phuretted hydrogen gases. (Elsner.) Tin heated in sulphuretted hydrogen gas produces protosulphuret of tin and hydrogen gas (Gay-Lussac & Thénard). Hydrogen gas passed over red-hot chloride of silver produces silver and hydrochloric acid gas; on the other hand, when silver is heated to redness in hydrochloric acid gas, chloride of silver is produced and hydrogen gas set free (Boussingault, Ann. Chim. Phys. 54, 260). The disposing cause in this case must be the adhesion between hydrochloric acid gas and hydrogen gas.-If carbonate of lime be heated in a tube till it begins to give off carbonic acid gas, and the heat be then lowered to such a degree that the evolution of gas shall cease, it will immediately recommence on passing vapour of water or common air through the tube (Gay-Lussac, Ann. Chim. Phys. 63, 219, also Ann. Pharm. 22, 52). In this case, the adhesion between vapour of water or atmospheric air and carbonic acid gas enables the affinity of heat for carbonic acid to overcome, even at a reduced temperature, the affinity of lime for carbonic acid.— While carbonate of lime gives up its carbonic acid when heated to redness in the open air, quick lime on the other hand absorbs carbonic acid in great abundance when strongly ignited in an atmosphere of that gas (Petzholdt, J. pr. Chem. 17, 464).-Carbonic acid gas passed through an aqueous solution of bi-hydrosulphate of potash drives out all the sulphuretted hydrogen in the form of gas and forms bicarbonate of potash; on the contrary, sulphuretted hydrogen gas expels carbonic acid from bicarbonate of potash and forms bihydrosulphate of potash.-Metallic fluorides are decomposed by aqueous solution of hydrochloric acid, and metallic chlorides by aqueous solution of hydrofluoric acid. Hydrochloric acid decomposes the acetates and acetic acid the metallic chlorides. The decomposing acid must always be added in very great excess, so that a mixture of the decomposing and separated acids may be evolved in the state of gas or vapour (Gay-Lussac, Ann. Chim. Phys. 30, 291, also N. Tr. 12, 2, 260). In these cases, therefore, a certain influence is always exerted by the mutual adhesion of the elastic fluids, or according to Dalton's theory (page 22) by the circumstance of each of the elastic fluids acting as a vacuum towards the other.

b. By Affinity. If to an aqueous solution of 2 atoms of sulphate of ammonia, potash, or soda there be added 1 atom of nitric acid (a larger quantity would remain uncombined) the smell of the acid is completely destroyed, and when the solution is left to evaporate spontaneously, nitrate of ammonia, potash, or soda crystallizes out, and the mother-liquid contains an alkaline bisulphate. If on the contrary to an aqueous solution of 1 atom of nitrate of ammonia, potash, or soda there be added 2 atoms of sulphuric acid and heat applied to the mixture, the whole of the nitric acid escapes and an alkaline bisulphate remains. One atom of sulphuric acid would take up only half the alkali and drive out only half the nitric acid, unless the action were assisted by an elevated temperature. Hence nitric acid takes from neutral sulphate of ammonia, potash, or soda, the half of its base and forms a nitrate; the nitrate on the other hand is decomposed by excess of sulphuric acid. This may be explained as follows. The above-named alkalis can combine either with 1 atom of sulphuric acid to form neutral sulphates, or with 2 atoms to form bisulphates with nitric acid they only combine according to equal numbers of atoms. Suppose the affinity of the alkali to nitric acid 5, to the first atom of sulphuric acid 6, and that of the neutral alkaline sulphate so formed to the second atom of sulphuric acid = 2. If then nitric acid be added to a neutral alkaline sulphate, either one atom of the sulphate will

:

=

=

remain undecomposed, affinity = 6; or its alkali will combine with the nitric acid, affinity 5, and its sulphuric acid with the other atom of neutral sulphate, affinity = 2; the sum of these affinities is 5 + 2 = 7, and since 67, decomposition takes place (Sch. 88). The alkaline bisulphate thus formed is likewise undecomposable by any excess of nitric acid, because it is held together by the affinities 6+2 8, and the nitric acid acts upon it only with an affinity = 5. If therefore on the other hand two atoms of sulphuric acid act upon one atom of an alkaline nitrate, the affinity of the nitric acid for the alkali, being 5, is overcome by that of the 2 atoms of sulphuric acid for the alkali which 6+2 = 8, and the whole of the nitric acid is set free while an alkaline bisulphate is produced (Sch. 89). Similar relations are exhibited by hydrochloric acid towards the sulphates of ammonia, potash, and soda, and by sulphuric acid towards sal-ammoniac and the chlorides of potassium and sodium (Richter, Stöchiometrie, 2, 237).

