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with arsenic and metallic arsenic.—Let A B be hydrochloric acid gas (hydrochloric acid + heat), and C water; the water combines with the acid forming aqueous solution of hydrochloric acid, and heat is disengaged. (Sch. 4).

Cases in which all three bodies are ponderable: Sulphuret of silver exposed to the air at a red heat yields sulphurous acid and metallic silver (Sch. 5).–Oxide of zinc heated to redness in contact with charcoal is resolved into carbonic oxide and metallic zinc (Sch. 6).-Oxide of copper in a similar manner into carbonic acid gas and copper (Sch. 7).—Chlorine gas expels oxygen gas from red-hot potash (KO) and produces chloride of potassium (Sch. 8).-Hydrochloric acid and zinc form chloride of zinc with disengagement of hydrogen gas (Sch. 9).-Iron decomposes chloride of silver in contact with water into chloride of iron and silver (Sch. 10). Cinnabar (Hg S) heated to redness in contact with iron yields sulphuret of iron and mercury (Sch. 11).—Carbonic acid gas is expelled by sulphuric acid from carbonate of lime, and sulphate of lime is formed (Sch. 12).Aqueous solution of nitrate of silver mixed with caustic potash yields oxide of silver and nitrate of potash (Sch. 13).—Water added to a solution of resin in alcohol forms dilute alcohol and precipitates resin. On the other hand, Glauber's salt dissolved in water is almost wholly precipitated by addition of alcohol : for the mixture of alcohol with a sarge quantity of water has scarcely any affinity for resin, and that of water with a large quantity of alcohol scarcely any affinity for Glauber's salt.

Sometimes C takes from the compound A B only a part of A, so that a compound of B with a smaller quantity of A is separated. Thus zinc in a state of incandescence robs carbonic acid of half its oxygen and converts it into carbonic oxide (Sch. 14).—The resulting compound A C may then combine with A B which has been deprived of half its original quantity of A. Thus zinc and solution of sulphurous acid form hyposulphito of zinc (Sch. 15).–Or C may rob the compound A B of the whole of A and part of B, and separate only the remaining part of B. Thus sulphuric acid heated with peroxide of manganese forms sulphate of protoxide of manganese

and drives out half the oxygen (Sch. 16). The decomposition of A B by C sometimes takes place in presence of another body D which is at first combined with A B and afterwards uuites with A C. Water (A B) mixed with sulphuric acid (D), yields when acted upon by zinc, sulphate of zinc and hydrogen gas (Sch 17).— The same kind of action takes place whenever hydrogen gas is given off during the solution of a metal in dilute oxygen acids or alkaline solutions. To this head likewise belong all precipitations of metals from solutions of their oxides in ox-acids or alkalis, by other metals in the metallic state: (in Sch. 17 a metal must be substituted for hydrogen). Thus zinc immersed in sulphate of copper produces sulphate of zinc and metallic copper (Sch. 18): similarly, copper and nitrate of silver give nitrate of copper and metallic silver (Sch. 19).—When carbonate of soda is heated to redness in contact with phosphorus, phosphate of soda and charcoal are produced (Sch. 20).

Sometimes only part of the compound A B is decomposed, and the other part combines with the newly-formed compound AC, taking the place of the substance D. Potassium heated in carbonic acid gas separates charcoal and forms potash, which combines with the undecomposed portion of the carbonic acid (Sch. 21).-Chlorine passed into solution of ammonia liberates the nitrogen of that compound in the gaseous form, and combines with the hydrogen forming hydrochloric acid, which then unites with the remaining portion of ammonia, forming sal-ammoniac (Sch. 22).

A similar action sometimes takes place, with this difference, that C takes only part of A from the decomposing portion of A B. Mercury heated with sulphuric acid produces sulphate of mercury and sulphurous acid (Sch. 23).-Copper treated with nitric acid yields nitrate of copper and nitric oxide gas (Sch. 24).—Zinc and dilute nitric acid yield nitrate of zinc and nitrous oxide gas (Sch. 25).

