each other. By suitable means chemical action may be made to take place between these two elements, whereby a complete rearrangement of the atoms takes place, resulting in the formation of molecules of water-molecules in which, as has been already mentioned, one atom of oxygen is associated with two atoms of hydrogen. The product of the chemical action is therefore water, while both the forms of matter of which it is composed are gaseous.

The air we breathe, and which is necessary to life, consists of a simple mixture of two colourless gases, viz., oxygen and nitrogen. When chemical action takes place between these substances, a brown-coloured gas is produced in which no animal or vegetable life could exist for many minutes, on account of its suffocating

nature.

Common salt, which is a white solid substance, and not only harmless, but even a necessary article of food, contains two atoms in its molecules--one an atom of chlorine, which is a yellow gas, intensely suffocating and poisonous; and the other an atom of sodium, a soft, silver-like metal, which takes fire in contact with

water.

Why it is that a molecule, consisting of an atom of chlorine and an atom of sodium held together by chemical affinity, should be endowed with properties so totally different from those of the contained elements, is altogether unknown; and similarly, it is quite impossible to predicate from the properties of any compound what are the particular elements of which it is composed. Thus, sugar is a white crystalline solid, soluble in water, and possessing a sweet taste; but no one would have ventured to predict that the molecules of this substance were composed of atoms of carbon (i.e. charcoal), a black, tasteless, insoluble solid; hydrogen, a colourless, tasteless gas; and oxygen, another colourless, tasteless gas.

Chemical Affinity.-When molecules, consisting of two atoms, say A B, come in contact with molecules consisting of other two atoms, CD, and a chemical change takes place resulting in the formation of new molecules, AC and B D, the question naturally arises, Why does the atom A leave the atom B and attach itself to C? In other words, what determines the rearrangement of the atoms into new molecules?

At present no exact answer can be given to this question. Chemists express the fact by saying that the chemical affinity

existing between A and C is greater than that exerted by B upon A. This remarkable selective power possessed by the atoms of different elements lies at the root of all chemical phenomena, and it differs between the various elements to an extraordinary degree. For example, the atom of chlorine possesses a very powerful chemical affinity for the atom of hydrogen: when hydrogen molecules, which consist of two atoms, are mixed with chlorine molecules, which are also aggregations of two atoms, at first a simple mechanical mixture is obtained, the two different kinds of molecules move amongst each other without undergoing change. On very small provocation, however, the affinity of the hydrogen atoms for the chlorine atoms can be caused to exert itself; by merely momentarily exposing the mixture to sunlight a complete redistribution of the atoms suddenly takes place with explosive violence and new molecules are formed, each containing one atom of hydrogen and one atom of chlorine.

Again, an atom of nitrogen is capable of associating itself in chemical union with three atoms of the element chlorine, forming a compound whose molecules therefore contain four atoms. The chemical affinity between the atoms of chlorine and nitrogen is so feeble, the system is, so to speak, in a state of such unstable equilibrium, that the very slightest causes are sufficient to instantly separate the atoms in the most violently explosive manner, and so break up the compound molecule into separate molecules of chlorine and nitrogen. In this case the affinity between one chlorine atom and another chlorine atom is greater than that between chlorine and nitrogen, consequently the redistribution that results is of the opposite order to that of the former example.

As a rule, those elements which the more closely resemble each other in their chemical habits have the least affinity for each other, while the greatest affinity usually exists between those which are most dissimilar.

Chemical Action. The actual process of redistribution of the atoms that takes place when molecules of different kinds of matter are brought together is called chemical action. In many cases chemical action takes place when the substances are merely brought together, while in others it is necessary to expose the bodies to the influence of some external energy: thus chemical action is brought about in a great number of instances by the application of heat to the substances. In some cases the influence

of light has the effect of causing chemical action to take place; for example, when the gases chlorine and hydrogen are mingled together, no chemical action takes place in the dark, but on exposing the mixture to light the hydrogen and chlorine combine, and form the compound hydrochloric acid. It is upon the effect of light in causing chemical action to take place that the art of photography depends.

