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this theory has been abandoned, and scientists now tell us that heat is a form of motion like the vibrations of a musical string. Whatever may be its real nature we know that nothing in this world of ours could be without it. And we have learned that it exists in two very different conditions; that each of these conditions is governed by a different set of laws, and gives out widely different results. For aught we know, every form of energy in the universe may depend upon this one tremendous activity.

The two conditions that we know are I called sensible and latent heat. The first of these is the most palpable and familiar of anything in nature. It is expressed by the words "temperature," "heat," and "cold." It manifests itself to the touch without any special organ for its perception. For the perception of light we require, as a special aid, the most delicate mechanical device ever turned out of nature's wonderful workshop. Sound, taste, and smell all come to us by the aid of almost equally high-class mechanical devices. Not so with heat. Every inch of the animal organism is keenly alive to sensible or free heat, whether it is given or taken away. Not only our bodies, but every material substance is more or less sensitive to this influence. It is only this condition of heat that we can measure by the thermometer.

Latent heat, or stored heat, as it might better be termed, is the heat of formation, without which no gas or liquid can exist. It cannot be measured by the thermometer or detected by any of the senses, but is a fact which, like the rotundity of the earth and the distance of the stars, must be accepted against the evidence of the senses. Until late in the last century nothing was known of this wonderful hidden energy. The great interest then aroused by the introduction of steam as a motive power directed scientific thought and investigation, as never before, to the nature of heat. It was then soon learned that gases and liquids contain, besides the ponderable matter of which they are composed, a large amount of hidden heat with which they cannot part without changing their forms. This rule was, however, thought to have exceptions, and it was believed that air, at least, could maintain its gaseous form without the aid of heat. Lavoisier, the father of modern chemistry, then declared that the difference between

But

gases, liquids, and solids was only a difference of heat, a statement that no scientist will now deny.

Although neither the thermometer nor the sense of touch can detect the presence of latent heat, yet its existence is easily proved. It can be accurately measured; not measured by degrees, but by units of quantity. The amount of heat required to raise the temperature of a pound of water one degree is taken as the heat unit. If we then take one pound of water at the freezing temperature and another pound heated to 100 degrees above this point, and mix the two together, we have two pounds 50 degrees above freezing. Or we may mix the one pound of hot water with three of the cold and have four pounds 25 degrees above. In these cases we obtain exactly the results that we should anticipate, because we are dealing with manifest heat. But if in place of the pound of cold water we take a pound of ice, at the same temperature, and mix it with a pound of the hot water, the conditions are changed. We shall then find that in place of two pounds 50 degrees above freezing, the mixture has dropped to the freezing-point; and that in some way we have lost every vestige of the 100 units of heat that was in the pound of hot water. And to add still more to the mystery, we shall find that almost one third of the ice remains still unmelted.

The explanation is, that in order to assume the liquid form a pound of ice must appropriate to its private use 142 units of heat. In this case there was but 100 in reach. Hence, but a little over two thirds of the ice was liquefied.

But the most striking example of latent heat is found in the formation of steam from boiling water. To raise the temperature of a pound of water from the freezing to the boiling point requires but 180 units of heat. But then to convert it into steam, without making it any hotter, requires 965 units. This heat is all stored as latent heat, but, by condensing the steam in a larger volume of cold water, it may be recovered as sensible heat.

The same law governs the conditions of atmosphere and liquid air, except that the degree of heat required to maintain the atmosphere in its aëriform state is much lower than that required to maintain steam in that condition. Air is the vapor of liquid air, and is maintained as a permanent gas just as steam would be in a

climate a few hundred degrees hotter than ours.

It was long ago observed that when a gas was compressed so as greatly to reduce its volume it became hot. This was called the heat of compression, and, strangely enough, was believed to be generated by the act of compression. But it is now understood that the rise in temperature is not caused by actual increase of heat, but by concentrating the manifest heat of a large volume into a small space. Experiments that proved this also suggested that the discovery could be turned to profit by cooling the heated gas down while under pressure, and then allowing it again to expand to its former volume. Experiments soon proved that a gas could be compressed and cooled, and then allowed to expand to its original volume, so that in the operation the temperature could be made to drop 200 degrees.

From this discovery have been developed all the methods of artificial refrigeration, including the manufacture of ice and cold storage. And this principle of cooling by expansion is the basis of every system of producing liquid air. Mr. Tripler, of New York, the most successful manufacturer of this new product, employs the air itself as the expanding agent to chill it down to the terrific degree of cold required. But this very low temperature is not obtained by a single expansion, but by a principle of accumulation, said to have been first employed by Solvay in 1885. The following statement will illustrate the principle though not the construction of any machine.

