212 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



the pi'oportion of the atoms of any clement which are ionized 

 depends not only on the temperature, but on the pressure (and also, 

 of course, on the ease of ionization of the element). 



We know the proportion for half a dozen elements whose strongest 

 arc and spark lines are both accessible, and so we can find the pres- 

 sure — or, at least, that part of it due to the free electrons. At the 

 level of the " j^hotosphere " (as defined above), this comes out 

 one twenty -thousandth of the " standard atmosphere." We can now 

 calculate that an element with an ionization potential of 8.5 volts 

 would be half ionized and half neutral. Most of the metals are more 

 easily ionized than this. Only one atom of sodium in a thousand is 

 neutral, and less than one in a hundred for calcium. One-fifth of 

 the iron atoms, and one-third those of silicon, are neutral; but 85 

 per cent are neutral for zinc, 99.6 per cent for carbon, and all but 

 one in 30,000 in the case of hydrogen. Making the appropriate 

 allowances, we find the total amounts of the various elements in the 

 sun's atmosphere. For the metals, the results should be fairly 

 reliable. They indicate that six elements, — sodium, magnesium, 

 silicon,^ potassium, calcium, and iron — furnish 95 per cent (by 

 weight) of all the metallic vapors, and six more nine-tenths of the 

 remainder, as is shown in Table 1. 



Table 1. — Metals in the sun's atmosphere 



[Tons per square mile] 



Magnesium 350 



Iron 2.50 



Silicon 150 



Sodium 100 



Potassium 50 



Calcium 50 



Aluminum 15 



Nickel 15 



Manganese 10 Total 1,008 



This is very much like the order of relative abundance that is 

 found on earth. The results of Clarke and Washington (based on 

 hundreds of careful analyses of typical rocks) show a greater abun- 

 dance of silicon, aluminum, and titanium. But these results repre- 

 sent the composition of the outer 10 miles of the earth's crust, which 

 is composed mainly of granitic rocks, richer in these three elements 

 than are the far thicker layers of dense rocks deeper down. 



Again, nickel and cobalt are much less abundant in terrestrial rocks 

 than in the sun. But in meteorites (which are probably more repre- 

 sentative of the general composition of the solar system than is the 

 siliceous slag, which forms the outer layers of the earth) they occur 

 in nearly the same proportions as in the sun. 



» SUicon behaves spectroscopically like a metal and is therefore included here. 



Cobalt 6 



Chromium 6 



Titanium 2 



Vanadium li^ 



Copper l^i 



Zinc 1 



All others 0.2 



