Yang Ping went on, "And another thing, did you notice this? In Professor Zhou’s lab notebook, he wrote ’synthesis of a novel chiral organophosphorus ligand.’ If A-8 is exactly that new compound he synthesized, then the whole incident might have been an accident: he created a highly toxic compound, accidentally came into contact with it during handling, suffered chronic poisoning, and ultimately died."

He paused and looked at Zhaxi. "This explanation is simpler than murder. No killer, no motive, no complicated conspiracy."

Zhaxi opened his mouth but couldn’t get any words out. Yang Ping was right; accidental poisoning was indeed simpler than murder. And it didn’t require explaining so many doubts: how did the killer get the keys? How did they know which books Professor Zhou would pull out? How did they get A-8 onto the books?

Yet he still felt that something was off.

He thought for a long time, then suddenly asked, "Professor Yang, if Professor Zhou synthesized A-8 himself, then he should have known how toxic it was. Thirty years ago he took part in the toxicology study of A-8; he knew its LD50, knew its percutaneous toxicity. Someone who knows all that—would he handle such a highly toxic compound bare-handed, without a fume hood, without any protection?"

Yang Ping looked at him, and his gaze sharpened.

Zhaxi went on, "And if he did synthesize A-8, why would he synthesize it? The lab notebook says ’a novel chiral organophosphorus ligand.’ His work is in asymmetric catalysis, not pesticides. Why would a chemist who does asymmetric catalysis synthesize an insecticide that was banned thirty years ago?"

Yang Ping nodded. "You’re slowly learning how to reason. You’re right, these two questions need answers."

He looked at Zhaxi. "So right now we don’t have one answer, we have two possible answers. One is murder, the other is accident. What we need to do is not pick one to believe, but find evidence to prove or rule out one of them. We’ll mainly start from the drug itself and the body’s reactions."

He put down his pen and glanced at his watch. "We’ll stop here for today. Go get some rest; after I finish the afternoon surgery, come with me somewhere."

Zhaxi asked, "Where to?"

Yang Ping said, "The Nandu University archives. I want to look up all the material on that project from thirty years ago."

At two in the afternoon, Yang Ping and Zhaxi arrived at Nandu University. The archives were on the top floor of the main building, a small room with a brass plaque on the door, its lettering already somewhat blurred. The archives were managed by a female teacher in her fifties, surname Sun, wearing reading glasses. When she heard they wanted to look up documents from thirty years ago, she frowned.

"Officer Yu from the Provincial Hall already came with his people. He called to say you’d be coming. The materials from thirty years ago are all in the old storeroom; no one’s touched them for a long time," Teacher Sun said. "What exactly are you looking for?"

Yang Ping said, "An internal report from 1992, titled ’Synthesis and Toxicological Study of a Novel Highly Effective Organophosphorus Insecticide A-8.’"

Teacher Sun thought for a moment and said, "That should have been a joint project between the Pesticide Institute of the Ministry of Chemical Industry and our department. I’ll help you look for it." She stood up, walked to the storeroom door in the back, and opened it. Inside were rows of sheet-metal cabinets, with the musty smell of old paper hanging in the air.

After about twenty minutes, Teacher Sun came out carrying a stack of yellowed folders. She set them on the table and patted the dust on top. "These are project files from 1990 to 1995. What you’re looking for should be in here."

Yang Ping and Zhaxi sat down and started going through the folders. Most of them were project applications, annual reports, funding usage reports and the like, without much valuable information.

When they got to the third folder, Zhaxi suddenly stopped. It was a handwritten experimental record, the handwriting scrawled, but he recognized the signature at the bottom: Zhou Huaixin, dated August 1991.

He began to read the record carefully. The content was about the synthesis and toxicology experiments of a compound coded "A-8." The record was very detailed: the conditions for each reaction step, the methods for purifying the products, the design and results of the toxicology experiments—everything was documented.

Zhaxi came to the most crucial page—the toxicity data summary. It read:

"Acute oral LD50 of A-8 (rat): 0.8 mg/kg. Acute percutaneous LD50 (rat): 2.5 mg/kg. Subchronic toxicity test (90 days): low-dose group (0.05 mg/kg/day) showed weight loss, reduced appetite, and cholinesterase activity inhibition; medium-dose group (0.1 mg/kg/day) showed fingertip pigment deposition and hepatocellular degeneration; high-dose group (0.2 mg/kg/day) showed focal necrosis of myocardial muscle fibers."

Zhaxi’s hands began to tremble; these data were exactly the same as what Director Zhang had read out that morning.

He kept reading. The last page was Zhou Huaixin’s handwritten concluding remarks:

"A-8 exhibits very high insecticidal activity. Its LC50 against agricultural pests such as cotton bollworm and cabbage worm is between 0.1–0.5 ppm, far superior to organophosphorus insecticides on the market at the time. But its mammalian toxicity is excessively high, especially percutaneous toxicity, making it unsuitable for use as a pesticide. It is recommended that further development of this compound be terminated."

After finishing, Zhaxi handed the record to Yang Ping. Yang Ping took it and read it carefully from beginning to end. When he finished, he said nothing.

Then he flipped to the last page, pointed at the concluding remarks written by Zhou Huaixin, and said, "Look at the last sentence of this paragraph."

Zhaxi leaned over to read it. "It is recommended that further development of this compound be terminated." 𝙛𝒓𝓮𝒆𝔀𝒆𝙗𝓷𝒐𝙫𝒆𝙡.𝒄𝓸𝓶

Yang Ping said, "This is a scientist’s conscience. He discovered this compound’s toxicity and recommended terminating its development. What happened thirty years ago may not have been so complicated; he might simply have done what he ought to have done."