of lime expels the carbonic acid, but produces no such effect when the mixture is heated in a strong closed iron vessel. In this case, it may with great probability be supposed that the affinity of silica for lime is weaker than that of carbonic acid, and that the formation of silicate of lime takes place only when the action is assisted by the affinity of heat for carbonic acid.

b. In other cases, difference of temperature appears to modify the result in consequence of the cohesion (or affinity?) of bodies increasing and diminishing in different degrees at lower and higher temperatures,-and here in particular Berthollet's law regarding the decomposition of salts by double affinity finds its application.

A solution of common salt and sulphate of magnesia evaporated at ordinary temperatures, or a little above, allows both salts to crystallize out unaltered (page 127); but at 0° C, or at lower temperatures, as was long ago observed by Scheele, hydrated sulphate of soda crystallizes out and the solution retains chloride of magnesium; on gently warming the whole, common salt and sulphate of magnesia are again obtained. But above 50° C. (122° Fah.) the solution again deposits sulphate of soda, though in the anhydrous state. (H. Rose, Pogg. 35, 180.) These results may be explained by the different solubility of sulphate of soda at different temperatures: at 0° C. one part of sulphate of soda requires 8.2 parts of water to dissolve it, at 33° the smallest quantity, viz. 0:33, and at 50-4° again 0:38 parts. Below 0° and above 50° the solubility must be considerably less. Since now, according to Berthollet's law, the least soluble salt is always produced, sulphate of soda separates both below 0° and above 50°, because at these extremes of temperature its solubility is less than that of common salt or sulphate of magnesia; at medium temperatures, on the contrary, at which sulphate of soda is more soluble than common salt or sulphate of magnesia, these salts remain unaltered.

In a similar manner, sulphate of soda and chloride of potassium dissolved together in water resolve themselves, at ordinary temperatures, into sulphate of potash and chloride of sodium, whereas, according to Hahnemann and Richter (Stöch. 2, 224) a solution of the last-named salts at -20°C (-4° Fah.) deposits sulphate of soda. At ordinary temperatures sulphate of potash, at low temperatures sulphate of soda is the less soluble salt. According to Constantini, alum and common salt yield crystals of Glauber's salt at freezing temperatures, and according to Hahnemann, Glauber's salt crystallizes at very low temperatures, even from a mixture of saturated solutions of gypsum and common salt.

The explanation of the following cases-by supposing a disproportionate alteration of cohesion to be produced by change of temperatureis not quite so satisfactory.

An aqueous solution of sulphate of lime gives with chloride of barium a precipitate of sulphate of baryta, while chloride of calcium remains in solution. (Sch. 52.) If, on the other hand, chloride of calcium be fused with sulphate of baryta, a mixture of sulphate of lime and chloride of barium is formed, the latter of which may be removed by rapidly boiling the powdered mass in water and filtering; but by longer standing under water the whole would again be converted into sulphate of baryta and chloride of calcium. Does the affinity of water for chloride of calcium contribute to this result?

Sulphate of baryta is decomposed both by fusion with carbonate of soda and by boiling with the aqueous solution of that salt (though but imperfectly), yielding carbonate of baryta and sulphate of soda: on the

contrary, as Kölreuter has shown, carbonate of baryta is decomposed by digestion with sulphate of soda at ordinary temperatures, the products being sulphate of baryta and carbonate of soda.

A dilute solution of nitrate of lime remains clear when mixed with a solution of sulphate of soda, but deposits sulphate of lime when warmed. (Persoz.)

A solution of alum does not become turbid when mixed with very small quantities of carbonate of lime or soda, but on evaporation at a gentle heat, yields crystals of cubic alum. But when more strongly heated, the solution becomes turbid and deposits basic sulphate of alumina which redissolves as the solution cools.

A solution of pure acetate of alumina does not become turbid on heating, but undergoes that change if it contains sulphate of ammonia, potash, soda, or magnesia; a fainter turbidity is produced by the addition of nitrate of potash, none by the addition of nitrate or acetate of baryta, chloride of calcium, or acetate of lead. The precipitate which consists of hydrate of alumina disappears when the solution is cooled, and appears again on heating. (Gay-Lussac, Ann. Chim. 74, 193, also Schw. 5, 49; further, Ann. Chim. Phys. 6, 201, also Schw. 21, 96.)

Persulphate of iron mixed with acetate of potash deposits hydrated peroxide of iron on boiling.

An aqueous mixture of borate of soda and sulphate of magnesia yields, on application of heat, a precipitate of borate of magnesia, which however disappears each time on cooling.

Metallic silver takes oxygen from persulphate of iron dissolved in water, when boiled in the liquid, so that a solution is formed containing sulphate of silver and protosulphate of iron; but on cooling, all the silver is reprecipitated in the metallic state, and the solution once more contains sulphate of peroxide of iron. (Sch. 94.)

In many other cases the occurrence of reciprocal affinity is only apparent.

