Complex sentence: Only the discovery of the rhenium was later confirmed, on the other hand the discovery of the element number 43 was doubted; this was discovered surely in 1937 and called Technetium, whereupon the name "Masurium" went into oblivion.
Split into simpler sentences: Only the discovery of the rhenium was later confirmed, and the discovery of the element number 43 was doubted. This was finally discovered in 1937 and called Technetium, whereupon the name "Masurium" went into oblivion.