Scientists compiled very detailed files on the mysterious strangers. Sherlock Holmes could detect a criminal by the ashes of a cigar he had smoked, by specks of clay adhering to his shoe leather. But his methods were nil compared to the delicate methods of chemists who had learned to identify the minutest amounts of unknown substances. 

   The clever detective was always lucky. The chemists were not. All their efforts to find the mysterious strangers and install them in their flats ended in failure.

   The strangers were sought everywhere: in cigar ashes and in plant remains; in the rarest and most exotic minerals, the pride of minerological museums; in the waters of the seas and oceans. All in vain!

   On the shelf of unsolved problems there appeared a new brief entitled "The Case of the Mysterious Disappearance of the Chemical Elements 43, 61, 85 and 87." A "disheartening case," some crime investigators might have called it. 

   Could nature have been up to the unsuspected trick of eliminating these elements from the list of simple substances existing on our planet. Could it be another of her strange whims?

   Indeed, it looked like magic. They say that miracles don't happen, but for reasons unknown the four flats of the Big House remained vacant. They were filled only after the scientists learned to make chemical elements artificially.

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