When ammonia is added to neutral sulphate (nitrate or hydrochlorate) of magnesia, it is taken up and magnesia precipitated; on the other hand, magnesia expels ammonia from the neutral sulphate (nitrate or hydrochlorate) of ammonia and is itself dissolved. In both cases however the decomposition is only half complete, in whatever excess the ammonia or magnesia may be added. In the first case, half of the sulphate of magnesia remains undecomposed and unites with the sulphate of ammonia in the form of a double salt containing 2 atoms of sulphuric acid, 1 atom of ammonia, and 1 atom of magnesia; in the second, half of the ammoniacal salt remains undecomposed, and forms the same double salt with the sulphate of magnesia produced. (Sch. 95 and 96.)

Nitric acid added to chloride of potassium forms nitrate of potash, and sets hydrochloric acid free; on the other hand, nitrate of potash is converted by excess of hydrochloric acid into chloride of potassium. The affinity of potassium for oxygen + that of chlorine for hydrogen + that of nitric acid for potash is undoubtedly greater than the affinity of potassium for chlorine that of hydrogen for oxygen; and thus the first case explains itself. (Sch. 97.) If, on the contrary, nitrate of potash is to be converted by hydrochloric acid into chloride of potassium, a great excess of the acid must be used and heat be applied: moreover the excess of hydrochloric acid does not expel the nitric acid in it unaltered state, but the two together are resolved into hyponitric O), chlorine, and water. (Sch. 98.) Thus it is not nitric acid weaker hypo

nitric acid that is driven out by the hydrochloric acid. Nitric acid separates arsenious acid from an aqueous solution of arsenite of potash; but when nitrate of potash is ignited in contact with arsenious acid, nitrous acid is evolved and arseniate of potash formed. In this case, the stronger nitric acid is converted by loss of oxygen into the weaker nitrous acid, while on the contrary, the weaker arsenious acid is by access of oxygen converted into the stronger arsenic acid. The affinity of arsenious acid for oxygen that of arsenic acid for potash overcomes the affinity of nitrous acid for oxygen + that of nitric acid for potash. (Sch. 99.)

The existence of reciprocal affinity has also been inferred from the fact that certain salts, which when dissolved in a small quantity of water decompose one another by double affinity, producing a precipitate of a difficultly soluble salt, give no precipitate in more dilute solutions,— although the quantity of water present would not be sufficient to hold in solution the less soluble salt which may be produced by the decomposition, if it existed in the separate state. For example, sulphate of lime requires about 400 parts of water to dissolve it; but chloride of calcium, dissolved in about 200 parts of water (which would produce more than an equal quantity of sulphate of lime) gives no precipitate with sulphate of potash. Many chemists conclude from this that when this large quantity of water is present, the chloride of calcium is not decomposed by the sulphate of potash, since if sulphate of lime were formed, more than half of it would be precipitated. But it is simpler to suppose that the formation of sulphate of lime takes place even in this case, but that the presence of the chloride of potassium, which is formed at the same time, renders it more soluble than it otherwise would be. Similarly, it was long since observed by Guyton-Morveau that lime-water rendered turbid by passing carbonic acid gas through it becomes clear again on the addition of sulphate of potash or chloride of potassium, as if the presence of these salts rendered the carbonate of lime more soluble: and Karsten (Schrift. d. Berl. Akad., 1841) has shown that many easily soluble salts are rendered more soluble by the addition of other salts. These mutual affinities of salts, and the greater solubility in water thereby produced, explain the occurrence of carbonate and sulphate of lime, carbonate of magnesia, &c., in mineral waters, in quantities greater than pure water could dissolve; so that this phenomenon by no means renders it necessary to suppose that such waters contain salts which are incompatible with each other, i. e., which at the given temperature would decompose one another and form a precipitate, if the quantity of water present were smaller. Comp. Berthollet (Statique Chim., 1, 103, and 129); Brandes (Schw. 43. 153; 46, 433.)

Many other facts relating to the theory of reciprocal affinity but requiring more accurate investigation may be found in Scheele (Opusc. 1, 223): Grotthuss (Scher. N. Bl. 275): N. Fischer (Pogg. 7, 263): Berthollet (Statique Chim. 1, 81, 99, 100, 401) and Dulong (Ann. Chim. 82, 273, also Schw. 5, 369). In the experiments of the last-named philosopher on the decomposition of insoluble by soluble salts, variation of temperature, which might have produced opposite results, does not seem to have been sufficiently attended to.

2. Circumstances and Results of Decomposition.

a. Change of temperature. Since heat is generally set free in the combination of bodies, an equal quantity of heat must also become latent

at their decomposition. Nevertheless most decompositions are accompanied by rise of temperature sometimes amounting to the most vivid combustion. With reference to this matter the following cases must be distinguished:

a. When peroxide of hydrogen is decomposed by heat or by the contact of pulverized bodies into water and oxygen gas, a great deal of heat and even light is developed, notwithstanding that a considerable quantity must be rendered latent in the formation of the gas. This remarkable instance leads us to suppose that in the formation of this very loosely united compound heat is not set free but rendered latent, that in fact oxygen gas heat enters into combination with the water.-A similar constitution may exist in certain metallic bromates which when heated are resolved with sudden incandescence into a metallic bromide and oxygen.

