There is more evidence to prove that saltness [of the sea] is due to the admixture of some substance, besides that which we have adduced. Make a vessel of wax and put it in the sea, fastening its mouth in such a way as to prevent any water getting in. Then the water that percolates through the wax sides of the vessel is sweet, the earthy stuff, the admixture of which makes the water salt, being separated off as it were by a filter.
Aristotle: Meteorology, Book IIFirst Posted October 15, 2012: It has been suggested that this is an example of Aristotle giving experimental proof of desalination by osmosis. In fact, it is an example of why science in ancient Greece never took off. Moreover, if the procedure Aristotle described had worked, it would have been an example not of osmosis but of reverse osmosis, whereby water from a salty solution is forced through a semipermeable membrane that prevents the passage of salts. But as described, the procedure would not have worked because the walls of a vessel of wax will not serve as a semipermeable membrane. Moreover, unless the vessel was immersed at depth, something Aristotle does not mention, there would have been no pressure gradient to drive the process.
So what was Aristotle talking about? Two possibilities suggest themselves. He may have been reporting a piece of nonsensical hearsay, something he was regrettably inclined to do, or the experiment yielded the results as claimed, but not in the way described. Considering the second possibility, Aristotle makes a point about fastening the mouth of the vessel "in such a way as to prevent any water getting in," which raises the question of how that might have been done? It is doubtful that Aristotle would have had a rubber bung of the right size to hand, so he may have tied something over the mouth of the vessel: a piece of cat gut, perhaps, or sheep's intestine (cling film had yet to be invented). In that case, he would have had an impermeable vessel covered at the mouth with a semipermeable membrane.
So now, if he had weighted the vessel with a rock then chucked it into the Mediterranean at a deepish spot, his experiment would have yielded the results as reported. But who knows? Apparently no one followed up on a momentous discovery. As a consequence, the inhabitants of the Aegean Islands have had to wait for the passage of more than two thousand years and a grant from the EU before receiving plentiful fresh water supplied by reverse osmosis.
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