'The nature of the powder,' Daniel said. 'Ground so exceedingly fine.' He nodded at the bag, for his hands were busy. Having weighed out five grains of powder from the keg, he poured them from the scale-pan into a cupped scrap of paper and carried it over to Hooke. 'Such powder burns much faster than this coarse stuff.' He shook the paper for emphasis and it made a sandy rasp. He handed it to Hooke, who poured it into the cylinder of the Rarefying Engine. Some of these engines were wrought of glass, but this was a heavy brass tube about the size of a tobacco-canister: a very small siege-mortar, in effect. Its piston fit into it like a cannonball.