Chemical action may sometimes be induced by the influence of pressure; thus, when the two gases, hydrochloric acid and phosphoretted hydrogen, are subjected to increased pressure they combine together to form a crystalline solid compound known as phosphonium chloride. In the same way, by very great mechanical pressure, a mixture of powdered lead and sulphur can be caused to combine together, when they form the compound, lead sulphide. There are also a number of chemical actions that are only able to proceed in the presence of small quantities (often extremely small) of a third substance, which itself remains unchanged at the conclusion of the action. These cases are generally included under the name of catalytic actions: in some of them the modus operandi of the third substance can be traced (see Oxygen, Modes of Formation; also Chlorine, Deacon's Process), while in others it is not understood. Thus it is found that a number of chemical actions are quite unable to take place if the materials are absolutely dry; for example, the element chlorine has a powerful affinity for the metal sodium, and when these substances are brought together under ordinary conditions, chemical action instantly takes place, and the compound known as sodium chloride (common salt) is produced. If, however, every trace of moisture be perfectly removed from both the sodium and the chlorine, no action between these elements takes place when they are brought together, and so long as they remain in this state of perfect dryness no chemical change takes place. The admission into the mixture of the minutest trace of the vapour of water, however, at once induces chemical action between the chlorine and the sodium, but the exact part that the trace of moisture plays in producing this effect is not known with certainty. (See also foot-note, page 89.)

A few interesting cases are known in which chemical action is brought about by the vibration caused by a loud sound or note; for example, the molecules of the gas acetylene consist of two atoms of carbon associated with two of hydrogen. When a quantity of this gas is exposed to the report produced by the detonation of

mercury fulminate, the mere shock of the explosion causes a redistribution of the atoms whereby solid carbon is deposited and hydrogen set free. We may suppose that the particular vibration produced by the detonation of the fulminate exercises a disturbing effect upon the motions of the atoms constituting the molecules of acetylene, and thereby causes them to swing beyond the sphere of their mutual attractions, and thus the system undergoes disruption and rearrangement.

All known instances of chemical action can be referred to one of three modes, in which the rearrangement of the atoms can take place.

(1.) By the direct union of two molecules to form a more complex molecule. Thus, if CO and CIC1 represent two molecules between which chemical action takes place according to this mode, they unite to form a molecule containing the four atoms COCICI.

(2.) By an exchange of atoms taking place between different molecules. In its simplest form this is illustrated in the action of one element upon another to form a compound. Thus, if HH and CIC stand for two elementary molecules between which chemical action takes place, the result is the formation of the two molecules HC HCl. Such a process as this, in which a compound substance is produced directly from the elements which compose it, is termed synthesis.

The same mode of chemical action may also be exemplified by the exact opposite to this process, namely, the resolution of a compound into its constituent elements. Thus, if OHH OHH represent two molecules of the same compound, when chemical action takes place it will result in the formation of the three elementary molecules OO, HH, and HH. Such a process as this, in which a compound is resolved into its elements, is known as analysis.

(3.) By a rearrangement of the atoms contained in a molecule. There are a number of instances of chemical change, in which the molecules of the substance do not undergo any alteration in their composition that is to say, no atoms leave the molecule, nor are any added to it. The molecule still consists of the same atoms after the change as it did before, but the chemical action has caused them to assume new relative positions, or different relative motions with respect to each other. For example, the substances known to chemists as ammonium cyanate and urea are two totally

different and distinct kinds of matter. These molecules, however, each contain the same atoms and in the same number; they each consist of aggregations of one atom of carbon, one atom of oxygen, two atoms of nitrogen, and four atoms of hydrogen. When ammonium cyanate is gently warmed, the eight atoms composing the molecules undergo this process of rearrangement, and the substance is changed into urea.