On the Corona of the Sun. 



121 



city, which rises to a height sufficient to afford an explanation of the 

 corona which streams several hundred thousand miles above the 

 photosphere. 



Gravitation on the sun is about twenty-seven times as great as on 

 the earth, and an atmosphere extending to a moderate coronal height, 

 even if it consisted of a gas thousands of times lighter than hydrogen, 

 would have more than metallic density at the sun's surface, a state of 

 things which spectroscopic and other observations show cannot be the 

 case. 



There is another consideration from the rapid increase of density 

 which would take place sunwards in such an atmosphere. Each 

 stratum would be compressed by the weight of all the strata above it, 

 and therefore in descending by equal steps the density of the atmo- 

 sphere would increase in geometrical ratio. Professor Newcomb gives 

 as an example an atmosphere of hydrogen ; such a gas, though heated 

 to as high a temperature as is likely to exist at a height of a hundred 

 thousand miles above the sun, would double its density every five or 

 ten miles.* There is no approach to so regular and so rapid an in- 

 crease of density to be observed in the corona. 



Another circumstance which puts a continuous gaseous atmosphere 

 out of question is the fact that comets have passed unscathed through 

 the coronal regions. Shooting stars passing with the relatively small 

 velocity of thirty or forty miles per second through our atmosphere, 

 rarefied as it is, at the height of fifty or sixty miles, are instantly burnt 

 up. Resistance and heat increase as the square [or more probably for 

 such high velocities as the cubef] of the velocity, yet the nucleus of a 

 comet has passed through several hundred thousand miles of coronal 

 matter with a velocity of 300 miles per second without suffering any 

 sensible loss of velocity. These considerations are amply sufficient to 

 show that the theory of a solar atmosphere of gas of the extent of the 

 corona held up by its own elasticity cannot be entertained. 



As we have reason to believe that the corona is an objective reality 

 about the sun, matter of some sort must exist wherever the corona 

 is seen to extend. The questions before us are — (1.) In what form 

 does the matter exist ? (2.) Whence does it come ? (3.) What are 

 the dynamical conditions under which it can exist at such great 

 heights above the sun ? 



(1.) On the first of these questions as to the condition of the 

 matter, the spectroscope has given us definite information. 



The spectrum of the corona is compound, and consists of three 

 superposed spectra. 



(a.) A bright continuous spectrum, which informs us that it comes 

 from incandescent solid or liquid matter. 



* " Popular Astronomy," p. 259. f See Basliforth, " Phil. Trans.," vol. 158, 

 p. 417. 



