The Snorkel (1958)

Starring Peter Van Eyck, Betta St. John, Mandy Miller, Gregoire Aslan, William Franklyn
Directed by Guy Green
(actor & director credits courtesy IMDB.com)

A clever sociopath murders his wife and makes it look like a suicide, fooling the police, but not his stepdaughter, who's convinced of his guilt, and tries to find a way to prove it.

Van Eyck makes a smooth villain in this suspense thriller from Britain's Hammer Films, and the filmmakers hook viewers early with the intricately planned and silently photographed murder, in which the killer uses a snorkel to breathe the harmless outside air while his wife asphyxiates on vapors from the gas lamps he turns on.  From there the race is on to see if the stepdaughter can find the evidence to expose him, or if he'll knock her off first, setting the stage for the finale in which one of them overplays their hand.  Although the rest of the film doesn't quite deliver as well as the opening scene does, this is enjoyable fare, with a story that avoids using the more relentless twists to be found in Hammer's thrillers to come.

The Brain (1962)

Starring Anne Heywood, Peter Van Eyck, Cecil Parker, Bernard Lee, Jeremy Spenser
Directed by Freddie Francis
(actor & director credits courtesy IMDB.com)

A scientist steals the brain of a dying financier for study and works to keep it alive, but the brain is so ruthless and powerful, it takes over his own mind.

According to Wikipedia, this was the third film adaptation of Curt Siodmak's novel "Donovan's Brain," after 1944's The Lady And The Monster and 1953's Donovan's Brain, and takes a somewhat different tack on the premise, focusing less on the novel's horror story, instead crafting a murder mystery as to who killed the financier in a plane crash.  Although well-done, it's a bit of a stretch that Holt's family and associates would answer any of Corrie's questions as he investigates the murder, driven by Holt's brain for answers.  For me, the 1953 adaptation is the best of the films, (and an earlier radio adaptation starring Orson Welles is very good as well).  While this version's mystery story is intriguing, it misses out by not focusing on the conflict between Corey/Corrie and the brain as it grows more powerful.