百草霜 The Frost Of A Hundred Grasses
沙僧道：「小弟不曾見藥內用鍋灰。」行者道：「鍋灰名為『百草霜』，能調百病，你不知道。」那獃子真個刮了半盞，又碾細了。沙僧道：「小弟不曾见药内用锅灰。」行者道：「锅灰名为『百草霜』，能调百病，你不知道。」那呆子真个刮了半盏，又碾细了。
逐句解 Line by line
Sha the monk said, "I have never seen soot used in a medicine."
曾 stays céng, "ever" — I have never in my life seen it. Sha the monk, who opened the scene reciting the pharmacopoeia correctly, is the right one to say this.
Pilgrim said, "Soot is named Frost of a Hundred Grasses, and can regulate a hundred ailments. You do not know about it."
And the answer is a name. Pot soot really is a recognised drug called Frost of a Hundred Grasses, so the reply is both perfectly true and a way of ending the conversation. 調 is tiáo, to regulate or bring into balance. 為 is wéi again.
The idiot really did scrape half a cupful, and ground that fine as well.
真個 — he really did — is the narrator's dry note on how far the objection got. 那 is nà, that.
Sha the monk raises the obvious objection, and Pilgrim answers it with a name. Soot scraped from a pan is a recognised drug called Frost of a Hundred Grasses — which is entirely true, and which turns a kitchen scraping into a pharmacopoeia entry without changing one thing about it.
This is the chapter's whole method in miniature. The diagnosis was sound medicine; the recitals were sound medicine; the name of the soot is sound medicine. What is being assembled out of all that soundness is another matter entirely.
The narrator's last word is 真個 — he really did. Eight Rules objected, was overruled, and scraped the soot anyway, which is how far any objection gets in this hostel.