120 UNIVERSITY OF MISSOURI BULLETIN 



Constitution of Our Sun. In recent years much atten- 

 tion has been paid to the structure and composition of our 

 sun, and some most ingenious methods have been devised for 

 this study. It is a very difficult subject, because we can only 

 see the surface of it, and its temperature, at least in the 

 interior, is much higher than any temperature we can reach 

 on the earth. The sun rotates about its axis like the earth, but 

 it is not a solid body like the earth. Physical experiments tell 

 us that for every substance there is what we call a critical 

 temperature above which the substance can exist only in the 

 form of a gas. Consequently at the very high temperature 

 which we know must exist there, we believe that the interior 

 of the sun must be gaseous, but at any rate the pressure is so 

 great that it is probably thick and viscous like a viscous liquid, 

 such as molasses, and differs from a liquid only in a technical 

 way which the physicist alone can understand. This mass of 

 fluid is constantly in motion like a boiling liquid, and probably 

 the currents which rise to the surface are there cooled suffi- 

 ciently for some of the materials to become liquid or even 

 solid, for we see on the surface dazzlingly bright patches which 

 give most of the light and act as if they were partly liquid or 

 solid. At times we see enormous spots on the sun, which 

 look like holes in the outer shell. The peculiar thing about 

 these sun spots is that, although they appear and disappear 

 somewhat irregularly, there are regular intervals, about eleven 

 years apart, when they are more abundant than at other times. 

 Another peculiar thing is that whenever we have a maximum 

 of sun-spots, the appearance of the aurora borealis becomes 

 more frequent, and also the disturbances of the compass- 

 needle. No satisfactory explanation for this coincidence 

 between the sun-spots and the earth's magnetism has ever 

 been made, but within the last year or two it was found by 

 some astronomers in California that in the center of the 

 sun-spots there is a very strong magnetic field, and that the 

 spots themselves are great vortices or whirlpools. 