2. Predisposing affinity of the solvent. Aqueous acetic acid added to carbonate of potash disengages carbonic acid and forms acetate of potash. But if the solution be evaporated to dryness, the remaining acetate of potash dissolved in alcohol, and carbonic acid gas passed through the solution, almost all the potash is thrown down in the form of carbonate, and the liquid contains acetic acid in combination with alcohol partly converted into acetic ether (Pelouze). It would appear from this that alcohol has a considerable affinity for acetic acid, which affinity together with that of carbonic acid for potash overcomes the greater affinity of acetic acid for potash. This result is usually explained according to Berthollet's theory, the more insoluble and coherent compounds being supposed to be most easily formed, and accordingly in this case, the carbonate of potash which is insoluble in alcohol. This theory is generally resorted to when it directly applies and passed over in silence in cases which contradict it. Why does not carbonic acid throw down carbonate of lime from acetate of lime dissolved in water, inasmuch as carbonate of lime is still less soluble in water than carbonate of potash is in alcohol? Moreover, according to Pelouze, carbonic acid does not decompose chloride of strontium, chloride of calcium, or nitrate of copper dissolved in alcohol, although the carbonates of strontia, lime, and oxide of copper are insoluble in alcohol as well as in water. This instance likewise shows that difficult solubility and great cohesion are two different things; otherwise carbonate of potash would be at the same time a very coherent salt (in relation to alcohol) and a very incoherent one (in relation to water).-The assertion of Berthollet (Statique Chim. 1, 401) that a moderately dilute solution of chloride of calcium in water, deposites sulphate of lime when mixed with sulphurous acid and afterwards with alcohol, is one which I have not found to be confirmed. Even a solution of chloride of calcium in absolute alcohol is not rendered turbid by saturation with sulphurous acid gas. (Gm.)

A solution of chloride of sodium and sulphate of magnesia when evaporated to a gentle heat, deposits both salts in crystals without alteration. But if the residue be pulverized and boiled in alcohol, the alcohol according to Grotthus (Scher. N. Bl. 273) takes up chloride of magnesium, and the residue therefore contains sulphate of soda (Sch. 90). The boiling must however be continued for a long time, and the quantity of chloride of magnesium obtained is very small (H. Rose). Since alcohol dissolves chloride of sodium and sulphate of magnesia very sparingly, but chloride of magnesium very abundantly, the formation of the last-named salt may be due to the predisposing affinity of the alcohol for it; but the

higher temperature produced by boiling with alcohol may also contribute towards the result.

While sulphate of soda and chloride of calcium dissolved in water are resolved by mutual decomposition into chloride of sodium and precipitated sulphate of lime, an aqueous solution of common salt and sulphate of lime evaporated to dryness, or a pulverized mixture of the two salts moistened with a large quantity of water and then dried, yields chloride of calcium when digested in boiling alcohol, while sulphate of soda may be extracted from the residue by water. (Sch 90, substituting Ca for Mg.) Unless the mixture of the two salts be moistened with water and then dried, no chloride of calcium will be extracted from it by alcohol (Döbereiner, J. pr. Chem. 1, 112). The reversing of the affinity is perhaps produced by the alcohol: possibly however, as Döbereiner supposes, the affinity of sulphate of lime for sulphate of soda (a combination of the two salts occurs in nature forming the mineral called Glauberite,) may cause the formation of a small quantity of the double salt. By melting the two salts together in equal numbers of atoms, we obtain a hard mass, which becomes soft and afterwards moist by exposure to the air, and consequently contains a small quantity of chloride of calcium. These two facts serve to show how it happens that in analyses of mineral waters, when the residue after evaporation is boiled in alcohol, chloride of magnesium or chloride of calcium and sulphate of soda are frequently obtained, though these substances undoubtedly existed in the water as sulphate of lime or magnesia and chloride of sodium.

