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Liebig's Chemical Letters

LETTER II

   

My dear Sir,

    In my former letter I reminded you that threeof the supposed elements of the ancients represent the forms or state in which allthe ponderable matter of our globe exists; I would now observe, that no substancepossesses absolutely any one of those conditions; that modern chemistry recognisesnothing unchangeably solid, liquid, or aeriform: means have been devised for effectinga change of state in almost every known substance. Platinum, alumina, and rock crystal,it is true, cannot be liquified by the most intense heat of our furnaces, but theymelt like wax before the flame of the oxy-hydrogen blowpipe. On the other hand, ofthe twenty-eight gaseous bodies with which we are acquainted, twenty-five may bereduced to a liquid state, and one into a solid. Probably, ere long, similar changesof condition will be extended to every form of matter.

    There are many things relating to this condensationof the gases worthy of your attention. Most aeriform bodies, when subjected to compression,are made to occupy a space which diminishes in the exact ratio of the increase ofthe compressing force. Very generally, under a force double or triple of the ordinaryatmospheric pressure, they become one half or one third their former volume. Thiswas a long time considered to be a law, and known as the law of Marriotte; but amore accurate study of the subject has demonstrated that this law is by no meansof general application. The volume of certain gases does not decrease in the ratioof the increase of the force used to compress them, but in some, a diminution oftheir bulk takes place in a far greater degree as the pressure increases.

    Again, if ammoniacal gas is reduced by a compressingforce to one-sixth of its volume, or carbonic acid is reduced to one thirty-sixth,a portion of them loses entirely the form of a gas, and becomes a liquid, which,when the pressure is withdrawn, assumes again in an instant its gaseous state - anotherdeviation from the law of Marriotte.

    Our process for reducing gases into fluids isof admirable simplicity. A simple bent tube, or a reduction of temperature by artificialmeans, have superseded the powerful compressing machines of the early experimenters.

    The cyanuret of mercury, when heated in an openglass tube, is resolved into cyanogen gas and metallic mercury; if this substanceis heated in a tube hermetically sealed, the decomposition occurs as before, butthe gas, unable to escape, and shut up in a space several hundred times smaller thanit would occupy as gas under the ordinary atmospheric pressure, becomes a fluid inthat part of the tube which is kept cool.

    When sulphuric acid is poured upon limestonein an open vessel, carbonic acid escapes with effervescence as a gas, but if thedecomposition is effected in a strong, close, and suitable vessel of iron, we obtainthe carbonic acid in the state of liquid. In this manner it may be obtained in considerablequantities, even many pounds weight. Carbonic acid is separated from other bodieswith which it is combined as a fluid under a pressure of thirty-six atmospheres.

    The curious properties of fluid carbonic acidare now generally known. When a small quantity is permitted to escape into the atmosphere,it assumes its gaseous state with extraordinary rapidity, and deprives the remainingfluid of caloric so rapidly that it congeals into a white crystalline mass like snow:at first, indeed, it was thought to be really snow, but upon examination it provedto be pure frozen carbonic acid. This solid, contrary to expectation, exercises onlya feeble pressure upon the surrounding medium. The fluid acid inclosed in a glasstube rushes at once, when opened, into a gaseous state, with an explosion which shattersthe tube into fragments; but solid carbonic acid can be handled without producingany other effect than a feeling of intense cold. The particles of the carbonic acidbeing so closely approximated in the solid, the whole force of cohesive attraction(which in the fluid is weak) becomes exerted, and opposes its tendency to assumeits gaseous state; but as it receives heat from surrounding bodies, it passes intogas gradually and without violence. The transition of solid carbonic acid into gasdeprives all around it of caloric so rapidly and to so great an extent, that a degreeof cold is produced immeasurably great, the greatest indeed known. Ten, twenty, ormore pounds weight of mercury, brought into contact with a mixture of ether and solidcarbonic acid, becomes in a few moments firm and malleable. This, however, cannotbe accomplished without considerable danger. A melancholy accident occurred at Paris,which will probably prevent for the future the formation of solid carbonic acid inthese large quantities, and deprive the next generation of the gratification of witnessingthese curious experiments. Just before the commencement of the lecture in the Laboratoryof the Polytechnic School, an iron cylinder, two feet and a half long and one footin diameter, in which carbonic acid had been developed for experiment before theclass, burst, and its fragments were scattered about with the most tremendous force;it cut off both the legs of the assistant and killed him on the spot. This vessel,formed of the strongest cast-iron, and shaped like a cannon, had often been employedto exhibit experiments in the presence of the students. We can scarcely think, withoutshuddering, of the dreadful calamity such an explosion would have occasioned in ahall filled with spectators.

