POTASSIUM, BUBIDIUM, CJESIUM, AND LITHIUM 569 



imagine that the sun, owing to the high temperature which is proper to it> 

 emits a brilliant light which gives a continuous spectrum, and that this 

 light, before reaching our eyes, passes through a space full of the vapours 

 of different metals and their compounds. As the earth's atmosphere 3a 

 contains very little, or no, metallic vapours, and as they cannot be sup- 

 posed to exist in the celestial space, 32 bi8 the only place in which 

 the existence of such vapours can be admitted is in the atmosphere 

 surrounding the sun itself. As the cause of the sun's luminosity must 

 be looked for in its high temperature, the existence of an atmosphere 

 containing metallic vapours is readily understood, because at that 

 high temperature such metals as sodium, and even iron, are sepa- 

 rated from their compounds and converted into vapour. The sun must 

 be imagined as surrounded by an atmosphere of incandescent vaporous 

 and gaseous matter, 33 including those elements whose reversed spectra 

 correspond with the Fraunhofer lines namely, sodium, iron, hydrogen, 

 lithium, calcium, magnesium, &c. Thus in spectrum analysis we find 

 a means of determining .the composition of the inaccessible heavenly 

 luminaries, and much has been done in this respect since Kirchhoffs 

 theory was formulated. By observations on the spectra of many 

 heavenly bodies, changes have been discovered going on in them, 34 and 



32 Brewster, as is mentioned above, first distinguished the atmospheric, cosmicol 

 Fraunhofer lines from the solar lines. Janssen showed that the spectrum of the atmo- 

 sphere contains lines, which depend on the absorption produced by aqueous vapour. 

 Egoreff, Olszewski, Janssen, and Liveing and Dewar showed by a series of experiments 

 that the oxygen of the atmosphere gives rise to certain lines of the solar spectrum, 

 especially the line A. Liveing and Dewar took a layer of 165 c.m. of oxygen compressed 

 under a pressure of 85 atmospheres, and determined its absorption spectrum, and found 

 that, besides the Fraunhofer lines A and B, it contained the following groups : 630-622, 

 581-568, 535, 480-475. The same lines were found for liquid oxygen. 



32 bis if tk e material of the whole heavenly space formed the absorbent medium, the 

 spectra of the stars would be the same as the solar spectrum ; but Huyghens, Lockyer, 

 and others showed not only that this is the case for only a few stars, but that the 

 majority of stars give spectra of a different character with dark and bright lines and 

 bands. 



35 Eruptions, like our volcanic eruptions, but on an incomparably larger scale, are of 

 frequent occurrence on the sun. They are seen as protuberances visible during a total 

 eclipse of the sun, in the form of vaporous masses on the .edge of the solar disc and 

 emitting a faint light. These protuberances of the sun are now observed at all times by 

 means of the spectroscope (Lockyer's method), because they contain luminous vapours 

 (giving bright lines) of hydrogen and other elements. 



54 The great interest and vastness of astro-physical observations concerning the sun, 

 comets, stars, nebulae, &c., render this new province of natural science very important, 

 and necessitate referring the reader to special works on the subject. 



The most important astro-physical data since the time of Kellner are those referring 

 to the displacement of the lines of the spectrum. Just as a musical note changes its 

 pitch with the approach or withdrawal of the resonant object or the ear, so the pitch of 

 the luminous note or wave-length of the light varies if the luminous (or absorbent) vapour 

 and the earth from which we observe it approach or recede from each other; this 



