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gas to be liquefied is introduced into a glass tube (Fig. 3), the narrow end of which consists of a strong capillary tube. The tube carries a metal collar, which enables it to be secured in position in the strong steel bottle (Fig. 4), by means of a nut, E' (Fig. 5), which screws into the mouth. The bottle, which is partially filled with mercury, is connected by means of a flexible copper tube of fine bore with a small hydraulic pump, by means of which water is forced into the steel bottle. The water so driven in forces the

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mercury up into the glass tube T, and thereby compresses the contained gas. In this way a pressure of several hundred atmospheres may be applied to the gas. In his earlier experiments Cailletet depended almost entirely for the refrigeration he required upon the fact, that when a gas is allowed suddenly to expand it undergoes a great reduction in temperature. This method of cooling may be termed internal refrigeration. In the case of oxygen, the gas was first subjected to a pressure of 300 to 400

atmospheres, and was then allowed suddenly to expand by a rapid release of the pressure. The result of the sudden expansion was to momentarily lower the temperature of the gas to such a point that the tube was filled with a fog, or mist, consisting of liquid particles of oxygen.

This principle, namely, the self-cooling of a gas by its own sudden expansion, has recently been applied for the liquefaction of oxygen in large quantities. When oxygen under considerable pressure, say 120 atmospheres, is allowed to escape from a fine orifice at the end of a long pipe, the issuing gas suddenly expands, and thereby its temperature is greatly lowered.

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If this self-cooled gas is made to sweep over the pipe from which it is escaping, it will cool the pipe, and therefore lower the temperature of the remaining gas before it issues. In this way the cooling effect becomes cumulative, the initial temperature of the gas before it escapes being continually brought lower and lower, until at last the

point is reached at which the oxygen is liquefied.*

If the oxygen be first cooled to about - 80° by means of solid carbon dioxide, then in a few

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minutes, by the further cooling due to its own expansion, the temperature will fall below the boiling-point of oxygen, and the liquefied gas be obtained.

The apparatus for the purpose is shown in Fig. 6. Oxygen *Linde, The Engineer, Oct. 4, 1895.

+ Designed by Dewar; constructed by Messrs. Lennox, Reynolds & Fyfe.

under a pressure of 120 to 140 atmospheres is passed through a series of spirals of fine copper pipe contained in the chamber C, which is encased in a non-conducting jacket of cork-dust. The gas enters by the pipe O (seen in the enlarged section), and passes through the spiral S S, which is immersed in a mixture of alcohol and solid carbon dioxide (the liquid carbon dioxide from the reservoir being admitted into the alcohol through the valve W, which is regulated by the screw B). The oxygen thus cooled passes through the double spiral pipe D D, which ultimately extends through the bottom of the chamber, and terminates in a stirrup, U, the short end of which is closed. In the bend of this stirrup there is a fine hole, which can be closed or opened at will by the pointed end of the rod V, connected to the screw A. On opening this valve, the oxygen, already cooled to about -80°, escapes from the hole under a pressure of 120 to 140 atmospheres. It instantly expands, and is thereby cooled still lower. This cold gas is prevented from escaping at once into the atmosphere by the glass tube G, but is compelled to rush upwards (as shown by the arrows), and, sweeping past the double spiral D D, cools this pipe, and therefore the succeeding portions of issuing oxygen. In a few minutes the temperature of this pipe is thereby brought so low, that the further cooling of the gas by its expansion causes the liquefaction of a portion of it, and a fine spray of liquid is seen to spurt out from the hole. This spray quickly increases in quantity, and rapidly collects as a clear liquid in the glass tube G. This tube is doublewalled, the space between the walls being perfectly vacuous. such a vessel the liquid oxygen may be kept for a considerable time, evaporating only very slowly in spite of its extremely low boiling-point, as it has been found that such a vacuous envelope forms the most perfect non-conductor.

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The instruments designed by Linde in Germany, and by Hampson in England, and known as air-liquefiers, are constructed on precisely similar principles. In this case, however, the preliminary cooling by means of solid carbon dioxide is dispensed with; for instead of a limited and comparatively small supply of compressed gas in a steel cylinder, an unlimited supply of air is delivered into the machine, under a pressure of 120 to 160 atmospheres, by means of powerful compression pumps driven by an engine.

