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Liebig's Chemical Letters

LETTER V

   

My dear Sir,

    Until very recently it was supposed that thephysical qualities of bodies, i.e. hardness, colour, density, transparency, &c.,and still more their chemical properties, must depend upon the nature of their elements,or upon their composition. It was tacitly received as a principle, that two bodiescontaining the same elements in the same proportion, must of necessity possess thesame properties. We could not imagine an exact identity of composition giving riseto two bodies entirely different in their sensible appearance and chemical relations.The most ingenious philosophers entertained the opinion that chemical combinationis an inter-penetration of the particles of different kinds of matter, and that allmatter is susceptible of infinite division. This has proved to be altogether a mistake.If matter were infinitely divisible in this sense, its particles must be imponderable,and a million of such molecules could not weigh more than an infinitely small one.But the particles of that imponderable matter, which, striking upon the retina, giveus the sensation of light, are not in a mathematical sense infinitely small.

    Inter-penetration of elements in the productionof a chemical compound, supposes two distinct bodies, A and B, to occupy one andthe same space at the same time. If this were so, different properties could notconsist with an equal and identical composition.

    That hypothesis, however, has shared the fateof innumerable imaginative explanations of natural phenomena, in which our predecessorsindulged. They have now no advocate. The force of truth, dependent upon observation,is irresistible. A great many substances have been discovered amongst organic bodies,composed of the same elements in the same relative proportions, and yet exhibitingphysical and chemical properties perfectly distinct one from another. To such substancesthe term Isomeric (from åo equal and æîopart) is applied. A great class of bodies, known as the volatile oils, oil of turpentine,essence of lemons, oil of balsam of copaiba, oil of rosemary, oil of juniper, andmany others, differing widely from each other in their odour, in their medicinaleffects, in their boiling point, in their specific gravity, &c., are exactlyidentical in composition, - they contain the same elements, carbon and hydrogen,in the same proportions.

    How admirably simple does the chemistry of organicnature present itself to us from this point of view! An extraordinary variety ofcompound bodies produced with equal weights of two elements! and how wide their dissimilarity!The crystallised part of the oil of roses, the delicious fragrance of which is sowell known, a solid at ordinary temperatures, although readily volatile, is a compoundbody containing exactly the same elements, and in the same proportions, as the gaswe employ for lighting our streets; and, in short, the same elements, in the samerelative quantities, are found in a dozen other compounds, all differing essentiallyin their physical and chemical properties.

    These remarkable truths, so highly importantin their applications, were not received and admitted as sufficiently established,without abundant proofs. Many examples have long been known where the analysis oftwo different bodies gave the same composition; but such cases were regarded as doubtful:at any rate, they were isolated observations, homeless in the realms of science:until, at length, examples were discovered of two or more bodies whose absolute identityof composition, with totally distinct properties, could be demonstrated in a moreobvious and conclusive manner than by mere analysis; that is, they can be convertedand reconverted into each other without addition and without subtraction.

    In cyanuric acid, hydrated cyanic acid, andcyamelide, we have three such isomeric compounds.

    Cyanuric acid is crystalline, soluble in water,and capable of forming salts with metallic oxides.

    Hydrated cyanic acid is a volatile and highlyblistering fluid, which cannot be brought into contact with water without being instantaneouslydecomposed.

    Cyamelide is a white substance very like porcelain,absolutely insoluble in water.

    Now if we place the first, - cyanuric acid,- in a vessel hermetically sealed, and apply a high degree of heat, it is convertedby its influence into hydrated cyanic acid; and, then, if this is kept for some timeat the common temperature, it passes into cyamelide, no other element being present.And, again inversely, cyamelide can be converted into cyanuric acid and hydratedcyanic acid.

    We have three other bodies which pass throughsimilar changes, in aldehyde, metaldehyde, and etaldehyde; and, again two, in ureaand cyanuret of ammonia. Further, 100 parts of aldehyde hydrated butyric acid andacetic ether contain the same elements in the same proportion. Thus one substancemay be converted into another without addition or subtraction, and without the participationof any foreign bodies in the change.

    The doctrine that matter is not infinitely divisible,but on the contrary, consists of atoms incapable of further division, alone furnishesus with a satisfactory explanation of these phenomena. In chemical combinations,the ultimate atoms of bodies do not penetrate each other, they are only arrangedside by side in a certain order, and the properties of the compound depend entirelyupon this order. If they are made to change their place - their mode of arrangement- by an impulse from without, they combine again in a different manner, and anothercompound is formed with totally different properties. We may suppose that one atomcombines with one atom of another element to form a compound atom, while in otherbodies two and two, four and four, eight and eight, are united; so that in all suchcompounds the amount per cent. of the elements is absolutely equal; and yet theirphysical and chemical properties must be totally different, the constitution of eachatom being peculiar, in one body consisting of two, in another of four, in a thirdof eight, and in a fourth of sixteen simple atoms.

