It is requisite here to remark that fusion does not necessarily imply combustion. It has been frequently asked, How can a volcanic theory of the lunar phenomena be upheld consistently with the condition that it possesses no atmosphere to support Fire? To this we would reply that to produce a state of incandescence or a molten condition it is _not_ necessary that the body be surrounded by an atmosphere. The intensely rapid motion of the particles of matter of bodies, which the dynamical theory shows to be the origin of the molten state, exists quite independently of such external matter as an atmosphere. The complex mixture of gases and vapours which we term “air,” has nothing whatever to do with the fusion of substances, whatever it may have to do with their combustion. Combustion is a chemical phenomenon, due to the combination of the oxygen of that air with the heated particles of the combustible matter: oxygen is the sole supporter of combustion, and hence combustion is to be regarded rather as a phenomenon of oxygen than as a phenomenon of the matter with which that oxygen combines. The greatest intensity of heat may exist without oxygen, and consequently without combustion. In support of this argument it will be sufficient to adduce, upon the authority of Dr. Tyndall, the fact that a platinum wire can be raised to a luminous temperature and actually _fused_ in a perfect vacuum. But while the mass of condensing cosmical matter was thus accumulating and forming the globe of the moon, the heat consequent upon the aggregation of its particles was suffering some diminution from the effect of radiation. So long as the radiated heat lost fell short of the dynamical heat generated, no effect of cooling would be manifest; but when the _vis viva_ of the condensing matter was all converted into its equivalent of heat, or when the accession of heat fell short of that radiated, a necessary cooling must ensue, and this cooling would be accompanied by a solidification of that part of the mass which was most free to radiate its heat into surrounding space: that part would obviously be the outer surface. With the solidification of this external crust began the “year one” of selenological history. The phenomena attendant upon the cooling of the mass we will consider in the next Chapter. CHAPTER III. THE SUBSEQUENT COOLING OF THE IGNEOUS BODY.