Liquid air and the liquefaction of gases; theory, history, biography, practical applications, manufacture

Front matter

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Preface

In- Gulliver’s veracious account of his travels we read of the work done in the famous Academy of Logado. In one department fifty men were at work under the superintendence of the universal artist, as one of the illustrious investigators was called. These men were engaged in various occupations. ‘Some were condensing air into a dry tangible substance by extracting the niter, and letting the aqueous or fluid particles percolate.” So says the great’ Dean, selecting the solidification of air as one of the impossibilities worthy of embodiment in his sarcastic romance.

During the present generation the triumphs in natural science have been most wonderful. The prosaic narration of what has been done sounds like the romancing of a Cyrano de Bergerac. We read of the hardest metals, such as iron and nickel, carried off in the gaseous state by carbon monoxide ; the surgeon uncgncernedly has the interior of his living patient’s body photographed ; the triumphs of chemical synthesis culminate in the production on the scale of manufacturing industry of a hydrocarbon from coal and water; Marconi follows a yacht race and telegraphs its phases to the distant shore over miles of water, without a wire; and to-day air is liquefied by the gallon, hydrogen and helium succumb to intense cold, becoming mobile liquids, and

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PREFACE.

the last miracles of science may figure among her greatest.

The present work aims to tell the history of the liquefaction of gases, wherein the physicist has ex. ceeded the fictitious achievements told of in Gulliver. The subject, extending overa century, is full of interest from the biographical as well as scientific standpoint, and it is hoped that the presentation of it with such scope will be acceptable to the reader.

For assistance in the compilation the author’s thanks are due to many. His requests met with quick response from such men as L. P. Cailletet, Henri Dufour, Charles E. Tripler and James Dewar. And a personal friendship brought about by this book has fully justified the labor of writing it—the friendship of that wonderfully endowed scientist Raoul Pictet, one of the fathers of liquid air, poet, musician, physicist, chemist and mathematician—a veritable Admirable Crichton.

The work is but begun, the future possibilities are great, and it is impossible to foresee the impending developments in the liquefaction of gases.

LAB EE Or CONLEN Ts.

CHAPTER I.—Puysics. Pages 9-36

What is liquid air?—The three states of matter: Solid, liquid and gaseous— Relations of pressure and heat to state assumed by matter—The critical state and its phenomena—Natterer’s tube—Physical units—Space, mass and time—Force and energy—Conservation of force an abandoned doctrine—Conservation of energy—Work a synonym for development of energy—Waste of energy and entropy—Possibility of utilizing the lower forms of energy of the universe.

CHAPTER II.—HEat. Pages 37-58

Heat and its measurement—Thermometers—The zero point—The Celsius or Centigrade thermometer scale—Fahrenheit’s thermometer scale—The absolute zero—Its basis—Coefficient of expansion of gases—Determination of temperatures in the liquefaction of gases—Different liquids used in filling thermometers—The air thermometer—The hydrogen thermometer—Details of its construction—Klectrolytic hydrogen—The hydrogen or air thermometer formula—The thermo-electric thermometer—Onnes’ instrument and details of its construction—Its calibration—The electric resistance thermometer—Calorimetric determination of temperatures.

CHAPTER III.—HEAT AND GASES. Pages 59-84

The perfect gas—The ultra-perfect gas—Energy expended in heating a gas— Specific heat at constant pressure and at constant volume—Atomic heats and variations of same from equality with each other—Adiabatic and isothermic expansion of gases—Carnot’s cycle—The perfect heat engine— Available and unavailable energy—Unavailable energy rendered available by liquid air—Latent heat of melting, of vaporization, of expansion—Boiling a cooling process—Expansiona cooling process—The spheroidal state —The Crookes layer—Experiments and illustrations—Utilization of the spheroidal state in low temperature work and in liquid air investigations.

CHAPTER IV.—Puysics AND CHEMISTRY OF AIR. Pages 85-91 The atmosphere as an ocean—What air is—Its constituents—Relations of air to living beings—The chemist’s and physicist’s view of air—Its constancy of composition—Carbon dioxide—Oxygen—Nitrogen, argon and other constituents.

