“Having obtained the liquid oxygen,” he continues, turning to the long table below the windows, “the question was how to store it for working purposes, with the least possible loss by evaporation. After various trials and experiments, we devised a set of vacuum vessels, each consisting of a tube or bottle for the liquid oxygen, sealed at the neck in a second tube or bottle, from which the air had been exhausted. I found the cheapest and best method of getting a vacuum to be the old Torricellian one of driving out the air with mercury vapor and then condensing the vapor. This had a further advantage. The tube containing the liquid oxygen was so cold that it froze the mercury vapor, and coated itself with a perfect metallic mirror, which by its reflection still further diminished the loss by radiated heat from the outside.” Without more ado, he lifted from a small frame one of the vacuum vessels referred to. It was a white glass jar, inside of which was what seemed to be a round metallic ball. Open at the neck, this ball was a bottle nearly filled with liquid oxygen, and by the light which reached it through the neck of the bottle it was a very clear pale blue liquid, which was evaporating quietly in a single thread of tiny bubbles, like a glass of champagne which has become nearly still. It was one of those moments which Faraday would doubtless have regarded as solemn. To behold, for the first time, a liquid which your professors of chemistry have assured you was a gas and always would be a gas, is an experience which does not occur many times in a lifetime. After that, a sight of perpetual motion or the square of the circle would leave you calm. To know, furthermore, that this strange gas, which is the prime agent of all life, which is eight-ninths of all water and three-fourths of the entire earth, has been laid captive by science, reduced to a form which cannot fail to shed a flood of light on any number of abstruse problems in chemistry and mechanics, excites a deeper feeling. The pale blue liquid, which is strangely lustrous, seems truly magical. Moreover, it is a great surprise to see the liquid, which you expect to find under great pressure and ready to blow its containing vessel to pieces, evaporating quietly in the air, protected from heat by a vacuum on one side and its own cold vapor on the other. And so you can do nothing but stare at it in amazement, and gently shake the bottle, and turn from it to its discoverer with a feeling which is not entirely dissociated from awe. It has lost all its impressiveness to the professor, however, for he is busy preparing to illustrate some of its properties--an interesting introduction in themselves to the conditions which prevail twice as far below the freezing point of water as its boiling point lies above. [Illustration: EARLY AND LATEST FORMS OF VESSELS FOR HOLDING LIQUEFIED OXYGEN.]