14 PLEASANT WAYS IN SCIENCE. 



effected between the various elements which exist in the 

 sun's substance. A separation of this sort is unquestion- 

 ably competent to modify the spectroscopic evidence re- 

 specting different elements. But it would be a mistake to 

 suppose that any such separation could occur as has been 

 imagined by some a separation causing in remote times the 

 planets supposed to have been thrown off by the sun to be 

 rarest on the outskirts of the solar system and densest close 

 to the sun. The small densities of the outer family of 

 planets, as compared with the densities of the so-called 

 terrestrial planets, must certainly be otherwise explained. 



But undoubtedly the chief circumstance likely to operate 

 in veiling the existence of important constituents of the 

 solar mass must be that which has so long prevented spec- 

 troscopists from detecting the presence of oxygen in the 

 sun. An element may exist in such a condition, either over 

 particular parts of the photosphere, or over the entire sur- 

 face of the sun, that instead of causing dark lines in the 

 solar spectrum it may produce bright lines. Such lines may 

 be conspicuous, or they may be so little brighter than the 

 background of the spectrum as to be scarcely perceptible 

 or quite imperceptible. 



In passing, I would notice that this interpretation of the 

 want of all spectroscopic evidence of the presence of oxygen, 

 carbon, and other elements in the sun, is not an ex post 

 facto explanation. As will presently appear, it is now ab- 

 solutely certain that oxygen, though really existing, and 

 doubtless, in enormous quantities, in the sun, has been con- 

 cealed from recognition in this way. But that this might 

 be so was perceived long ago. I myself, in the first edition 

 of my treatise on "The Sun," pointed out, in 1870, with 

 special reference to nitrogen and oxygen, that an element 

 " may be in a condition enabling it to radiate as much light 

 as it absorbs, or else very little more or very little less ; so 

 that it either obliterates all signs of its existence, or else 

 gives lines so little brighter or darker than the surrounding 

 parts of the spectrum that we can detect no trace of its 



