The first artificial cold was produced by a mixture of salt and ice, such as we now use in an ice-cream freezer. In time men learned other ways of producing great cold and even to manufacture ice in large quantities. The cold of liquid air is far greater than that of ice or even a freezing mixture of salt and ice. Liquid air is simply air that is so cold that it becomes a liquid just as steam when cooled forms water. Steam has only to be cooled to the temperature of boiling water, while air must be cooled to 314 degrees below zero on the Fahrenheit scale. If it were possible for us to live in such a climate, and the world were cooled to the temperature of liquid air, we should have a curious world. Watch-springs might be made out of pewter, bells of tin, and piano wires of solder, for these metals are made stronger by the extreme cold of liquid air. There would be no air to breathe. Oceans and rivers would be frozen solid, and the air would form a liquid ocean about thirty-five feet deep. This ocean of liquid air would be kept boiling a long time by the heat of the ice beneath it, for ice is hot compared with liquid air. The ice would cool as it gave up its heat to the liquid air and in time become as cold as the liquid air itself. Liquid air has been shipped thousands of miles in a double walled tin can, the space between the two walls being filled with felt. The felt protects the liquid air from the heat of the air without. The liquid air evaporates slowly, and escapes through a small opening at the top. Professor Dewar, a successor of Faraday in the Royal Institution, invented the Dewar bulb, by means of which the evaporation of the liquid air is prevented. This bulb is a double-walled flask. In the space between the two walls of the flask is a vacuum. Now a vacuum is the best possible protection against heat. If we were to take a bottle full of air and pump out from the bottle all except about a thousandth of a millionth of the air it contained at first we should have such a vacuum as that of the Dewar bulb. With such a vacuum around it ice could be kept from melting for many days even in the hottest weather, for no heat can go through a vacuum. But the greatest cold is not the cold of liquid air. Liquid hydrogen is so cold that it freezes air. When a flask of liquid hydrogen is opened there is a small snow-storm of frozen air in the mouth of the flask. But even this is not the greatest cold. The liquid hydrogen may be frozen, forming a hydrogen snow whose temperature is 435 degrees below zero. This is nearly equal to the cold of the space beyond the earth's atmosphere, which is the greatest possible cold. The Electric Furnace and the Greatest Heat