We have seen in the last chapter that some conception can be made regarding the form of molecules, supposing them to occupy space of three dimensions. It is further imagined that the atoms in the molecule, unlike those in the diagrams given, are not quiescent, but are in motion relatively to each other, and that the molecules themselves also change their relative places; both atoms and molecules contain what is termed "energy," in virtue of this motion. When a chemical reaction takes place, energy may be lost or gained —lost, when atoms or molecules assume a more stable condition ; gained, when the state of a resulting compound is a less stable one than that of the substances from which it is formed.

We must now consider what is meant by this term " energy." Energy can exist under various forms; for example, when a stone falls to the ground under the influence of the earth's attraction, it loses energy after its fall; when a billiard-ball is set in motion, for instance, by the tension of a spring, the spring loses and the billiard-ball gains energy. Energy can also be communicated to substances in the form of heat when their temperature is raised ; it may be imparted to a body in the form of an electrical charge, and in various other ways.

We have already seen (p. 6) that Lavoisier laid down as a maxim that matter can neither be created nor destroyed. This same doctrine holds as regards energy ; but there is a difference in kind between matter and energy, for while one form of matter, e.g. iron, cannot be changed into another kind of matter, such as lead, one kind of energy is convertible into all other kinds of energy quantitatively, so that no loss of energy occurs during the conversion.

An example will suffice to make this clear: In a coalmine the steam-engine serves to raise the coals from the pit to the surface. The engine expends energy in overcoming the attraction of the earth for the weight. Whence does the engine obtain its energy ? Obviously from the expansion of the steam in the cylinder, for steam (or any other gas) loses energy in expanding. The steam is produced by boiling water in the boiler ; water absorbs energy in changing into steam. And this energy reaches the water in the form of heat from the boiler fire ; the heat is produced by the combustion of coal; and the coal, which is the product of the decay of wood buried under the surface of the earth, must originally have derived its energy from the sun, the rays of which are essential to the growth of plants.

We have here a long chain of transformations of energy; the chemical energy of the coal is transformed into heat, the heat causes the expansion of the water into steam, the steam overcomes the resistance of the piston in the cylinder, the motion of the engine raises the weight. In all this chain there is no loss of energy ; it is only transformed from one kind to another. But it must not be imagined that each kind of energy is quantitatively transformed into the other ; for example, when the steam urges the piston forward in the cylinder, some of the energy is lost by the friction of the piston against the walls of the cylinder, and is converted into heat; and, indeed, energy tends to be degraded, that is, to be transformed into heat-energy.

In almost all chemical reactions which take place, either of their own accord or on rise of temperature, heat is spontaneously evolved. When that is the case the reaction is termed " exothermic ; " but " endothermic " reactions are also known, in which he-L is absorbed. Such reactions, however, do not take place spontaneously at ordinary temperatures. All the phenomena of combustion are exothermic reactions. We are familiar with many examples of this, as when coal burns, when hydrogen and oxygen explode, when gunpowder is fired—all these are examples of exothermic reactions. The interaction of any two or more elements which spontaneously unite to form a compound is of the same nature as combustion.

Endothermic reactions—those in which heat is absorbed —are usually only possible at ordinary temperatures when an exothermic reaction proceeds at the same time. But one point must be noticed here; it is necessary that both the exothermic and the endothermic reaction should be part of the same chemical process. For example, the formation of chloride of nitrogen by the action of chlorine upon a concentrated solution of ammonia is an exothermic reaction. Chloride of nitrogen is a fearfully explosive body, detonating with the least shock into its elements, chlorine and nitrogen ; but while it is being formed there is formed at the same time ammonium chloride, a substance which is produced with great evolution of heat. These two reactions are part of the same chemical process, and they are expressed by the equation 4NH3+3Cl₂ = NCl₃+3NH₄Cl. It is essential that both ammonium chloride and the chloride of nitrogen should be produced by the same chemical reaction. The combination of nitrogen and chlorine would not take place were any other exothermic reaction unconnected with the formation of nitrogen chloride to be going on in the same vessel. The elements nitrogen and chlorine do not form nitrogen-chloride when mixed, even under the influence of a high temperature, nor would they if another exothermic reaction were proceeding simultaneously in contact with the nitrogen and the chlorine. Moreover, in order that an endothermic compound may be formed, it is not sufficient that an exothermic reaction take place simultaneously; the heat evolved during the exothermic reaction must usually exceed that absorbed by the formation of the endothermic compound. •

Endothermic compounds readily decompose, often with explosion; when they do so heat is evolved ; the compound loses energy. This implies that the elements in the free state or any other products of the decomposition of the endothermic compound contain less energy than the compound before decomposition. On the other hand, in order to decompose exothermic compounds, heat must be imparted to them. The example given on p. 31 of ammonium chloride is a case in point. It will be remembered that in order to decompose ammonium chloride into ammonia and hydrogen chloride the temperature must be raised, and heat is absorbed by the chloride ; hence its products ammonia and hydrogen chloride in the uncombined state contain more energy than their compound, ammonium chloride. Other substances similar to ammonium chloride are known which dissociate more gradually than that compound, and the characteristic of all such dissociating bodies is this—that the higher the temperature the less stable they are. Even water when raised to a temperature approaching 2000⁰ C. dissociates partially into hydrogen and oxygen. Indeed, the rule for all exothermic compounds is that they become less and less stable the higher the temperature.