IN CONNEXION WITH THE SPECTRUM OF THE SUN. 
491 
observation referred to in my paper, I have taken the longest lines as thus approximately 
determined ; for it seemed desirable, in view of the very large number of unnamed lines, 
to search at once for the longest elemental lines in the solar spectrum without waiting 
for a complete set of maps. 
A preliminary search having been determined on, I endeavoured to get -some guidance 
by seeing if there was any quality which differentiated the elements already traced in 
the sun from those not traced; and to this end I requested my assistant, Mr. It. J. 
Friswell, to prepare two lists showing broadly the chief chemical characteristics of the 
elements traced and not traced. This was done by taking a number of the best known 
compounds of each element (such, for instance, as those formed with oxygen, sulphur, 
chlorine, bromine, or hydrogen), stating after each whether the compounds in question 
were unstable or stable. Where any compound was known not to exist, that fact was 
indicated. 
Two Tables were thus prepared, one containing the solar, the other the more important 
non-solar elements (according to our knowledge at the time). 
These Tables gave me, as the differentiation sought, the fact that in the main the 
known solar elements formed stable oxygen-compounds. 
I have said in the main, because the differentiation was not absolute ; but it was suf- 
ficiently strong to make me commence operations by searching for the outstanding strong 
oxide-forming elements in the sun. 
The result up to the present time has been that strontium , cadmium , lead , copper, 
cerium , and uranium *, in addition to those elements in Thalen’s last list, would seem 
with considerable probability to exist in the solar reversing layer. Should the presence 
of cerium and uranium be subsequently confirmed, most of the iron group of metals will 
thus have been found in the sun. 
As another test, certain of those elements which form unstable compounds with oxygen 
were also sought for, gold, silver, mercury being examples. None of these were found. 
The same result occurred when the lines due to the jar-spark taken in chlorine, 
bromine, iodine, and those of some of the other non-metals were sought, these being 
distinguishable as a group by formation of compounds with hydrogen. 
Now other researches, not yet completely ready for publication, have led me to the 
following conclusions : — 
I. The absorption of some elementary and compound gases is limited to the most 
refrangible part of the spectrum when the gases are rare, and creeps gradually into the 
visible violet part, and finally to the red end of the spectrum, as the pressure is increased. 
II. Both the general and selective absorption of the photospheric light are greater (and 
therefore the temperature of the photosphere of the sun is higher) than has been supposed. 
III. The lines of compounds of a metal and iodine, bromine, &c. are observed gene- 
rally in the red end of the spectrum, and this holds good for absorption in the case of 
aqueous vapour. 
* Potassium has since been added. 
