page thirty-seven

The Assay

Thirty-six rooms have measured, dated, bred and forecast things, and not one of them has ever asked what something is made of. Today's number washes the labels off four bottles in an old chemist's cellar and leaves you the stock book, a rack of test tubes and very little to spare.

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The bench

Pick a bottle, then a reagent. Every test takes one portion of that bottle for good, and each bottle has four. The colour of the solution is free; everything else costs.

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The rack

Every tube you have run, bottle by bottle, with what the bench saw.

The bench card

Pinned above every sink in the laboratory: what each ion does with each reagent. Each bottle holds one salt, so one cation and one anion.

What the ions do

The stock book

Seven lines for the bottom shelf. Four are in the bottles; each line fills one bottle at most.

The labels

Write each bottle back up from the stock book. The shop will dispense from them in the morning.

Shelf
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The arrow keys walk to other days. Every day is a different shop, a different flood and a different shelf.

What is real here and what is not

The tests are the ones still taught at school benches, most of them older than the shop. The chemist, the town, the flood and the shelf are the day's number.

  1. 01

    Colour in a flame

    Heat a metal's salt and its electrons give light back at wavelengths only that element uses. Robert Bunsen and Gustav Kirchhoff turned the observation into spectroscopy in 1859 and found two new elements with it within two years. Sodium is the nuisance: a trace of it swamps everything else in yellow.

  2. 02

    Hydroxides

    Most metal hydroxides will not stay dissolved, and their colours give the metal away: blue for copper, green for iron(II), rust for iron(III). Zinc and aluminium hydroxides are amphoteric and dissolve again in excess alkali. Ammonia separates them, because zinc forms a soluble complex with it and aluminium does not. Copper does too: that deep blue is the tetraammine ion.

  3. 03

    The anions

    Silver halides come out white, cream and yellow; barium sulfate is one of the least soluble salts there is; acid drives carbon dioxide out of a carbonate. Nitrates form no precipitate with anything, so they are reduced to ammonia instead, here with aluminium foil, classically with Devarda's alloy. Any ammonium has to be boiled off first, or it gives a false positive.

  4. 04

    The rest is simplified

    Each bottle holds one pure salt at a sensible strength. Nothing is contaminated, every precipitate is unmistakable and nobody has to tell cream from white by gaslight. A real unknown is usually a mixture, and the order of tests starts to matter a great deal.

Thirty-seven rooms, and one number doing all the work. Today it floods a cellar, washes four labels off and waits to see how few portions you can spend getting them back.