So likewise the elasticity of nitrogen may prevent that substance from combining with metals, inasmuch as by entering into such combination, it would lose its gaseous form. If a gas be regarded as a compound of a ponderable body with heat, the explanation just given will amount to this, that nitrogen is prevented from combining with metals in consequence of its greater affinity for heat.

Compounds resulting from the union of two simple substances or Compounds of the First Order, to which belong the inorganic acids, bases, metallic chlorides, &c., are themselves for the most part capable of combining-sometimes though rarely-with simple substances, but much more frequently with other compounds of the same order. In this manner are formed Compounds of the Second Order, the most important of which are the simple salts. These compounds again are capable of uniting both with each other and with compounds of the first order, thus forming compounds of the higher orders: and so on. But the more complicated the constitution of any such substance may be, the more nearly will the combining tendencies of its elements be satisfied, and the less therefore will be the inclination of those elements to enter into further combinations. In this manner chemistry reaches its limit. In compounds of the second order, a distinction may be made between Proximate and Ultimate elements (Principia proxima et remota); in those of the third order, between Proximate elements of the first order, Proximate elements of the second order and Ultimate elements. Thus in sulphate of potash, sulphuric acid and potash are the proximate elements; and since sulphuric acid consists of sulphur and oxygen, potash of potassium and oxygen, we say that oxygen, sulphur and potassium are the ultimate elements. Since compounds generally exhibit affinities different from those of their components, it follows that the affinities of the components in their character of primitive or elementary substances will sometimes differ from the resulting affinity of the compound. The older chemists distinguished such cases by particular names. Thus if there be a substance A with which another substance B is capable of uniting, while a third substance C is not capable by itself of entering into such combination but becomes so by uniting with B, the affinity thus manifested was called Mediating Affinity (verittelnde Verwandtschaft, Affinitas approximans, appropriata s. adjuta.) For instance, alumina (C) by combining with sulphuric acid (B) becomes uble in water (A). If neither B nor C can combine with A, but the mbination BC can form such a union, the affinity is called Produced or Developed Affinity, (erzeugte Verwandtschaft, Affinitas producta). Thus ither carbon nor nitrogen can combine with mercury; but their comMund, cyanogen, has a powerful affinity for it. An example of the ability of four liquids to combine is afforded by the so-called four elements ercury, solution of carbonate of potash, dilute alcohol and rock-oil).

III. FORMATION OF CHEMICAL COMPOUNDS.

The case in which 2 or more bodies combine without causing the struction of any previously existing chemical compound was called by e older chemists Affinity of composition or of mixture, zusammensetzende er vermischende Affinität, Affinitas compositionis s. mixtionis.

1. Conditions under which Chemical Combination takes place. A. The affinity of the combining bodies must be sufficient to overme all opposing forces, such as gravitation, cohesion, and elasticity.

B. The substances must be brought into immediate contact, for affinity does not act at sensible distances.

C. Generally speaking, one at least of the combining bodies must be either in the liquid or gaseous state, and if it be not so at ordinary temperatures it must be brought into that state by elevation of temperature.

Hence the old rule: Corpora non agunt nisi fluida from the erroneous supposition that the fluid or menstruum was the only active body, and the solid or solvendum a resistance to be overcome. Solid bodies either do not combine at all, or their combination is attended with great difficulty, because from the immobility of their particles their points of immediate contact are but few, and the exceedingly thin film of compound which may be formed at such points acts as a partition to prevent further contact and consequently further combination. But by continued rubbing, which renews the points of contact, more complete combination may often be effected in this manner finely divided copper may be made to combine with sulphur, the combination being even attended with rise of temperature. If, on the other hand, the compound formed by the two solids is itself fluid, its mobility gives rise to continually renewed contact, and combination goes on. Thus ice under 0° unites with chloride of sodium and other salts, and solid amalgam of lead with solid amalgam of bismuth. Crystallized oxalic acid and lime may be made to combine by rubbing them together, because the acid contains more water of crystallization than the oxalate of lime produced is able to take up: hence at the beginning of the action a little water is set free and dissolves the oxalic acid, &c., &c. In some cases it is sufficient to heat one of the solid bodies till it softens: thus iron surrounded with charcoal and heated to whiteness is slowly penetrated by the charcoal (Cementation). When in consequence of one or both bodies being in the fluid state, combination takes place at the ordinary temperature or a little above it, it is called Solution in the wet way, (Solutio via humida); if a higher temperature is required, the process is called Solution in the dry way, Fusion (Solutio via sicca, Confusio.)

D. Even if one or both of the bodies be in the fluid state, a high temperature is often necessary to effect the combination.

