The swing of his nature took him from extreme languor to devouring energy; and as I knew well, he was never so truly formidable as when, for days on … - Arthur Conan Doyle

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The swing of his nature took him from extreme languor to devouring energy; and as I knew well, he was never so truly formidable as when, for days on end, he had been lounging in his armchair amid his improvisations and his black-letter editions. Then it was that the lust of the chase would suddenly come upon him, and that his brilliant reasoning power would rise to the level of intuition, until those who were unacquainted with his methods would look askance at him as on a man whose knowledge was not that of other mortals. When I saw him that afternoon so enwrapped in the music of St. James's Hall I felt that an evil time might be coming upon those whom he had set himself to hunt down.