B. When bodies separate in the solid form from their solution in a liquid or a gas, in consequence of their cohesion being increased by cooling, a development of heat takes place, the heat of fluidity which the bodies had previously absorbed in the act of solution being set free.

7. Most decompositions take place in consequence of weaker affinities being overcome by stronger ones. Now even if heat be rendered latent by the destruction of the compounds produced by the weaker quiescent affinities, a still greater quantity must nevertheless be set free in consequence of the neutralization of the stronger decomposing affinities, and the rise of temperature attending the decomposition marks the difference between these two quantities of heat.-If A develope a quantity of heat =2 in combining with B, and = 3 in combining with C, then in the decomposition of AB and formation of A C, a quantity of heat must be developed 3 − 2 = 1.

Lowering of Temperature takes place when gaseous products of decomposition are evolved from solid or liquid compounds, and the absorption of heat thereby produced is not compensated by a development of heat resulting from the formation of new compounds.-The escape of carbonic acid gas from water charged with it, on diminishing the pressure, is accompanied by diminution of temperature; on the contrary, when this gas is disengaged from a solution of carbonate of soda by the addition of sulphuric acid, a slight rise of temperature is produced.

[On the development of Electricity by chemical decomposition, vid. Electricity.]

b. The time in which decomposition takes place depends chiefly on the circumstances noticed on page 39. If one of the products of decomposition is gaseous and has to make its escape from a liquid, the decomposition is accelerated by the presence of angular bodies.

c. Qualitative alteration. Every decomposition results in the production of at least two heterogeneous substances or products of decomposition, which may be either solid, liquid or gaseous, and-so long as they do not separate by virtue of their different specific gravities-produce a cloudy, opaque mixture.

If the decomposition is attended with the formation of gaseous products, effervescence and explosion may ensue.-Effervescence or frothing is produced when a gaseous body is continually developed and rises up in bubbles during the decomposition of- a liquid;-Carbonate of potash and sulphuric acid. ise is fainter) a 'most instantly

In Explosion, Detonation (or Puffing, wher gaseous product of decomposition (or several)

from a solid (e.g. fulminating silver), a liquid (chloride of nitrogen) or a gas (oxide of chlorine)-and in endeavouring to expand itself into the much larger volume corresponding to its elasticity (often greatly increased by rise of temperature) forcibly presses back the air and other objects, producing detonation and breaking the solid bodies in its neighbourhood.Even gaseous compounds, such as oxide of chlorine, may be decomposed with explosion, if the separated elements, such as chlorine and oxygen gases, take up a larger volume than that occupied by the compound.The production of light which accompanies so many of these explosions often proceeds from this circumstance, that great heat is developed in the decomposition in consequence of the formation of more intimate compounds, as in the explosion of gunpowder, detonating powder, &c. and this higher temperature imparts to the gases and vapours at the moment of their production a proportionately increased elasticity, and thereby strengthens the explosion. In the decomposition of oxide of chlorine, chloride of nitrogen, iodide of nitrogen, &c., which appears not to be attended with a rise of temperature amounting to ignition, the development of light may perhaps be explained by the violent compression of the air around the exploding body.-The theory of the detonation of an explosive mixture propounded by Brianchon (Bibl. univ. 28, 393) has been shown by Gay-Lussac (Ann. Chim. Phys. 29, 53) to be untenable. R. Böttger has also shown (Ann. Pharm. 29, 75) that the explosive force of fulminating silver is equally strong in all directions.

When several liquid products result from a decomposition, they form a turbid mixture, until they have taken up the determinate relative positions corresponding to their different specific gravities:-Volatile oil dissolved in alcohol with water.

When in the decomposition of a liquid or gaseous body, solid products are formed and sink to the bottom in consequence of their greater specific gravity, they are called Precipitates, and a decomposition of this kind is called Precipitation,-Spontaneous Precipitation, (Præcipitatio spontanea) when the separation of the solid body takes place merely from change of temperature (page 113);-Forced Precipitation, (Præcipitatio coacta) when it is brought about by the addition of another ponderable body, the Precipitant (Præcipitans). If the solid product of decomposition is specifically lighter than the liquid, it is separated as a Scum (Cremor).The precipitate and the scum may be either educts or products.-Lime precipitate from lime-water by alcohol is an educt; oxalate of lime precipitated from lime-water by oxalic acid is a product.

The atoms of the solid product of decomposition unite themselves at the moment of their separation or formation into larger masses, which however possess various magnitudes and forms depending chiefly on the nature of the bodies, so that to a certain extent the nature of the precipitate may be inferred from its outward appearance: considerable influence is however exerted by the time in which the precipitation takes place and the degree of dilution of the liquid. The following forms of precipitates may be particularly distinguished,-and of these the first two must be regarded as anarphous, the others as crystalline: Flocculent; aggregation in large, loose, threadlike masses: alumina, hydrated peroxide of iron, phosphate of lime.-Curdy; in this form the masses are still larger and more dense and solid, but still uncrystalline: chloride of silver as it is precipitated from solutions of silver-salts by hydrochloric acid; casein as precipitated from milk by an acid.-Pulverulent; the atoms are united in small, imperfectly crystalline masses: sulphate of baryta as precipitated