The Romance of Modern Chemistry

BELOW ZERO and the reader will see that boiling does not necessarily mean a high temperature. That liquid carbon dioxide, kept in an open vessel, is very cold can be simply shown by thrusting a piece of metal into it. There is a hissing and a bubbling exactly similar to what is observed when a red-hot poker is thrust into water; so that, relatively to the piece of metal, which is at the ordinary tempera- ture, liquid carbon dioxide is exceedingly cold. For purposes of refrigeration, in ice-making and cold storage, liquid ammonia is very largely used nowadays; rapid evaporation of this liquid under a suction pump gives a very low temperature, and if brine is circulated round the pipes in which the evaporation is taking place, it is rendered so cold that water may be frozen by it in large quantities. The success of chemists in liquefying such gases as carbon dioxide and ammonia is now overshadowed by the greater achievements of the last ten or fifteen years, during which period liquid air and liquid hydrogen have been produced in quantity. This has become possible by the introduction of an altogether new principle in gas-liquefying machines—a principle which deserves a few words of explanation. We regard a gas as consisting of an enormous number of separate particles or molecules moving rapidly in all directions; under ordinary conditions the total volume of the molecules is very much less than the space in which they move—in other words, the molecules are, relatively and on the average, not very close to each other. When, however, the gas is compressed, the mole- cules are crowded together, and they come within range of each other’s attraction. Each molecule exerts an attractive force on its neighbours, and is in turn attracted 186