CHAPTER V.—THE RovaL INSTITUTION OF ENGLAND. Pages 92-99

The Royal Institution—Its origin and objects—Count Rumford—Sir Humphry

Davy—The Pneumatic Institute—Davy’s experiments in inhaling poison-

ous gases—His engagement as director of the Royal Institution—His views on the utility of liquefying gases.

CHAPTER VI.—MIcCHAEL FARADAY. Pages IOI-I1I5

Michael Faraday--His early life—Early devotion to science—His introduction to Humphry Davy—Attendance at scientific lectures—Engagement at the Royal Institution—Injuries from explosion in the laboratory—Kuropean tour with Davy—Rivalry of scientific men—Davy and Faraday as rivals— The liquefaction of chlorine—Davy’s share in the experiment—Davy’s opposition to Faraday’s election as fellow of the Royal Society—Dr. Paris and the liquefaction of chlorine—Faraday’s descriptions of his liquefactions—Explosions—Northmore’s priority published by Faraday—Notes on Faraday’s liquefaction of gases—His exhibit’on of Thilorier’s apparatus— His later work in liquefying gases—Discovery of the magnetism of oxygen gas—His death—Bent tubes as used by Faraday—Experiments with use of bent tubes—The Davy-Faraday Laboratory.

‘CHAPTER VII.—EARLY EXPERIMENTERS AND THEIR METHODS. Pages I16-151

Perkins’ claim to have liquefied air—Its absurdity—Northmore’s liquefaction of chlorine—Rumford’s experiments as commented on by Faraday—Babbage’s experiment in a drill hole in limestone rock—Monge and Clouet’s alleged liquefaction of sulphurous oxide—Faraday’s liquefaction of chlorine—Stromeyer’s liquefaction of arseniureted hydrogen—Faraday’s bent tubes for liquefaction of gases—Manometer for use with them—Experiment in a straight sealed tube on the liquefaction of chlorine—Davy’s suggested method—Cagniard de la Tour—His bent tube experiments—D. Colladon—His apparatus as still preserved—Thilorier—His discovery of solid carbon dioxide—A fatal explosion—The improved Thilorier apparatus— Johann Natterer’s apparatus—His experiments—Loir and Drion’s solidification of carbon dioxide—Thomas Andrews, of Belfast.

The life of Raoul Pictet—His education—His ice machines—Disputed priority —Honors awarded—His apparatus for liquefying gases—Description of its operation—Temperatures of the cycles of operation—His dispatch of December 22, 1877, to the French Academy—Regnault’s statement—Hydrogen —His dispatch of January 11, 1878, to the French Academy - Olszewski’s comments on the hydrogen experiment—Pictet’s arrangement of pumps—

~ His desire to produce liquid oxygen in quantity—Comments on his work — . The liquide Pictet. ,

CHAPTER IX.—Louis-PavuL CAILLETET. Pages 173-202

The life of L.-P. Cailletet—His education—Honors received—His modification

TABEEOF CONTENTS.

of Colladon’s apparatus—Accidental liquefaction of acetylene by release— Description of his apparatus—How the apparatus was filled—The full apparatus with hydraulic press—Liquefactions of nitrogen oxide—Of carbon monoxide and oxygen mixed—Liquefactions of the same separately —His letter of December 2, 1877, to the French Academy—Ljiquefaction of nitrogen—Of hydrogen—Rival claims of Cailletet and Pictet—Mercury stopper method—Manometers—Original methods of testing—Hiffel tower manometer—Carbon dioxide experiments—Mercury pump—High pressure gas reservoir—Ethylene as a refrigerant —Closed cycle method—Accelerated evaporation—Electric conductivity at low temperatures—Comparison of thermometric methods—La Tour’s experiment repeated.

