"Gay Lussac's work is of a different kind. Less broad and striking, it is of the most thorough and comprehensive kind. Though he did not discover potassium, he invented a mode of preparing it in practical quantity. He did not at once grasp the significance of the elemental characters of iodine and chlorine, and was not at once prepared to accept the hydrogen acid theory, but he did at last give in his adherence, and his monograph on iodine and its compounds is a classical research, and exhausted the subject."