progress. The vapor, rendered highly luminous by intense heat or other causes, is shot out from the sun's interior with great velocity. Consequently there are rapid changes in the forms of these brilliant regions, whereas the more extensive flocculi, which occupy the greater part of the photograph, change slowly, and represent a much less highly disturbed condition of affairs. The brilliant eruptive flocculi always occur in active regions of the solar surface, and doubtless correspond with the eruptive prominences sometimes photographed projecting from the sun's limb. A remarkable instance was recorded on the Kenwood photographs, which showed four successive stages in an eruption of calcium vapor on an enormous scale. A vast cloud thrown out from the sun's interior completely blotted from view a large sunspot, and spread out in a few minutes so as to cover an area of four hundred millions of square miles.
As already remarked, these eruptive flocculi probably correspond in many instances with the eruptive prominences observed at the sun's edge. But it must not therefore be concluded that the quiescent calcium flocculi correspond with the quiescent, cloud-like prominences. As a matter of fact, we have good evidence for the belief that the flocculi shown in these photographs represent in most instances comparatively low-lying vapors, which, if observed at the sun's edge, would hardly project appreciably above the level of the chromosphere.
In a few cases, represented, perhaps, by the dark calcium flocculi found on certain photographs, the quiescent calcium prominences have been recorded in projection on the disk. But such instances will remain exceptional until a spectroheliograph has been constructed of such high dispersion as to permit photographs to be made with the light of the K3 line exclusively.
So far we have considered the photography of the sun with the light of the H and K lines of calcium. But it must naturally occur to any one familiar with the solar spectrum that it should be possible to take photographs corresponding to other lines, and thus representing the vapors of other substances. In the solar spectrum some 20,000 lines have been recorded in the great photographic map and catalogue of Rowland, representing almost all the elements known on the earth, and doubtless including many of the radiations of substances which are as yet unknown to terrestrial chemistry. Just as the gas helium was discovered in the sun long before it was round by Ramsay in the laboratory, so we may confidently expect that many other substances represented by lines in the solar spectrum will ultimately be detected on the earth.
Now these lines, like the H1 and A1 hands, are dark, and at first sight it might be supposed that for this reason the vapors corresponding with them could not he photographed with the spectroheliograph. But