Combeferre sees Enjolras's expression change. He wants nothing more than to say something, anything, that might console. But he also knows exactly what consolation Enjolras is asking for.

"Catalytic distillation," Combeferre says. "Specifically, to purify gasoline."

He explains the chemical process, watching Enjolras's face carefully all the while. He will wait for Enjolras to bring the conversation back to Valjean. Combeferre's more than happy to provide a distraction until then, if that's what Enjolras wants.