From his sleeve he at once drew a square of goose-yellow silk handkerchief, and tied to its fringe was a small key.

Here is a nice one. 匙 has two readings and this chapter uses both. In 鑰匙, a key, it is shi — light, worn down, part of a compound. In 茶匙, a teaspoon, a few passages back, it is chí, a full syllable. Same character, two words, and only the company it keeps tells you which. 繫 is jì, to tie, and 著 is zhe, keeping the key tied there rather than tying it once. 鵝黃, goose-yellow, is a real colour name — the pale warm yellow of a gosling.

He opened the lock and brought out a packet of medicine.

藥 is medicine, drug, preparation — the word covers all three, and Chinese does not force you to choose. That ambiguity is the joke the next twelve lines are about to make.

And this medicine was:

Four characters, and the verse switch again. Whenever this book stops to sing about an object, the object matters.

Droppings of a hundred birds out of the hills, swept up and heaped to a thousand catties.

斤 is the catty, a traditional weight of about half a kilogram, so a thousand of them is around half a tonne. The recipe opens on bird droppings, swept off the hillside by the cartload — which is either the least dignified opening any set-piece in this book has, or exactly the point. Real Chinese alchemy did work with such substances; saltpetre, the making of which involves nitrogen-rich waste, is the ancestor of gunpowder.

Boiled up then in a copper pot, decocted at an even heat.

火候 is the fire-timing: how hot, how long, and when to change it. It is the central technical term of both alchemy and Chinese cooking, and it has since become a metaphor for judgement in any craft — to say someone's 火候 is not there yet is to say they have not learned timing. 煎熬, to decoct by long simmering, is also the ordinary word for suffering through something drawn out.