At any rate, Leslie Orgel did a number of elegant experiments of which I will describe the simplest. He took some of the basic constituents which are sure to have been present in the atmosphere of the earth at any early time: hydrogen cyanide is one, ammonia is another. He made a dilute solution of them in water, and then froze the solution over a period of several days. As a result, the concentrated material is pushed into a sort of tiny iceberg to the top, and there the presence of a small amount of colour reveals that organic molecules have been formed. Some amino acids, no doubt; but, most important, Orgel found that he had formed one of the four fundamental constituents in the genetic alphabet which directs all life. He had made adenine, one of the four bases in DNA. It may indeed be that the alphabet of life in DNA was formed in these sorts of conditions, and not in tropical conditions.
The problem of the origin of life centres not on the complex but on the simplest molecules that will reproduce themselves. It is of the same molecule that characterises life; and the question of the origin of life is therefore the question, whether the basic molecules that have been identified by the work of the present generation of biologists could have been formed by natural processes. We know what we are looking for at the beginning of life: simple, basic molecules like the so-called bases (adenine, thymine, guanine, cytosine) that compose the DNA spirals which reproduce themselves during the division of any cell. The subsequent course by which organisms have become more and more complex is then a different, statistical problem: namely, the evolution of complexity by statistical processes.
It is natural to ask whether self-copying molecules were made many times and in many places. There is no answer to this question except by inferences, which have to be based on our interpretation of the evidence provided by living things today. Life today is controlled by a very few molecules – namely the four bases in DNA. They spell out the message for inheritance in every creature that we know, from a bacterium to an elephant, from a virus to a rose. One conclusion that could be drawn from this uniformity in the alphabet of life is, that these are the only atomic arrangements that will carry out the sequence of replication of themselves.
However, there are not many biologists who believe that. Most biologists think that nature can invent other self-copying arrangements; the possibilities must surely be more numerous than the four we have. If that is right, then the reason why life as we know it is directed by the same four bases is because life happened to begin with them. On that interpretation, the bases are evidence that life only began once. After that, when any new arrangement came up, it simply could not link to the living forms that already existed. Certainly no one thinks now that life is still being created from nothing here on earth.
Biology has been fortunate in discovering within the span of one hundred years two great and seminal ideas. One was Darwin’s and Wallace’s theory of evolution by natural selection. The other was the discovery, by our own contemporaries, of how to express the cycles of life in a chemical form that links them with nature as a whole.
Were the chemicals here on earth at the time when life began unique to us? We used to think so. But the most recent evidence is different. Within the last years there have been found in the interstellar spaces the spectral traces of molecules which we never thought could be formed out in those frigid regions: hydrogen cyanide, cyano acetylene, formaldehyde. These are molecules which we had not supposed to exist elsewhere than on earth. It may turn out that life had more varied beginnings and has more varied forms. And it does not at all follow that the evolutionary path which life (if we discover it) took elsewhere must resemble ours. It does not even follow that we shall recognise it as life – or that it will recognise us.
CHAPTER TEN
WORLD WITHIN WORLD
There are seven basic shapes of crystals in nature, and a multitude of colours. The shapes have always fascinated men, as figures in space and as descriptions of matter; the Greeks thought their elements were actually shaped like the regular solids. And it is true in modern terms that the crystals in nature express something about the atoms that compose them: they help to put the atoms into families. This is the world of physics in our own century, and crystals are a first opening into that world.
Of all the variety of crystals, the most modest is the simple colourless cube of common salt; and yet it is surely one of the most important. Salt has been mined at the great salt mine at Wieliczka near the ancient Polish capital of Cracow for nearly a thousand years, and some of the wooden workings and horse-drawn machinery have been preserved from the seventeenth century. The alchemist Paracelsus may have come this way on his eastern travels. He changed the course of alchemy after AD 1500 by insisting that among the elements that constitute man and nature must be counted salt. Salt is essential to life, and it has always had a symbolic quality in all cultures. Like the Roman soldiers, we still say ‘salary’ for what we pay a man, though it means ‘salt money’. In the Middle East a bargain is still sealed with salt in what the Old Testament calls ‘a covenant of salt forever’.
In one respect Paracelsus was wrong; salt is not an element in the modern sense. Salt is a compound of two elements: sodium and chlorine. That is remarkable enough, that a white fizzy metal like sodium, and a yellowish poisonous gas like chlorine, should finish up by making a stable structure, common salt. But more remarkable is that sodium and chlorine belong to families. There is an orderly gradation of similar properties within each family: sodium belongs to the family of alkali metals, and chlorine to the active halogens. The crystals remain unchanged, square and transparent, as we change one member of a family for another. For instance, sodium can certainly be replaced by potassium: potassium chloride. Similarly in the other family the chlorine can be replaced by its sister element bromine: sodium bromide. And, of course, we can make a double change: lithium fluoride, in which sodium has been replaced by lithium, chlorine by fluorine. And yet all the crystals are indistinguishable by the eye.
