

374 POPULAR LECTURES AND ADDRESSES. 



few hours, after which all would be darkness. 

 Observation confirms this conclusion so far as 

 the outward appearance of the sun is concerned, 

 but does not suffice to disprove the idea which 

 was so eloquently set forth by Sir John 

 Herschel, and which prevailed till thirty or 

 forty years ago, that the sun is a solid nucleus 

 inclosed in a sheet of violently agitated flame. 

 In reality, the matter of the outer shell of 

 the sun, from which the heat is radiated 

 outwards, must in cooling become denser, and 

 so becoming unstable in its high position must 

 fall down, and hotter fluid from within must 

 rush up to take its place. The tremendous 

 currents thus continually produced in this great 

 mass of flaming fluid constitute the province 

 of the newly-developed science of solar physics, 

 which, with its marvellous instrument of research 

 the spectroscope is yearly and daily giving 

 us more and more knowledge of the actual 

 motions of the different ingredients, and of the 

 splendid and all-important resulting phenomena. 

 To form some idea of the amount of the heat 



