Physics and chemistry had made tremendous progress by the twenties of our century. In a matter of two decades these sciences had scored no less achievements than throughout the preceding history of mankind.

   But the discovery of new elements suddenly
came to a standstill. There remained several "blank" spaces in the Periodic Table which had to be filled. These were the boxes corresponding to the atomic numbers 43, 61, 85, and 87. What strange elements were these which refused flatly to settle in the Periodic Table?
 

Stranger No.1. An element of Group Seven with the atomic number 43, situated between manganese and rhenium, and probably similar in properties to these elements. It was to be sought in manganese ores.

Stranger No.2. A companion of the rare-earth elements resembling them in all aspects. Atomic number 61.

Stranger No.3. The heaviest halogen, iodine's elder brother. It could be a great surprise to chemists, for it was not impossible that its properties would be weakly metallic. Halogen and metal what a splendid example of a two-faced element! Flat 85 of the Big House was kept in readiness for it.

Stranger No.4. Now this is an interesting element. The most furious, the most active metal, and it would melt if just held on the palm of your hand. The heaviest of the alkali metals. Its atomic number is 87.

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