22 POPULAR SCIENCE MONTHLY. 



— a most important matter in work of this nature. The hydrogen 

 lines employed were HP, Hy and Hfi ; H$ is perhaps the best of these 

 three lines for the purpose. 



On developing the first plate we were surprised to find evidences 

 of a mottled structure covering the sun's disk, resembling in a general 

 way the structure of tlie calcium flocculi, but differing in the important 

 fact that whereas the calcium flocculi are bright those of hydrogen 

 are dark. This result was confirmed by subsequent photographs, and 

 it was found that in general the hydrogen flocculi are dark, although 

 in certain disturbed regions bright hydrogen flocculi appear. Some of 

 these are eruptive in character, and correspond closely with the brilliant 

 eruptive calcium flocculi. But in other cases, in regions where no 

 violent eruptive disturbances seem to be present, the hydrogen flocculi 

 sometimes appear to be bright instead of dark. Such regions are 

 usually in the immediate vicinity of active sun-spots, where it is prob- 

 able that the temperature of the hydrogen vapor is considerably higher 

 than in the surrounding regions. The spectroheliograph thus seems 

 to afford a method of distinguishing between regions of higher and 

 lower temperatures — an additional property which should prove of 

 great value in investigations on the vapors associated with sun-spots. 

 It is possible, of course, that the increased brightness is due not merely 

 to an increase of temperature, but to other causes, perhaps of a chem- 

 ical or electrical nature, which are not yet understood. But in any 

 event, the method serves to differentiate these regions from others in 

 which these conditions are not fulfilled, and the possibility of making 

 such a differentiation is of value, even if we do not as yet understand 

 the actual cause of the increased brightness. 



The comparative darkness of the hydrogen flocculi evidently indi- 

 cates that this gas in the flocculi for some reason radiates less light 

 than the hydrogen gas which, probably after diffusing from the flocculi, 

 has spread in a nearly uniform mass over the entire surface of the sun. 

 For the present the simplest hypothesis is to assume that the dimin- 

 ished brightness of the flocculi is due to a lower temperature at these 

 points, perhaps caused by the rapid expansion of the gas as it rises from 

 the interior of the sun into the region of greatly reduced pressure 

 above the chromosphere. On such an assumption it would seem prob- 

 able that the hydrogen flocculi really represent the hydrogen promi- 

 nences, which lie at a considerable height above the chromosphere, in a 

 region of very low pressure, where the effect of expansion should have 

 produced the greatest fall in temperature. It may ultimately appear 

 that some other explanation must be adopted, especially since the 

 hydrogen in the upper part of the chromosphere seems to be repre- 

 sented by the smaller hydrogen flocculi which form a network over 

 the entire surface of the sun. It already seems probable, in spite of 



