PRODUCING THE COLDEST COLD.

Pelorus Guardian and Miners' Advocate., Volume 30, Issue 23, 22 March 1918, Page 2

PRODUCING THE COLDEST COLD.

CASES SQUEEZED AND COOLED UNTIL THEY LOOK LIKE WATER !

Each substance on our earth has individual properties, both chemical and physical, which it can retain only at a specific temperature —a temperature which nature set for it. I! we alter this temperature by artificial means the substance will gradually assume a different physical state. This change of state is "forced” upon it, and when vie- withdraw the artificial means of changing its temperature, nature promptly transforms the substance into its original state.

As an example, water at ordinary temperature is a liquid. If we heat it to '212 deg. F. it becomes steam, and~ifwe cool it to 32 deg. F. it becomes a solid.

What is known as the kinetic theory of matter tells us that all molecules are in perpetual 1 ■ 'ration at the tremendous velocity ami are continually colliding with one another. This rate of, molecular vibration produces i the temperature of matter —the higher the mto of motion the greater the temperature and 1 vice versa. The molecules are incessantly giving out their energy of motion and at > the same time arc receiving these mechanical Impulses from other particles of matter. .With these simple facts in mind, wo may. continue more intelligently.

Wh-en \vc boil water we merely impart energy in the form of heat to molecules. If the source of heat is intense enough, the particles become bo wild in their vibration that they come out of the range of their natural mutual attraction and pass of! u vapour. It this vapour is cooled. It again assumes the liquid state, because we have taken energy from the molecules and have caused them to return to their natural degree of vibration, If wo continue to cool the liquid, we still further paralyse the motion of the molecules, until they become so crowded together that we have a solid—ice. HOW GAS FREEZING IS ACCOMPLISHED.

Now, then, in the light of the knowledge imparted in the foregoing paragraphs, if we wish to change a gas to a liquid we must cool it. This is true, If sulphur dioxide (a gas abtained by burning sulphur) is cooled to a few degrees below zero, it condenses into a liquid. As soon as the artificial means of cooling the gas is withdrawn, it rapidly assumes its natural state, as gas, by evaporation. Now to get back- to its natural state, it needs a specific amount of heat to make its molecules vibrate at a definite rate, that which nature determined.

Where does it get this heat ? It abstracts it from its surroundings so rapidly that a still further degree of coldness is realised as the gsis is formed from the liquid and passes off carrying with it its natural amount of heat which it has greedily robbed from material in contact with it. For commercial purposes liquid carbon dioxide is stored under great pressure in durable*steel cylinders. If the jet on the cylinder is opened the liquid evaporates so rapidly that the temperature of the container is soon lowered far below zero, and a solid formation of carbon dioxide appears on the mouth of the jet. Professor Dewar liquefied hydrogen and helium in the laboratory of the Roj'al Society by a different method from that of rapid, evaporation. The principle a pplied by him -.is based on the fact that a compressed gas allowed to expand freely greatly lowers its own temperature. Lord Kelvin made known this fact early in his career, and it was commercially utilised by Linde, a German scientist, and by Hampeon, an English physician. Both workers were labouring independently of each other. It was found that if the compressed gas was allowed to expand through a small opening its temperature was still further lowered.

Working with these' facts in mind, Linde and Hampson perfected a process by which they were able not only • to obtain far lower temperatures than with the old' evaporative method, but to liquefy gases that had hitherto resisted all efforts.

The apparatus used consists of a coil of pipe, through which the compressed gas is permitted to pass and expand through a small opening at the end. First, the air is brought to a pressure of 200 atmospheres by means of the compressor. It is discharged from this through a valve and into a water-cooled jacket, where the best of compression is abstracted. From there is flows through a smaller coiled pipe which is concentrically arranged within a larger one. As the air reaches an expansion valve, and flows into a heat-insulated chamber, its temperature is greatly lowered. The cooled air then rushes back through the larger pipe, in which the small pipe already referred to is coiled, and lowers the temperature of the succeeding air coming through the smaller pipe. It will be seen, then, that the air emanating at the expansion valve will gradually become colder until a liquid state is reached, and it can he made to conne out as a liquid at the nozzle in approximately six minutes.

When attempts we e made to liquefy hydrogen by this means it was found that instepd of being cooled by expansion its temperature was actually raised. Later it was discovered that hydrogen obeyed this law only when its substance was first cooled by contact with some refrigerating medium. In the apparatus employed ! to-day for the liquefaction of hydrogen, the gas is first reduced to a low temperature by means of solid car-

bonic acid and. liquid air. By this means Dewar also brought helium to a liquid state. The fact that these liquid gases cannot be kept in ordinary containers should he readily appreciated when it is understood how rapidly they abstract heat from their surroundings. If liquid air is poured into an ordinary glass vessel it immediately starts to boil and will reduce the container to bits. It must be remembered that liquid air has a boiling point, about 180 degrees Centigrade below zero. It liquid:gases, then, are to be kept any length of time they must in some way i be insulated from the beat of their surroundings.

It has been known for a long time that nothing but tangible matter will conduct heat waves. Dewar ingeniously took advantage of this fact in a method by means of which he can preserve liquid gases over a considerable period of time. He uses a glass vessel with two walls between which a high vacuum prevails. If a small amount of mercury vapour is left between the walls, it will be solidified and deposited upon the walls of the vessel upon the entrance of a liquid gas. In this manner it acts as a mirror and reflects heat waves that impinge upon the outer surface of the container. Thus was the familiar commercial vacuum bottle created. — "Popular Science Siftings."