Air compressed to about 2,500 pounds to the inch, and cooled by being passed in pipes through a bath of running water while thus compressed, is carried through coils. of pipes to a receiver several feet away. Into this it is discharged through pinholes not large enough to reduce the pressure in the coils. As fast as set free in the receiver, the air expands to nearly its original volume, falling in temperature perhaps a hundred degrees or more. From the receiver the expanded air flows back through a large jacket that surrounds the incoming coils, and returns to the compressor, where it is again compressed, cooled, returned through the coils, and discharged at the pinholes. Thus it will be seen that as soon as the operation is started the coils are enveloped in an intensely cold atmosphere that

greedily snatches heat from every inch that it touches. In this condition the air in the coils is every moment growing colder, and is thus discharged from the pinholes at a temperature more reduced, and filling the jacket with expanded air ever more and more eager to devour the last remaining vestige of heat in the coils. This cannot long continue. The cold becomes so intense that the expanding air gives up its latent heat, forms a cloud, and rains down a liquid shower to the bottom of the receiver. From this moment the condenser must draw a part of its supply from the outside, as every drop of the liquid takes up 750 times its volume of the expanded air.

Since liquid air has come to the front, with a promise of becoming an important factor in the world's progress, its history assumes an additional importance. The limits of this article will, however, admit of only a brief mention of some of the important experiments and discoveries that have led up to the final success.

Near the end of the last century, Von Marum, a Dutch physicist, while experimenting with ammonia gas, accidentally reduced it to a liquid. This happened when the gas was under a heavy pressure; and, although not what was looked for, revealed a possibility that had perhaps not been thought of before. It appears, however, to have had but little effect on the scientific thought of the world. It was regarded rather as a curious fact than as a great discovery. But the experiment was frequently repeated and varied in method; and the fact was developed that ammonia gas would liquefy at a temperature of about 54 degrees below zero without great pressure.

A little later Gaspard Monge, a French physicist, by combining cold with pressure, liquefied sulphuric-acid gas. But by far the most careful, extended, and persistent experiments in liquefying gases were conducted by Michael Faraday, first as assistant to Sir Humphry Davy, and afterward on his own account, Faraday liquefied many gases that had defied the efforts of other experimenters; but still there were several that he could not liquefy. Illuminating gas, swamp gas, carbon dioxide, nitrogen, and hydrogen resisted all his efforts. These were then called permanent gases.

But experimenters would not be satisfied with this. Others followed Faraday,

and the struggle went on. The effect of greatly increased pressure was tried. Air in elastic vessels was sunk in the ocean to a depth where the pressure corresponded to four hundred atmospheres, but no signs of liquid appeared. Berthelot succeeded in bringing a pressure of seven hundred and eighty atmospheres on some oxygen in a strong glass tube. At this point the tube burst, with no sign of liquefaction. Again, in another experiment, the pressure was increased to one thousand atmospheres with no results. Natterer, a physicist of Vienna, subjected hydrogen and nitrogen to a pressure of thirty-six hundred atmospheres, yet the gases refused to liquefy.

Speaking of this last experiment, Ernesto Mancini recently remarked:

«The interesting part of the experiment was that nitrogen under this tremendous pressure attained a density greater than water, while, as verified later on, liquid nitrogen is much lighter than water.»

These experiments appeared to prove that pressure alone could not liquefy the permanent gases. The theory was then suggested and afterwards accepted as a law that there is a point of temperature, different in different gases, above which it is impossible for them to assume the liquid form. This is now called the critical point, and has been determined in most of the gases.

This law having been recognized, experiments were directed toward the production of lower temperatures. The efforts in this direction were productive of great and valuable results. They brought to the notice of inventors and practical men the principle of cooling by expansion, out of which have grown all practical methods of producing artificial cold.

By the employment of the expansion principle, Callette, in 1879, produced the first liquid air at a very great cost. In 1885 Solvay successfully employed the principle of accumulation, as already described. But these were only laboratory experiments, producing but a few drops of the liquid. In 1892 Professor James Dewar produced liquid air in some quantities by this process, and actually froze it into ice.

