The Ascent of Man (30 page)

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.

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
.

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 decided 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.

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.