    The discovery of these facts immediately ledto many most beautiful and interesting results; they furnished us with a satisfactoryexplanation of observations which were before veiled in mystery, - a key to manyof Nature's most curious recesses.

    Again; solid bodies, whether simple or compound,are capable of existing in two states, which are known by the terms amorphous andcrystalline.

    When matter is passing from a gaseous or liquidstate slowly into a solid, an incessant motion is observed, as if the molecules wereminute magnets; they are seen to repel each other in one direction, and to attractand cohere together in another, and in the end become arranged into a regular form,which under equal circumstances is always the same for any given kind of matter;that is, crystals are formed.

    Time and freedom of motion for the particlesof bodies are necessary to the formation of crystals. If we force a fluid or a gasto become suddenly solid, leaving no time for its particles to arrange themselves,and cohere in that direction in which the cohesive attraction is strongest, no crystalswill be formed, but the resulting solid will have a different colour, a differentdegree of hardness and cohesion, and will refract light differently; in one word,will be amorphous. Thus we have cinnabar as a red and a jet-black substance; sulphura fixed and brittle body, and soft, semitransparent, and ductile; glass as a milk-whiteopaque substance, so hard that it strikes fire with steel, and in its ordinary andwell-known state. These dissimilar states and properties of the same body are occasionedin one case by a regular, in the other by an irregular, arrangement of its atoms;one is crystalline, the other amorphous.

    Applying these facts to natural productions,we have reason to believe that clay-slate, and many kinds of greywacke, are amorphousfeldspar, as transition limestone is amorphous marble, basalt and lava mixtures ofamorphous zeolite and augite. Anything that influences the cohesion, must also ina certain degree alter the properties of bodies. Carbonate of lime, if crystallisedat ordinary temperatures, possesses the crystalline form, hardness, and refractingpower of common spar; if crystallised at a higher temperature, it has the form andproperties of arragonite.

    Finally, Isomorphism, or the equality of formof many chemical compounds having a different composition, tends to prove that matterconsists of atoms the mere arrangement of which produces all the properties of bodies.But when we find that a different arrangement of the same elements gives rise tovarious physical and chemical properties, and a similar arrangement of differentelements produces properties very much the same, may we not inquire whether someof those bodies which we regard as elements may not be merely modifications of thesame substance? - whether they are not the same matter in a different state of arrangement?We know in fact the existence of iron in two states, so dissimilar, that in the one,it is to the electric chain like platinum, and in the other it is like zinc; so thatpowerful galvanic machines have been constructed of this one metal.

    Among the elements are several instances ofremarkable similarity of properties. Thus there is a strong resemblance between platinumand iridium; bromine and iodine; iron, manganese, and magnesium; cobalt and nickel;phosphorus and arsenic; but this resemblance consists mainly in their forming isomorphouscompounds in which these elements exist in the same relative proportion. These compoundsare similar, because the atoms of which they are composed are arranged in the samemanner. The converse of this is also true: nitrate of strontia becomes quite dissimilarto its common state if a certain proportion of water is taken into its composition.

    If we suppose selenium to be merely modifiedsulphur, and phosphorus modified arsenic, how does it happen, we must inquire, thatsulphuric acid and selenic acid, phosphoric and arsenic acid, respectively form compoundswhich it is impossible to distinguish by their form and solubility? Were these merelyisomeric, they ought to exhibit properties quite dissimilar!

    We have not, I believe, at present the remotestground to suppose that any one of those substances which chemists regard as elementscan be converted into another. Such a conversion, indeed, would presuppose that theelement was composed of two or more ingredients, and was in fact not an element;and until the decomposition of these bodies is accomplished, and their constituentsdiscovered, all pretensions to such conversions deserve no notice.

    Dr. Brown of Edinburgh thought he had convertediron into rhodium, and carbon or paracyanogen into silicon. His paper upon this subjectwas published in the Transactions of the Royal Society of Edinburgh, and containedinternal evidence, without a repetition of his experiments, that he was totally unacquaintedwith the principles of chemical analysis. But his experiments have been carefullyrepeated by qualified persons, and they have completely proved his ignorance: hisrhodium is iron, and his silicon an impure incombustible coal.



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