The Romance of Modern Chemistry

BELOW ZERO leading to the vacuum pump. When the latter is turned on, the liquid air in the vacuum vessel begins to boil vigorously under the reduced pressure, and in consequence of the low temperature thus produced, air gradually con- denses and collects as a liquid in the previously empty tube A. That, of course, is the natural result of bringing the temperature of the air in A below its boiling-point. The composition of liquid air is not quite the same as that of gaseous air, for the simple reason that oxygen is rather more easily condensed than nitrogen, so that liquid air contains a higher proportion of the former. Further, if liquid air is allowed to evaporate slowly, it becomes very much richer in oxygen, for the nitrogen is the more volatile constituent, and passes off more readily, leaving behind a liquid with a higher proportion of oxygen. On this fact is based a method for the extraction of oxygen from the atmosphere. More wonderful even than liquid air is liquid hydrogen. It is more difficult to prepare, for in applying the regenerative cooling process to hydrogen, it is necessary first of all to cool the compressed gas to a low temperature by means of liquid air before it is allowed to issue from the nozzle of the apparatus. Dewar, however, has made considerable quantities of liquid hydrogen, and on one occasion over a gallon of the substance, made in his laboratory, was carried through the streets of London to the rooms of the Royal Society. This quantity would weigh only about eleven ounces, for liquid hydrogen is by far the lightest liquid known to the chemist; bulk for bulk, it is only one-fourteenth as heavy as water. Some very interesting experiments have been made at these extremely low temperatures on the vitality of bacteria and seeds. Typical bacteria were exposed for 190