Prior to its discovery, which chemical element was referred to as 'eka-caesium'?

As early as 1870, chemists thought there should be an alkali metal beyond caesium, with an atomic number of 87. It was then referred to by the provisional name eka-caesium, which was discovered on 7 January 1939 by Marguerite Perey of the Curie Institute in Paris. Later it was named Francium (Fr) after France, the homeland of Marguerite.

To give provisional names of the predicted elements, Mendeleev (discoverer of periodic table) used the prefixes eka-, dvi- or dwi, and tri-, from the Sanskrit names of digits 1,2, and 3 depending upon whether the predicted element was one, two,or three places down from the known element of the same group in his table.

Fr is extremely radioactive; its most stable isotope is Fr-223. It is the second most electropositive element behind only caesium, and is the second rarest naturally occurring element (after astatine). The element is classed as an alkali metal.

The Fr was the last element first discovered in nature, rather than by synthesis. The chemical properties of Fr mostly resemble those of caesium. It is a heavy element with a single valence electron; it has the highest equivalent weight of any element.