"Oh, the webbing," he translates, enlightened. "Yes, precisely--there's only a small number of spells that cause damage so gross and unsubtle as to be noticeable in the short term and unmistakable over time, given the soul's intrinsic strength and resiliency, but a stream of any liquid will eat through wood in time, not just acid. And in difficult times, life can leave a soul bruised to the point where even less dramatic exposures to dark magic can be deeply damaging."
He picks up one of the books he hadn't grouped for Sirius, fingers caressing the pages as he sits. "You'll remember Francis and Alice Longbottom; they're the Cruciatus victims I was referring to. There are others. The hypothetical patient I first mentioned would, hypothetically, not have given me permission to disclose his difficulties."