The Evolution of the Sciences 



of an attenuated solid dust, which sends out 

 light to us, either by direct emission or by diffus- 

 ing light received from the photosphere. 



The study of the corona constitutes at times 

 of total eclipse one of the principal occupations 

 of physicists and astronomers; we shall have 

 occasion to consider it more in detail in a future 

 chapter. Enough has, however, already been 

 said to give a summary idea of the general 

 constitution of the sun ; we look on it as a fluid, 

 and probably, almost entirely, gaseous mass, 

 animated with a rotary motion, whose speed 

 decreases from the centre towards the peripheral 

 zone, and raised to a very high temperature, 

 varying in general in a similar manner to the 

 speed. Possibly such bodies as gold and 

 platinum, which have the highest vapour- 

 densities, are concentrated towards the centre, 

 while the lightest bodies, such as hydrogen and 

 sodium, are arranged towards the periphery; 

 this would explain the absence of any lines 

 characteristic of the heavy metals in spectro- 

 scopic observations of the surface-zone. None 

 but isolated elements could exist in the nucleus 

 owing to its elevated temperature; but cease- 