If 1 part of carbonate of potash be dissolved in at least 10 parts of water, and the solution shaken up with lime, the carbonic acid is taken up by the lime: with 4 parts of water however no decomposition takes place: on the contrary, a strong solution of caustic potash takes carbonic acid from carbonate of lime (Liebig, Pogg. 24, 365). The affinity of potash for carbonic acid is probably greater than that of lime; but when the quantity of water is increased, the affinity of that liquid for potash perhaps increases more rapidly than its affinity for carbonate of potash, and thus the first-mentioned result is brought about.

Aqueous sulphurous acid dissolves iodine, forming a mixture of sulphuric and hydriodic acids; but if the quantity of water in the solution be diminished by evaporation, sulphurous acid is evolved and hydriodic acid containing iodine in solution is left behind. Similarly, concentrated sulphuric acid and hydriodic acid are resolved into sulphurous acid and iodine (Sch. 91). Hence the affinity of water for sulphuric and hydriodic acids gives rise to their formation.

The following experiment of Chevreul also shows the influence of the solvent. Excess of water abstracts half the potash from neutral stearate of potash, and forms bi-stearate of potash. Ether dissolves stearic acid from neutral stearate of potash, and separates a compound of stearic acid with excess of potash. Water has a more especial affinity for potash, and ether for stearic acid.

3. Difference of temperature may give rise to reciprocal affinity in two ways:

a. At high temperatures, the affinity of heat for that substance simple or compound which is most disposed to form a gaseous compound with it, often comes into play and determines the result. Heat, in such cases, acts like a fourth body introduced.

Peroxide of manganese mixed with hydrochloric acid at ordinary or slightly elevated temperatures, gives up its second atom of oxygen to the

hydrogen of the acid, so that water, chlorine, and chloride of manganese or hydrochlorate of protoxide of manganese are produced. (Sch. 64, 73.) If on the other hand chlorine gas be exposed to light or to a red heat in contact with water, hydrochloric acid is formed and oxygen gas evolved. At one time therefore oxygen (in peroxide of manganese) takes hydrogen from hydrochloric acid, setting chlorine free; at another, chlorine takes hydrogen from water and evolves oxygen gas. We may with probability suppose that the affinity of oxygen for hydrogen is greater than that of chlorine; on this hypothesis the explanation of the first case is evident. On the other hand, heat has a greater affinity for oxygen than for chlorine; for chlorine gas has been liquefied by strong pressure, which oxygen has not. When therefore heat acts with great intensity, its affinity for oxygen that of chlorine for hydrogen effects the decomposition of water.

Potassium at a red heat decomposes black oxide of iron forming potash and metallic iron: at a white heat on the contrary potash is decomposed by metallic iron, the products being black oxide of iron and vapour of potassium. In this case it must be supposed that the affinity of potassium for oxygen is greater than that of iron; nevertheless, at a white heat the affinity of heat for potassium, with which it combines and forms a vapour, comes into play and determines the result.

At a red heat potassium decomposes carbonic oxide, forming potash and charcoal; at a low white heat charcoal decomposes potash, producing carbonic oxide gas and potassium vapour. (Sch. 6, substituting K for Zn.) In the latter case, the weaker affinity of carbon for oxygen is assisted by that of heat for carbonic oxide and potassium.

When potash (or soda) is in combination with phosphoric acid, boracic acid, or silica, sulphuric acid will separate these substances at ordinary temperatures and combine with the potash by virtue of its greater affi nity. If on the contrary sulphate of potash be ignited in contact with phosphoric acid, boracic acid, or silica, these acids will take hold of the potash and separate the sulphuric acid in the state of vapour. In this case it is the affinity of heat for the volatile sulphuric acid, with which it forms a vapour, that enables the much weaker affinity of the above-mentioned acids for potash to gain the mastery.

