,
and hence not to be trusted out of sight, it was, like them, liable to
break down at still higher elevations. Radiation by this new prescription
increases as the _square_ of the _absolute_ temperature--that is, of the
number of degrees counted from the "absolute zero" of -273 deg. C. Its
employment gave for the sun's radiating surface an effective temperature
of 20,380 deg. C. (including a supposed loss of one-half in the solar
atmosphere); and setting a probable deficiency in emission (as compared
with lamp-black) against a probable mutual reinforcement of superposed
strata, Professor Rosetti considered "effective" as nearly equivalent to
"actual" temperature. A "law of cooling," proposed by M. Stefan at Vienna
in 1879,[715] was shown by Boltzmann, many years later, to have a certain
theoretical validity.[716] It is that emission grows as the fourth power
of absolute temperature. Hence the temperature of the photosphere would
be proportional to the square root of the square root of its heating
effects at a distance, and appeared, by Stefan's calculations from
Violle's measures of solar radiative intensity, to be just 6,000 deg. C.;
while M. H. Le Chatelier[717] derived 7,600 deg. from a formula, conveying
an intricate and unaccountable relation between the temperature of an
incandescent body and the intensity of its red radiations.

From a series of experiments carefully conducted at Daramona, Ireland,
with a delicate thermal balance, of the kind invented by Boys and
designated a "radio-micrometer," Messrs. Wilson and Gray arrived in
1893, with the aid of Stefan's Law, at a photospheric temperature of
7,400 deg. C.,[718] reduced by the first-named investigator in 1901 to
6,590 deg.[719] Dr. Paschen, of Hanover, on the other hand, ascribed
to the sun a temperature of 5,000 deg. from comparisons between solar
radiative intensity and that of glowing platinum;[720] while F. W. Very
showed in 1895[721] that a minimum value of 20,000 deg. C. for the same
datum resulted from Paschen's formula connecting temperature with the
position of maximum spectral energy.

A new line of inquiry was struck out by Zoellner in 1870. Instead of
tracking the solar radiations backward with the dubious guide of
empirical formulae, he investigated their intensity at their source. He
showed[722] that, taking prominences to be simple effects of the escape
of powerfully compressed gases, it was possible, from the known
mechanical laws of heat an