At the present day Arago would hesitate, for science has made a great advance in the question of the physical constitution of the sun. The new method invented by MM. Kirchhoff and Bunsen, and known as analysis of the luminous spectrum, being applied to the solar rays, has given rise to an entirely new conception of the nature of the sun. We have returned to the opinion of the physicists of the middle ages, who regarded the sun as a globe of fire, a sort of gigantic torch. It would be impossible to enter into the details of the optical experiments which rendered accurate analysis of the solar rays possible, and enabled us to deduce a new theory of the constitution of the sun from their properties. We shall confine ourselves to explaining this theory, as it evolves itself from the experiments of M. Kirchhoff. According to the German philosopher, the sun is not, as it has hitherto been supposed, a cold, dark, and solid body, surrounded by a burning atmosphere; it is a globe, a sphere, probably liquid, which burns throughout its whole mass, and in all its parts. This incandescent globe is surrounded by a very heavy atmosphere, formed of the vapours which proceed from the incandescent globe, and which are themselves kept burning in consequence of the high temperature of all those masses of fire. How are the spots on the sun to be explained according to this theory? M. Kirchhoff admits that, owing to unknown causes, a cooling process may take place in the vaporous atmosphere which surrounds the body of the sun. This cooling process would form at certain points condensations of vapour analogous to the condensation of the vapour of water, which on our globe produces clouds and rain. These agglomerations of condensed vapours would form a species of cloud in the atmosphere of the sun, and those clouds, which would intercept the light of the solar disc from us, would produce the effect of a spot on this disc. The cloud, once formed, cools portions of the neighbouring vapours, and, by provoking a partial condensation, gives rise to the _penumbra_ which surround the _umbra_. Thus, according to M. Kirchhoff, the solar spots are clouds suspended in the sun's atmosphere. Galileo had previously propounded an analogous hypothesis. Without abandoning M. Kirchhoff's theory we may mention another explanation of the spots. A German physicist considers the spots, not as clouds in the sun's atmosphere, but as partial solidifications of the liquid matter which forms the body of the sun; a kind of scoria, analogous to those which may be observed in crucibles containing matters in a state of fusion, and which come from particles of metal not yet melted, or which are beginning to solidify. The penumbra of the spots would be the half-molten, and consequently, half-transparent pollicule which always surrounds the edges of metallic scoria with a semi-liquid ring.