Through all these explanations, and others too elaborate and too technical for these pages, he had spoken in the clear, emphatic way which is characteristic of men who deal with abstruse subjects, and desire, from long habit, to present them with the maximum of clearness and the minimum of words. His speech is incisive, the utterances of an energetic and concentrated mind. Over a cup of tea upstairs, however, he spoke more slowly and dwelt with interest upon some of the many strange results which have already met his eyes in the region of −200° Centigrade. “As we approach the zero point of absolute temperature,” said he, “we seem to be nearing what I can only call the death of matter. Pure metals undergo molecular changes which cannot yet be defined, but which entirely alter their characteristics as we know them. Tensile strength, electrical resistance, in fact, the whole character of the metal as we are acquainted with it, appears to change. At −200°, for instance, iron becomes as good an electrical conductor as copper, while it is more than probable that at the zero of absolute temperature, if not before, the electrical resistance of all metals reaches its zero point. The alloys do not follow the same rule, being much less affected. Carbon is a strange exception, its electrical resistance increasing with cold and decreasing with heat. The effect upon colors is also remarkable, and opens up a wide field for experiment and investigation. In fact, the most marked and immediate effect of my experiments will appear, I think, in the field of magneto-optics. You have seen a red oxide of mercury turn yellow when cooled to the temperature of liquid oxygen, and regain its original color upon returning to the temperature of the air. In the same way, sulphur becomes white. Bichromate of potash becomes also white. A solution of iodine in alcohol becomes colorless, as does ferric chloride, a deep red at the temperature of the air. They all regain their colors upon returning to the ordinary temperature. At these low temperatures chemical action ceases, as you have seen. I supposed the rule was invariable, but found that a photographic plate placed in liquid oxygen was still acted upon by energy from the outside, and at even −200° C. was sensitive to light.