2. The action of C on the compound A B produces two new compounds, AC and BC (Sch. 26). When oxide of mercury is heated to redness, vapour of mercury (mercury + heat) and oxygen-gas (oxygen + heat) are produced. Sulphuret of carbon burnt in oxygen gas produces sulphurous acid and carbonic acid (Sch. 27).-Sulphuret of antimony heated in contact with air yields sulphurous acid gas and antimonious acid (Sch. 28).—A similar action takes place with other metallic sulphurets.-Chlorine gas converts sulphuret of antimony into chloride of sulphur and chloride of antimony (Sch. 29), and produces similar effects on other metallic sulphurets.—Sometimes the two new compounds A C and BC combine together. Thus sulphuret of copper when heated in the air is converted into sulphate of oxide of copper (Sch. 30).

The same decomposition often takes place in presence of a fourth body D combined with A B, one of the new compounds A B and A C, or each of them separately, uniting with this fourth body.--Mercury placed in a solution of nitrate of silver forms subnitrate of mercury and silveramalgam (Arbor Dianæ, Sch. 31).-When phosphorus is boiled in water holding potash in solution, phosphate of potash and phosphuretted hydrogen gas are produced (Sch. 32). -Chlorine gas passed into aqueous solution of potash yields chlorate and hydrochlorate of potash (Sch. 33). -If in this process we suppose that potash instead of water is the body decomposed, and that not hydrochlorate of potash but chloride of potassium is produced, then Sch. 34 must be substituted for Sch. 33. The action of bromine or iodine upon potash is precisely similar to that of chlorine. Sulphur exerts a similar action on lime (or potash) when water is present; the action will be represented by Sch. 35 or Sch. 36 according as we suppose hydrosulphate of lime or quinto-sulphuret of calcium to be the body formed. În Sch. 34 and 36, the fourth body is a part of the compound A B, which remains undecomposed. The same is the case in the conversion of potash by excess of sulphur at a red-heat into quintosulphuret of potassium and sulphate of potash (Sch. 37).

Sometimes a fourth body D exists before the decomposition in combination with A B, and is set at liberty by the decomposition. Sulphate of ammonia dissolved in water is converted by chlorine into hydrochloric acid, chloride of nitrogen, and free sulphuric acid (D) (Sch. 38).

3. The compound A B is acted upon by the compound CD, and there are formed two new compounds A C and BD. This very frequent and important case is called decomposition by Double Elective Affinity, Double Affinity, Attractio electiva duplex (Sch. 39).

Perchloride of phosphorus (P Cl) and water produce hydrochloric acid and phosphoric acid (P 0) (Sch. 40).-Sulphuretted hydrogen with oxide of lead yields sulphuret of lead and water (Sch. 41); with peroxide of tin, bisulphuret of tin and water (Sch. 42); with arsenious acid, tersulphuret of arsenic and water (Sch. 43); with arsenic acid, quinto-sulphuret of arsenic and water (Sch. 44).-Other hydracids act upon metallic oxides in a similar manner. Tersulphuret of antimony heated with protochloride of mercury yields terchloride of antimony and proto-sulphuret of mercury (Sch. 45).

The most frequently occurring decompositions by double affinity are those of salts (containing a base and an acid as proximate elements). Two salts which contain different acids and different bases often interchange elements in such a manner that the acid of the first combines with the base of the second, and the acid of the second with the base of the first. This decomposition is seldom observed when salts are melted together, because in case of an exchange taking place the two newly fornied salts often fuse together; but it very frequently occurs when solutions of the salts in water or other liquids are mixed together. In the latter case if the two newly-formed salts are likewise soluble, the decomposition may be discovered from the constitution of the crystals obtained on evaporation or cooling; but in very many cases one of the newly-formed salts is but slightly or not at all soluble in the menstruum, and consequently affords evidence of the decomposition by separating from the solution in the solid form. When aqueous solutions of carbonate of potash and sulphate of soda are mixed, the liquid yields on evaporation and cooling, first crystals of sulphate of potash, afterwards of carbonate of soda (Sch. 46).Nitrate of baryta and sulphate of soda mixed in the state of aqueous solution yield nitrate of soda which remains dissolved, and sulphate of baryta which immediately precipitates in the form of an insoluble white powder (Sch. 47).—Similarly, aqueous solutions of carbonate of potash and nitrate of lime yield a thick precipitate of carbonate of lime, while nitrate of potash remains dissolved in the liquid (Sch. 48).—A soluble salt may likewise interchange elements with an insoluble one; e. g. carbonate of soda and sulphate of lead yield sulphate of soda and carbonate of lead.

