# Page:Popular Science Monthly Volume 45.djvu/735

The Vacuum Jacket and Liquid Oxygen.—Prof. Dewar protects his liquefied gases, in order to keep them in that state, from the heat of convection, by inclosing them in a vacuum jacket; and from the heat of radiation by silvering the surface of the containing vessel. He is thus able to keep liquid air for thirty or forty hours. The vacuum used contains a little mercury vapor, which, though present in very minute quantities, can be condensed into a bright mirror by cooling the outside surface of the vessel with liquid air. Among the experiments made in one of Prof. Dewar's lectures to illustrate the properties of liquid oxygen, alcohol, which freezes at ${\displaystyle -}$120º, solidified when dropped into it, and in that state would not take fire. So-