Stevenson, Robert Louis-Jekyll's letter
At the end of the novel, Jekyll decides to kill himself because he has lost control over Hyde. As a matter of fact, he doesn't need the potion anymore to turn into hyde.
Jekyll tells everything that has happened to him in a farewell letter which may be read as a spiritual will.
In the first part of the passage Jekyll explains that he hesitated a lot before putting his experiment into practice because he was aware that it was a dangerous experiment. But then the perspective of such a particular discovery won his fears and doubts. So he bought the last ingredient he needed, prepared and drank the potion.
Then the outer and inner effects of the potion are described: at first he felt sick and experienced pain; but then he felt lighter and happier but also more wicked then before. He then realized that he was shorter.
Dr Jekyll realized that he had given his evil side a body. As a matter of fact, as opposed to other human beings, who are a mixture of good and evil, Mr. Hyde was pure evil. On the one hand, Jekyll is attracted by Hyde’s ugliness, which to him seems to be livelier than his divided self; on the other hand, he’s afraid of losing his respectability and, for this reason, he has to keep his dualism.
Jekyll faced the second part of the experiment: he drank the potion again and turned back into his usual self.