To this case belongs Richter's Law of Neutralization (Beiträge, 4, 66). Richter found that when decomposition takes place between two salts which are neutral to vegetable colours (page 97) the two newly-formed salts are likewise neutral. From this he concluded that if the acid of the first salt, by combining with a certain quantity of the base of the second, sets free a certain quantity of acid belonging to it, this quantity of acid is exactly sufficient to form a neutral salt by combining with the disposable quantity of the base of the first. Of this important principle Richter availed himself in his stoichiometrical calculations: the result can now be easily explained by the atomic theory as above developed. Salts are commonly neutral when they contain one atom of acid for every atom of base. If now two salts thus constituted decompose each other, precisely 1 atom of acid of the first combines with 1 atom of base of the second; and 1 atom of base of the first with 1 atom of acid of the second: hence both the new compounds are neutral. In those cases, however, in which the newly formed insoluble salt contains a number of atoms of a particular acid different from that which occurs in the former salt, the law of neutralization is subject to exceptions. Thus ordinary phospbate of soda contains 1 atom of phosphoric acid and 2 atoms of soda: now when this is decomposed by nitrate of silver (1 atom of acid + 1 atom of base) a compound is precipitated containing 1 atom of phosphoric acid and 3 atoms of oxide of silver; but these were originally combined with 3 atoms of nitric acid, and they now come in contact with only 2 atoms of soda, and since 2 atoms of soda require only 2 atoms of nitric acid to neutralize them, the liquid becomes acid (Sch. 49).

The following are cases of decomposition by double atfinity, in which a fifth body E also comes into play. The compound A B is in combination with È and the latter subsequently unites with A C. Oil of vitriol (sulphuric acid + water) mixed with chloride of sodium yields sulphate of soda and hydrochloric acid gas (Sch. 50).-Sulphate of mercury heated with chloride of sodium yields sulphate of soda and chloride of mercury (Sch. 51).—By fusing together sulphate of baryta and chloride of calcium we obtain sulphate of lime and chloride of barium (Sch. 52). - In a similar manner may be explained all other decompositions of a salt consisting of an oxacid and a base by any metallic sulphuret, iodide, bromide, chloride, fluoride, or cyanide, whether water be present or not. Thus aqueous solutions of sulphate of soda and chloride of barium yield, on mixing, a precipitate of sulphate of baryta, while chloride of sodium remains in solution (Sch. 53); and solutions of carbonate of potash and chloride of calcium yield precipitated carbonate of lime and soluble chloride of potassium (Sch. 54). If however we suppose that the metallic sulphurets, chlorides, &c., are converted on solution into salts consisting of hydracids combined with metallic oxides, then Sch. 54 must be altered into Sch. 55.-The mutual decomposition of ferrocyanide of potassium and sulphate of copper may be represented by Sch. 56 or Sch. 57, accordingly as it is supposed that the former dissolves in water without alteration, or becomes changed into prussiate of protoxide of tin and potash; in the former case both A B and C D are combined, the one with a fifth, the other with a sixth body, the former of which combines with A C, the latter with BD.

The decomposition of common salt by heating it with silica, and allowing vapour of water to have access to the mixture, presents this peculiarity—that E the silica is not combined with A B the water, but acts by itself and combines with A C the soda (Sch. 58).

In some cases of decomposition by double affinity, part of the compound AB remains undecomposed and enters into combination with the new compound A C. When 2 atoms of baryta at a red heat are acted upon by 1 atom of bisulphuret of carbon, there are formed 2 atoms of sulpburet of barium and 1 atom of carbonic acid which unites with the undecomposed atom of baryta (Sch. 59).— In other cases the undecomposed portion of A B unites partly with A C and partly with BD. Thus when tersulphuret of antimony in excess is fused with potash, a compound of sulphuret of antimony with oxide of antimony is formed, and also a compound of sulphuret of antimony with sulphuret of potassium (Sch. 60).

