From a swollen pig's bladder in 1748 to the membranes that water cities.
People noticed water creeping through animal membranes long before anyone knew what molecules were. Over two centuries, experimenters named the effect, measured its surprising strength, found it follows the same law as a gas, and then put it to work: in artificial kidneys, in purifiers and desalination plants, and in a cheap salt-and-sugar drink that has saved millions of lives.
J. H. van 't Hoff, for osmotic pressure and chemical dynamics
1965
First reverse osmosis drinking-water plant
Coalinga, California, with the Loeb–Sourirajan membrane
1748A bladder that swells
1740 – 1860
A bladder that swells
Curious experimenters notice that water sneaks through animal membranes and gases mix by themselves, and give the effects their first names and laws.
1748
A pig's bladder swells and bursts
Abbé Jean-Antoine NolletParis, France
Nollet filled a jar with spirit of wine (alcohol and water), tied a piece of pig's bladder over the top and sank it in pure water. Over a few hours the bladder bulged outwards, and when he pricked it, liquid spurted up more than a foot. Water had got in through the bladder, but the alcohol could not get out.
Why it mattered. It is the first recorded experiment on osmosis, and it already showed the key idea: a membrane that lets water through but not everything else builds up a pressure.
The French physician Henri Dutrochet studied how water moves into living tissues and through membranes. He called the inward flow endosmose and the outward flow exosmose, from Greek words for pushing. He built simple osmometers, tubes closed by a membrane, to watch liquid climb.
Why it mattered. His words gave us the name osmosis, and he suspected it was central to how plants and animals move water.
Thomas Graham measured how fast different gases leak and mix through porous plugs. Lighter gases were quicker: the rate went as one over the square root of the gas's density. We still call it Graham's law.
Why it mattered. It was one of the first measurements showing that diffusion follows simple rules set by the molecules themselves.
The young German physiologist Adolf Fick, working in Zurich, argued that salt spreads through water just as heat spreads through a metal bar. The flow of stuff is proportional to how steeply its concentration changes. He tested it with salt diffusing up tall columns of water.
Why it mattered. Fick's law is still how we calculate diffusion everywhere, from oxygen crossing the lungs to dye spreading in a glass.
Chemists make artificial membranes strong enough to measure osmotic pressure, find it obeys a gas-like law, and use diffusion to prove that atoms are real.
1861
Dialysis is invented
Thomas GrahamLondon, England
Graham noticed that small molecules like salt and sugar pass through parchment paper quickly, while gluey substances like gum and gelatin hardly pass at all. He called these colloids and named the separation method dialysis.
Why it mattered. Dialysis became a standard lab tool, and eighty years later the basis of the artificial kidney.
Moritz TraubeBreslau, Prussia (now Wrocław, Poland)
Moritz Traube found that when copper sulphate meets potassium ferrocyanide they form a thin film of copper ferrocyanide that lets water through but holds back many dissolved substances. It was a membrane made in a test tube instead of taken from an animal.
Why it mattered. Traube's membranes were far more selective than bladders, which made real measurements of osmotic pressure possible.
The botanist Wilhelm Pfeffer grew a Traube membrane inside the pores of an unglazed clay pot, so it could stand high pressure. Filled with sugar solution and sealed to a pressure gauge, it gave the first reliable numbers: the pressure grew with the concentration and with the temperature. He published them in his book Osmotic Investigations.
Why it mattered. These were the numbers van 't Hoff would turn into a law. A weak 1% sugar solution already pushed with about two-thirds of an atmosphere.
Hugo de Vries dipped plant cells in solutions of different strengths and watched under the microscope. In strong ones the living contents pulled away from the cell wall, which he called plasmolysis. By finding the strength that just started it, he could compare how hard different substances pulled. He called solutions that balanced a cell isotonic.
Why it mattered. He showed that living cells obey the same osmosis rules as clay pots, and his data helped van 't Hoff.
Jacobus Henricus van 't HoffAmsterdam, Netherlands
Using Pfeffer's and de Vries's numbers, van 't Hoff showed that a dissolved substance pushes on a membrane with exactly the pressure it would have as a gas in the same space: π = MRT. Salts pushed more than expected, and he added a factor i to count that. Svante Arrhenius soon explained why: salts split into ions.
