The Flash (vol 1) #147 GD

Second appearance of Professor Zoom/Reverse Flash.

Police Forensics Scientist, Barry Allen, and his girlfriend, Picture News Reporter, Iris West, are meeting Al Desmond, and his fiancée, Rita, for dinner. Desmond once terrorized Central City as both the villainous Mr. Element, as well as the nefarious Doctor Alchemy. Having paid his debt to society, Desmond has reformed. At dinner, Desmond complains of a sudden headache, and departs for home. Trancelike, he finds himself, instead, returning to his criminal lair. Uncovering his old laboratory equipment, Desmond mixes up a batch of chemicals, with volatile results. As the smoke clears, Desmond finds himself facing Professor Zoom, the Reverse Flash. Having taken control of Desmond's mind, Professor Zoom telepathically explains his presence in the 20th century. After losing the Flash costume that gave him his incredible super-speed, Professor Zoom became obsessed with regaining his powers. Professor Zoom discovered an element, that he dubbed "Element Z", that gave him super-human speed. Element Z, however, proved to be too unstable to maintain his powers for more than a few moments. Searching the history books, Professor Zoom learned that Desmond is the only chemist brilliant enough to stabilize Element Z. Desmond, though, refuses. His brilliance is too tied to the criminal side of his mind. Undeterred, Professor Zoom telepathically forces Desmond to don his Mr. Element costume, and to resume his criminal career. Desmond, as Mr. Element, strikes at the Egyptian Museum, intent on stealing the Horus Scarab. After taking out the guards with a block of pure sodium, Mr. Element reaches for the Horus Scarab, but finds he cannot take it. The Flash arrives on the scene, crestfallen to discover that Desmond has resumed his criminal career. Mr. Element stops the Flash with a web of crystal iron, which impedes the Scarlet Speedster's forward motion, before snapping him back, hard, into the wall. When the Flash revives, Mr. Element is gone, but the Horus Scarab remains.