At the cyanamid plant at Niagara Falls, in Canada, there are seven of these great carbide furnaces, each about fifteen feet long and half as wide and one-third as deep. We all have some idea of how much heat is generated in the ordinary electric arc light such as is used for street lighting. In the carbide furnace the carbon pencil, instead of being six or eight inches long and as large around as your finger, is six feet long and two feet in diameter. There are three of these in each furnace, and when the furnace is in full action it can be imagined that there is a terrific heat generated; in fact, when the fused lime and coke come out of the furnace in the form of molten carbide the brightness of the molten material is so dazzling that one cannot look at it with the naked eyes without injury. [Illustration: ONE OF THE CARBIDE MILLS] Then there is the problem of producing pure nitrogen gas, that is, separating the eighty per cent of nitrogen in the air from the twenty per cent of oxygen. The latter is the element that we breathe and which passes into the body, there to combine with the impurities resulting from the various life activities. If the nitrogen and the oxygen were both allowed to act upon calcium carbide the oxygen would burn up the carbide before the nitrogen could be fixed in it, hence these two elements must be separated and all other impurities removed so that only chemically pure nitrogen is brought to the calcium carbide for fixation. The separation is accomplished by means of liquid air machines. This industry, therefore, not only utilizes the greatest heat obtainable on a practical scale, but it also utilizes the greatest cold. While the electric furnaces produce a temperature of over 4000° F., or about twice as hot as molten cast-iron, the liquid air machines work at a temperature of 372° F. below zero. The air must first be purified and dried. It is then compressed, cooled while under pressure, and then expanded. The expansion lowers its temperature considerably. If this extra cool air is used for cooling another batch of air under pressure, the latter upon expansion becomes still colder than the first batch expanded. By repeating this operation the final temperature of 372° below zero is reached, at which the air liquifies. How cold this is can be seen from some simple experiments. For instance, if a dipper full of the liquid air is drawn, in an instant the outside of the dipper is covered with a coating of frost deposited upon it from the surrounding atmosphere. The surrounding air is so much hotter than the liquid air that the liquid boils violently. If a piece of rubber hose is held in the liquid air for eight or ten seconds and then struck with a hammer the rubber flies into pieces just like glass. To dip one’s finger into this liquid air would freeze it solid in a second and would be as disastrous as dipping it in red-hot iron. [Illustration: LIQUID AIR PLANT]