The Romance of Modern Chemistry

BELOW ZERO By the combined application of cooling and compres- sion both ammonia and carbon dioxide are readily obtained in the liquid form, and they are now commercial articles, sold in steel bottles or cylinders. With the aid of liquid ammonia and liquid carbon dioxide we are able to go a long step farther in realising low temperatures, for the cold produced by their rapid evaporation is very intense. This is well shown by what happens when the tap of a liquid carbon dioxide bottle is opened. The liquid is forced out in a fine jet by the high pressure which pre- vails in the bottle, and the cold produced by the eva- poration of the outer portion of the jet is so great that the inner portions are solidified to a white, snow-like powder. If a coarse canvas bag is tied over the nozzle of the bottle while the liquid is escaping, a quantity of this curious solid carbon dioxide may be collected. “ Carbonic acid snow,” as we may call it, can be placed on the hand without danger, but if pressed into the skin a serious blister is produced, the effect being pretty much the same as that caused by a red-hot metal rod. A number of interesting experiments can be made with solid carbon dioxide; if, for example, some of it is placed on the top of a little mercury in a dish, and some methylated spirit or ether is added, the mercury is very quickly frozen to a hard mass. In fact the temperature reached in this way is as low as —112° Fahrenheit; and if a mixture of ether and carbonic acid snow is made to evaporate very rapidly by connection with a suction pump the temperature reached is considerably lower still. The temperature of —112° Fahrenheit just mentioned is the boiling-point of liquid carbon dioxide under atmospheric pressure; this substance is a conspicuous example of what may be called “cold boiling liquids,” 185