60 Mr. li. Meldola on a Cause for the Appearance 



It is well known to spoctroscopists that the solar spectrum 

 is never absolutely free from the so-called " telluric " lines, 

 which have been shown to owe their existence to the aqueous 

 vapour of our atmosphere. It is possible from the present 

 point of view that these lines may be partly caused by aqueous 

 vapour in the higher regions of the sun's atmosphere *. Should 

 there be any connexion between the activity of combustion 

 and the formation of sun-spots, a rigorous comparison of the 

 " telluric " lines in the solar spectrum carefully observed (or 

 still better, photographed) at different periods of the spot- 

 cycle would be of the highest possible interest. Thus it may 

 be suggested that the solar combustion varies periodically in 

 activity — combination being in excess of dissociation during 

 one half of the cycle, and dissociation being in the excess 

 during the other half, when the heat resulting from the com- 

 bustion, having reached its maximum, tends to decompose the 

 compounds formed. This view points to the belief that the 

 connexion between the sun-spot period and the period of va- 

 riation of magnetic declination may be due to a common 

 cause — the activity of combustion in the sun's atmosphere and 

 the resulting variation either in the amount of free oxygen, or 

 in the magnetic characters of this gas consequent on variation 

 of temperature. 



Sir William Thomson's theory of the dissipation of energy 

 leads to the belief that the sun, like other stars, is gradually 

 cooling down. Thus we should be led to infer a priori that 

 there must be a period in the life of a star when compounds 

 can begin to form. Such combination would begin in the 

 outer and cooler portions of the star's atmosphere, as required 

 by the present hypothesis, and would be attended with the 

 development of the heat representing the energy of chemical 

 separation. As the star goes on cooling down, the zone of 

 combustion, at first a mere shell, would gradually encroach 

 upon the central regions, and a star having permanently 

 bright lines in its spectrum would result. In the earlier 

 stages of what may be called the " chemical period " of a star's 

 history — a period into which our sun may be supposed to have 

 entered — the lines of the non-metallic elements would alone 

 appear bright, for the reasons detailed in the foregoing por- 

 tions of this paper (paragraph 11), and, owing to their compa- 

 rative faintness, would be lost at the enormous distances which 



* I may here recall the mucli-discussed observation of Secchi, who 

 ted the existence of water-vapour in the neighbourhood of sun- 

 t pots ( Compt. Rend, lxviii. p. 238). Janssen also, in 1 864, observed aque- 

 ous vapour in the atmosphere of Antares, and, in 1868, in the atmosphere 

 of many other stars (Compt, Rend, lxviii. p. 181o). 



