Bolghai looked amused. ‘Oh, get up, madam. Overwhelmed even though you understand not a jot of it, I suppose?’

Pyxeas snorted. ‘Uzzia, Bolghai is studying properties of the air. We are all at the mercy of the weather, yes? And though I, Pyxeas, and the generations who went before me, have shown that the great cycles of the weather are dominated by astronomy, by the dipping and nodding of the world as it orbits the central fire, it is nevertheless the air that delivers that weather to us. So we study it too.

‘After all, invisible though it may be the air is real; it has weight and substance. You can feel it dragging into your lungs, you can use pumps to evacuate it from a chamber – we all felt its lack up on the roof of the world. And the air is made up of several component parts, which can be separated with sufficient ingenuity. This was first achieved by Northlander scholars. We know there is vital air, an air full of energy, which we suspect is the agent that supports combustion – and indeed the slower fire that burns in our bodies to sustain life. And then there is fixed air. This was first identified by a scholar at Etxelur called Cleomedes, of Greek descent, who studied the burning of charcoal in a closed vessel.’

Bolghai said, ‘The scholars of Cathay have long shared their knowledge with Northland, a tradition I have sought, in my time, to maintain, or rather revive. I with my party was the first to travel to Northland after the conquests of the Great Khans. And, given the importance of the air to the weather which shapes all our destinies, as Pyxeas points out, we have continued its study here. Although Cathay scholars are perhaps of a more practical bent than those of Northland.’

Pyxeas sighed. ‘True, true, but the first Wall-builders would cringe to hear you say it. That’s the legacy of Pythagoras and his Greeks, who could be a bit contemplative.’

Bolghai gestured at his apparatus. ‘We have found ways to measure the presence and concentration of fixed air more precisely. For instance here, you see, the air from this chamber is fed through lime dissolved in water; it precipitates a kind of chalk whose weight we can determine . . . The details are unimportant.’

‘Not to me, they’re not!’ thundered Pyxeas. ‘I want to examine every tube and valve, every seal and measuring gauge. Excellent experimental design,’ he said now, walking around the boxes. ‘Can you see it, Avatak? Why these empty boxes, for instance?’

That was easy; Avatak had seen similar set-ups in Pyxeas’ own studies. ‘They are for comparison. A horse in this box, not in that box that’s otherwise the same; you can subtract one from the other to see what difference the horse makes.’

‘Exactly!’