What makes these family likenesses among the elements? In the 186os everyone was scratching their heads about that, and several scientists moved towards rather similar answers. The man who solved the problem most triumphantly was a young Russian called Dmitri Ivanovich Mendeleev, who visited the salt mine at Wieliczka in 1859. He was twenty-five then, a poor, modest, hardworking and brilliant young man. The youngest of a vast family of at least fourteen children, he had been the darling of his widowed mother, who drove him through science by her ambition for him.
What distinguished Mendeleev was not only genius, but a passion for the elements. They became his personal friends; he knew every quirk and detail of their behaviour. The elements, of course, were distinguished each by only one basic property, that which John Dalton had proposed originally in 1805: each element has a characteristic atomic weight. How do the properties that make them alike or different flow from that single given constant or parameter? This was the underlying problem and Mendeleev worked at this. He wrote the elements out on cards, and he shuffled the cards in a game that his friends used to call Patience.
What distinguished Mendeleev was not only genius but a passion for the elements.
Dmitri Ivanovich Mendeleev.
Mendeleev wrote on his cards the atoms with their atomic weights, and dealt them out in vertical columns in the order of their atomic weights. The lightest, hydrogen, he did not really know what to do with and he sensibly left it outside his scheme. The next in atomic weight is helium, but luckily Mendeleev did not know that because it had not yet been found on earth – it would have been an awkward maverick until its sister elements were found much later.
Mendeleev therefore began his first column with the element lithium, one of the alkali metals. So it is lithium (the lightest that he knew after hydrogen), then beryllium, then boron, then the familiar elements, carbon, nitrogen, oxygen, and then as the seventh in his column, fluorine. The next element in order of atomic weights is sodium, and since that has a family likeness to lithium, Mendeleev deci
ded this was the place to start again and form a second column parallel to the first. The second column goes on with a sequence of familiar elements: magnesium, aluminium, silicon, phosphorus, sulphur, and chlorine. And sure enough, they make a complete column of seven, so that the last element, chlorine, stands in the same horizontal row as fluorine.
Evidently there is something in the sequence of atomic weights that is not accidental but systematic. It is clear again as we begin the next column, the third. The next elements in order of atomic weights after chlorine are potassium, then calcium. Thus the first row so far contains lithium, sodium, and potassium, which are all alkali metals; and the second row so far contains beryllium, magnesium, and calcium, which are metals with another set of family likenesses. The fact is that the horizontal rows on this arrangement make sense: they put families together. Mendeleev had found, or at least had found evidence for, a mathematical key among the elements. If we arrange them in order of atomic weight, take seven steps to make a vertical column, and start afresh after that with the next column, then we get family arrangements falling together in the horizontal rows.
So far we can follow Mendeleev’s scheme without a hitch, just as he set it out in 1871, two years after the first conception. Nothing falls out of step until the third column – and then, inevitably, the first problem. Why inevitably? Because, as you can see from the case of helium, Mendeleev did not have all the elements. Sixty-three out of the total of ninety-two were known; so sooner or later he was bound to come to gaps. And the first gap he came to was where I stopped, at the third place in the third column.
Mendeleev’s Game of Patience. The cards are arranged in order of atomic weight: the elements group themselves in families.
The sequence of atomic weights is not accidental but systematic.
An early draft of Mendeleev’s Periodic Table of the Elements of 1869.
I say that Mendeleev came to a gap, but that abbreviated form of words conceals what is most formidable in his thought. At the third place in the third column Mendeleev came to a difficulty, and he solved the difficulty by interpreting it as a gap. He made that choice because the next known element, namely titanium, simply does not have the properties that would fit it there, in the same horizontal row or family with boron and aluminium. So he said, ‘There is a missing element there, and when it is found its atomic weight will put it before titanium. Opening the gap will put the later elements of the column into the right horizontal rows; titanium belongs with carbon and silicon’ – and indeed it does in the basic scheme.
The conception of the gaps or missing elements was a scientific inspiration. It expressed in practical terms what Francis Bacon had proposed in general terms long ago, the belief that new instances of a law of nature can be guessed or induced in advance from old instances. And Mendeleev’s guesses showed that induction is a more subtle process in the hands of a scientist than Bacon and other philosophers supposed. In science we do not simply march along a linear progression of known instances to unknown ones. Rather, we work as in a crossword puzzle, scanning two separate progressions for the points at which they intersect: that is where the unknown instances should be in hiding. Mendeleev scanned the progression of atomic weights in the columns, and the family likenesses in the rows, to pinpoint the missing elements at their intersections. By doing so, he made practical predictions, and he also made manifest (what is still poorly understood) how scientists actually carry out the process of induction.