Or again, the undecomposed portion of A B unites with the new compound BD, and the undecomposed portion of C D with the new compound A C. Thus on fusing together 8 atoms of tersulphuret of antimony and 7 atoms of potash, we obtain a compound of sulphuret of potassium with sulphuret of antimony and a compound of oxide of antimony with potash (Sch. 61).—Bisulphuret of carbon dissolved in aqueous solution of potash yields sulphuret of potassium which unites with sulphuret of carbon, and carbonic acid which combines with potash (Sch. 62).

The compound A B may also be in combination with a fifth body E, which is set free by itself simultaneously with the formation of A C and BD. The compound of hydrochloric and ammonia (E) is resolved when mixed with lime into chloride of calcium and water, while ammonia is set free (Sch. 63.)

Lastly, a portion of B may remain uncombined: 2 atoms of hydrochloric acid and 1 atom of peroxide of manganese yield 2 atoms of water and 1 atom of chloride of manganese, while 1 atom of chlorine is disengaged (Sch. 64). If we suppose that hydrochlorate of protoxide of manganese is formed instead of the chloride of manganese, we must represent the action by Sch. 73.

4. The mutual action of AB and CD produces only the compound A C, while both B and D are separated in the free state (Sch. 65). Carbonate of potash mixed in solution with sulphate of alumina formis sulphate of potash, while carbonic acid is disengaged as gas, and alumina precipitated, this earth not being able to combine with carbonic acid (Sch. 66). Nitric acid and hydrochloric acid heated together form water, hyponitric acid, and chlorine (Sch. 67). In this case the compounds A B and C D may exist in combination one with the other, and suffer decomposition when heated. Thus sulphate of ammonia passed through a red-hot tube is resolved into water, nitrogen gas and sulphur (Sch. 68). Moreover the element B of the compound A B and the element D of the compound CD may be identical: thus sulphurous acid and sulphuretted hydrogen form water and sulphur (Sch. 69); iodic acid and hydriodic acid yield water and iodine (Sch. 70); nitrite of ammonia dissolved in water is resolved by gentle warming into water and nitrogen gas (Sch. 71). If in these cases the body C does not take up the whole of A, part of the latter remains in combination with the separated element which existed in both the original compounds: thus nitrate of ammonia is resolved by heat into water and protoxide of nitrogen. (Sch. 72.)

A portion of A B may also remain undecomposed and combine as a fifth body with either D or AC. Hydrochloric acid and peroxide of manganese resolve themselves by their joint action into water, chlorine, and hydrochlorate of protoxide of manganese (Sch. 73). Anhydrous sulphuric acid acting with the aid of beat on chlorine of sodium produces sulphate of soda, sulphurous acid gas, and chlorine gas (Sch. 74).

5. The two bodies C and D act separately on the compound A B, and produce the compounds A C and B D. The two electricities flowing into water on opposite sides produce oxygen gas, which may perhaps be regarded as a compound of positive electricity with oxygen, and hydrogen gas, which should perhaps be regarded as a compound of negative electricity wite hydrogen (Sch. 75); and a similar explanation may be given of the decompositions of other ponderable compounds by the electric current. Silica mixed with charcoal and acted upon at a red heat by chlorine gas, yields chloride of silicium and carbonic oxide (Sch. 76). Similar results are obtained with many other metallic oxides.

6. There are two compounds A B and C D independent of each other: a body E combines with A, brings B into combination with D, and sets C at liberty (Sch. 77). When

vapour

of water is passed over a mixture of chloride of silver and charcoal at a red heat, carbonic oxide, hydrochloric acid, and metallic silver are produced (Sch. 78). In the following instance another portion of E combines at the same time with BL. Chloride of sodium, peroxide of manganese, and sulphuric acid yield sulphate of soda, sulphate of protoxide of manganese, and chlorine (Sch. 79).

7. From a compound of A B with A D, E takes the whole of A and separates B and D either in the free state or combined together (Sch. 80). When hydrate of potash is brought into contact with iron at a white heat, oxide of iron, hydrogen gas, and potassium are produced (Sch. 81). Charcoal decomposes carbonate of soda, producing 3 At. carbonic oxide gas and 1 At. sodium (Sch. 82). Charcoal at : rnd heat decomposes

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