Combustion is a chemical process, it is said, and this definition may be true, although it may also be misleading. In daily speech combustion is generally identified with the phenomena of light and the generation of heat, which we immediately observe, but chemical processes can neither be seen nor felt, because they take place in the inner world of matter which hitherto has proved inaccessible to human observation. Yea, chemical processes are so foreign to the experiences of our senses that chemistry, the science of these processes, is entirely founded on the deductions of our reason. The premises that our reason uses for its conclusions belong to the physical world which is the outer side of matter that faces us. The phenomena that accompany combustion belong to this world and are, therefore, strictly speaking, not chemical but physical phenomena. But even if these phenomena of light and heat, of which the latter especially interests us here, belong to the world comprehensible to our senses, they must nevertheless be intimately connected with the inner chemical process because heat is developed in nearly every chemical reaction. Heat is not created from nothing; there must be a cause for this force, and the cause cannot be anything but the chemical energy which in the chemical process is transformed into heat. In few words: What we generally term combustion cannot be identical with the actual chemical process. The light and the heat must, on the contrary, be considered as the external results of the chemical process, its physical effect. By a close study of this physical effect we have also been able to explain what happens within matter itself. As it is necessary to understand this in order to comprehend how heat is developed, we will endeavor shortly to outline the present scientific conception of the chemical process called combustion. From the qualities of matter we have concluded that the bodies we see are composed of extremely tiny particles called molecules, which, however, are so small that with our optical resources we never shall be able to observe them. Even the smallest particle of dust visible to the eye must be considered as containing an enormous number of them. With molecules, however, we have not reached the limit of the divisibility of matter. They may themselves be divided by chemical forces into smaller material units called atoms, and these latter are therefore the building stones of which matter is ultimately composed. Now neither the atoms within the molecule, nor the molecules within the visible body, are packed closely together. They are separated by comparatively great spaces. But if these building stones are separated from each other we might expect that they would behave like the grains in a sand heap.