Hydrogen was discovered by the famous English physicist the Hon. Henry Cavendish. He was the richest of the learned and the most learned of the rich, as one of his contemporaries put it. We might add that he was the most punctilious of scientists. It is said that whenever Cavendish borrowed a book from his own library he always signed his name on the book card. The most sedate of scientists, devoted entirely to scientific research, always engrossed in his science, he got the reputation of an eccentric recluse. But it was just these qualities that enabled him to discover the new gas hydrogen. And, believe us, it was no easy task!

He made his discovery in 1766, and by 1783 the French Professor Charles had flown the first hydrogen-filled balloon.

Hydrogen was a most valuable find to chemists, too. It helped them to get an insight into the structure of acids and bases, these most important classes of chemical compounds. It became an indispensable laboratory reagent for precipitating metals from solutions of salts, and for reducing metallic oxides. And paradoxical though it seems, had hydrogen not been discovered in 1766, but, say, half a century later (such a thing could really have happened) the progress of chemistry, both in theory and practice, would have been retarded for a long time.

When the chemists had come to know hydrogen well enough, and practical workers had begun to utilize it for the production of important substances, this gas drew the attention of physicists. And they found out a great deal of information which enriched science many times over.

Do you need more evidence? For one thing, hydrogen solidifies at a lower temperature than any other liquid or gas (except helium), at minus 259.1� Celsius. Secondly, the hydrogen atom enabled the Danish physicist Niels Bohr to work out a theory of the arrangement of electrons around the atomic nucleus, without which the physical sense of the Periodic Law could not have been understood. And these facts laid the foundation for other very important discoveries.

And hydrogen takes up its abode in the seventh group. But not for long. After getting to know its new relation a little better one of the halogens remarks a little disappointedly: "See here, brother. You don�t seem to have many electrons in your outer shell, do you? Only one as a matter of fact - like them blokes in group one. Hadn�t you better get back to the alkali metals?"

See what difficult straits hydrogen is in: there are plenty of rooms but none it can occupy permanently, with full rights. But why? What is the reason for this surprising two-facedness of hydrogen? What makes hydrogen behave so eccentrically?

The specific properties of any chemical element become evident when it combines with other elements. It then yields or accepts electrons which either leave its outer shell or enter it. When an element loses all the electrons of its outer shell, the rest of its shells usually remain unchanged. Such is the case with all the elements except hydrogen. When hydrogen parts with its only electron, all that remains is its atomic nucleus. What is left is a proton, this being, as a matter of fact, all the hydrogen atom nucleus consists of. (Actually it does not always consists of only a proton, but we shall come to this important point later.) Hence the chemistry of hydrogen is the only chemistry of its kind, as it were, the chemistry of an elementary particle, the proton. Thus, reactions involving hydrogen proceed under the influence of protons. And that is why hydrogen behaves so inconsistently.

Another remarkable property of hydrogen is that its atom emits radiations having a wavelength of 21 centimetres. This is called a universal constant because it is the same throughout the universe.

And scientists have taken up the problem of organizing radio communications with other inhabited worlds on the hydrogen wave. If these worlds are inhabited by intelligent creatures, they should have an idea of what 21 centimetres is.

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