Smoke had jumped down from the back of the armchair and now occupied the middle of the carpet, where, with tail erect and legs stiff as ramrods, it was steadily pacing backwards and forwards in a narrow space, uttering, as it did so, those curious little guttural sounds of pleasure that only an animal of the feline species knows how to make expressive of supreme happiness. Its stiffened legs and arched back made it appear larger than usual, and the black visage wore a smile of beatific joy. Its eyes blazed magnificently; it was in an ecstasy.

At the end of every few paces it turned sharply and stalked back again along the same line, padding softly, and purring like a roll of little muffled drums. It behaved precisely as though it were rubbing against the ankles of someone who remained invisible. A thrill ran down the doctor’s spine as he stood and stared. His experiment was growing interesting at last.