If this increased strength and hardness of a contracted metal are what one would expect on molecular principles, the decreased chemical activity at low temperatures is no less natural-seeming, when one reflects how generally chemical phenomena are facilitated by the application of heat. In point of fact, it has been found that at the temperature of liquid hydrogen practically all chemical activity is abolished, the unruly fluorine making the only exception. The explanation hinges on the fact that every atom, of any kind, has power to unite with only a limited number of other atoms. When the "affinities" of an atom are satisfied, no more atoms can enter into the union unless some atoms already there be displaced. Such displacement takes place constantly, under ordinary conditions of temperature, because the vibrating atoms tend to throw themselves apart, and other atoms may spring in to take the places just vacated--such interchange, in fact, constituting the essence of chemical activity. But when the temperature is reduced the heat-vibration becomes insufficient to throw the atoms apart, hence any unions they chance to have made are permanent, so long as the low temperature is maintained. Thus it is that substances which attack one another eagerly at ordinary temperatures will lie side by side, utterly inert, at the temperature of liquid air. Under certain conditions, however, most interesting chemical experiments have been made in which the liquefied gases, particularly oxygen, are utilized. Thus Olzewski found that a bit of wood lighted and thrust into liquid oxygen burns as it would in gaseous oxygen, and a red-hot iron wire thrust into the liquid burns and spreads sparks of iron. But more novel still was Dewar's experiment of inserting a small jet of ignited hydrogen into the vessel of liquid oxygen; for the jet continued to burn, forming water, of course, which was carried away as snow. The idea of a gas-jet burning within a liquid, and having snow for smoke, is not the least anomalous of the many strange conceptions that the low-temperature work has made familiar. PRACTICAL RESULTS AND ANTICIPATIONS