TEMPERATURE AND STRUCTURE OF THE SUN. 
85 
law more exactly than was expected, considering (he enor- 
mous difficulty of measuring high temperatures and black 
radiation . 
Thus we were enabled to conclude from our observations 
that the hypothesis is correct on which Boltzmann based his 
theoretical development of the fundamental law of black 
radiation. Boltzmann’s deduction is based upon the second 
law of thermodynamics and on Maxwell’s theory, according 
to which the radiation pressure measured in dynes is equal 
to the energy in ergs which is contained in the unit of vol- 
ume of the medium through which the radiation passes. 
There can be no doubt of the applicability of the second law 
of thermodynamics to pure temperature radiation, where wc 
have to deal with disordered radiation, containing waves of 
all lengths, phases, and amplitudes, such as the radiation 
emitted by a black body. Later we will show that Kirchhoff’s 
law is not at all applicable to luminescent bodies, and there- 
fore that Kirchhoff’s conclusions with reference to the sun’s 
temperature and to spectrum analysis in general are not 
correct. 
Since the second law of thermodynamics is applicable to 
black radiation, the astonishing agreement between theory 
and experiment can be considered as a proof that the other 
hypothesis used by Boltzmann is also correct. In other 
words, that by our experiments not only the existence of the 
ether pressure, but also the magnitude of this pressure is 
established. The pressure produced by the sun’s radiation 
upon our earth is represented by a weight of only a milli- 
gram per square meter. Therefore we admire still more the 
excellent work of Lebedew and of Nichols and Hull, who 
experimentally demonstrated the pressure of a light source 
in a very convincing and striking manner. 
Here we have the first result of practical importance which 
includes within its scope even the stars. Indeed, from the 
existence of the radiation pressure we must conclude that two 
hot bodies like the sun and our earth exert on each other not 
only an attractive force, but also a repulsive one. This result 
introduces into the processes of nature quite a new factor and 
