ORIGIN OF THE SUN's HEAT. 279 



made with an artificial and luminous source of heat, it is 

 found that, though the focus be very hot when the screen is 

 away, the interposition of the latter cuts off nearly all the 

 heat ; moreover, the focus will not recover its former temper- 

 ature when reflector and screen are placed sufficiently near to 

 the source of heat to make the focus appear brighter than it 

 did in the former position without the glass screen. 



The empirical law, that the diathermic energy of heat in- 

 creases with the temperature of the source from which the 

 heat is radiated, teaches us that the sun's surface must be 

 much hotter than the most powerful process of combustion 

 could render it. 



Other methods furnish the same conclusion. If we ima- 

 gine the sun to be surrounded by a hollow sphere, it is clear 

 that the inner surface of this sphere must receive all the heat 

 radiated from the sun. At the distance of our globe from the 

 sun, such a sphere would have a radius 215 times as great, 

 and an area 46,000 times as large as the sun himself; those 

 luminous and calorific rays, therefore, which meet this spheri- 

 cal surface at right angles retain only 4^oob tn P art f their 

 original intensity. If it be further considered that our at- 

 mosphere absorbs a part of the solar rays, it is clear that the 

 rays which reach the tropics of our earth at noonday can 

 only possess from ^th to ^ m t}i of the power with which 

 they started. These rays, when gathered from a surface of 

 from 5 to 6 square metres, and concentrated in an area of 

 one square centimetre, would produce about the temperature 

 which exists on the sun, a temperature more than sufficient 

 to vaporize platinum, rhodium, and similar metals. 



The radiation calculated in Chapter III. likewise proves 

 the enormous temperature of the solar surface. From the 

 determination mentioned therein, it follows that each square 

 centimetre of the sun's surface loses by radiation about 80 

 units of heat per minute an immense quantity in compari- 

 son with terrestrial radiations. 



