THEORY OF EXCHANGES. 24S 



dark pattern absorbs most of them. On heating the porcelain to incan- 

 descence, the pattern is still the best absorber, and it is, therefore, also 

 the best radiator, and the pattern stands out as bright on a less bright 

 ground. 



Blue glass absorbs red more freely than blue rays. If heated to 

 incandescence, it radiates red more freely than blue, and therefore it 

 appears a very bright red. Transparent glass absorbs little, if any, of 

 the light rays, and therefore it radiates poorly, and a piece of colourless 

 glass-tubing heated white-hot only gives out a faint white light as com- 

 pared with a wire heated to the same temperature. 



We have a converse case with sodium vapour in a Bunsen flame, 

 which emits yellow light, its spectrum consisting of the two well-known 

 D lines. It, therefore, also absorbs yellow light of the same wave-lengths. 

 If, then, we place a cooler sodium flame that of a spirit lamp in front 

 of a hotter sodium flame given by a Bunsen burner, the cooler flame will 

 absorb from the hotter flame behind it, but will not restore to the out- 

 going radiation as much as it absorbs. Hence it appears smoky, especially 

 round the edges, where the absorbing vapour is coldest. 



This is applied to explain the existence of the dark lines in the 

 spectrum of the sun. The body of the sun is intensely hot, and so dense 

 that it sends out radiations of all kinds, forming a continuous spectrum. 

 But round it is an atmosphere consisting of a mixture of gases and 

 vapours cooler than the interior mass, though they are still exceedingly 

 hot, sufficiently so to send out their own radiations. But they absorb 

 from the stream passing outwards more than they restore, each selecting 

 the kind of radiation it emits. Thus, sodium vapour absorbs the two D 

 wave-lengths, while the light on each side passes on unabsorbed. On 

 examining the solar spectrum, then, these two D lines appear dark, not 

 from absence of sodium light, but because the sodium light is weakened 

 relatively to the neighbouring radiation. We may quote as another 

 example the cases already mentioned of absorption by cold rock-salt of 

 the radiation from hot rock-salt, and of the absorption by water of the 

 radiation from a hydrogen flame. 



