How can material bodies then be solid, hard, tough, etc.? The reason is that the spacing in question is regulated by other forces of essentially different kind. We have attracting as well as repelling forces, such as tend to increase as well as to reduce the distances between the particles. We shall first consider the attracting forces, and these are called cohesion and adhesion when exerted between molecules. The mutual attraction between the atoms within the molecules has been named affinity or chemical energy. Turning again to the form of energy acting in the opposite direction, we find just the force we are in search of—heat, which is the physical source of energy of all living beings. That heat increases the distances between molecules is already evident from the fact that all bodies increase in volume when heated, a process which may be continued by further supply of heat until the solid becomes a fluid, and the fluid a gas. In solid bodies the attracting forces have predominance. The molecules are arranged with definite spacing and in definite positions so that the body assumes a certain external shape. If such a body is exposed to heat the molecules are removed from each other and the cohesion becomes correspondingly feebler. Finally a point is reached when the molecules are so far unfettered that they are at liberty to move with respect to each other. The solid has then become a fluid and may through continued heating enter the gaseous state. The cohesion is then entirely conquered so that the molecules move freely in all directions independent of each other. Similarly, heat influences the atoms of which the molecules are composed. Even chemical attraction gives way to heat so that all bodies at sufficient temperature are decomposed into free atoms or elementary constituents. We have seen that heat performs mechanical work in so far as it separates masses from each other. But heat not only performs this work but is the work itself, or is identical with the movement of these particles. Consequently a certain quantity of mechanical work is equivalent to a certain quantity of heat and _vice versa_, and it is this transformation from one form of energy into another that takes place during a chemical reaction. The mechanical energy of the atoms is here converted into heat which may again be used for the other forms of mechanical activity. Through the chemical reaction that heat is regained which previously was utilized in separating the atoms or sustaining their movement, and this explains why heat is developed in chemical processes. If this development of heat is increased to a certain point, or, which is the same, if the reaction takes place with greater violence, the common phenomena of fire and light appear. But even without these, every chemical process may be called combustion in a wider sense, that is, if we consider the production of heat as the characteristic external effect of the chemical force.