Franklin first spends two hours anxiously pacing around and postponing his planned meetings with Mrs. Ablewhite and Betteredge. When the time is up, Franklin meets Jennings in a surgery room at Mr. Candy’s house. Jennings has papers out and repeats his question to Franklin about opium, before adding that Franklin is merely “not aware of ever having taken opium” and asking about Franklin’s argument with Mr. Candy on the night of Rachel’s birthday (Franklin summarizes that he “had attacked the art of medicine,” frustrating Mr. Candy). Finally, Jennings asks whether Franklin had “any special anxiety about the Diamond,” and Franklin declares that he “had the strongest reasons for feeling [such] anxiety.”
Jennings continues to build suspense, gathering important preliminary information before he dares to reveal his theory (and, in doing so, violate Mr. Candy’s privacy). All the circumstances he cites show the complexity of the causality behind whatever happened that night. Jennings sees that the confluence of diverse circumstances and actors, rather than one person’s individual malice, led to the theft. At last, Franklin’s animosity towards science and medicine returns to hurt him—which is perhaps Collins’s way of encouraging his readers to trust the emerging medical establishment in Victorian England.
Jennings declares that he has the solution: Franklin “took the Diamond, in a state of trance, produced by opium […] given to [him] by Mr. Candy” in order to prove his point about the capacity of medicine. Jennings admits that Candy’s “dreadful mischief” was “innocent,” and that Candy was planning to return to the Verinder estate the next morning to admit his trick—a plan that, of course, Candy’s illness thwarted. Jennings produces his transcription of the words and phrases Candy repeated, and then his new filled-in version, in which Candy appears to admit having slipped Franklin laudanum. Although Franklin does not know much about Laudanum, he admits that he “feel[s] convinced” that it accounts for his behavior.
Jennings’s remarkable solution to the theft perfectly explains both how Franklin could have stolen the jewel in front of Rachel and how he could have had no recollection of it the next morning. The course of events appears to be nobody’s fault in particular, but rather the result of various small responsibilities added together: Candy’s pride, Franklin’s anxiety about the Diamond, and the unexpected rain, plus Rachel’s insistence on leaving the stone unlocked and whatever circumstances left Franklin without the Diamond in the morning.
“The next question,” Jennings continues, is how “to carry our conviction to the minds of other people.” His notes are insufficient, since they “represent a medical and metaphysical theory” and could have been falsified. Rather, they “must put [their] conviction to the proof.” Jennings proposes “a bold experiment” involving “personal inconvenience” on Franklin’s part: they will replicate the circumstances of the last year and Franklin “shall steal the Diamond, unconsciously, for the second time, in the presence of witnesses whose testimony is beyond dispute!” The first step is for Franklin to quit smoking (again), and subsequently they must return him to the same house and same obsession with the Diamond that entranced him the year before. Jennings points Franklin to some scientific literature showing that people could repeat or remember their past actions when given the same substances as in the past. Franklin agrees that this proposal carries scientific weight.
In a brilliant twist, the investigation now will require the crime to be committed, again—just as Franklin has broken the thief’s cardinal rule (acting intentionally), he and Jennings will now disrupt the very distinction between crime and investigation, as well as guilt and innocence: Franklin must prove his innocence by showing that he did, indeed, commit the theft. Jennings’s distinction between theoretical and demonstrable knowledge again illustrates the novel’s argument about knowledge and evidence—mere persuasion is meaningless in The Moonstone, and only verifiable proof does any good, whether for investigative purposes or for the sake of legitimating a theory. Jennings’s “bold experiment” is actually the novel’s second: the first was Cuff’s decision to tell Rachel about Rosanna’s death (which did nothing but reveal that Rachel and Rosanna were not, in fact, working together).
Jennings admits that it will be impossible to “exactly reproduce […] the conditions as they existed last year,” but he thinks the experiment should be enough to prove Franklin’s innocence if it goes correctly. Franklin’s only question is why laudanum would have made him walk around, not go straight to sleep, and Jennings declares Franklin mistaken—not only is Jennings on laudanum right now, but he also points Franklin to the famous book Confessions of an English opium Eater, whose author Thomas De Quincey wrote about exploring London on the drug, which has “a stimulating influence first, and a sedative influence afterwards.” This “stimulating influence,” Jennings suggests, might have turned Franklin’s anxiety about the Diamond into a quest to preserve it. Under the “sedative influence,” Franklin would have “fall[en] into a deep sleep” and in the morning he would have forgotten everything.
Beyond all the inversions and ironies already present in the experiment, Jennings’s description of the opium’s effect reveals one more irony: Franklin might have stolen the Diamond precisely out of an effort to protect it from being stolen. While Collins’s experiment itself is more theatrical than precise, his understanding of opium and reference to its literature are in line with both his own experiences and the fervor surrounding the drug (and its accompanying literature) in his day, as opium treatments and addictions spread rapidly in England.
Franklin asks if Jennings can figure out what happened after he took the Diamond, and Jennings suggests he might have hidden it for apparent safekeeping after taking it—in a place he might be able to recall during the experiment. Franklin points out that the Diamond is pledged in London with Mr. Luker, but Jennings suggests that this theory “rests on a mere assumption” and could be completely wrong—perhaps the Indians wrongly searched Luker and Godfrey Ablewhite, and Luker was telling the truth when he said he knew nothing of the Moonstone.
Jennings’s experiment actually has two goals: to prove that Jennings took the Diamond, of course, but also to figure out what happened to the Diamond afterward. Currently, both of these questions remain up in the air; although Bruff already has plans to trace the Diamond from the bank, the reader truly cannot know if it will turn up there, and must—like the novel’s characters—rely on their best hunches.
Franklin proposes contacting Bruff, and Jennings tells him to do so and quit smoking at once. Jennings emphasizes that it is crucial to rearrange the house exactly as it was before, and he offers to write Rachel on behalf of Franklin (since she likely has “a strong interest in the attempt to prove [his] innocence”). They part, but not before Jennings reiterates that “this little service” would be “like a last gleam of sunshine” for him. Franklin writes that the record of this experiment continues in Ezra Jennings’s journal.
Ultimately, Franklin finds an excellent way to pass the two weeks before the Moonstone’s removal from the bank, as well as to jumpstart and finally repair his relationship with Rachel, depending what she thinks of the experiment.