CHAPTER X.—SIGMUND VON WROBLEWSKI AND KARL OLSZEWSKI. - Pages 203-229

Wroblewski’s life--Banishment from his native country—Early scientific work—His association with Olszewski—Study of Cailletet’s methods—Their apparatus—Defective position of the hydrogen thermometer—Liquefactions of oxygen, carbon monoxide and nitrogen—Ethylene data—Solidification of carbon disulphide and alcohol—Determination of the critical pressure and temperature of oxygen—Ljiquefaction of hydrogen—Use of a thermo-electric thermometer—Electric resistance of metals at low temperatures—Two liquids from air—Olszewski’s individual work—Apparatus for producing liquid oxygen in quantity—Comparison of platinum resistance and of hydrogen thermometers—Determination of hydrogen constants.

CHAPTER XI.—JAMES DEWAR. Pages 231-285

Dewar’s life and education—His associates—Controversies with Cxilletet as to priority—Karly liquefaction apparatus—Solid nitrous oxide as a refrigerant—Royal Institution apparatus—Cooling cycles employed—Laboratory apparatus —Vacuum vessels—Air as a heat conveyer—Experiments with incandescent lamps—Reflection of ether waves from vacuum vessel—Keeping power of vacuum vessels—The Dewar vacuum—Its extraordinary perfection—Analogy with population of earth—Experiment in slow diffusion of mercury vapor—Incidental production of vacuum vessels—Elasticity and strength of metals at low temperatures—Apparatus used—Hlongation of metals when stressed at low temperatures—Determination of specific and latent heats of liquefied gases—Gas-jet experiments—Low temperatures thus obtained—Freezing air—Large jet apparatus—Analysis by liquefaction —Ljiquefaction of fluorine—Liquefaction of hydrogen and helium—Experiments to show the intense cold of liquid hydrogen.

CHAPTER XII.—CHARLES E. TRIPLER. Pages 287-296

The life of Charles EK. Tripler—His early experiments with gas motors— Mechanical difficulties encountered—His electrical experiments —Chemistry

*—His work in fine art—Exhibition of his paintings—Return to the investigation of compressed gases—Liquefaction of air—He endeavors to utilize the low grade heat of the universe—Simplicity of his apparatus—The plant —The compressor—General plan of operations—Capacity of his plant— How he transports liquid air--His lectures—Raoul Pictet in Charles E. Tripler’s laboratory.

TABLE OF CONTENTS.

CHAPTER XIII.—THE JOULE-THOMSON EFFECT. Pages 297 -306

First attempts at liquefying gas—Joule and Thomson and their discovery— Coal a cheap chemical—Substitution of mechanical for chemical energy- Sir William Siemens’ regeneration of cold—Self-intensive refrigeration— Negative Joule-Thomson effect—Mathematics of the theory—Conditions of pressure for economical application.

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CHAPTER XIV.—THE LINDE APPARATUS. Pages 307-319

Linde’s apparatus—The simplest form of apparatus—Its operation—Its storing of air at atmospheric pressure—Avoidance of atomization and waste— Subdivision of pressure-drop—Laboratory apparatus—A feature of inefficiency in it—Its power of liquefaction—Continuous oxygen-producing apparatus—Date of Linde’s first successful use of his apparatus.

CHAPTER XV.—THE HAMPSON APPARATUS. Pages 320-324 Hampson’s apparatus—Its general features of construction—The jet and regulating device—Thermal and mechanical advantages—Data of its operation—Use of cylinders of compressed gas instead of pumps—Application of preliminary cooling to the air or gas to be liquefied.

CHAPTER XVI.—EXPERIMENTS WITH LIQUID AIR. Pages 325-337

Experiments with liquid air—Formation of frost on bulbs—Filtering liquid air—Dewar’s bulbs—Liquid air in water—Tin made brittle as glass—India rubber made brittle Descending cloud of vapor—A tumbler made of frozen whisky—Alcohol icicle—Mercury frozen—Frozen mercury hammer— Liquid air as ammunition—Liquid air as basis of an explosive— Burning electric light carbon in liquid air—Burning steel pen in liquid air—Carbon dioxide solidified—Atmospheric air liquefied—Magnetism of oxygen.