Why it mattered. It tied osmosis to the gas laws and helped found physical chemistry.
Pollen specks in water jiggle for ever, which Robert Brown had seen in 1827. Einstein showed this Brownian motion is caused by water molecules knocking the specks about, and that the distance they wander grows as the square root of time. He linked it to the diffusion coefficient D.
Why it mattered. It explained diffusion from the bottom up, as random molecular kicks, and gave a way to count molecules.
Jean Perrin tracked tiny beads through a microscope and measured their Brownian wandering. The numbers matched Einstein's theory and gave the number of molecules in a mole. He won the 1926 Nobel Prize in Physics.
Why it mattered. After Perrin, few scientists still doubted that atoms and molecules exist.
Osmosis and diffusion become tools: an artificial kidney, membranes that turn seawater into drinking water, and a salt-and-sugar drink that saves millions of lives.
1943
1943–1945
The artificial kidney
Willem KolffKampen, Netherlands
During the German occupation, the Dutch doctor Willem Kolff wound cellophane sausage casing around a rotating drum in a bath of salt solution. Blood ran through the casing, and waste like urea diffused out while blood cells and proteins stayed in. In 1945 a patient in kidney failure survived thanks to it.
Why it mattered. Dialysis by diffusion now keeps millions of people with failed kidneys alive.
Sidney Loeb and Srinivasa SourirajanUniversity of California, Los Angeles, USA
Loeb, an American engineer, and Sourirajan, a chemist from India, made a cellulose acetate membrane with a very thin, dense skin on a spongy support. Water could be pushed through fast while salt stayed behind. The breakthrough was announced in August 1960.
Why it mattered. It made reverse osmosis practical. In 1965 a plant in Coalinga, California, began supplying drinking water with it.
During the Bangladesh Liberation War, cholera broke out among refugees near the border. Intravenous fluid ran out, so Dilip Mahalanabis, a paediatrician from Kolkata, had families give a simple salt-and-glucose drink themselves. In over 3,000 patients deaths fell to about 3.6%, against about 30% among those treated the usual way.
Why it mattered. It proved that oral rehydration works on a huge scale, in the worst conditions, given by ordinary people.
The most important medical advance of the century?
The LancetLondon, England
An editorial in The Lancet, Water with sugar and salt, said the discovery that sodium and glucose are absorbed together in the gut, pulling water with them, was potentially the most important medical advance of this century.
Why it mattered. It is a striking claim for a drink that costs almost nothing, and oral rehydration has since saved many millions of children.
John Cadotte made a polyamide skin only a fraction of a micrometre thick by letting two chemicals react right at the boundary between water and oil, on top of a porous support. His patent was granted in 1981, and FilmTec made it into the FT-30 membrane.
Why it mattered. Nearly every RO purifier and desalination plant today uses this kind of thin-film composite membrane.
Biologists find the protein pores that let water through cells, and reverse osmosis grows into a worldwide supply of fresh water.
1992
1992 (Nobel Prize 2003)
Water's own doorway: aquaporins
Peter AgreJohns Hopkins University, Baltimore, USA
Peter Agre's team found a protein in red blood cell membranes that forms a pore just for water. Cells with it swelled and burst in dilute solution; cells without it did not. These aquaporins are how the kidney's water channels work under the control of ADH.
Why it mattered. It explained how water crosses cell membranes so fast, and won the 2003 Nobel Prize in Chemistry.
Trials showed that a slightly weaker solution works better: less sodium and glucose, 245 mOsm/L instead of 311. The WHO and UNICEF recommended it worldwide. It cut stool volume by about a quarter and the need for drips by about a third.
Why it mattered. A slightly hypotonic drink lets water into the body more easily, which is osmosis doing its job.
By about 2019 some 16,000 desalination plants were running around the world, making roughly 95 million cubic metres of fresh water a day, most of it by reverse osmosis. Seawater plants push at 40–80 bar to beat the sea's osmotic pressure.
Why it mattered. The pig's bladder effect, run backwards, now waters cities.