But the first really practical machine for liquefying air was exhibited by Dr. Carl Linde, at Munich, in 1895. His machine was a better development of the accumulative principle, and produced several litres of liquid air per hour. It remained,

however, for Mr. Charles E. Tripler, of New York, to construct a machine that will produce this wonderful liquid on a commercial scale. It is understood that a company has been chartered to manufacture and introduce Mr. Tripler's machine, which, it is said, will produce liquid air in any quantity and at almost no cost, and, what is more startling, that liquid air can be used as the motive power to generate itself, and will produce a much larger quantity than is consumed in the production. Then, of course, this surplus can be used to drive other machinery, and may be increased so as to supply motive power to drive the machinery of the world. A writer in "McClure's Magazine » for March quotes Mr. Tripler as saying:

"I have actually made about ten gallons of liquid air in my liquefier by the use of about three gallons in my engine. There is therefore a surplusage of seven gallons that has cost me nothing, and which I can use elsewhere as power.»

That the expansive power of liquid air is tremendous, and that it may be used as a motive power, is undoubtedly true. But that it can be made thus self-generating so as to multiply itself indefinitely, like vegetable and animal life, is a proposition that no scientist can entertain.

It appears, however, not only possible, but altogether probable, that this child of the last decade of this wonderful century will revolutionize and greatly cheapen the world's motive power. This result must not, however, be looked for in the direction suggested by Mr. Tripler. He leads us beyond the realm of science, where our feet find no support but faith and fancy. But there is solid ground within the domain of dynamic law on which we may now hope for an improvement of economy in the production of power, amounting to several hundred per cent.

Here is a mine of great richness to be worked by our inventors, a mine that ought to yield much to benefit mankind. But this benefit cannot be obtained by simply expanding the cold liquid to seven hundred and fifty times its bulk of air. This action can do no more than give back the energy expended in its liquefaction. But there is an immense power in this liquid that is not brought out at all by its expansion. This is its power to promote combustion.

It is well known that in the use of steam we lose about nine tenths of the energy

that is stored in the fuel. Liquid air may enable us to utilize most of this loss so as to multiply six or eight times the efficiency of a ton of coal.

The atmosphere is almost wholly composed of oxygen and nitrogen. About twenty-three per cent of the weight is oxygen. In liquid air the percentage of oxygen is greater because it is the first to liquefy; and the nitrogen, being most volatile, is first to evaporate and escape. Oxygen is the vital principle of the air, without which there could be no life and no fire, no growth and no decay. In an atmosphere of only nitrogen nothing could live and nothing would burn. In an atmosphere of oxygen, almost every substance on the earth would burn with fervent heat.

When liquid air is allowed to stand in an open vessel and boil gently, it grows

richer in oxygen, as it is the nitrogen that first evaporates. If this is continued until the liquid contains fifty per cent of oxygen, then, if mixed with a substance containing carbon, as cotton, petroleum, or powdered coal, and set on fire, it explodes with great force. In this way it could be automatically served to a furnace within a boiler so as to obtain perfect combustion and add the whole energy of the fuel to the expansive force of the air. This product, if worked in an engine, like steam, might give us much more work from a ton of coal than we now obtain.

It may require a decade or possibly a century of experiment and invention to obtain a thorough knowledge and perfect control of this new power. But there are few scientists who will deny that liquid air contains potency and promise.

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"FAUST" AND "THE GOLDEN LEGEND"

O STUDY the "Golden Legend" in order to compare or contrast it with a creation like "Faust" is to receive much the same impression as that gained at the Columbian Exposition when we stood before two statues, one of the Venus of Milo in her perfection, the other constructed from measurements taken to show the form of the average American woman, so unlike, in its frailty and checked development, to the ideal womanbody, and yet like enough to show that it was conceived in the same spirit and that in it lay possibilities of suppleness, strength, and grace which, realized, would make it, too, a thing of beauty. So the "Golden Legend," though infinitely little and meagre in both its poetry and philosophy when measured by "Faust" as a standard, yet shows plainly that Longfellow has basked in the warmth of Goethe's divine fire and inspiration, and in many ways it may be compared with the masterpiece of the modern Olympian.

In comparing them, however, we can never forget the relative merits of the two poems. One is the greatest work of a poet, so great that we place him among the immortals who come to us but rarely, to leave behind them works the beauty and worth of which are only increased by time. The other is the minor

work of a poet who sang so sweetly, so simply, so gently, that he touched the hearts of the people, but whom his most ardent admirer could never think of placing near, much less with, the great ones,

Homer, Dante, Shakespeare, Goethe. The one is a world poem so great, so universal in its significance, that it typifies the spirit and tendency of all modern thought, and is instinctively claimed by all moderns, whether Germans or not. The other is little known, even by those who, through love and recognition, have adopted as their own, "Hiawatha," "Miles Standish," "Evangeline," and the innumerable shorter poems of Longfellow. The one is a whole of parts which to the critics seem too loosely connected; the other, the part of a whole against which the same charge-lack of external unity— may be brought.

