It is to be remembered that the class of minute planetary nebulæ make themselves known solely by the fact that they exhibit the bright line indicative of gaseous spectra. If these objects (though still nebulæ) had not displayed gaseous spectra, it is certain they would have escaped detection, at least by the process which has actually proved so successful. The continuous band of light which they would then have presented could not be discriminated from the band of light from a star. It is therefore not improbable that among the star-like bodies which have been represented on our photographs, there may be some which are really minute spiral nebulæ. In general a star is a minute point of light which no augmentation of telescopic power and no magnification will show otherwise than as a point, granted only good optical conditions and good opportunity so far as the atmosphere is concerned. It has, however, been occasionally noted that certain so-called stars are not mere points of light; they do possess what is described as a disc. It is not at all impossible that the objects so referred to are spiral nebulæ. We may describe them as formed on a small scale in comparison with the great spiral or the nebula in Andromeda. But the smallness here referred to is only relative. They are in all probability quite as vast as the primæval spiral nebula from which the solar system has been evolved, though not so large as those curious ring-shaped nebulæ of which the most celebrated example lies in the constellation Lyra (Fig. 38). Such is an outline of what we believe to have been the history of our solar system. We have already given the evidence derived from the laws of heat. We have now to consider the evidence which has been derived from the constitution of the system itself. We shall see how strongly it supports the belief that the origin of sun and planets has been such as the nebular theory suggests. ------------------------------------------------------------------------ CHAPTER XIII. THE UNITY OF MATERIAL IN THE HEAVENS AND THE EARTH. Clouds—Fire-Mist—Vapour of Platinum—Components of Chalk—Constituents of the Primæval Fire-Mist—Objections—Origin of the Mist—Remarkable Discovery of the Century—Analysis of the Sun—Spectroscopic Analysis—Simplicity of Solar Chemistry—Potassium—A Drop of Water—The Solar Elements—Calcium—The Most Important Lines in the Solar Spectrum—Photograph of the Sun—Carbon in the Solar Clouds—Function of Carbon—Bunsen’s Burner Illustrates Carbon in the Sun—Carbon Vapours in the Sun—The Supposed Limit to our Knowledge of the Heavens—Characteristics of Spectroscopic Work—Bearing on the Nebular Theory.