vapours formed the relatively cool envelope of a still hotter internal
mass. Kirchhoff, accordingly, included in his great memoir "On the Solar
Spectrum," read before the Berlin Academy of Sciences, July 11, 1861, an
exposition of the views on the subject to which his memorable
investigations had led him. They may be briefly summarised as follows:

Since the body of the sun gives a continuous spectrum, it must be either
solid or liquid,[432] while the interruptions in its light prove it to
be surrounded by a complex atmosphere of metallic vapours, somewhat
cooler than itself. Spots are simply clouds due to local depressions of
temperature, differing in no respect from terrestrial clouds except as
regards the kinds of matter composing them. These _sun-clouds_ take
their origin in the zones of encounter between polar and equatorial
currents in the solar atmosphere.

This explanation was liable to all the objections urged against the
"cumulus theory" on the one hand, and the "trade-wind theory" on the
other. Setting aside its propounder, it was consistently upheld perhaps
by no man eminent in science except Spoerer; and his advocacy of it
proved ineffective to secure its general adoption.

M. Faye, of the Paris Academy of Sciences, was the first to propose a
coherent scheme of the solar constitution covering the whole range of
new discovery. The fundamental ideas on the subject now in vogue here
made their first connected appearance. Much, indeed, remained to be
modified and corrected; but the transition was finally made from the old
to the new order of thought. The essence of the change may be conveyed
in a single sentence. The sun was thenceforth regarded, not as a mere
heated body, or--still more remotely from the truth--as a cool body
unaccountably spun round with a cocoon of fire, but as a vast
_heat-radiating machine_. The terrestrial analogy was abandoned in one
more particular besides that of temperature. The solar system of
circulation, instead of being adapted, like that of the earth, to the
distribution of heat received from without, was seen to be directed
towards the transportation towards the surface of the heat contained
within. Polar and equatorial currents, tending to a purely superficial
equalisation of temperature, were replaced by vertical currents bringing
up successive portions of the intensely heated interior mass, to
contribute their share in turn to the radiation into space which might
be called