CHAPTER XVII.—SoOME OF THE APPLICATIONS OF LOW TEMPERATURES. Pages 338-356

Frigotherapy—The frigorific well—Pictet’s experiment—Effects of the first trial of the system—Medieal-uses of liquid air—Critical point as test of purity of chemicals—Purification of chemicals by low temperature crystallization—Low temperature distillation—Regulation of chemical reactions by cold—Liquid air explosives—The principle of their action—Liquid air in electric power transmission—Liquid air as a reservoir of energy. ad

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LIQUID AIR

ANDY PEs

GHAPTER: Tf.

PHYSICS.

What is liquid air?—The three states of matter: Solid, liquid, and gaseous—Relations of pressure and heat to state assumed by matter—The critical state and its phenomena—Natterer’s tube—Physical units—Space, mass, and time—Force and energy—Conservation of force an abandoned doctrine—Conservation of energy—Work a synonym for development of energy—Waste of energy and entropy—Possibility of utilizing the lower forms of energy of the universe.

A question has often been asked latterly; it is,

*, ‘‘What is liquid air?” The subject has been so much discussed, and so much has been made of it, that it is hard to believe that there isnot some . occult mystery attending it. Liquid air is simply aix which is so cold that it assumes the liquid state. The fact that the question has been so often asked » suggests the need for a thorough answer; for back of it there lies a great region of physics and chemistry, a summary exploration of which in the light of the knowledge of to-day cannot but be interesting. In it are concerned the great doctrine of the conservation of energy, the laws of heat, the three states of matter, and the chemistry of air, and it is not expecting too much of the reader of to-day to hope that the theory of the subject presented within the compass of an hour’s reading will interest him.

The account of the liquefaction of gases includes a period of about one hundred years, and with it is bound up the history of the Royal Institution of London. In its laboratory Faraday worked with bent tubes, liquefying gases and blowing the tubes to pieccs and nearly blinding himself in his efforts. This was half a century ago and more. And now within its walls, with elaborate machinery based upon Pictet’s circuits of 1877, James Dewar, the successor of Faraday, liquefies hydrogen and helium and ends the century’s work.

In Switzerland and France, toward the end of 1877, the beginning of the cnd appeared when oxygen was liquefied. Pictet and Cailletet were the rivals, separated only a few days in their liquefaction of this gas, discovered by Priestly and Lavoisier almost exactly one hundred years betore the date of its reduction to the liquid state.

America was not idle. Tripler working away @ privately, with no institution or association to back him, has surpassed the dreams of the most enthusiastic visionaries and has made liquid air by the barrel, and has sent it all over a wide range of country 4f tin cans.

The long record should not be read until the® answer to the query cited above has been given; the *

LIQUEFACTION OF GASES. II reader should know accurately what liquid air is, what constitutes a gas, what the relations of heat and pressure to state of matter are, and how heat is treated by the modern scientist. |

Matter is generally stated to exist in three forms or states—the solid, liquid and gaseous. An attempt has been made to assert the existence of a fourth state—the ultra-gaseous or radiant state. There is a certain objection, however, to this. The first three states are broadly differentiated. As a rule, there is little question of the form or state being solid, liquid or gaseous, but the ultra-gaseous state is only recognizable by rather refined tests and may perhaps be better considered as the extreme carrying out of the gaseous condition.

Water is the most convenient substance to cite to illustrate the three states. In ice we have solid water. The masses are of fixed contour, and, even if ice is subject to a species of flow, the masses of ice definitely hold their shape. The molecules of solid water are in constant vibration back and forth over the same path, under any conditions of temperature we are familiar with. At the absolute zero this motion would cease. The paths are inconceivably short. Wecannot and probably never will acquire any direct knowledge or sight of these vibrations. All we know is that ice at mundane temperatures is hot. It will be seen that, dropped into liquid air, it makes it boil as if the ice were a red hot poker thrust into it. By the kinetic theory of heat all hot bodies are held to have their molecules in constant vibration. Molecular attraction holds the particles of the ice firmly together in spite of this vibration.

