Liquid oxygen at atmospheric pressure boils at −184° C. (−229.2° F.). By evaporating it under a diminished pressure, he gets much higher degrees of cold, and these have enabled him to both liquefy and solidify nitrogen and air. The experiment illustrating this was not only interesting; it was difficult to believe. In a double vacuum vessel the centre of which was an open test tube, and the second compartment a reservoir of liquid oxygen connected with an exhaust pump, he so lowered the pressure that the oxygen boiled tumultuously. As it did so, drops of clear liquid began to form on the sides of the test tube and gather at the bottom. It was liquid air, the oxygen and nitrogen of the atmosphere liquefying together at a temperature of −197.2° (−322.9° F.). He poured some of the liquid air into a second tube, and then showed how the nitrogen, which liquefies at a temperature fourteen degrees below oxygen, boiled off first. A smouldering splinter of wood held at the mouth of the tube was extinguished. A few moments later when it was again held there, it burst into brilliant flame. The nitrogen had all evaporated, and the oxygen was coming off. He explained that air became solid under pressure at −207° C. (−340.6° F.). It was a structureless glass, and he had not determined whether or not the oxygen in it was solid or was held suspended as a jelly. Nitrogen solidified under pressure at −210° C. (−346° F.). It was a white crystalline substance. He had no knowledge as yet whether oxygen crystallized in solidifying, but his belief was that it would not. [Illustration: THE “COMPRESSORS.”] Concerning hydrogen, most elusive of all the gases, he had no present expectation of attaining liquefaction. Its critical point was below −210° C., and its boiling point −250° C. He had no means as yet of attacking the problem. In fact, the only thermometer he was able to use at these low temperatures was one which used hydrogen expansion as a measure of temperature. His main reliance in measuring low temperatures was a thermo-electric junction. Deeply interesting also was his description of liquid ozone, that strange form of oxygen which though identical with it in constitution is different in molecular arrangement. He obtains twenty per cent. of ozone from liquid oxygen by electrical stimulation, the ozone being of a very dark blue color, as dark as concentrated indigo. It is highly unstable, a beam of light having caused it to explode on one occasion, and its study even in small quantities requires all the delicacy of manipulation which is one of the special directions in which Professor Dewar as a chemist occupies the foremost rank.