This time, though, there is an alley running down the flank of the building, and a side door. In the mud outside, footprints are visible, amongst them, unmistakable, the heel of a woman’s boot. The door is closed but not locked. It is a carelessness that speaks of haste, of having one’s hands full. Beyond, a gaslight has been left burning. A corridor connects a string of offices, shabby-looking despite their once-decent carpets. It is a place of business, not of reception. Before they venture farther, Thomas turns on his heel and returns to the alley to relieve himself against the factory wall. It is a moment’s bliss in the middle of a nightmare.

Inside again, he finds Charlie and Livia have run ahead. The main floor holds the facilities associated with the sale and manufacture of ordinary tobacco. A front desk, an orders department; a workshop floor for packaging; and across the cobbled inner courtyard, a warehouse smelling like a giant pipe.

It is in the basement that the other factory is located. To enter one has to negotiate another, sturdier door, also left unlocked, and descend two revolutions of a carpeted spiral staircase. At its bottom a cavernous storehouse opens, suffused in a smell at once floral and rotting, emanating from a row of giant copper vats that loom like pillars in an old cathedral. Slender copper pipes emerge from these vats and move in coils towards a separate tank, minute by comparison, and holding the results of some long process of distillation. A cloying, wet heat stands in the room. Charlie has climbed the brass rungs riveted into one of the vats to get to its open top, has stuck a hand beyond the rim and is scooping up a palmful of its contents. Not far from him, Livia is examining a glass cabinet that holds a series of pharmaceutical bottles, each carefully labelled with dates.

“Flowers,” Charlie calls surprised, holding aloft a delicate stem crowned by a papery bloom, not unlike a poppy but of veined purple-grey. “The vat is filled to the rim with some kind of flower, submerged in warm liquid. There must be a million flowers here. And the smell. .” He sniffs his hand. “Like cigarettes, but faint.”

Livia, five steps from him, has uncorked one of the bottles and sniffed its contents.

“Sebastian mentioned flowers. He said: ‘A field of flowers for a dozen cigarettes.’ It must be how they produce the solvent: that which quickens Soot.” She lets her gaze wander between the giant vats and the small glass bottles with their labels, clearly struck by the misproportion. Almost reverently she accepts the flower Charlie holds out to her.

“Can you picture the size of the plantations, Charlie? Half of the Empire must be kept busy growing these.”