By an extension of the same principles hydrogen was first successfully liquefied in 1898. In this case, however, the gas requires

to be previously cooled to about - 200° before expansion is allowed to take place. By utilising the low temperatures which can be obtained by means of boiling liquefied gases, it has now become possible to liquefy all the known gases by cold alone, that is, without the application of pressure; in other words, their temperatures can be brought down below their boiling-points, under which circumstances they must obviously assume the liquid state. For example, liquefied ethylene boils at - 103.5°; if, therefore, a stream of nitrous oxide is passed through a tube immersed in a bath of liquid ethylene, the nitrous oxide will be cooled below its boiling-point (-89.8°), and will consequently be reduced at once to the liquid state.

Again, liquid oxygen boils at -182.5°. This boiling liquid therefore is sufficiently cold to cool marsh gas below its boilingpoint, namely, 164.7°, and therefore to cause its liquefaction.

Moreover, by the rapid evaporation of liquid oxygen the temperature may readily be lowered to the point at which air will liquefy. Thus, if a quantity of liquid oxygen in the glass tube O (Fig. 7), which is provided with a vacuous envelope, V, be made to boil -N rapidly by putting the pipe P in connection with an exhaust - pump, the temperature quickly falls to 200°, when air itself becomes liquefied without the application of pressure; and drops of liquid air quickly. collect upon the walls of the inner empty tube, N, which is freely open to the atmosphere. In this way considerable quantities of liqueFIG. 7. fied air can be collected in a few minutes. By means of boiling liquid hydrogen the low temperature of -253° has been reached, at which temperature all other known gases, except perhaps helium, are frozen to the solid state. The lowest temperature yet obtained by the rapid evaporation of solid hydrogen is 260° (Dewar).

The Critical Point.-As far back as the year 1869, it was shown by Andrews that when liquid carbon dioxide was heated to a particular temperature, it passed from the liquid to the gaseous state, and that no additional pressure was able to condense it again so long as the temperature remained at or above that point. This

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particular temperature is called the critical point, or the critical temperature of the gas. In the case of carbon dioxide this critical temperature is 31.35°, and in order that this gas may be liquefied by pressure, it is an essential condition that the temperature be below that point; above 32° no pressure is capable of bringing about liquefaction. All gases have a critical temperature, which is special for each gas, and until the temperature of the gas be lowered to that point, liquefaction is impossible. The critical temperatures of the different gases vary through a very wide range: thus, the critical temperature of hydrogen is as low as - 238°, while that of sulphur dioxide is +155.4°. In the third column of the table of physical constants on page 80 the critical temperatures of a number of the more common gases are given.*

The gases in this list, from ethylene downwards, all have their critical temperatures so high that there is no difficulty in cooling them below these points. These are the gases which were first reduced to the liquid state. The first five upon the list have very low critical temperatures; these are the very gases which for so long resisted all attempts to liquefy them, and which were on that account called permanent gases. We now know that the failure to obtain them in the liquid state was owing to the fact that the relation between the critical temperature and the point of liquefaction was not fully realised. Just as carbon dioxide cannot be liquefied unless its temperature be brought down to 31.35°, so oxygen resists liquefaction under the highest possible pressures, until its temperature be lowered to -118.8°, the critical temperature of oxygen.

The critical temperature of a gas is sometimes spoken of as the absolute boiling-point.

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Critical Pressure. The particular pressure that is required to liquefy a gas at its critical temperature is called the critical pressure. Thus the pressure necessary to liquefy oxygen, when the temperature has been lowered to 118.8°, is 58 atmospheres ; while that required to condense chlorine at its critical point, viz., +141°, is 84 atmospheres. At temperatures below the critical temperatures a gas liquefies under less pressure than the critical

*For the constants for the gases of the Argon family see page 271. It may be well to remind the student that such constants as are here tabulated are obtained from measurements involving very great experimental difficulties, and that consequently they are always liable to revision. The values here given are from the most recent determinations.

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