402 Professors Liveing and Dewar on Sun-spots 
mean temperature of the photosphere, which converts the 
clouds into the gaseous condition, as sunshine dissipates the 
aqueous clouds in our atmosphere. If the spot is due only to 
a downrush of the upper atmosphere of the sun, we should be 
merely looking through a greater quantity of the outer atmo- 
sphere, increased in density and in temperature but consisting 
mainly of the same materials as before, and driving the photo- 
sphere, the chief source of light, downwards. We should 
expect to see the ordinary absorption-lines strengthened, and 
perhaps some new lines, due to a higher temperature, developed. 
For we must remember that a vapour is capable of absorbing 
the same kind of radiation as it emits; and that as its emission- 
spectrum varies with change of circumstances, of which tem- 
perature is one, its absorption will vary too. If the clouds of the 
photosphere were not merely mechanically depressed, but partly 
vaporized at the spot, we should expect to see new absorption- 
lines ; not only on account of the higher temperature of the 
vapours previously existing as such, but because matter which 
had before been cloud, and emitted a continuous spectrum, 
had now become gaseous, with a discontinuous spectrum. 
And here we would say a word about an a priori objection 
to the supposition that a spot may be a region of a temperature 
generally equal to, or even higher than, that of the photosphere 
at the same level. Messrs. De La Rue, Stewart, and Loewy, in 
their ' Researches on Solar Physics '(1st ser. p. 31), have argued, 
very cautiously and in a questioning manner, from the known 
laws of radiation and absorption, that we must be looking 
through a stratum of atmosphere at the spot cooler than the 
photosphere, because from such a thickness of matter as the 
sun we must at all points get the total radiation due to the 
temperature. We do not at all question the general principle 
of this argument, but we question its application in this case, 
because the total radiation is not the same thing as the lumi- 
nosity.- There may be more radiation on the whole from the 
darker spot than from the brighter photosphere, if the excess 
be in the ultra-violet or the infra-red. It is a general rule 
with solids that the radiations of short wave-length increase 
with the temperature more rapidly than those of longer wave- 
length ; and some of the terrestrial elements which are abun- 
dant in the sun, such as iron and magnesium, have an emis- 
sion-spectrum which appears to be much stronger in the ultra- 
violet than in the visible region. The substance we call iron, 
whether it be one element or many, emits in the electric arc 
ultra-violet rays which are extraordinary both in number 
and intensity. We are not in a position to test whether the 
total radiation from a spot is as great as from an equal portion 
