The elementary bodies forming a compound are frequently called its constituents, or components, and the act of union is called chemical combination, whilst the separation of a compound is called decomposition. CHEMICAL AFFINITY. The power or force by which bodies chemically unite is called affinity. It differs from the force of gravitation, which tends to draw all bodies towards the centre of the earth, and operates at all known distances. It also differs from cohesion, which binds like particles of matter together. The rain falls by gravitation, the particles of iron are held together by cohesion; but in iron rust (or oxide of iron) the oxygen is united to the iron by the force of chemical affinity. Chemical combination may be distinguished from mere mechanical mixture by the formation of compounds having properties different from the bodies which have entered into combination. And what is most remarkable, bodies of opposite chemical characters evince the strongest tendency to combination. Chemical affinity may be regarded as that force by which bodies unite, and form compounds having properties totally different to the constituents which have entered into combination. This combination is usually attended with an elevation of temperature. Chemical affinity acts with different degrees of force. If sulphuric acid be united with ammonia, and magnesia added to the compound, the ammonia is set free, and the sulphuric acid combines with the magnesia. This union of magnesia and sulphuric acid may be again decomposed with lime; and this compound may be decomposed with soda, then potash, then strontia, then baryta. From this it appears that the affinity of sulphuric acid for baryta is strongest, and for ammonia the weakest. The force of chemical affinity is very much influenced by heat. Some chemical compounds are entirely decomposed by heat; the particles seem to be removed beyond the sphere of chemical attraction by the repulsive force of heat. Heat destroys all organic substances, but other compounds are formed of a more permanent character. Light, also, modifies the force of affinity if equal volumes of chlorine and hydrogen be mixed in the dark they do not unite, but under the influence of sun-light combination takes place with great violence. The particles of matter attracted are assumed to be in different electrical states, and this is regarded as the basis of all chemical action. Electrical currents are among some of the most powerful causes of chemical decomposition. As chemical combination can only take place when the particles or molecules are brought within the sphere of chemical attraction, which acts at insensible distances, the force of cohesion must, first, be overcome; and this is done by dissolving the substance usually in water: so that the particles may be brought into closer contact. Substances in a solid state rarely combine, even if reduced to powder. Phosphorus with Iodine is an exception. When elements are liberated at the same time, in what is called their nascent condition, compounds are sometimes formed which cannot be effected under other circumstances. If nitrogen and hydrogen be mixed in the same vessel, combination does not take place; but when these gases are set 'free at the same time, as in the case of manure heaps, ammonia is formed. LAWS OF CHEMICAL COMBINATION. CATALYSIS. 7 Chemical combination is also influenced by what is called catalysis; that is, a body by its mere presence induces changes, in which the body itself seems to act by mere contact. If starch be boiled in a little weak sulphuric acid, it is converted into sugar. This acid, if examined after the process has terminated, is found unaltered; so that the smallest quantity of acid is sufficient to convert any quantity of starch into sugar. The acid seems to act by its presence. The binoxide of manganese undergoes no change when heated with the chlorate of potash to obtain oxygen, but the oxygen comes off at a much lower temperature than when the salt is used alone. LAWS OF CHEMICAL COMBINATION. Chemical combination takes place according to certain definite laws.. These laws are called the laws of combining proportion: no circumstances can alter the proportional quantities in which bodies chemically unite. The same chemical compound invariably consists of the same elements united in the same proportion by weight. Pure water, no matter where obtained, contains 8 parts of oxygen by weight united to 1 part of hydrogen. Common salt, whether from Poland or Cheshire, contains 23 parts by weight of sodium, to 35.5 parts by weight of chlorine. Carbonate of lime, whether crystallised or amorphous, has the same elements united in the same proportion by weight. When one body is capable of uniting with another, in several proportions, these proportions bear a simple relation to each other. This is called the law of multiple proportions. The best illustration of this law are the five oxygen compounds of nitrogen. N=14. 0,= 24 NO, = 14+8 +8 +8 = 38 Nitrous acid. 01 = 32 NO1 = 14 +8 +8 +8 +8 = 48 Hyponitric acid. 0,- 40 NO, 14+8 +8 +8 +8 +8 = 54 = Nitric acid. In this example, while the nitrogen remains constant, the oxygen increases by multiples of eight. Sometimes the members of a group are wanting: if these members should be discovered, they will, doubtless, follow the law just illustrated. Chlorine affords a good example. Cl = 35.5. O, 24 CIO, = 35′5+8+8+8=59'5 == Chlorous acid. Cl=35.5. 0,=32 CIO1 = 355 +8 +8 +8 +8=67.5 Cl=355. 0,40 CIO,=355 +8 +8 +8 +8 +8=75.5 Chloric acid. Cl=355. 0,=56 CIO,=35·5+8 +8 +8 +8 +8 +8+8=91.5 Perchloric acid. Here the compounds between the first and second, and the fourth and fifth are wanting. While the chlorine is a constant quantity, the oxygen increases by multiples of eight. The oxygen compounds of sulphur afford another illustration of this law. S1 = 32 S=16 0,= 40 S205 − 16 + 16 + 8 + 8 + 8 + 8 + 8 = 72 = Hyposulphuric acid. S = 16 01 = 24 SO, = 16 +8 +8 +8 = 40 Sulphuric acid, If one body, A, unite with other bodies, BCD, the quantities of B CD which unite with A represent the proportions in which they unite among themselves, in the event of union taking place. No element has a greater range of affinity than oxygen, and many of its compounds are capable of exact analysis. Its combining equivalent is eight. Let us take a few illustrations. These numbers represent the proportions in which these elements unite: thus, 8 of oxygen unites with |