same atoms would give 1 atom of cyanate of ammonia and 1 atom of water, viz., N H3, C2 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 heat, 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 fluid; 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; consequently, when the fluid is heated up to a certain point, a further solution usually takes place, till by new addition of the solid

body the affinity of the fluid for it is so far diminished that equilibrium between that force and the cohesion of the solid is again established. If now a solution thus saturated while warm be cooled down to its former temperature, the solid body regains its original cohesive power, and a portion of it separates from the fluid in order to unite in larger and usually crystalline masses, the quantity remaining in solution being only just so much as the fluid would directly have taken up at this lower temperature. This separation is called Spontaneous or False Precipitation, Præcipitatio spontanea, in so far as it takes place without the addition of a foreign body to the solution.

This precipitation by cooling is exhibited by the solutions of most salts in water and alcohol, of many kinds of camphor and fat in alcohol and ether, &c. Aqueous solutions containing excess of water often when cooled below 0° C. deposit a portion of the water in the form of ice, the remaining liquid being a concentrated solution of the salt; for at low temperatures the cohesion of ice may overcome its affinity for the salt. Whereas therefore a saturated solution when cooled deposits salt, so on the other hand a dilute solution, when its temperature is sufficiently reduced, yields ice. Lastly, a saturated solution of common salt solidifies at 20° C. to a mixture of ice and common salt containing water of crystallization.-Glacial acetic acid solidifies at +15° C. (59° Fah.); a mixture of this substance with water deposits glacial acetic acid at a lower temperature, the remaining liquid being a compound of glacial acetic acid with water; when the quantity of water is somewhat greater nothing solidifies; when it is still greater, part of it freezes leaving a more concentrated acid behind. When the mixture of glacial acetic acid with water, instead of being cooled, is subjected at 15° C. to a pressure of 1100 atmospheres, of it crystallizes in a few minutes in the form of glacial acetic acid. (Perkins, Schw. 39, 161.) It appears from this that increased pressure has the same effect as cold in increasing cohesion.

Some solid

The following are exceptions to the law just considered. bodies, as lime and citrate of lime, are more soluble in cold water than in hot; so that a solution of either of them saturated in the cold becomes turbid when warmed, and clear again on cooling. If to a solution of chloride of calcium or nitrate of lime in absolute alcohol, there be added as much ether as will throw down only a portion of either of these salts, the mixture of alcohol and ether will become turbid, even to opacityfrom precipitation of the limesalt still remaining in solution-every time the liquid is warmed, if only by the hand, but will regain its transparency on cooling. (Döbereiner, Ann. Pharm. 14, 249.) A solution of caustic potash in water dissolves in the cold a large quantity of tartrate of lime; the clear solution coagulates to a pasty mass when heated, but becomes clear and fluid again on cooling. (Lassonne, Osann.)-The solubility of Glauber's salt in water increases rapidly with rise of temperature up to 33° (92° Fah.), but diminishes when the temperature is raised above this point; water saturated with Glauber's salt at 33°, yields hydrated crystals when cooled, and anhydrous crystals when further heated.

Similar anomalies occur in solutions of liquids in other liquids. Coniin, agitated with water at ordinary temperatures, takes up a small portion of it: the clear liquid becomes turbid, from separation of water, every time it is warmed even by the hand, and clear again on cooling. (Geiger.) Animin dissolves in 20 parts of water: the solution becomes turbid when heated, from separation of animin, which is redissolved on cooling. (Unverdorben.)-When a solution of chloride of calcium in a

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mixture of water and acetone is heated, the acetone separates forming a film on the surface.--(Liebig and Pelouze, Ann. Pharm. 19, 287.)

The following observations of Gay-Lussac (Ann. Chim. Phys. 70, 407, also, J. Pr. Chem. 18, 193) render it doubtful whether cohesion plays so important a part in spontaneous precipitations as has been hitherto supposed, and raise a suspicion that this kind of precipitation is affected by difference of temperature in a manner which has not yet been explained. Cetine, paraffin, and stearic acid melt below the boiling point of alcohol, so that the quantities of them dissolved in that liquid at temperatures at which they are solid, can be compared with the quantities dissolved at temperatures at which they are liquid. From what precedes we should expect that these bodies would be much more soluble in the melted state than below their melting points, because the cohesion of a solid body is much greater than that of a liquid. But, according to Gay-Lussac, the solubility of these substances increases regularly as their temperature rises, without any sudden augmentation at the melting point. GayLussac therefore considers that their solubility is unaffected by cohesion, and determined solely by temperature. Bodies exhibit the same relations in dissolving as in evaporating; vapour of water at 0° has the same tension, whether it be raised from water or from ice.

