19 



gave the same result. It thus became clear that 

 sun-spots actually are regions of reduced tempera- 

 ture in the solar atmosphere. 



The next step bore out this conclusion. If the 

 solar vapors are cooler in spots than in the general 

 atmosphere of the sun, then it may be possible for 

 some of them to unite chemically. Thousands of 

 faint lines in the spot spectra were measured and 

 identified as band lines dueto chemical compounds. 

 Fowler, who had also worked with success on the 

 strengthened and weakened lines, found magne- 

 sium hydride. Titanium oxide and calcium hy- 

 dride were identified in our laboratory. Thus we 

 began to form a new picture of these regions of the 

 solar atmosphere and to recognize the chemical 

 changes at work in the spot vapors. 



SOLAR METEOROLOGY. 



Meanwhile systematic work was in progress 

 with the spectroheliograph, which gives images of 

 the sun in monochromatic light, showing the dis- 

 tribution of some one vapor in its atmosphere. 

 In the favorable California climate it is possible 

 to photograph the sun on about 300 days of the 

 year (in one season on 113 successive days). 

 Every clear morning, and frequently in the after- 

 noon, the instrument was at work, making pictures 

 of the great gaseous clouds in the solar atmosphere. 

 These were first observed many years ago as solar 

 prominences, rising high above the sun's limb at 

 total eclipses, when the bright light of the disk was 



