Thanks for your reply! & the great YT video-link "MURDER AHOY MISS MARPLE EXPERIMENTS.wmv" (www.youtube.com/watch?v=YH6WSfCFgIg).
Yes, I too! thought it might be the experiment bit in 'Murder Ahoy!' but I have found several relevant segments of that Miss M. film at YouTube, & have discovered that it is NOT that film as that experiment reveals another poison - strychnine, not cyanide. Plus, there is (of course) no mention/enactment of the Prussian Blue experiment/test.
Then I thought it might be the science bit in the Miss M. film 'Murder Most Foul' but I have found the relevant segments for that film also at YouTube:
"A. Christie's Murder Most Foul 7 10.flv"
"A. Christie's Murder Most Foul 8 10.flv"
& THAT segment of the Miss M. film is about Miss M. scenting almonds = cyanide; again NO mention/enactment of a Prussian Blue test. Also, no experiment at all, in this film!
Maybe I am muddling, in my mind/memory, the 2 M. Rutherford films' respective 'science bit'?! Although if that is the case then where exactly I am 'remembering' the "Prussian Blue" bit from is unclear! :-/
I agree with you: I too thought it was "definitely one of the Rutherford ones".
Hey ho! Maybe ONE day I will find out in which film/TV prog that Prussian Blue test was demonstrated! ,-)
TV(UK): Terrestrial&Freeview (no cable/sat wanted here! )
Hmmm, right, I was thinking of the Murder Ahoy one when you first mentioned it. I'm not sure I actually know the answer now!
Those 4 films tend to be on Film 4 quite a lot and TCM. So, you could record them there and eliminate them...