“So when your explorers landed on the West coast of Africa in 1850, the local tribe presented them with the… magic rocks?”

“Rocks that stay warm without fire or sunlight, yes.” Burton unrolled a hand-drawn map. “They were found more than 200 miles inland, here, so it took another two years before we could get a team through the jungle and collect more.”

“And then you had that chemist look at the rocks, and try to extract some essence or element from them. Did he make any progress before that illness took him?”

A few moments passed before Burton responded. He stared into the distance as the train carriage gently rocked on its suspension. “Yes. I knew Monsieur Laurent.”

“I’m sorry.”

“Thank you. It was hard seeing him go like that. But yes. He found that the more the rocks were purified, the more heat they produced.”

“So it’s some chemical trick, then. How long does it last?”

“Did you notice anything odd about our train today? The lack of a coal car, in particular. Thus far… fourteen years.”