When carbonate of ammonia is added to an aqueous solution of nitrate of lime, nitrate of ammonia and precipitated carbonate of lime are formed. But when a mixture of nitrate of ammonia and carbonate of lime is heated above 100° C. carbonate of ammonia volatilizes and nitrate of lime remains. (Sch. 92.) Here the result is determined by the affinity of heat for the volatile carbonate of ammonia. Similar relations are exhibited between carbonate of ammonia and chloride of calcium in the cold, and between sal-ammoniac and carbonate of lime at a higher temperature. For a similar reason, borate of ammonia and common salt act upon each other only at a high temperature, evolving sal-ammoniac in the state of vapour.

To the same category may also be assigned the following facts, so far as they may prove to be correct. When sulphuric acid acts upon zinc under the ordinary pressure of the atmosphere, sulphate of zinc and hydrogen gas are evolved (Sch. 17); but according to Babinet (Ann. Chim. Phys. 37, 183; also Pogg. 12, 523) this decomposition ceases when the process is conducted in a strong copper vessel closed by a stop cock, as soon as the disengaged hydrogen gas has attained a certain pressure; at 10° C, the decomposition and evolution of gas cease when the gas presses with a force of 13 atmospheres; at 25°, when the pressure amounts to 33

VOL. I.

K

atmospheres. This seems to show that the affinity of zinc for oxygen + that of sulphuric acid for oxide of zinc is less than the affinity of hydrogen for oxygen that of sulphuric acid for water, and therefore the decomposition does not take place under strong pressure;-but at lower pressures, when the affinity of heat for hydrogen, with which it forms a gas, likewise comes into play, the action goes on. On the contrary, Faraday has observed (Qu. J. of Sc. 3, 474) that at this increased pressure, the decomposition is not arrested but merely retarded, because the effervescence ceases and with it the motion of the liquid, by which the chemical action is materially assisted. I have also obtained the following results at a summer heat of from 20° to 30° C. (68° to 86° Fah.), using very thick and narrow glass tubes 5 inches in length. On filling the tube full of moderately strong hydrochloric acid, introducing a piece of zinc just above the acid, sealing the tube, and laying it in a horizontal position, it burst after four hours with a violent explosion. Now since a tube of equal strength is capable of holding liquid carbonic acid at 25° C. without bursting, and the elasticity of carbonic acid at that temperature amounts to 50 atmospheres, that of the hydrogen gas in the experiment just described must have exceeded 50 atmospheres. When a similar experiment is made with a mixture of 1 part of oil of vitriol and 8 of water, the tube does not burst even when left in the horizontal position for several weeks and placed upright every day. On cutting off the end, the gas escapes with a slight detonation without bursting the tube, and the acid is found to be nearly saturated with zinc. This seems to show that the decomposition is not arrested by strong pressure but only retarded.

The following experiment (recorded in Berzelius's Lehrbuch, 5, 9) is also connected with this matter. When pieces of carbonate of lime are placed in a strong glass vessel, a somewhat dilute acid poured upon them, and the vessel closed air-tight, the solution ceases after a time, and the lime is no longer attacked for whatever length of time it may be left in the acid; but on opening the vessel the lime is in a few minutes completely dissolved. From this it might be inferred that the affinity of carbonic acid for lime is greater than that of sulphuric, nitric, or hydrochloric acid, that these acids decompose carbonate of lime only under comparatively small pressure, when the action is assisted by the affinity of heat for carbonic acid with which it forms a gas,-that at strong pressures, on the contrary, carbonic acid would expel these acids from their combinations with lime. (Sch. 12.) But hydrochloric acid of moderate strength exhibits a different relation from this, at least according to my experiments. A quantity of acid like that above mentioned having been sealed up in a tube together with an excess of calcspar, the tube laid horizontally but turned upright every day in order to renew the points of contact, the liquid was found after 14 days to be covered with a highly moveable film of liquid carbonic acid, 2 lines in thickness. On cutting off the point, the upper half of the tube burst with a loud report, and the remaining liquid was neutral to litmus-paper. This experiment shows that hydrochloric acid decomposes carbonate of lime even under a pressure at which carbonic acid becomes liquid, and therefore that the affinity of hydrochloric acid for lime is greater than that of carbonic

acid.

Lastly, with reference to this subject, we may mention an experiment of Petzhold (J. pr. Chem. 17, 464), according to which pulverized quartz heated to whiteness in an open vessel with an equal weight of carbonate