By the end of her shift she’d made slots that allowed her to remove and replace three of the bars at will. Two more, and she’d be able to squeeze through.

Agata packed up her tools and forced herself to swallow them again. Beyond the half-disassembled grille and the last segment of the tunnel, there’d be another grille just like it guarding the outlet from the chamber itself. She’d need more time to transit that segment, but the task no longer seemed impossible.

Agata ground the roots with the mortar and pestle she normally used for spices, feeling like some kind of demented kitchen alchemist. The flowers she’d cut up were all decorative species that had been planted around the beds of travellers for generations, in a relic of the old folk belief that their petals’ light had health-giving properties. She had never gone in for the custom herself, but at the garden no one had questioned her. The assistant had invited her to pick whatever she liked.

Every plant root contained substances that bound to a range of minerals. Over the generations, chemists had painstakingly tabulated their properties, species by species, and the appendices to Agata’s school chemistry textbooks were full of such quaint pre-photonic compendia.

The effect she was hoping for was not dramatic; she didn’t need a liberator to set the sunstone in the cooling chamber on fire. All she had to do was ‘poison’ a large enough portion of the surface of the rock. The size of the air particles that the sunstone produced was sensitive to the amount of time the decomposing agent spent in contact with it; if she could weaken the agent’s binding, some lighter air than normal would be produced. The total volume didn’t have to be enormous; the giant centrifuge of the Peerless itself would separate out the less massive component, concentrating it preferentially around the axis.

If she could get enough disruptor into the cooling chamber, the next step would be verification. She would need to be able to quantify the effect of her intervention on the refractive index of the air near the axis, not least to be able to convince Ramiro and Tarquinia to abandon their own efforts. The shift ought to be measurable in principle, but it would require specialised, high-precision instruments. Agata had no idea how she could get her hands on equipment like that without attracting attention – but she was sure that, like every other aspect of the plan, if she remained resolute it would fall into place.

‘You get keener every day,’ Celia observed. ‘I can move you to an earlier shift if you like.’