Beth Perry, age 12, of Kearny, Nebraska,for her question:

How do they make liquid oxygen?

The spaceman's term for liquid oxygen is LOX and LOX is a space age fuel agent. If our weather ever dropped to temperatures of minus 183 degrees centigrade, the gaseous oxygen in the atmosphere would rain down in pale blue drops of watery liquid. Then we would have all the LOX we need. However, at this temperature everyday life as we know it would be frozen to a halt    including our space program. Fortunately, such a global chill is not likely to occur    and besides there is an easier way to reduce gaseous oxygen to its liquid form.

Liquid oxygen looks like palest blue water    but it is cold enough to freeze your finger solid. It remains liquid only when sealed under pressure in a heavy metal tank. If a saucer of this LOX is left in a room, it quickly boils away and evaporates back into its gaseous form. The oxygen in the air is a gas because our normal temperatures are always way above its particular boiling point    which is 183 centigrade degrees below the freezing point of water. When the gas is chilled to this temperature it becomes a pale watery liquid.

Substances are gases because their molecules have enough heat energy to separate and zoom around at fantastic speeds. Removing heat removes their energy. They slow down and come closer together. At minus 183 degrees centigrade, oxygen molecules cling together and form LOX. If more heat is removed and the temperature reduced to minus 218.4 degrees centigrade, the clinging molecules close ranks and freeze together as solid oxygen.

Instead of using a special sort of refrigerator, scientists trick the gaseous oxygen into giving up some of its heat. Vhen a gas is compressed, or squeezed into a smaller space, it becomes hotter. If this warm gas touches cooler surroundings, it shares some of its heat. Also, when a gas expands, it automatically loses some of its heat. All gases must obey these rules    and the rules can be used to prepare liquid oxygen at minus 118 degrees centigrade. This is still a shudderingly cold temperature. But it is 65 centigrade degrees warmer than the normal liquifying stage of oxygen. The apparatus for the job is a sturdy plumbing system. It has a heavy compressor linked to coils of pipes set inside two metal tanks. After the oxygen has gone through this plumbing circuit several times, enough heat is removed to turn the gas into liquid.

First, gaseous oxygen is pumped into the compressor. There it is squeezed under a pressure of about 700 pounds and becomes warm. Then it is led through coiled pipes in a tank of cool, circulating liquid. This removes some of the heat and the cooler gas is led to the second tank. There it expands through wider pipes    and loses more heat. Then it is returned to the compressor and sent on another cooling trip through the tanks. After several round trips, the chilled gas begins to condense into drops of liquid. At the bottom of the second tank, a fine valve is waiting to drain the liquid into a sturdy storage tank.

The key to the process is the correct, or critical pressure. And extra pressure is wasted. The critical pressure for liquifying oxygen is 49.7 times the standard pressure of the atmosphere. When used with suitable cooling systems, this critical pressure can cause oxygen to liquify at a critical temperature of minus 118 degrees centigrade.