�It never occurred to me that the Periodic System should begin precisely with hydrogen.� Whose words do you think these are? They would be most likely to come from one of the legion of investigators or amateurs who set themselves the task of drawing up a new periodic system all of their own or of rearranging it to suit themselves. The diversity of �periodic systems� that made their appearance was no smaller than of notorious perpetual motion machine projects.

Well, the phrase in quotation marks was written by none other than D. Mendeleyev. It appeared in his famous �Foundations of Chemistry,� a textbook used in its time by tens of thousands of students. Now why was the author of the Periodic Law mistaken? In his time there were all grounds for such mistakes. The elements were then arranged in the Table in order of increasing atomic weight. The atomic weight of hydrogen is 1.008, of helium is 4.003. Hence, why not assume that there might be elements with atomic weights of 1.5, 2, 3, and so on? Or elements lighter than hydrogen, with atomic weights smaller than unity?

Mendeleyev and many other chemists thought this to be quite possible. And they were supported by the astronomers, representatives of a science very far removed from chemistry. We daresay, their support was involuntary. It was the astronomers that first proved new elements to be discoverable not only in the laboratory and not only by analysing terrestrial minerals.

In studying the total solar eclipse of 1868 the English astronomer Lockyer and the French astronomer Janssen passed the blinding light of the solar corona through a spectroscope prism. In the dense palisade of spectral lines they observed some that could belong to none of the elements known on Earth. Thus was discovered helium, the name coming from the Greek for �solar.� Only twenty-seven years later did the English physicist and chemist William Crookes discover helium on Earth.

His discovery proved contagious. Astronomers began to point their telescopes at distant stars and nebulae. Their findings were scrupulously published in astronomical yearbooks, and some even found their way into chemical journals. These were findings which treated of alleged discoveries of new elements in the boundless space of the universe. The elements were given pompous names such as coronium and nebulium, archonium and protofluorine. Apart from their names, chemists knew nothing about them. But bearing in mind the happy end of the helium story, they hurried to place these celestial strangers in the Periodic System. They put them before hydrogen or in the space between hydrogen and helium. And they cherished the hope that at some time in the future new Crookeses would prove coronium and its no less mysterious fellows to exist on Earth.

But as soon as the physicists tackled the Periodic System these hopes were shattered. Atomic weights were found to be an unreliable footing for the Periodic Law. They were replaced in this function by the nuclear charge, or the atomic number of the element. In passing from element to element in the Periodic System this charge increases by one unit each time.

Time passed and more precise astronomic instruments scattered the myth of the mysterious nebulae. They turned out to be atoms of long known elements, atoms that had lost some of their electrons and therefore gave unfamiliar spectra. The �business cards� of the celestial strangers proved false.

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