It is pretty interesting as to the details and the techniques mentioned being used to uncover certain crime details. This story just shows that money is the root of all evil. And if you want to find the criminal, you have to look at all the money trails. I will not describe about the story, as it is like a 2 crimes in 1 book.
Catherine ran a moist swab across the stain, then dipped a combination of Brentamine Fast Blue and alpha-naphthyl phosphate on the swab. Within 20 seconds, it turned bright purple, an almost certain indication that there was semen on the sheet. Just in case, she swabbed a second time and tested this one with a periodic acid-Schiff reagent. The magenta color confirmed the presence of vaginal fluids as well. Wow, what a technique, that the cops can find eventually who has been sleeping with who.
Acid phosphate is found in high levels of semen. Under the black light technique, invisible semen stains appear visible to the human eye.
Damp or wet items found at the crime scene were put into paper bags rather than plastic. This is because the more airtight confines of a plastic blag will result in the moisture causing bacterial growth or molds that would then contaminate precisely what the investigator was trying to preserve. Paper wasn’t airtight, and that was exactly the point. Even if the blood on the lighter seemed dry to the touch by the time he was ready to go, he would bag it in paper, just in case.
In the case, Helena Cameron and her daughter Dania were all diagnosed with heart problems by their family doctor. But the forensic trained doctor was able to diagnose that they were actually not sick, but were being poisoned – selenium poisoning. A general practitioner would be most concerned about the heart and might, for a while, see the skin discoloration as jaundice, until the orange colour became more pronounced. But it would take a while for anyone without forensic training to get to selenium poisoning.
I am impressed after the reading that there may be some people who were poisoned but they thought that they were sick by their family doctors, as the symptoms may not be that obvious. Agatha Christie also like to use poison to murder in her stories.