond which I observed one day to be very rough with the wind, I fetched out a cruet
of oil and dropped a little of it on the water. I saw it spread itself with surprising
swiftness upon the surface; but the effect of smoothing the waves was not produced;
for I had applied it first on the leeward side of the pond where the waves were greatest;
and the wind drove my oil back upon the shore. I then went to the windward side where
they began to form; and there the oil, though not more than a teaspoonful, produced
an instant calm over a space several yards square which spread amazingly and extended
itself gradually till it reached the lee side, making all that quarter of the pond,
perhaps half an acre, as smooth as a looking glass."
Oil on the Waters, by Benjamin Franklin, 1773