If we apply heat, we diminish this attraction, we increase the repulsive forces, and the ice reaches a temperature where the two opposing forces about balance each other, the attractive ones slightly preponderating. Now there is no longer a powerful set of forces in operation binding the molecules together. They begin to slide about on each other, their vibrations continue with energy, but the paths vary. Amolecule bounces back and forth like a billiard ball, recoiling to right or left from its neighbor, so that sooner or later it travels through the entire mass and néver ceases its travels: } As the molecules slide about without true friction the ice loses all tendency to preserve its shape and falls to pieces, literally speaking. In other words, the ice melts, and we have water-—a representative of the liquid state of matter.

Let us apply more heat. Our water is already shapeless. We have to keep it in a containing vessel. Evenadropof water hanging from the window shutter ona rainy day is held ina little sack of water-film. Later on we shall see what an important bearing the liquid film has inthe manipulation of liquid air. So we put our water in a kettle and heat it. Soon a white cloud issues from the spout, and we may say that we see the steam. If we make such an assertion, it 1s an erroneous One, as the white cloud is really composed of little balls of liquid water, each held in its own little sack of water-film. Asthe kettle boils harder, we find that the white cloud ‘does not begin its existence until it is a few inches from the mouth of the spout, and a space apparently void of all matter intervenes between spout and white cloud. This space is filled with the substance we are in search of; it is occupied by a column of gaseous water or steam rushing out of the spout and as invisible as air itself.

By applying heat to our water, we have made the molecules vibrate through paths many times longer than the old paths; a cubic inch of water gives us approximately a cubic foot of steam. The molecules travel about through the mass with greater rapidity than ever. The mass loses all pretensions to shape or cohesion. A vessel will not hold it unless it is elosedsevcnywuere. dite third state” ot matter is formed—the water exists as a gas.

By refinement of observation and experiment most interesting and captivating views are formed concerning these states of matter. Their individual properties are not so sharply cut off and defined as might be supposed. <A body is said to be solid when it is practically unchanging in the shape imparted to it. But many solids flow under pressure. The suffering “continuous deformation under the action of a continuous force” is not a certain criterion of .a liquid, but it is good enough to define it or identify it by.

A barrel of asphalt opened and thrown on its side in the street seems to be filled with a black solid, yet by the end of the day it will have flowed -and changed shape. A stick of sealing wax supported at its ends slowly and continuously bends. Some authorities consider these as examples of liquids.

A soft jelly pressed by a spoon yields considerably, but, when the pressure ceases, springs back into its original shape. Jelly, therefore, is treated as a solid.

All this seems to cast confusion on. the subject.

14 LIQUID AIRSAND: THe

But nothing very critical hinges on the sharp separation of solid, liquid and gas. It would perhaps be better to assume a continuity of state between solids and liquids, and to consider asphalt, sealing wax and the like as being on the border line. If sealing wax is to be considered a liquid, then lead and most other metals could be considered such; for metals, asa rule, are more or less malleable and ductile, and the qualities of malleability and ductility depend upon the flow of the material composing them.

We are confronted with the old property of nature expressed in the adage, Natura non facit saltum, Nature does not jump. The air we breathe is in the gaseous condition. The water we drink is in the liquid condition. The glass which holds the water is in the solid condition. Yet we can indicate many cases where an intermediate state exists and where a substance cannot well be termed one thing or the other. Even air is not a perfect gas, and hydrogen is an ultra-perfect gas.

For want of correct understanding of such things as these, confusion in ideas results and an obscurity bordering upon complication is introduced into our conception of the laws and system of nature. Thus moist air is generally considered heavier than dry air, presumably because a wet cloth is heavier than a dry one. Popularly, people would say that the air is damp and heavy. Now air is wet because of the mixture with it of another gas, gaseous water or literally steam. Water from rain, from the ground and from the immense cvaporating surface of the leaves of the vegetable world assumes the gaseous form and mixes with the air. The specific gravity of water in the gaseous condition is less than that of air. It is about two-thirds as heavy only. Wet air, therefore,is lighter than dry air. <A balloon would rise better on a dry day than on a wet day, not only because there would be no moisture with which to dampen the cordage and cloth, and thereby increase the weight, but because the dry air is a better floating medium than wet air, because it is heavier.

