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Perhaps also the fact of gypsum dehydrated by gentle heat becoming hard when mixed with water, and the non-production of this effect if the gypsum has been strongly burnt, may be explained by supposing that this substance when deprived of its water by a moderate heat is in the amorphous, but after strong ignition in the crystalline state. 6. Differences in the properties of compounds, probably arising from the
different grouping of the Simple Atoms which make up a compound atom.
In considering the different properties of compound bodies resulting from dimorphism and amorphism, the structure of the compound atoms has been supposed to continue unaltered, and the occurrence of this or that crystalline form, or of the amorphous state, to depend solely on the manner in which these compound atoms are arranged amongst themselves. Hence it follows that these dimorphous and amorphous conditions may occur in simple bodies also, since simple atoms as well as compound ones may be disposed amongst themselves in various ways,-moreover that these several conditions (with perhaps some exceptions requiring further investigation) may be destroyed by fusion, evaporation, or solution of the solid body in which they exist; and it will then depend upon circumstances in what particular state the body will resume its solid form.But it is otherwise with the differences now to be considered, the cause of which we shall assume to be that the manner or number in which simple atoms combine to form a compound atom may differ in different substances. It is easy to see that the various conditions hereby produced can occur only in compound bodies, and may remain unaltered by the passage of a body from the solid to the fluid state or vice versá; for the compound atoms once constructed in this or that particular way may, without disturbance of internal structure, form fluid compounds with heat or with ponderable solvents. With this difference of grouping of simple atoms in the formation of compound atoms are connected many striking differences not only in the physical properties but also in the chemical relations of the bodies concerned.
a. Isomerism. When two or more compounds which exhibit different physical and chemical relations, are so constituted that their compound atoms must contain the same elements combined according to the same numbers of simple atoms, and there is no ground for supposing that their proximate elements are of different natures, these compounds are said to be isomeric (using the word in its narrowest sense). It is supposed that the simple atoms which form a compound atom are put together in different ways.
Many of the compounds formerly classed under this head are now regarded as polymeric; it is only with regard to phosphoric acid, tellurous acid and telluric acid, peroxide of tin, and tartaric acid, that isomeric conditions are at present recognized; and even these bodies may with more or less probability be regarded as polymeric.
Phosphoric acid, P О', exhibits the three isomeric states of ordinary, pyro—and meta-phosphoric acid. Among the many diversities exbibited by these three acids, the most remarkable is that they saturate different quantities of a salifiable base. P OS in the state of ordinary phosphoric acid saturates 3 atoms of a base, in the state of pyrophosphoric is situates 2 atoms, and in the state of metaphosphoric acid only 1 atom. If i atom of P Oʻ, in whichever of the three states it may
happen to be, is ignited with three atoms of any base, soda for instance, an ordinary phosphate is produced; but when P O is ignited with 2 atoms of soda, the result is a pyrophosphate, and with 1 atom a metaphosphate. By long digestion or boiling with a large quantity of water, which itself acts as a base, the pyrophosphates and metaphosphates are converted into ordinary phosphates. The particular quantity of base with which P O is in contact, seems then at certain high temperatures to affect the manner in which 1 atom of phosphorus arranges itself with respect to 5 atoms of oxygen, so that the compound is capable of saturating sometimes 3 atoms of a base, sometimes 2, and sometimes only one. If we would explain these remarkable relations of phosphoric acid first discovered by Graham on the hypothesis of Polymerism, we might consider ordinary phosphoric acid as P О', pyrophosphoric acid as P2O, and metaphosphoric acid as P2 015; then P Os would saturate three, D2010 four, and POS three atoms of a base. (Vid. Phosphorus.)
In the case of tellurous acid, Te 0%, a more soluble modification A and a less soluble modification B are to be distinguished: the latter is produced by the action of nitric acid upon A, and is again converted into A by fusion with canstic potash. Telluric acid, Te 0°, exbibits two modifications perfectly similar to the above; the more soluble of the two is converted into the less soluble, when two or more atoms of it are fused with one atom of potash; this is analogous to the transformations of phosphoric acid above noticed. If these compounds are regarded as polymeric, they must be supposed to exist as Te 0?, Te* О', Te 0% and Te O. (Vid. Tellurium.)
Antimonious and antimonic acid may possibly pass from one such modification to the other when their salts are ignited.
Peroxide of tin, when precipitated by caustic alkalis from a solution of the bichloride, is much more easily soluble in acids than the anomalous variety of it produced by the action of nitric acid upon nietallic tin; the latter when dissolved exhibits also very different relations. Possibly the soluble oxide may be Sn O’ and the anomalous variety Sn® Oʻ.
