I Vampiri

In 1957 Paris, a series of mysterious killings are committed against young women of the same blood type who are found dead and drained of their blood. These killings are reported in the press as being performed by a murderer coined The Vampire. The journalist Pierre Lantin (Dario Michaelis) begins to investigate, and becomes more involved when his fiancee, the dancer Nora Duval is kidnapped. As Inspector Chantal (Carlo D'Angelo) examines the crime scene, Lantin arrives predicting that the crime was committed by The Vampire. Lantin investigates the school that the latest murder happened at to search for clues and finds that the woman was being followed by a tall man before the murder. Elsewhere, a man named Joseph (Paul Muller) begs for "his fix" in a dark room is told to go after a woman named Lorette (Wandisa Guida) and that he "know what to do" at Rue Saint Etienne. Joseph arrives at the location and is spotted by Lantin but manages to get away from him. Joseph arrives at the clinic of Professor Julian Du Grand (Antoine BalpÃªtrÃ©) and demands money to leave town or he will report what's happening to the police. He is strangled by Du Grand's assistant when a shadowed woman named Marguerite arrives and states that if the police track them down, it will be the end of Du Grands career. A newspaper headline later reveals that Professor Julian Du Grand has died unexpectedly.
After a funeral procession for Julian, a group of men arrive and reveal that the body buried was that of Joseph. Joseph's corpse is taken to a castle where he is experimented on by Julian who is attempting discover the secret for eternal life. Later, Lorette meets a blind man in the street who asks her to drop off a letter. On dropping the letter off, she is kidnapped and finds herself locked in a bedroom with the skeletons of the Vampire's previous victims. As the police try and track down Lorette's kidnappers, Lantin is reassigned from following the Vampire story and is set to cover a ball at the castle of Du Grand. At the castle, he meets Gisele (Gianna Maria Canale) who expresses admiration for Lantin as he reminds her of his father. Lantin leaves the party and is pursued by the photographer Donald. Lantin states he does not want to lead on Gisele with her emotions which leads to Donald re-entering the castle to profess his love for Gisele. Gisele turns him down as her face begins to grow old before his eyes as she reveals that each person killed restores her youth for a short time. Knowing her secret, she reaches for a pistol and murders Donald. Gisele then calls upon Professor Julian to make her eternally young. Julian states that under her fragile emotional state it may not work, but begins an experimental transfer Lorette's youth and beauty to Giselle.
Gisele meets Pierre the next day when she is picking up a painting where he spots odd behavior in her such as writing with the wrong hand, which leads him to return to her castle to investigate further. Gisele begins growing ill from her previous experiment and calls upon Professor Julian to aid her. As he leaves, Joseph awakens in Julian's lab. Pierre triggers an alarm which has him race out the castle where he meets the disoriented Joseph. Pierre takes Joseph to the police station where he reveals he was the kidnapper of the young women but the people in the castle are the real murderers. The police arrive looking for Marguerite but only find Gisele who denies any knowledge of Joseph. Pierre and the police explore the castle without finding clues. On leaving, Gisele begins transforming back into Marguerite before their eyes prompting for an emergency search of the castle. A gun battle ensues between Professor Du Grands assistant and the police leaving the assistant and Professor Du Grand shot. This leads the police to open his grave where they find Lorette. Lorette is sent home and Inspector Chantal reveals that Giselle confessed to the crimes and died shortly after.

Question: Julian experiments on Joseph's corpse to discover what secret?
Answer: eternal life