OXYGEN IN THE SUN. 5 



elements which exist in our earth. Some of these elements 

 might indeed escape discovery, because existing only in 

 small quantities; and others (as platinum, gold, and lead, 

 for example), because but a small portion of their vaporous 

 substance rose above the level of that glowing surface which 

 is called the photosphere. But that oxygen, which con- 

 stitutes so large a portion of the solid, liquid, and vaporous 

 mass of our earth, should not exist in enormous quantities, 

 and its presence be very readily discernable, seemed amaz- 

 ing indeed. Nitrogen, also, might well be expected to be 

 recognizable in the sun. Carbon, again, is so important a 

 constituent of the earth, that we should expect to discover 

 clear traces of its existence in the sun. In less degree, 

 similar considerations apply to sulphur, boron, silicon, and 

 the other non-metallic elements. 



It was not supposed, however, by any one at all com- 

 petent to form an opinion on the subject, that oxygen, nitro- 

 gen, and carbon are absent from the sun. It was perceived 

 that an element might exist in enormous quantities in the 

 substance of the sun, and yet fail to give any evidence of its 

 presence, or only give such evidence as might readily escape 

 recognition. If we remember how the dark lines are really 

 caused, we shall perceive that this is so. A glowing vapour 

 in the atmosphere of the sun absorbs rays of the same colour 

 as it emits. If, then, it is cooler than the glowing mass of 

 the sun which it enwraps, and if, notwithstanding the heat 

 received from this mass, it remains cooler, then it suffers 

 none of those rays to pass earthwards.* It emits rays of the 

 same kind (that is, of the same colour) itself, but, being cooler, 

 the rays thus coming from it are feebler ; or, to speak more 



* More strictly, it plays the same part as a glass screen before a 

 glowing fire. When the heat of the fire falls on such a. screen (through 

 which light passes readily enough), it is received by the glass, wanning 

 the glass up to a certain point, and the wanned glass emits in all direc- 

 tions the heat so received ; thus scattering over a large space the rays 

 which, but for the glass, would have fallen directly upon the objects 

 which the screen is intended to protect. 