Among organic compounds the following may be regarded as isomeric: Tartaric and racemic acid (C? Hạ 0);—mucic and paramucic acid (CH'0);-maleic and paramaleic acid (CH 0).
B. Polymerism. Two or more compounds possessing different physical and chemical properties, and composed of the same elements in the same proportions are said to be polymeric, when their differences may be explained by supposing that their compound atoms contain different numbers of simple atoms, varying however in such a manner that the numerical ratio of the several kinds of simple atoms remains unaltered. If for example one of a group of polymeric compounds contains 1 atom of a substance A and 3 atoms of a substance B, then the second may contain 2 atoms of A and 6 atoms of B, the third 3 atoms of A and 9 atoms of B and so on. such cases the weight of the compound atom varies, but the proportions between its elements remains the same.
Besides the instances mentioned under Isomerism, which ought all perhaps to be included under this head, the following among organic combinations must be particularly noticed.
Polymeric compounds all containing 1 part of hydrogen united with 6 parts of carbon, and therefore containing C H: Olefiant gas, the more volatile oil of oil-gas, rock-oil, eupion, oil distilled from bees’-wax, caout
choucine, heveene, oil of wine, stearoptin of oil of roses, paraffin, cetine, &c. Olefiant gas is probably C H', cetine ('32 H 32.
Benzin, oilgas-camphor, and scheererite are C H.
The oils of turpentine, juniper, copaiba, lemons, and black pepper are CSH.
Naphthaline and paranaphthaline are 0 H”.
Cyanic acid is C N O, fulminic acid probably C' N? O’, cyanuric acid C N 03.
Volatile chloride of cyanogen is C N CI, the fixed chloride probably CN Cl.
9. Metamerism. This term is applied by Berzelius to the case in which the compound atoms of two chemical compounds containing the same elementary atoms, and for the most part in the same proportions, are nevertheless made up of different proximate elements. According to this definition metameric bodies must always belong to the higher orders of compounds. In some cases one only of the bodies concerned is a compound of a higher order.
It is only among organic bodies that metameric compounds occur. The following are the most important:CH 0
с но Acetic acid
2 1 Water 1 1 Methylic ether.....
2 3 1
6 6 4 Acetate of methylic ether 6 6 4 Formic ether and acetate of methylic ether have the same specific gravity when in the form of vapour, and nearly the same specific gravity and boiling point when in the liquid state: but in other respects they are totally different; the former, when treated with caustic potash, is resolved into formiate of potash and alcohol, the latter into acetate of potash and wood-spirit.
The differences between the several compounds produced by the action of sulphuric acid upon alcohol, viz. sulphovinic acid, ethionic acid, isethionic acid, &c. are probably dependent on metameric conditions.
[For some examples adduced by Laurent, vid. Ann. Chim. Phys., 66, 175.)
Aldehyde is CH 02, and its vapour weighs 1.5317; acetio ether, which is composed of acetic acid (Č" H0) and ether (C4 H: O‘) is CH* O'; and both its atomic weight and the spec. grav. of its vapour are twice as great as those of aldehyde.
When cyanic acid is mixed in the cold with aqueous solution of ammonia, cyanate of ammonia is produced, as proved by the fact that the liquid yields cyanic acid when treated with sulphuric acid, and ammonia when treated with potash. But warming, or even spontaneous evaporation, is sufficient to convert the salt into urea, which does not exhibit these reactions with sulphuric acid and potash. Urea is C NH' 0%; the same atoms would give 1 atom of cyanate of ammonia and 1 atom of water, viz., NH, C NO,HO. Cyanate of ammonia is, therefore, converted into urea merely by a change in the arrangement of its atoms.
IV. DECOMPOSITION OF CHEMICAL COMPOUNDS. Every chemical compound may, as far as we know, be resolved into its elements. Nevertheless it is possible that many substances hitherto undecomposed may be compounds of so intimate a nature that they have resisted all attempts which have as yet been made to decompose them. The resolution of a chemical compound into its elements is called Decomposition, the compound is said to be decomposed; it is resolved into heterogeneous substances, which might be called Decomposition-substances. (Zersetzungs-stoffe.) These are either Educts or Products of decomposition. They are called educts when they exist in the compound before decomposition, and form part of it; products, when they are generated during decomposition. Carbonic acid which is disengaged by the action of hydrochloric acid upon carbonate of lime, is an educt; but the same acid, when evolved by heating charcoal with red oxide of mercury, is a product. Products are always compound bodies; educts may be either simple or compound,--the latter, when the decomposing body (e. g., carbonate of lime) contains proximate as well as ultimate elements. According to the mode of decomposition, sometimes only educts are obtained (water decomposed by the electric current), sometimes only products water decomposed by phosphuret of calcium), sometimes both together (water decomposed by potassium).