Very well: the points of greatest interest are the gaps that lie in the third and fourth columns. I will not go on building the table beyond there – except to say that when you count the gaps and go on down, sure enough, the column ends where it should, at bromine in the halogen family. There were a number of gaps, and Mendeleev singled out three. The first I have just pointed to in the third column and third row. The other two are in the fourth column, in the third and fourth rows. And of them Mendeleev prophesied that on discovery it would be found, not only that they have atomic weights that fit into the vertical progression, but that they would have those properties that are appropriate to the families in the third and fourth horizontal rows.
For instance, the most famous of Mendeleev’s forecasts, and the last to be confirmed, was the third – what he called eka-silicon. He predicted the properties of this strange and important element with great exactitude, but it was nearly twenty years before it was found in Germany, and called not after Mendeleev, but germanium. Having begun from the principle that ‘eka-silicon will have properties intermediate between silicon and tin’, he had predicted that it would be 5.5 times heavier than water; that was right. He predicted that its oxide would be 4.7 times heavier than water; that was right. And so on with chemical and other properties.
These forecasts made Mendeleev famous everywhere – except in Russia: he was not a prophet there, because the Tsar did not like his liberal politics. The later discovery in England of a whole new row of elements, beginning with helium, neon, argon, enlarged his triumph. He was not elected to the Russian Academy of Sciences, but in the rest of the world his name was magic.
The underlying pattern of the atoms is numerical, that was clear. And yet that cannot be the whole story; we must be missing something. It simply does not make sense to believe that all the properties of the elements are contained in one number, the atomic weight: which hides – what? The weight of an atom might be a measure of its complexity. If so, it must hide some internal structure, some way the atom is physically put together, which generates those properties. But, of course, as an idea that was inconceivable so long as it was believed that the atom is indivisible.
And that is why the turning-point comes in 1897, when J. J. Thomson in Cambridge discovers the electron. Yes, the atom has constituent parts; it is not indivisible, as its Greek name had implied. The electron is a tiny part of its mass or weight, but a real part, and it carries a single electric charge. Each element is characterised by the number of electrons in its atoms. And their number is exactly equal to the number of the place in Mendeleev’s table that that element occupies when hydrogen and helium are included in first and second place. That is, lithium has three electrons, beryllium has four electrons, boron has five, and so on steadily all through the table. The place in the table that an element occupies is called its atomic number, and now that turned out to stand for a physical reality within its atom – the number of electrons there. The picture has shifted from atomic weight to atomic number, and that means, essentially, to atomic structure.
That is the intellectual breakthrough with which modern physics begins. Here the great age opens. Physics becomes in those years the greatest collective work of science – no, more than that, the great collective work of art of the twentieth century.
I say ‘work of art’, because the notion that there is an underlying structure, a world within the world of the atom, captured the imagination of artists at once. Art from the year 1900 on is different from the art before it, as can be seen in any original painter of the time: Umberto Boccioni, for instance, in The Forces of a Street, or his Dynamism of a Cyclist. Modern art begins at the same time as modern physics because it begins in the same ideas.
Since the time of Newton’s Opticks, painters had been entranced by the coloured surface of things. The twentieth century changed that. Like the X-ray pictures of Röntgen, it looked for the bone beneath the skin, and for the deeper, solid structure that builds up from inside the total form of an object or a body. A painter like Juan Gris is engaged in the analysis of structure, whether he is looking at natural forms in Still Life or at the human form in Pierrot.
The Cubist painters, for example, are obviously inspired by the families of crystals. They see in them the shape of a village on a hillside, as Georges Braque did in his Houses at L’Estaque, or a group of women as Picasso painted them in Les Demoiselles d’Avignon. In Pablo Picasso’s famous beginning to Cubist painting – a single face, the Portrait of Daniel-Henry Kohnweiler – the interest
has shifted from the skin and the features to the underlying geometry. The head has been taken apart into mathematical shapes and then put together as a reconstruction, a re-creation, from the inside out.
This new search for the hidden structure is striking in the painters of Northern Europe: Franz Marc, for example, looking at the natural landscape in Deer in a Forest; and (a favourite with scientists) the Cubist Jean Metzinger, whose Woman on a Horse was owned by Niels Bohr, who collected pictures in his house in Copenhagen.
There are two clear differences between a work of art and a scientific paper. One is that in the work of art the painter is visibly taking the world to pieces and putting it together on the same canvas. And the other is that you can watch him thinking while he is doing it. (For example, Georges Seurat putting one coloured dot beside another of a different colour to get the total effect in Young Woman with a Powder Puff and Le Bec.) In both those respects the scientific paper is often deficient. It often is only analytic; and it almost always hides the process of thought in its impersonal language.
I have chosen to talk about one of the founder fathers of twentieth-century physics, Niels Bohr, because in both these respects he was a consummate artist. He had no ready-made answers. He used to begin his lecture courses by saying to his students, ‘Every sentence that I utter should be regarded by you not as an assertion but as a question’. What he questioned was the structure of the world. And the people that he worked with, when young and old (he was still penetrating in his seventies), were others who were taking the world to pieces, thinking it out, and putting it together.