Melted sulphur will not combine with carbon; the sulphur must brought in the state of vapour into contact with red-hot charcoal, althoug the elasticity of the vapour might rather be expected to interfere wi the combination. Neutral carbonate of soda in the efflorescent sta absorbs carbonic acid very slowly at first, but more and more quickly it gets heated by the absorption, and ultimately with great violenc (Mohr, Ann. Pharm. 29, 268.) Charcoal requires to be heated before will burn in oxygen gas, that is, before it will combine with the oxyge At ordinary temperatures, oxygen may be mixed with hydrogen an other inflammable gases without combining with them, but at a red he combination takes place immediately. In this case both bodies are flui and we might expect that heat by increasing their elasticity would rath oppose than favour the combination. The manner in which heat ad in such cases is not precisely understood. If for instance we suppose th the affinity between oxygen and hydrogen in the cold is not sufficient overcome their elasticity but becomes greater at a higher temperatur then the resulting compound ought, on cooling, when the affinity betwed its elements is again diminished, to be resolved into those elements h the action of elasticity. If again, with Monge and Berthollet, we su pose that the portion of the gaseous mixture first heated presses by i

expansion on the neighbouring particles, and thereby causes them to combine, it may be argued on the other hand that such pressing together of the particles can have but little effect, and that no combination ensues when the gaseous mixture is gradually subjected to a pressure of 50 or even 150 atmospheres.

E. In some cases, light has the same effect as an elevation of tempc

rature.

Chlorine and hydrogen or carbonic oxide.

F. Electricity likewise favours the combination of many substances, acting chiefly by elevation of temperature, but also by the compression which the electric spark exerts upon the gaseous mixture through which

it

passes.

G. In some instances, the expansion of gaseous bodies favours their combination with others. Phosphorus undergoes slow combustion in oxygen gas however low the temperature may be, the action going on more quickly as the gas is more rarefied; a mixture of oxygen and nouinflammable phosphuretted hydrogen gases explodes on expansion.

H. The presence of a heavy solid body, particularly a metal, having a great extent of surface, likewise causes, sometimes at ordinary, sometimes at slightly elevated temperatures, the combination of oxygen with inflammable gases and vapours, which would otherwise take place only at

a red heat.

This property is most strikingly exhibited by platinum; the more finely divided the platinum, the stronger is its action. When the combination of oxygen with inflammable gases takes place at its surface, the heat developed raises its temperature and thereby increases its activity, till at length the metal becomes red-hot and then sudden combination (Vid. Oxygen and Hydrogen.) Platinum appears to condense gases particularly oxygen on its surface by adhesion (page 26), so that the heterogeneous atoms, being deprived of their heat-spheres, are able to approach one another and combine.

ensues.

I. Many bodies, particularly those which are very elastic, or very cohesive, combine together only when aided by the chemical co-operation of other bodies.

a. Formation of chemical compounds by Substitution.

One, or both, of the combining bodies is previously contained in another ompound which is less elastic or less coherent than the body itself, and from which it passes over to the new combination in the so-called nascent rate, before it has time to reassume the highly elastic or highly coherent ate which belongs to it. Nitrogen and hydrogen will not combine to form ammonia by the action of either heat or electricity; but if tinings be placed in contact with water and binoxide of nitrogen, the tin vill rob both these bodies of their oxygen, and the hydrogen disengaged rom the water will combine at the moment of liberation with the Mitrogen set free from the nitric oxide, and form ammonia. Tin acts in he same manner on dilute nitric acid. Ammonia is also produced on eating nitre with gum, and likewise from azotized organic substances then heated alone. Oxygen and nitrogen will not, without great diffiulty, combine directly to form nitric acid: this substance is however btained when ammoniacal gas is passed over red-hot oxide of mangaese, or a mixture of ammoniacal gas and oxygen passed through a redhot tube. The nitrogen being the less elastic of the two elements of mmonia, is more disposed to combine with oxygen when the hydrogen has been separated from it by the action of that substance, than when it

is in the free state. Similarly the combinations of nitrogen with chlorine, bromine, iodine, sulphur, and phosphorus are not obtained directly from nitrogen gas itself, but from the decomposition of ammonia. Iodine will not combine directly with oxygen gas to form iodic acid: this acid is however produced on heating iodine with nitric acid. Similarly iodic acid is produced, as also bromic and chloric acid (which latter cannot be formed by means of nitric acid) by bringing iodine, bromine, or chlorine in contact with solution of potash. The compound of water and oxygen, called peroxide of hydrogen, is obtained not from water and oxygen gas, but from water, peroxide of barium, and hydrochloric acid, the acid abstracting baryta and leaving the excess of oxygen of the peroxide of barium to go over to the water.