1. Conditions of Chemical Decomposition, In order that a compound may be decomposed, the forces which bind its elements together must be overcome by stronger forces. The greater number of decompositions are brought about by the action of stronger affinities; other natural forces may, however, concur in producing the effect.
A. No chemical combination of ponderable bodies can be overcome by pressure: but compounds of ponderables with imponderables, as beat, may be decomposed by that kind of force. Water may be pressed out of a sponge, a proof that pressure can overcome combination produced by adhesion : but the strongest pressure fails to separate water from gypsum and other salts containing water of crystallization, provided the temperature does not rise to the melting point of the salt. It has, indeed, been affirmed that lead amalgam, and some other amalgams, give up a portion of their combined mercury when subjected to pressure ; but the mercury thus pressed out is only that which is in excess, and remains in the liquid state adhering to the particles of the solid compound. On the other hand, vapour of water is resolved by pressure into liquid water and heat: perhaps also the development of heat, light, and electricity by pressing and rubbing various substances is an effect of a similar nature.
B. Gravitation.—When a light and a heavy substance are contained in a fluid compound, it might be supposed that after long standing the former would settle at the top and the latter towards the bottom, so that even if complete separation did not take place, the upper part of the fluid would be richer in the lighter material, and the lower in the heavier: no such effect however is actually observed.
It is said that in the vessels used to hold the concentrated liquid of the salt-works, the upper portions are found to be less rich in salt than the lower. Since, however, these vessels are not always filled with one and the same liquid, but are charged from time to time with solutions of various degrees of strength, the less concentrated portions dispose themselves above the more concentrated, and the liquor being left at rest, uniform mixture does not take place for a long time. Similarly, brandy kept in casks is said to contain a greater proportion of spirit in the upper, and of water in the lower part. Here, again, the question may be raised whether the cask has not been filled with successive portions of brandy of different strengths, which have disposed themselves in layers one above the other. Leblanc (J. Phys., 33, 376) found that if in a saturated solution of any salt crystals of the same salt are placed, some in the upper part of the liquid and others at the bottom, the former gradually dissolve while the latter increase in the same ratio, and ultimately the crystals at the bottom of the liquid decrease at their upper and increase at their lower part. This effect is attributed by Berthollet (Stat. Chim. 1, 49,) to a sinking of the particles of the salt by their own weight; it may, however, without difficulty, be explained by observing that the upper strata of air surrounding the vessel are warmer than the lower, and, consequently, that the upper portions of the liquid become warmer than the lower, and dissolve the salt immersed in them: hence these portions of the liquid become heavier, and sink to the bottom, where they become cooler, and deposit part of their salt in crystals. Lastly, it is very difficult to obtain flint-glass of perfectly uniform constitution, the lower part is generally much richer than the upper in oxide of lead. But this, again, does not prove the sinking of the heavier material out of a perfectly homogeneous mixture. For when a mixture of oxide of lead, alkali, and silica is heated, the oxide of lead melts first, and sinks before it has entered into uniform combination with the other ingredients. These latter subsequently melt: but since liquids of different specific gravities mix but slowly when at rest, and in this case, moreover, the great viscosity of the melted mass presents a further obstacle to the mixture, uniformity can only be produced by repeated and careful stirring. But when this end has once been attained, it is probable that the glass will continue uniform, even when kept for a long time in a state of fusion. That such is the case appears from Faraday's directions for the preparation of flint-glass; as also from the statement of Frauenhofer, that he obtained a mass of flint-glass weighing 400 lbs., of perfectly uniform constitution throughout. Now, when we consider the long time which such a mass would occupy in cooling, such a result could scarcely be credited if it were admitted that oxide of lead could sink to the bottom of a mass once obtained in a state of uniformity.
C. Cohesion appears to exert a much more decided influence on the decomposition of chemical compounds, at least of the less intimate kind.
The hitherto received theory on this matter is as follows. When a solid body dissolves in a liquid, the cohesion of the solid acts in opposition to the dissolving power of the Auid; the two forces tend to equilibrate each other; and in proportion as the fluid takes up more and more of the solid, its tendency to dissolve a further quantity-or, in other words, its affinity for the solid—diminishes and ultimately becomes no greater than the cohesion of the solid or the tendency of its particles to remain united amongst themselves,—and then the process of solution stops. But the cohesion of a solid body is generally diminished by elevation of temperature; consequeutly, when the fluid is heated up to a certain point, a further solution usually takes place, till by this new addition of the solid