Wet air is not air soaked like a sponge with water.| It is simply a mixture of dry air with gaseous water, The truth is here far simpler than fiction.

The sequence followed by a substance in passe from state to state is not always the same, as a solid on heating is often vaporized or gasified directly without passing into the liquid state at all. This occurs in slow vaporization very often. Thus ice in the open air below the freezing temperature wastes away by volatilization and is gasified slowly, without liquefying, and contributes water vapor to the air, although far below the solidifying temperature. fodine volatilizes in the same way, and those who have used camphor or naphthaline for preserving clothes from moths have observed the same mysterious diminishing of the lumps of preservative used. In druggists’ windows the shrinkage of camphor there exposed is sometimes quite striking. Now it is less often exposed than formerly, as naphthaline has largely supplanted it in the trade.

Carbon dioxide, the gas which escapes from soda water and other effervescent beverages, when subjected to cold and pressure, liquefies. When the pressure is released and it is allowed to escape into the open air, it solidifies and produces a true carbon dioxide snow. This snow exhibits surprising permanency, disappearing quite slowly in the open air- In disappearing it evaporates and produces gas directly without passing through the intermediate liquid state.

Such direct transition from a solid into a gaseous state is termed often sublimation ; an expression, perhaps, too limiting, covers the extreme case where a solid on application of heat sublimes vigorously before melting. It is to the effect that the substance boils at a lower temperature than that at which it liquehes— that the temperature of boiling is lower than that of liquefaction. The idea of a solid boiling seems rather odd.

It is not only the change of temperature which brings about change of state. Change of pressure allects it greatly. The greater the pressurcierae higher is the temperature at which a liquid becomes agas. A gas just hot enough to hold that form may, under some conditions, be converted into a liquid by applying pressure, without any change in temperature being required to effect the change of state. This, too, is very natural. Foralquid, under ordinary conditions, being of smaller volume than the same molecules gasified, is naturally brought to the liquid condition by mechanical reduction of volume as well as by thermai reduction.

Pressure will not always do it, and by combining the effects of great heat and great pressure, conditions foreign to the ordinary status of matter are brought into existence which complicate the problem. Heat is the great and all-controlling agent. Heat is what establishes the critical state, and pressure is quite a

. LIQUEFACTION OF GASES. Leys secondary matter. [or every gas there is a critical temperature and a critical pressure, but the latter is quite a subsidiary thing, and is not critical in the full sense that the temperature is.

Pressure tends to liquefy a solid, if the latter grows smaller on liquefaction. So that it is quite conceivable that a point might be reached where pressure would help to convert aliquid into a gas. As such a phenomenon, uncomplicated by other factors (page 24), has never been observed, it is better to set it aside and consider pressure as invariably on the side of cold in liquefying gases.

A gas must be pictured to the imagination as a very active thing. In a room full of air the molecules are moving about rapidly, colliding with each other, and bounding about like billiard balls. We know that, if we turn on the gas without lighting it, in a very few minutes the odor of gas will be perceived in all parts of the room. This can only be so because in those few minutes the gas has penetrated every corner.. Its molecules have traveled about until some of them are everywhere present, and the activity of their operations may be judged by the amount of gas and the size of the room. An ordinary burner delivers one cubic foot of gas in about ten minutes, and in that time a room of over a thousand times that volume would be pervaded with it. Hence it will be seen how active the molecules of a gas are. |

If there were no wind, if the air were absolutely motionless, its molecules would be as active as ever in their own spheres. The air which on one day would be in America would be scattered the next day far and wide, and its molecules would find their way sooner or later all over the world.

The same is true in a lesser degree of liquids: The water of a tideless, currentless lake is in molecular motion. The water which beats against the coast of America is in constant process of change, and its molecules are changing and moving about all the time. Sooner or later some of them will be in the waves which break upon the Irish cliffs and English beaches, nearly three thousand miles away. They would travel thus were there no oceanic currents and no waves.

