194 J. R. Mayer on Celestial Dynamics. 
respect, their diathermic energy is found to be far superior to 
that of all artificial sources of heat. The temperature of the 
focus of a concave metallic reflector in which the sun’s light has 
been collected is only diminished from one-seventh to one-eighth 
by the interposition of a screen of glass. If the same exper 
ment be made with an artificial and luminous source of heat, it 
is found that, though the focus be very hot when the screen 18 
away, the interposition of the latter cuts off nearly all the heat; 
moreover, the focus will not recover its former temperature when 
reflector and screen are placed sufficiently near to the source of 
ered from a surface of from 5 to 6 square metres, and concentra 
ut the 
sufficient to vaporize platinum, rhodium, and similar metals. the 
The radiation calculated in Chap. III. likewise proves *” 
enormous temperature of the solar surface. From the nag 
nation mentioned therein, it follows that each square centume’™ 
of the sun’s surface loses by radiation about 80 units of a 
minute—an immense quantity in comparison with ter 
A correct theory of the origin of the sun’s heat must explait 
the cause of such enormous temperatures. This-explana ‘let, 
be deduced from the foregoing statement. According oe 
temperature at which bodies appear intensely white-Ao 
