Silver bromide particles on any positive print leave minute traces on any surface when they're combined with compression and heat,like,say, a wallet being sat on for ten hours a day in a hot taxi.

What's more, his magnetism or whatever they call it was such that the dashed animal, instead of pinning him by the leg, calmed down as if he had had a bromide, and rolled over on his back with all his paws in the air.