And yet, though occupying such different positions in the world of readers, each poem has filled much the same place in the thought of its creator. Each has dominated the literary life of its writer. Each has been the loved and cherished idea of the poet who breathed into it life. As sixty years passed between the conception of "Faust" in the brain of the proud, passionate boy poet and its completion by the serene old man to whom

life had given wisdom, so in Longfellow's mind the conception of a great poem, the theme of which would be the various aspects of Christendom in the apostolic times, the middle ages, and the present, lay for many years, formless, shapeless. In the latter's diary for November, 1841, we find a note stating his intention of writing such a poem, and in the following summer appears this memorandum:

CHRISTUS: a dramatic poem in three parts.
PART I. The Time of Christ (Hope).
The Middle Ages (Faith).
PART 3. The Present (Charity).

PART 2.

It was not until thirty-two years had elapsed that the poem as it now stands, and as he from the beginning planned it, appeared. During these thirty-two years the subject of the trilogy was never long absent from him, and though he published each of its three parts separately, yet he never once lost sight of the one of which these three were parts, of the unity into which this trinity was to resolve itself.

In the same way the composition of "Faust" and the life of Goethe had an intimate connection and ran in parallel lines. It is his biography, though not, however, that it tells the external events of his life, but in the sense that it mirrors his spiritual development. When a boy he had seen a puppet-play derived from Marlowe's "Dr. Faustus," and had been so impressed by it that he decided to write a poem of Faust, a drama which would present the life of man showing his progress from heaven across the world to hell, and back through the world to heaven again. Years passed, and in 1790 appeared "Faust, a Fragment," showing by its name that it was incomplete. In 1808 the First Part, as we now have it, was published, but not until 1832 was the Second Part finished. So the poem began with his spiritual life and closed with his bodily. Without the Second Part, which many critics denounce and think superfluous, the work would have been terribly incomplete; for, though not finished until his old age, the design for it was drawn, and some of the figures in it were nearly completed, in the master's best days. If it had been omitted, the poem would not have been the perfect whole which it is, "spanning Goethe's long and eventful career like a rainbow bridge, revealing in brilliant colors the tumultuous passion of his youth, the struggles and aspirations of

his manhood, and the wisdom of his old age.»

The Faust legend itself is as old as the human race. The idea of a man necessarily a strong one-storming against his limitations; tempted, through the pride of his intellect and his eagerness to exert its powers, to scale the unattainable heights of heaven and gain by sheer strength and pride of will the celestial fires - is embodied in more than one myth. It was the Faust spirit which led Prometheus, through recognition of the god in him, to declare his independence of the gods without.

It was the Faust spirit giving to the first pair a "desire to be as gods, knowing good and evil," that led to the Fall. The Faust of Goethe is the greatest of them all, as he embodies the thought of our age,an age of spiritual doubt and scepticism as opposed to an age of faith and reverence to things traditional.

This Faust has also an historical basis. We are told that during the sixteenth century there was at Wittenberg where Hamlet was educated, and where, judging by these two illustrious ones, seeds of doubt were sown a Dr. Faustus. He had attained great renown as a student, being deeply versed in philosophy, astrology, and magic. At that time demonology and angelography were recognized sciences, and the people believed that his skill and learning were derived from Satan, to whom he had promised his soul in payment for knowledge. This is the Dr. Faustus depicted by Shakespeare's predecessor, Marlowe, and it was from this germ that Goethe's Faust came. But as

it is always miraculous that from a thing so small and brown as an acorn can come the magnificent strength and sweep and sway of the oak, so it is equally wonderful to see how the vivifying thought and imagination of a great poet can take a theme already used or abused by many and make it his own forever.

Longfellow's poem takes us farther back in time. It is based on the story of Dr. Arme Heinrich, as told by a minnesinger of the twelfth century. The belief-in itself so poetic-in the power of a pure woman to heal, strengthen, and create, goes still further back to the very beginnings of time, and in one form or another is to be found in the folk-lore of almost all peoples. An old Oriental legend has it that the touch of a maiden's hand causes the trees to bloom. Agamemnon

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