Issue 8, 15th April 1996: John Dalton, Colour-Blind Atom Scientist
By Adam Hart-Davis.
[Computing, Law, Music, Science and Technology, SciFi, Sport, The Unexplained, UP, icons]
When I've made a gin-and-tonic, I want the remainder of the tonic to stay as fizzy as possible for tomorrow. In other words, I want to maintain the highest possible concentration of carbon dioxide in solution in the bottle. Before I put it in the fridge, should I (a) simply screw the top on, (b) blow into it to insert some carbon dioxide, then screw the top on, or (c) squeeze the bottle as flat as possible to minimize the air space, and then screw the top on? Which of these options will keep the most carbon dioxide in solution? The first person who could have sorted that one out was born in the Lake District, at Eaglesfield, near Cockermouth, and his name was John Dalton.
John Dalton was born on 6 September 1766. His first job was as a teacher; his own teacher retired, and he took over the school for a couple of years---when he was only 12 years old!
In 1793 he went to Manchester to be Professor of Mathematics and Natural Philosophy at the New College, and while he was there he did two things that made him world famous.
First he scientifically described colour-blindness for the first time. He himself was what we now call red-green deficient, and he described elegantly from his own observations what the problem was, and realized that it must be hereditary, since his brother saw the same things as he.
He was convinced that he could not distinguish between red and green because his eyeballs were tinted blue, and in a rather bizarre way asked that after his death his eyes should be removed to prove him right. Unfortunately he was wrong; they weren't tinted at all. We now know the cause is a missing gene. Nevertheless, for many years colour-blindness was called Daltonism, after him.
A few years later, he laid the foundations for the entire system of chemical atomic weights, by realizing that different elements combine together in fixed proportions.
His scientific work started, however, six years before he went to Manchester, on 27 March 1787, when he saw the northern lights---aurora borealis---and wrote an account of his observations in a meteorological journal. He kept up this journal for 57 years, and in all recorded 200,000 observations. His neighbours in Manchester used to set their clocks by the time he opened his window to read the temperature every morning.
This passion for meteorology began in the Lakes, and he climbed the majestic 3100-foot Helvellyn 40 times, partly for pleasure, for he did love the Lake District, and partly for meteorology. He said himself in later life that his book of meteorological observations and essays contained the germs of most of the later ideas he worked on.
In one essay he tried to decide whether the quantity of rain and dew is equal to the quantity of water carried off by the rivers and raised by evaporation, and also showed that springs are fed by rain---which some people didn't believe.
That essay contained one crucial definition---of dew-point. The dew-point is the temperature of a surface at which water vapour just begins to condense on it---when you get a cold lager in a pub and the glass mists up---that means the lager is below the dew point.
So for many years John Dalton measured and recorded the pressure of the water vapour, rainfall, wind-speed, and temperature, and all the time he thought about the atmosphere and the gases that go to make it up, and that is how he had his great breakthrough. Imagine a balloon inflated with a bit of atmosphere---mainly nitrogen, plus oxygen, plus a bit of carbon dioxide and quite a bit of water vapour, because it's just come out of some lungs.
A few years earlier, Priestley had discovered oxygen---though he called it "de-phlogisticated air!"---and Lavoisier had shown there were different gases in the air---but people thought they were chemically combined. And people were also puzzled about how much space they occupied.
Liquids don't occupy the same space inside a container. Put oil and vinegar into a jar to make salad dressing, and the oil sits on top of the vinegar. When you shake them to make an emulsion, the volume stays the same as when they were quite separate.
But gases are different, as Dalton came to see by thinking about the atmosphere. In my imaginary balloon I have a mixture of at least four gases, and they do all occupy the same space. Carbon dioxide is the most dense---the heaviest---but it doesn't sink to the bottom of the balloon. Water vapour is the least dense, but it doesn't rise to the top. Each one of the gases in there occupies all the volume. They just ignore one another.
Dalton realized the gases must all exert their own separate pressures, too. So the oxygen in my balloon is pushing out on the rubber with a pressure of about an eighth of an atmosphere---say 2 pounds per square inch. And it would do exactly the same if all the other gases werent there at all. In other words the total pressure inside the balloon is the sum of the pressures of all the individual gases inside. And that's what came to be called "Dalton's Law of Partial Pressures".
His thinking about the air let eventually to all his other brilliant insights into the world around him---and made him internationally famous. He wrote about his ideas in a meteorology book which was published in 1793, the year he went to Manchester.
Most people didn't believe his theories, and argued with him fiercely. He went and delivered some lectures at the Royal Institution in London, but he and Humphry Davy hated one another on sight. Davy said "His aspect and manner were repulsive. His voice was harsh and brawling," and so on, and yet he thought Dalton was a genius. Dalton won the Royal Society's first Royal Medal in 1825. The French scientists also thought he was wonderful, and gave him loads of medals and honours.
In Manchester he began to think about gases dissolving in water. He found that a fixed amount of water did not dissolve the same amount of all gases, but that about the same weight of nitrogen as of oxygen would dissolve in a pint of water. Light gases like hydrogen seemed to be much less soluble than heavy gases like nitrogen and oxygen---or at least less weight would dissolve.
And then he had a thought---perhaps there were sort of "holes" in the water, and only a fixed number of particles of gas would fit in...
... and suppose those particles were the ultimate particles of matter---atoms. Then the weight of any particular gas that would dissolve would be proportional to the weight of its atoms.
One of the great moments in chemistry came on 21 October 1803, when John Dalton read a paper to the Manchester Literary and Philosophical Society, about the solubility of gases in water.
He wasn't quite sure yet, but he said "I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases: Those whose particles are lightest and single being least absorbable and the others more as they increase in weight and complexity. An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new.."
And to the end of his paper he added the first ever list of atomic weights.
The way he got them was intriguing. The lightest element was hydrogen, and he gave hydrogen the atomic weight of 1. He knew 1 oz of hydrogen reacted with 8 oz of oxygen to make water. He assumed water was a one-to-one compound; so he gave oxygen an atomic weight of 8.
Unfortunately as we now know water is not HO but H2O; so Dalton got most of his values wrong in that first list. But the point was that he had had the idea and sown the seed. The atomic theory was born, and quantitative chemistry was on its way.
When Dalton died in 1844, his body was placed in Manchester Town Hall, and 40,000 people filed past it to pay their respects. A hundred carriages formed his funeral procession.
Dalton could easily have solved the tonic problem. Simply screwing the top back on the tonic bottle will leave ordinary air inside, and the pressure of oxygen and nitrogen will have no effect on the carbon dioxide in solution; so some of it will come out.
Blowing into the bottle will help a little, because there is a little carbon dioxide in your breath; that will make a small partial pressure of CO2 inside the bottle, which will keep a little bit more in the tonic.
But much the best is to flatten the bottle as much as you can, and minimize the volume of space inside, and so very little CO2 needs to escape to create the correct partial pressure inside.
So please drink a toast---a tonic---to the colour-blind philosopher who invented partial pressures and the system of atomic weights---to John Dalton!