and Terrestrial Elements in the Sun. 405 
rents; and the same remark will apply to all those substances 
which do not ordinarily exist in the condition of saturated 
vapour in the sun's atmosphere. 
But not only are lines sometimes widened and sometimes 
not widened in spots, but sometimes lines usually seen as dark 
lines disappear, or appear as bright lines. This has been satis- 
factorily explained as the effect of ascending currents bearing 
vapours into the upper regions at such a high temperature 
that their em'ission is equal to or exceeds their absorption. In 
fact it is just those lines which have been observed as bright 
lines above the sun's limb in solar storms which are absent or 
are reversed in spots. There is still the question why some 
lines of such elements as iron and calcium should belong to 
the category of lines seen extending to considerable heights in 
the solar atmosphere while others do not so extend. We are 
inclined to the opinion that this appearance of certain lines at 
high elevations to the exclusion of others is dependent more 
on the tension of the vapour than on its temperature. Of 
course there is a relation between the tension of a vapour and 
its temperature, and this relation is by no means the same for 
saturated as it is for unsaturated vapour ; and there may be 
unsaturated low-tension vapour at a very high temperature 
when but little of the material of the vapour is present, as well 
as saturated low-tension vapour at a low temperature. Now 
the distinction of long and short lines in the electric discharge, 
first introduced by Thalen, to which Mr. Lockyer has since 
drawn more particular attention, corresponds, as he has noted, 
with difference in the density of the vapour ; the short lines 
being seen only near the poles or in the central portion of the 
discharge, while the long lines extend further. We know 
but little about the temperatures of different parts of an elec- 
tric discharge; but in this case temperature and density of 
vapour may very well go together. It is otherwise when one 
element is present in very small quantity, as in the alloys on 
which Mr. Lockyer has experimented. He found that in 
alloys containing only a very small percentage of one metal, it 
was the long lines of that metal which were persistent in the 
spectrum. This seems conclusive that long lines are the lines 
of vapour of low tension rather than of low temperature ; the 
lines corresponding to the vibrations which the particles take 
up most readily when they are in the most unfettered state, in 
their least complicated aggregations. The short lines will on 
this supposition correspond to vibrations either of more con- 
densed particles, or to vibrations induced by the constrained 
condition of a dense gas. This view is in harmony with the 
fact that only the long lines usually appear in the higber 
