Type 4 (Osmotic)

Type 4 (Osmotic) is the classification given to holdings whose defining property is the passive migration of moisture across a boundary that the holding itself maintains. Unlike the active generators catalogued under Floor 770 and the Guild of Theoretical Mechanics, a Type 4 object does not produce water; it arranges for water to arrive, and then declines to release it without written cause. The category was first set out in Fourth Concordance, Annex 2, and has been amended eleven times, twice in the same afternoon.

Definition and boundary conditions

A Type 4 holding is recognised by three concurrent signs: a discernible membrane, a measurable gradient, and an indifference to the direction of that gradient. The third sign is the troublesome one. Ordinary osmotic bodies move moisture from the dilute side to the concentrated side and are content to be described that way. Type 4 holdings have been observed to reverse without external stimulus, and the Bureau of Gradients has ruled, in its circular of 14 Frostfall, that this reversal "shall not be treated as an event but as a habit, and catalogued accordingly" (Bureau of Gradients, Circular 14-F).

The membrane need not be physical. The celebrated Type 4 (Osmotic) holding known as the Salt Index of the Ninth Reading Room consists of nothing but a list of names of persons who have borrowed umbrellas from that room; moisture nonetheless accumulates on the list, and the list is stored in a tray.

Relation to the Great Desiccation Event

The Great Desiccation Event obliged the Tower to reconsider the Type 4 category entirely. When the fungal archives of Floor 770 entered suspended animation and their osmotic moisture generators were requisitioned by the Guild of Theoretical Mechanics for remediation, a question arose that had not previously been asked: whether a holding that had been drained of its gradient remained a Type 4 holding or became a holding of Type 4, held in trust. The Fifth Concordance holds that it became the latter. The Sixth Concordance, Annex 9, holds that it became the former and always had been. Both are cited in the standing instructions to the Floor 770 custodians, which resolve the matter by instructing the custodians to cite whichever is shorter.

"Where a membrane is preserved but its gradient is withdrawn by lawful requisition, the holding shall be entered in the register as Type 4 (Osmotic), Dormant, and shall not be watered."

— Sixth Concordance, Annex 9, clause 4

Measurement

Gradient strength is recorded in librations, one libration being the moisture movement across a standard parchment membrane over the interval between two bells of the Hours of the Middle Stair. Readings are taken with a Type-2 Laminar Flow Gauge where the boundary is gaseous, and with the older wick-and-slate method where it is not. Disputes over which instrument governs are heard by the Office of Instrumental Precedence, which has not sat since its third member was lost in a stairwell survey and whose decisions are therefore issued by posthumous majority.

Type 4 (Osmotic) readings are classified as Type 4 holdings, and a Type A (Evidentiary Hold) is used to temporarily isolate them, exactly as with any other Type 4 material. This has caused difficulty: isolating a Type 4 (Osmotic) holding from its surroundings also isolates it from the moisture that constitutes its evidence, and the resulting reading is, in the words of the Seventh Concordance, "true, unhelpful, and admissible" (Annex 1, marginal note).

The Vellum Dispute

Between the winters of 41 and 44 the Scriptorium of the Lower Reaches and the Office of Weights and Membranes conducted a dispute over the Vellum Sheets of Stack 19, which the Scriptorium held to be Type 4 (Osmotic) and the Office held to be an unusually damp floor. The Office's position was that a holding must be about something; the Scriptorium's was that the sheets were about water and had said so for six hundred years.

The matter turned on the testimony of the clerk Anselm Vey, who had maintained the stack for thirty-one years and who submitted a sworn statement that the sheets "take the damp out of a room and put it somewhere I am not permitted to know." Vey's statement was accepted as a Type 4 (Evidentiary) holding on the strength of a gauge reading taken the same morning. He was subsequently reassigned to the Registry of Withheld Gradients and is understood to have requested the transfer.

The Eighth Concordance settled the dispute by declaring Stack 19 a Type 4 (Osmotic) environment rather than a holding, a distinction that satisfied neither office and required the construction of a door.

Handling and prohibitions

Type 4 (Osmotic) holdings are stored in dry rooms of established gradient and are not to be moved between floors whose relative humidity differs by more than four librations. Where a holding must be moved, it travels in a sealed case lined with Ersatz Adhesive, Type VII, which the Ninth Concordance describes as "adequate, and no more than adequate" (Annex 3). Custodians are reminded that a Type 4 (Osmotic) holding left unattended in a corridor will find the corridor's moisture and keep it, and that the corridor will then be catalogued as a Type 1 (Drift) site under the provisions of Type 1.

The archive's standing inventory of confirmed Type 4 (Osmotic) holdings is short and frequently revised:

• The Salt Index of the Ninth Reading Room (Dormant since the Desiccation).
• The Vellum Sheets of Stack 19, held as an environment.
• One sealed jar, accession unrecorded, which has never been opened and whose label reads only do not.

The jar is the subject of an open enquiry by the Office of Unresolved Accessions, which reports annually that it is making progress.