Constitution of the Sun and Stars. 305 



some intimate relation to the true theory ; 2 will indicate that we 

 have good ground to conclude our hypothesis to he the true theory, 

 although at the same time the evidence is too scanty or conflicting 

 to free us from hesitation ; 3 will indicate a proof so strong that we 

 should be very much surprised if anything were eventually to dis- 

 turb it ; 4, as has been already stated, will mark a conclusion fully 

 made out ; and, to complete the series, 5 may be used for that demon- 

 strative proof of which few subjects of inquiry are susceptible. 



Observations with the spectroscope have made known to us that the 

 sun's outer atmosphere (that is, the part of the atmosphere which 

 extends outside the photosphere) is a mixture of many gases, amongst 

 which hydrogen, sodium, magnesium, calcium, chromium, manganese, 

 iron, nickel, cobalt, copper, zinc, and barium — all of them permanent 

 gases in consequence of the temperature — have been detected. Now 

 it is shown to be a necessary consequence of the molecular constitu- 

 tion of gases, that in such an atmosphere, decreasing in temperature 

 from within outwards, the various constituent gases are not everywhere 

 equally mixed, but that in the upper regions those which have the 

 lightest molecules rise the furthest, so that the gases overlap one an- 

 other in the order of the masses of .their molecules (probability 5). 

 It also follows from a consideration of the vapour-densities and atomic 

 weights of the chemical elements, with probabilities which range 

 from 4 to 1, that those which are present in the sun's atmosphere 

 have molecules with masses increasing in the order in which their 

 names have been printed above, the molecules of hydrogen being the 

 lightest. This, then, is the order in which the boundaries of these 

 gases would be met with in descending from the surface of the sun's 

 atmosphere downwards. 



This result is abundantly confirmed, and in its main features raised 

 to probability 4, by observations with the spectroscope. Each con- 

 stituent of the solar atmosphere is opake to those rays which it emits 

 when incandescent, and which constitute its spectrum. In this way 

 all the light of these particular wave-lengths which has been emitted, 

 either by the photosphere, or by the lower and more intensely heated 

 strata of a gas in the solar atmosphere, is stopped in its passage out- 

 wards, and the gas substitutes for it the much more subdued light 

 which emanates from its own upper and therefore coolest stratum. 

 Now, if the view enunciated in the last paragraph be true, these outer 

 layers of the respective gases, from which the rays as we see them 

 come, must be at very various temperatures, that of hydrogen being 

 the coldest, and the others in order after it. This is precisely in 

 conformity with the observations. The rays of hydrogen, sodium, 

 and magnesium emanate from a region so cold that the lines of these 

 elements in the sun's spectrum are intensely black in whatever part 

 of the spectrum they may occur ; in other words, the light pro- 

 ceeding from the upper layers of these gases is so feeble that it is 

 not in any perceptible degree luminous when placed in contrast 

 with the intense background of light from the photosphere. On 

 the other hand, calcium, iron, and the rest, while they produce 

 only black lines in the violet and indigo, give rise to lines which are 



