The Romance of Modern Chemistry

BELOW ZERO FIG. 10.—The apparatus sketched here serves to show the intense cold which is produced by boiling liquid air under diminished pressure. for over twenty-four hours, and the examination of its properties is thus rendered possible. Not only is it possible to study the properties of liquid air itself, but we can see how other substances behave when cooled to the temperature of liquid air. Their behaviour then is frequently quite different from what it is under ordinary conditions. Grass, leaves of plants, and indiarubber, for ex- ample, become so brittle when kept for a short time at the tempera- ture of liquid air that they can easily be powdered in a mortar. An egg, after immersion in this wonderful medium, becomes so “hard-boiled” that it may be severely knocked about without being damaged. Chemicals, too, which react vigorously at the ordi- nary temperature, become mutually callous when cooled to the boiling- point of liquid air. It has just been stated that liquid air boils at — 347° Fahrenheit, but by boiling under reduced pressure the temperature is lowered to a point at which the air of the atmosphere will condense straight away. This may be very simply and very beautifully shown in the following manner. A Dewar vacuum tube (see Fig. 10), filled to the extent of two-thirds with liquid air, is provided with a cork. Through this cork there are passed (1) an empty glass tube, A, closed at the bottom and dipping into the liquid air; (2) a bent tube, B, open at both ends and 189