560 PRINCIPLES OF CHEMISTRY 



quantity of heat rays and also absorbs a considerable quantity, 

 whilst a polished metallic surface both absorbs but few and emits but 

 few, so a flame coloured by sodium emits a considerable quantity 

 of yellow rays of a definite refrangibility, and has the property of 

 retaining a considerable quantity of the rays of the same refractive 

 index. In general, the sphere which evolves definite rays also retains 

 them. 



Thus the bright spectral rays characteristic of a given metal may 

 be reversed that is, converted into dark lines by passing light which 

 gives a continuous spectrum through a space containing the heated vapours 

 of the given metal. A similar phenomenon to that thus artificially pro- 

 duced may be observed naturally in sunlight, which shows dark lines 

 characteristic of known metals that is, the Frauenhofer lines form an 

 absorption spectrum or depend on a reversed spectrum ; it being pre- 

 .supposed that the sun itself, like all known sources of artificial light, 

 gives a continuous spectrum without Frauenhofer lines. 31 We must 

 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 32 

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

 posed to exist in the heavenly space, therefore 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, therefore the existence of an 

 atmosphere containing metallic vapours is readily understood, because 

 at its 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 



emitted at the same temperature by the same substance, then Kirchhoff s law, the ex- 

 planation and deduction of which must be looked for in text books of physics, states that 

 the fraction A/JE is a constant quantity depending on the nature of a substance (as A 

 depends on it) and determined by the temperature and wave length. 



31 Heated metals begin to emit light (only visible in the dark) at about 420 (vary- 

 ing with the metal). On further heating solids first emit red, then yellow, and lastly 

 white light. Compressed or heavy gases (see Chap. III. Note 44), when strongly heated, 

 also emit white light. Heated liquids (for example, molten steel or platinum) also 



give a white compound light. This is readily understood. In a dense mass of matter 

 the collisions of the molecules and atoms are so frequent that waves of only a few 

 definite lengths cannot appear ; the reverse is possible in rarefied gases or vapours. 



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

 Frauenhofer 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 determines certain lines of the solar spectrum, 



especially the line A. 