D. Some experiments seem to show that the feebler chemical combinations may likewise be overcome by Adhesion.

When vinegar is filtered through pure quartz-sand, the first portion of liquid that runs through is robbed of almost all its acid, and the vinegar does not pass through unchanged till the sand has become well charged with acid. Potato-brandy diluted with water and filtered through quartz-sand, yields at first pure water, then a mixture of water and alcohol deprived of its fusel-oil, and lastly the original mixture unaltered. Wood-shavings also at first deprive vinegar of nearly all its acid; charcoal acts still more powerfully. (Wagenmann, Pogg. 24, 600.) The last two substances may perhaps act by affinity.-Sömmering's experiment (vid. Alcohol), in which a mixture of water and alcohol enclosed in a bladder yielded on evaporation scarcely anything but water, may likewise be referred to the same class of phenomena, provided we suppose that the water is taken up by the bladder, not in consequence of affinity but of adhesion, and thus transferred to the outer surface, where it evaporates into the air.

E. A mode of decomposition not yet completely understood is that called Action by Contact (action de présence), or Catalytic action. These terms are applied by Berzelius and Mitscherlich to the case in which a solid or liquid body, when brought into contact with a compound, excites a decomposition of that compound (Catalysis)—without itself undergoing any alteration, mechanical or chemical, or at all events, if a chemical alteration does take place, without entering into combination with either of the elements of the compound. The Contact-substance or Catalytic body awakens by its mere presence, not by its affinity, the slumbering affinities of the elements, and causes them to assume a new arrangement involving more complete electro-chemical neutralization. Berzelius regards catalytic force as a peculiar manifestation of electro-chemical action.

The following instances may be referred to this mode of action. In peroxide of hydrogen, H O2, the second atom of oxygen is retained by a very feeble affinity only, and escapes with slow effervescence even at ordinary temperatures. Many metals and metallic oxides, when brought

in the state of powder in contact with this liquid, produce a violent disengagement of oxygen gas without themselves taking up oxygen or suffering any other alteration, excepting that some oxides, as that of silver, part with their own oxygen at the same time. This, according to Berzelius, is a case of catalytic action. The following explanation by Liebig is more probable (Ann. Pharm. 2, 22). Pulverulent and angular bodies accelerate the disengagement of a gas absorbed by a liquid (vid. Heat); they likewise exert this action on peroxide of hydrogen; this rapid escape of gas produces a rise of temperature, and this again accelerates the disengagement of gas,-and thus these actions continue to augment each other in intensity till the effervescence amounts to a slight explosion.— The following decompositions are also regarded as catalytic. The rapid decomposition of the aqueous solution of nitrosulphate of ammonia into nitrous oxide gas and sulphate of ammonia (which also takes place slowly by itself) by the action of spongy platinum, oxide of silver, &c.;-the separation of persulphuret of hydrogen, S H, into sulphuretted hydrogen, SH, which escapes as gas, and sulphur which remains behind, by contact with alkalis, chloride of calcium, &c.;-the resolution of alcohol into ether and water by the action of sulphuric acid;-that of sugar dissolved in water into carbonic acid and alcohol by ferments;-the conversion of alcohol into acetic acid by ferments;-the conversion of starch into sugar by dilute sulphuric acid or by diastase; the conversion of urea dissolved in water into carbonate of ammonia by the action of animal mucus; -(See these substances.)—But the mode of action in these transformations sometimes admits of other explanations; and when this is not the case, our conception of it is by no means sufficiently clear to justify the positive assumption of this so-called contact-action or catalytic force, which, after all, merely states the fact without explaining it.

F. Just as a body in the act of combination may induce another to enter into combination at the same time (page 38), so likewise a compound in the act of decomposition may impart this decomposing activity to another. How and wherefore? we are unable to explain.

When peroxide of hydrogen by contact with oxide of silver gives up its second atom of oxygen, it likewise induces the oxide of silver to part with its oxygen. Vinous fermentation may be explained consistently with this view by supposing that the decomposing ferment brings the sugar into a state of decomposition, and the sugar is then resolved into carbonic acid and alcohol. A similar explanation may be applied to the decomposition of urea into carbonic acid and ammonia by animal mucus, of asparagin into aspartate of ammonia by yeast, and of amygdalin into hydrocyanic acid and other products of decomposition by the action of yeast and sugar (Liebig). G. The vital force of plants and animals likewise exerts a decomposing action on chemical compounds.

The most remarkable instance of this is the action of light on the green parts of plants, causing them to decompose carbonic acid into oxygen and carbon, the latter of which elements combines with the hydrogen and oxygen of the vegetable juice producing numerous organic compounds.

H. But the most numerous and important decompositions of chemical compounds are those which are brought about by the action of other bodies, whose superior affinity produces new compounds at the same time that it destroys the old ones. For the production of these decompositions the same conditions are required as for the formation of chemical compounds (page 36); viz. immediate contact and the fluid state, at least of one of the bodies concerned,-whence fusion or vaporization by elevated