1867.] Constitution of the Sun and Stars. 27 



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 proceeding 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 sensibly less dark in the blue, and to lines which emit a still 

 more considerable amount of light in the green, yellow, orange, and red, 

 those colours in which a body gradually heated begins to glow. 



A detailed scrutiny of the lines emitted by the various gases leads to 

 several interesting results. Hydrogen and iron are the two most abundant 

 constituents of the sun's outer atmosphere, and play in it the same part 

 which nitrogen and oxygen do in the earth's. There is but the merest 

 trace of sodium present. The other gases are met with in intermediate 

 quantities. Again, barium cannot have a vapour-density so high as would 

 appear as first from its atomic weight, and therefore probably belongs to the 

 same class of elements as cadmium and mercury, which have vapour-densi- 

 ties half of what correspond to their atomic weights. To these several re- 

 sults we may attribute the probability 3. 



The photosphere consists of two strata which may be distinguished. 

 The outer of these is shown to be cloud in the ordinary sense of the word, 

 that is, solid or liquid matter in a state of minute division, and denser than 

 the part of the atmosphere in which it is dispersed (probability 3). This 

 cloud is precipitated from its vapour by the chill produced by its own 

 abundant radiation towards the sky, a chill which constitutes the shell of 

 clouds a surface of minimum temperature considerably cooler than either 

 the layer above it or the layer beneath (probability 3). The hotter layer, 

 which is outside the luminous clouds, seems to have a depth somewhat 

 greater than the length of the earth's radius (probability 2). Just outside 

 it there is a second layer of luminous clouds, but so excessively thin that 

 they can be seen only during a total eclipse, on which occasions a portion of 

 them has been seen under the form of two arcs of cloud extending for some 

 distance on either side of the points of first and l^st contact, where alone a 

 sufficiently low part of the sun's atmosphere was disclosed (probabiHty 3). 

 Above these there soar other clouds raised by causes which will be referred 

 to further on. 



About the middle of the hot stratum over the photosphere there is a sur- 

 face of maximum temperature, outside which the temperature decreases 

 almost continuously to the limit of the iron atmosphere. A little outside 

 this there is a second very feeble maximum, the temperature of which falls 

 short of the heat of the flame of a Bunsen's burner ; and outside this, 

 through the immense height which is tenanted by sodium, magnesium, and 



