The Romance of Modern Chemistry

BELOW ZERO The importance of removing the air from the space between the walls will be realised when it is remembered that under ordinary circumstances that space is filled with molecules of oxygen and nitrogen rushing hither and thither. With the outer wall near the temperature of the atmosphere, and the inner one in con- tact with liquid air, these molecules act like an army of heat-car- riers. Each molecule as it strikes the outer wall will take up so much heat, which sooner or later it de- livers up to the inner wall, only to return for a fresh supply. Fig. 9—Double-walled glass vessels—so- When the a1^ ls left in called vacuum vessels. Owing to the intervening space absence of air between the walla, hot the transfer of heat is liquids put in such vessels remain hot. a™ p .,, COM liquids remain cold, tor a romart therefore very »Pldly ably long time. effected, and the liquid air in the vessel soon evaporates. When this space, however, is rendered free from air, the heat-carrying molecules are removed, and the inner tube is more perfectly cut off from any heat exchange with the atmosphere. The insulation of the inner tube is made still more complete by silvering the inside of the outer one, for at such a bright surface the heat rays are reflected. In a Dewar vacuum flask, surrounded by a non-con- ducting material like cotton-wool, liquid air may be kept 188