Text 01 -- HOW WE MEASURED THE ATOM
Text 1
HOW WE MEASURED THE ATOM
George Sinfield, the Sporting Editor of the Daily Worker, recently asked me how we knew the size and weight of atoms. I told him the answer would take about twelve hours. But I must try to give some of it in this article.
Over a century ago Dalton laid the foundations of our knowledge. Hydrogen and oxygen form two quite different compounds, water and hydrogen peroxide. In water there are eight parts of oxygen by weight for each one of hydrogen; in peroxide there are sixteen. It is a reasonable guess that oxygen is made up of atoms, either eight or sixteen times as heavy as those of hydrogen. When a few thousand compounds had been analysed in this way, the relative values of the atomic weights became pretty certain.
But nobody knew how small the atoms were. And some philosophers said they were only conventions to help our thinking, like parts of speech in grammar or the decimal system in arithmetic. Chemical changes occurred as if there were atoms, but we could never know what matter was really made of. You can read quotations from them in Lenin's Selected Works.
The first measurements which led to anything like the right figure were made by J. J. Thomson at Cambridge about 1896 on the electron, the smallest electrical charge. Later on Millikan measured it more accurately by a simple method. He made a spray of very fine oil drops. Some of them had an electrical charge which could be calculated from the speed at which they moved between two charged metal plates. The charge on a drop was just one, two, three or some whole number of Thomson's units, never a fraction such as one-fifth or two and a half.
But, according to chemical theory, one of these electrical units went through a wire for each silver atom deposited in electro-plating. So the number of silver atoms in an ounce could be calculated, and hence the sizes of all other atoms.
Radium told the same story. It shoots out nuclei of helium atoms with such speed that a single one makes a visible flash when it hits a screen of the right sort of material. The "flashes per minute produced by a small amount of radium can be counted, and also the amount of helium gas produced by a much larger quantity in a month. Thus the size of a helium atom is known.
It turned out that the distance between neighbouring atoms in a solid is always about a hundred millionth of a centimetre, generally rather more, but never as much as five times more. In particular, the distance between layers of atoms in a crystal can be calculated. Now, if a series of very fine lines, say, a ten-thousandth of an inch apart, are ruled on glass it has long been known that this glass reflects light in a different way. White light gives a rainbow effect, as with a pearl or a pigeon's breast. But light of a single colour-- for example, the yellow light from a sodium street lamp -- is only reflected in certain directions.
In these directions the difference between the lengths of the paths travelled by light reflected from two neighbouring lines is just one, two, three, or some other whole number, times the wavelength of the light, and successive waves help one another. In this way, the wavelength of light can be measured.
Now, a crystal behaves in this way to X-rays, reflecting those which strike it at a particular angle. When the Braggs discovered this, they were able not only to calculate the structure of crystals, but the wavelength of X-rays, given the size of an atom. The calculated wavelength agreed well with other properties of X-rays -- for example, the voltage in the discharge needed to produce them.
But the finishing touch was put by Siegbahn, who used soft X-rays -- that is to say, rays of long wavelength and low penetrating power. He could reflect the same rays at a steep angle from a very finely divided ruled grating, and at a glancing angle from a crystal. The figures which he calculated for the distances between atoms agreed to 1 part in a 1,000 with the earlier figures.
When soap is added to water, it affects its surface properties much more than its bulk properties. The soap molecules form a layer on the surface of the water. Most water has a surface layer of some impurity. Indeed, the first person to make really clean water was a Miss Pockels about 1895. Clearly no layer of a substance can be less than 1 molecule thick. So by cleaning water thoroughly, and then seeing how far a drop of oil will spread, we can measure the thickness of 1 molecule.
If the surface layer consists of protein -- for example, egg-white -- it can be skimmed off on to a clean glass or metal plate and dried. Then this is done again, and so on. In this way Miss Blodgett got a layer of dried egg-white 1,764 molecules thick on a metal plate. You notice how women beat men on this very fine work. She stripped the layer off, foK ded it up, measured it with a micrometer such as is used in fine gauging, and found the same thickness, within 1 per cent, as that given by X-ray measurements.
The size of molecules, and therefore of atoms, can also be calculated from the rate at which they settle in a very powerful centrifuge, such as I described in an earlier article, and in literally hundreds of other ways, which all agree pretty well, and give a coherent account of the structure of matter.
It is such a convincing account that if anyone produced evidence which overthrew it I should certainly give up writing for the Daily Worker or supporting its policy. For if all this practical and theoretical work was meaningless, scientific thinking would be no use, and among other scientific thinking, that of Marx, Engels and Lenin.
And if that is so, I may as well start believing the B.B.O. bulletins, sell my articles to Lord Rothermere, and spend the money in night clubs. But as long as I believe that one can get somewhere by rational thinking and action, I shall do my best to persuade others that this is so.
1946
Література
John B.C. Haldane. Reader of Popular Scientific Essays. - Изд-во«Наука», М., 1993. - 235 с.