Answer:
The rat poison is Arsenic and is found in the title of the murder mystery.
Step-by-step explanation:
Arsenic was the king of poisons during the 19th century. It was widely used in everyday life and was part of kills, pigments, wallpapers, pesticides and even medications. Its use in agriculture and medicine intensified in the early twentieth century. Currently, its presence in groundwater transforms it into one of the ten most dangerous substances for human health, according to WHO reports. And yet, he has lost his position of honor as king of poisons.
The ease with which it could be obtained in commerce was one of the keys that made arsenic the king of poisons during the nineteenth century. But there were also other factors that are related to the physical, chemical and medical properties of arsenic. With this name, what modern chemists call diarsenic trioxide, arsenious oxide and arsenic oxide (III) were commonly known, and which they usually represent with the formula As2O3. But we will keep here the most traditional denomination: arsenic, without more. It is a white, crystalline powder, which does not differ excessively from substances in everyday life such as flour or sugar, so it is easy to mix it with them. It barely tastes, so it can be supplied at meals without raising too much suspicion. Faced with these rather bland physical and organoleptic traits, arsenic has very notable poisonous properties. It acts on various parts of the human body and leads to a great diversity of symptoms that depend on the dose, the mode of administration and the particular characteristics of the victim. Many of these symptoms resemble other diseases. In the 1830s, when cholera reached European cities, many of the first victims thought they had been poisoned with arsenic and various people were persecuted for these alleged crimes.
As for the title of a mystery and murder movie, "Arsenic and Old Lace" was about women who poisoned and killed the men who visited them by adding arsenic to the wine.