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LXVI. 
A THEORY OF SUN-SPOTS. By J. JOLY, M.A., Sc.D., F.R.S., 
Hon. Sec. R.D.S., Professor of Geology and Mineralogy in the 
University of Dublin. 
[Read Ducemzer 22, 1897; Received for Publication DrecEmBerR 23 ; 
Published Frprvary 4, 1898]. 
Tue temperature of the Solar Photosphere hag been variously 
estimated. The recent estimate of Messrs. Wilson and Gray! 
affords 8000° C. as about the probable temperature. 
If this temperature is above the critical temperature of the 
mixture of elements present in the hotter parts of the photosphere, 
we are led to consider the great mass of the Sun as gaseous ; for it 
is very certain that the temperature beneath the photosphere is 
still higher. In fact the actual seat of the evolution of heat must, 
on the hypothesis of the dynamical origin of solar heat, be in the 
denser body of the Sun lying within the photosphere. 
Although the gaseous constitution of the Sun must on this 
assumption be admitted as a matter of definition (and the low mean 
density suggests that the effect of temperature is to maintain his 
- exterior layers for great depths in a true gaseous state) the interior 
parts of the Sun are at pressures so enormous that the density 
probably rises to that of the solid state, and the matter within 
assumes or approximates to the physical properties of solid matter. 
It would further appear that a true surface cannot be ascribed to 
the Sun. The highly compressed gaseous matter within gives place 
to rarer matter as the photosphere is approached, till the density has 
diminished so much as to permit of enormous convection currents, 
and finally of the great explosive out-rushes observed extending 
from the photosphere. Nowhere would a true surface exist, any- 
more than in the experimental tube containing carbon dioxide at a 
temperature above 31°C. 
1 Phil. Trans. A., vol. 185, 1894. 