    When we had ascertained the fact of gases becomingfluid under the influence of cold or pressure, a curious property possessed by charcoal,that of absorbing gas to the extent of many times its volume, - ten, twenty, or evenas in the case of ammoniacal gas or muriatic acid gas, eighty or ninety fold, - whichhad been long known, no longer remained a mystery. Some gases are absorbed and condensedwithin the pores of the charcoal, into a space several hundred times smaller thanthey before occupied; and there is now no doubt they there become fluid, or assumea solid state. As in a thousand other instances, chemical action here supplants mechanicalforces. Adhesion or heterogeneous attraction, as it is termed, acquired by this discoverya more extended meaning; it had never before been thought of as a cause of changeof state in matter; but it is now evident that a gas adheres to the surface of asolid body by the same force which condenses it into a liquid.

    The smallest amount of a gas, - atmosphericair for instance, - can be compressed into a space a thousand times smaller by meremechanical pressure, and then its bulk must be to the least measurable surface ofa solid body, as a grain of sand to a mountain. By the mere effect of mass, - theforce of gravity, - gaseous molecules are attracted by solids and adhere to theirsurfaces; and when to this physical force is added the feeblest chemical affinity,the liquifiable gases cannot retain their gaseous state. The amount of air condensedby these forces upon a square inch of surface is certainly not measurable; but whena solid body, presenting several hundred square feet of surface within the spaceof a cubic inch, is brought into a limited volume of gas, we may understand why thatvolume is diminished, why all gases without exception are absorbed. A cubic inchof charcoal must have, at the lowest computation, a surface of one hundred squarefeet. This property of absorbing gases varies with different kinds of charcoal: itis possessed in a higher degree by those containing the most pores, i.e. where thepores are finer; and in a lower degree in the more spongy kinds, i.e. where the poresare larger.

    In this manner every porous body - rocks, stones,the clods of the fields, &c., - imbibe air, and therefore oxygen; the smallestsolid molecule is thus surrounded by its own atmosphere of condensed oxygen; andif in their vicinity other bodies exist which have an affinity for oxygen, a combinationis effected. When, for instance, carbon and hydrogen are thus present, they are convertedinto nourishment for vegetables, - into carbonic acid and water. The developmentof heat when air is imbibed, and the production of steam when the earth is moistenedby rain, are acknowledged to be consequences of this condensation by the action ofsurfaces.

    But the most remarkable and interesting caseof this kind of action is the imbibition of oxygen by metallic platinum. This metal,when massive, is of a lustrous white colour, but it may be brought, by separatingit from its solutions, into so finely divided a state, that its particles no longerreflect light, and it forms a powder as black as soot. In this condition it absorbseight hundred times its volume of oxygen gas, and this oxygen must be contained withinit in a state of condensation very like that of fluid water.

    When gases are thus condensed, i.e. their particlesmade to approximate in this extraordinary manner, their properties can be palpablyshown. Their chemical actions become apparent as their physical characteristic disappears.The latter consists in the continual tendency of their particles to separate fromeach other; and it is easy to imagine that this elasticity of gaseous bodies is theprincipal impediment to the operation of their chemical force; for this becomes moreenergetic as their particles approximate. In that state in which they exist withinthe pores or upon the surface of solid bodies, their repulsion ceases, and theirwhole chemical action is exerted. Thus combinations which oxygen cannot enter into,decompositions which it cannot effect while in the state of gas, take place withthe greatest facility in the pores of platinum containing condensed oxygen. Whena jet of hydrogen gas, for instance, is thrown upon spongy platinum, it combineswith the oxygen condensed in the interior of the mass; at their point of contactwater is formed, and as the immediate consequence heat is evolved; the platinum becomesred hot and the gas is inflamed. If we interrupt the current of the gas, the poresof the platinum become instantaneously filled again with oxygen; and the same phenomenoncan be repeated a second time, and so on interminably.

    In finely pulverised platinum, and even in spongyplatinum, we therefore possess a perpetuum mobile - a mechanism like a watch whichruns out and winds itself up - a force which is never exhausted - competent to produceeffects of the most powerful kind, and self-renewed ad infinitum.

    Many phenomena, formerly inexplicable, are satisfactorilyexplained by these recently discovered properties of porous bodies. The metamorphosisof alcohol into acetic acid, by the process known as the quick vinegar manufacture,depends upon principles, at a knowledge of which we have arrived by a careful studyof these properties.



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