Among the instances in which cohesion is diminished by the action of a pre-existing compound, the following may be mentioned: Anhydrous baryta does not absorb carbonic acid gas, but the hydrate takes it up readily, water being set free at the same time. Crystallized alumina (Sapphire), and many other weak bases in the crystallized or ignited state do not dissolve in hydrochloric acid; but after being heated with a large quantity of caustic potash with which they combine, they become soluble in that acid. If the insolubility of crystallized alumina arose from its cohesion being greater than its affinity for the acid, it ought not to dissolve in the acid after being ignited with potash, but to separate, in consequence of its greater cohesion, after the potash had been dissolved in the acid: it appears then to be only the peculiar kind of aggregation belonging to the crystalline state that prevents the alumina from acting in obedience to its affinity for the acid.

b. Induction of chemical combination by communication of chemical energy. A body in the act of chemical combination has the power of inducing the same kind of activity in another body and causing it to combine with a third body, thereby forming a compound which, under the existing circumstances, would not have been formed without the presence of the first body. (Liebig, Ann. Pharm. 30, 262.) Wet peat-earth gradually absorbs oxygen gas; if the latter be mixed with hydrogen portion of the hydrogen enters into combination with the oxygen, whic it would not do in the absence of the peat-earth. (Saussure.) Nitroge gas does not by itself combine with oxygen, even when heated; but if mixture of nitrogen and hydrogen be set on fire, the hydrogen burn producing water, and a portion of the nitrogen combines at the sam time with oxygen, producing nitric acid. Pure copper does not oxidat in water mixed with sulphuric acid, but when combined with zinc an nickel (in German silver), metals which decompose acidulated water, when combined with three times its weight of zinc only, it oxidates an dissolves completely together with the other metals. Platinum when alon does not oxidate and dissolve in nitric acid, but when alloyed with silvo it becomes soluble in that acid.

2. Circumstances and Results of the chemical combination of Ponderab Bodies.

A. Emission and absorption of imponderable substances.

C

In all cases of the combination of ponderable bodies, a change and geuerally a rise of temperature is produced, sometimes amounting to the most intense heat. This rise of temperature is generally greater in pro portion to the strength of the affinity by which the combination is brought

about; a fall of temperature is less frequent. (Vid. Heat.) Some combinations are attended with a slight development of electricity. (Vid. Electricity.)

B. Time in which combination takes place.

a. With the same two bodies, the rapidity of combination is increased by diminishing the quantity of a solid body in proportion to that of a liquid, or of a gaseous body in proportion to that of a liquid or a solid; by diminishing the cohesion of a solid by heating, or the elasticity of a gas by cooling and compression; and lastly, by comminuting a solid body and increasing the number of points of contact by agitation and friction.

A salt placed at the bottom of a quantity of water dissolves very slowly when at rest, because the heavy film of saline solution which is gradually forming remains above the salt and prevents its contact with the rest of the water: the same salt placed at the upper part of the water in a muslin bag or a filter dissolves very rapidly, because the solution as it is formed sinks to the bottom and allows the rest of the water to come in contact with the salt. Ammoniacal gas directed upon the surface of water is very slowly absorbed, because the new compound (solution of ammonia) is lighter than water, and therefore forms a layer on the surface preventing the further contact of the water and gas; but if the gas is directed through a tube to the bottom of the water, the absorption takes place very quickly. Hydrochloric acid gas, on the contrary, is rapidly absorbed when directed on the surface of water because its solution in that liquid being heavier than water sinks to the bottom, and fresh water comes to the surface. A metal slowly cooled after fusion dissolves more rapidly in acids than it would if it had been hammered.

b. With different bodies, the rapidity of combination is greater, in proportion as their affinity is greater, their cohesion less, their difference of specific gravity smaller, their diffusion through one another more easy, and the fluidity of the new compound more complete. The combination of solids with fluids takes place much more slowly in consequence of the ofreater cohesion of the former than that of fluids with fluids. Liquids f different specific gravity combine slowly when at rest and disposed one bove the other in layers, but quickly when shaken. Gases combine most quickly of all, because they diffuse themselves through each other pontaneously by adhesion (page 20). If the new compound is solid at rdinary temperatures, it places itself between the new bodies and hinders heir further combination; e. g., zinc and sulphur.

C. Proportions in which bodies combine.

This forms the subject-matter of Stoichiometry, or the Doctrine of Chemical Proportions or Chemical Equivalents.

Ponderable bodies generally combine in definite proportions, which come out with greater distinctness as the affinity between the combining substances is stronger. With respect to the proportion in which two bodies combine, the following cases present themselves:

a. Two bodies may be mixed in any proportion whatever, and in no ase does the mixture present any peculiar properties: e. g., Water and alcohol, alcohol and ether, ether and volatile oils.

b. One body A may take up any quantity whatever of another body B; but B, after having combined with a certain quantity of A, takes up no more of it.-B is then said to be saturated with A; the point of satu