This molecular travel is termed diffusion.

We have seen that the motions of the molecules _ are increased in vigor by heat, that, if heat is withdrawn, they decrease in intensity. The obvious question arises, What would happen if there were no heat? The molecular motions would cease, and molecular death would ensue.

The passage of a substance from the solid to the liquid state or from the liquid to the gaseous state involves generally a change in dimension or size, and in the case of many substances the liquid state is the one of smallest size. Thisis the case with water. In round numbers, a pint of water gives nearly a pint and two ounces of ice, if it freezes, and if converted into steam, gives nearly two hundred gallons. We are most concerned with the liquid and gaseous states, and under ordinary circumstances there is a very great reduction of volume incident to the passage of a substance from the gaseous to the liquid State.

It follows that, to produce liquefaction of a gas, the first thing we should naturally try to do would be to reduce it in volume, and the simplest way to do this would be by pressure. Early experimenters adopted this plan. Natterer attained pressures of many thousand pounds to the square inch, yet gases compressed to a small fraction of their volume staid gases and refused to yield.

At last Andrews, of Belfast, made his classic discoveries, and the existence of a critical state was established. This stateis very easy to understand. It depends on the fact that for every gas there is a temperature called its critical temperature, and a corresponding pressure called the critical pressure. When hotter than this temperature, no compression, however great, will liquefy it. Below this temperature, a compression easy of attainment is enough to effect the change to the liquid state.

The critical pressure is a term which is often misunderstood. It may be said that the pressure is never Critical in the full sense in which temperature becomes critical. There is no pressure which can be defined as so low that liquefaction would be impossible init. There is a theoretical point of cold never yet attained, which is termed the absolute zero. At this point heat ceases, the molecules no longer vibrate, and absolute cold exists. If a body were reduced to the absolute zero, where the motions of the molecules cease, pressure would be without effect upon it, as its only power is to shorten the paths of vibration of the molecules. The term critical pressure is used to describe the pressure required to liquefy a gas when it is at the critical temperature.

When a gas is at the critical temperature and at the critical pressure also, the least increase of pressure or decrease of temperature will convert it into a liquid. When in this condition, ready to bea gas or a liquid, it is said to be in the critical state.

It will be seen how very well the term critical state applies when a substance is at the critical pressure and temperature, the least change will so profoundly modify its state.

A law relating to the critical state is known as La Tour’s law, and expresses very succinctly the phenomenon of the critical temperature. It is the following : |

There is for every vaporizable liquid a certain temperature and pressure at which it may be converted into the aeriform state in the same space occupied by the liquid.

It will be evident how strikingly this puts the fact. that, above a certain temperature, a gas can be squeezed down to the volume of its mass as a liquid without liquefying. If a gas rigorously followed Mariotte’s law and changed in volume in inverse proportion to the pressure exerted upon it, and if pressure sufficient to reduce it to the absolute volume, as it may be termed, or the volume it should have at the absolute zero, were exerted upon it, it is hard to say what would become of it.

The condition of a substance in the neighborhood of the critical state is sometimes termed the intermediate state. The expressions are almost synonymous—the first is the more abstract, the latter the more concrete expression.

The reduction from the gaseous to the liquid state is usually a reduction of volume. <A cubic foot of

LIQUEFACTION OF GASES. BY steam gives about a cubic inch of water; eight hundred cubic inches of ordinary air give about a cubic inch of liquid air. But owing to the phenomenon of the critical temperature, or, what is the same thing, to La Tour’s law, this is not always true. The existence of a gas of no greater volume than the liquid it could be converted into is asort of scientific riddle. It has its counterpart in the inexplicably great power of expansion by heat possessed by some liquefied gases without departure from the liquid state.

The passage of a substance from the liquid to the gaseous state is marked by a change of appearance A liquid has always a defined limit. It lies in the containing vessel and its upper surface forms a visible boundary. If the vessel is of large diameter, the surface is level and flat, except along the edges, where it curves up ordown alittle. If the diameter is small, it curves throughout its whole extent, forming a little cup or a little hill, as the case may be.