Neville continues to explore the mystery of why vampires are repulsed by crosses. He wonders what a Mohammedan (i.e., Muslim) vampire would do if faced with a cross, and laughs. It's the first time Neville has laughed in months, if not years. Later in the day, Neville tries to see if the “running water bit makes sense,” and sprays a hose into a wooden trough, so that the water flows through the trough and down into the soil below.
Like any good scientist, Neville conducts further experiments to answer his questions and learn more about his subjects. Here, he tries to test the old superstition that vampires fear (or cannot pass over) running water.
That night, Neville looks out through his peephole and sees Ben Cortman on his front lawn. Cortman has a beard now, and his face seems thinner. Cortman and Neville once worked at the same plant, and used to be good friends; now, Cortman is totally alien to Neville.
The passage confirms that Cortman was once Neville’s good friend, but now he’s Neville’s sworn enemy.
Neville runs into the kitchen and turns on the water, so that the hose flows on the front lawn. Neville goes back to looking through the peephole, and sees Cortman grinning and laughing—Cortman jumps over the “running water” of the hose without any hesitation. Furious, Neville get his pistol, opens the front door, and shoots Cortman in the shoulder. Cortman collapses, and suddenly, a female vampire stands in front of Cortman and begins “jerking up her dress.” Neville goes back inside before he can see anything—he doesn’t want to feel temptation. Later that night, Neville looks outside and sees Cortman standing again, calling, “Come out, Neville!” Suddenly, Neville realizes that Cortman reminds him of Oliver Hardy (the black-and-white comedy star). He laughs hysterically, but later, he cries.
Evidently, not all vampires are afraid of running water. As we’ve already seen (and as Neville already knows) Cortman and the other vampires seem impervious to gunfire, meaning that Neville’s attack is a pointless act of violent rage, stemming from Neville’s isolation and depression. Neville’s hysterical laughter could be another symptom of his plummeting mental health: in times of extreme isolation or stress, people have been known to respond with uncontrollable laughter.
In the following days, Neville continues experimenting with vampires. He takes female vampires and stabs them with wooden stakes, sometimes in the heart, sometimes elsewhere. No matter where he stabs, the vampires die, spurting dark blood everywhere. But Neville discovers that some vampires have a different reaction to being stabbed. He stabs a female vampire, and in mere seconds, she disintegrates into “salt and pepper” ashes. Neville then remembers talking to a “Negro” at the plant, who told him that corpses eventually decompose until they look like a pile of salt and pepper. Neville concludes that this particular vampire must have been dead for a very long time—perhaps she was one of the vampires who originally started the plague. He wonders if Virginia, buried in her crypt, looks like a pile of salt and pepper, too.
As Neville continues experimenting with vampires, he becomes more aware of the distinction between the two main kinds of vampires, living and dead. Humans who contract the vampire virus while they’re alive react to wooden stakes by spurting blood everywhere; those who contract the virus after they’re buried in the ground react by disintegrating into “salt and pepper.” The passage foreshadows the connection between Virginia and the vampires—as we’ll see later, that connection is important to Neville’s backstory.