SUNSHINE ON THE FAEM. 381 



intensity. They show that there has been enormous activity 

 in vegetable production; and then came ice, which covered 

 the earth's surface, except spaces along the line of the equa- 

 tor. We have no absolute knowledge whether the sun is 

 growing hotter or colder, but it is probable that at present 

 it is receiving as much heat as it throws off into space, 

 and there is no occasion for alarm as to a failure of the 

 supply. 



As regards the nature of the fuel of the sun we have no posi- 

 tive knowledge. Inquiries in this direction are beset with 

 difficulties. There are nearly as many theories as there have 

 been distinguished astronomers and mathematicians. It may 

 be due to meteoric matter raining upon the sun's surface, or 

 it may be a peculiar universal atmosphere through which the 

 sun is passing, or it may be that no fuel (in any sense in 

 which that term is used by man) is needed by the great cen- 

 tral orb. It is quite possible to conceive of a body losing 

 heat for ages without changing its temperature, although 

 changes go on in the molecular constitution of its materials. 

 There must, however, be an end to the sun ; its final extinc- 

 tion cannot be a matter of doubt ; but at our epoch we need 

 not give ourselves any anxieties on that account. 



If asked as regards the chemical processes going forward 

 at the sun, which result in the evolution of the stupendous 

 quantities of light and heat received, I would say that no 

 processes known to our chemistry are adequate to explain 

 the problem. The hottest of all our chemical changes or 

 combustions are as ice currents compared with the heat of the 

 sun. It cannot be that any processes corresponding with what 

 is known among us as combustion, can occur at the sun, or 

 at least they fail to account for observed phenomena upon 

 the solar disk. The heat of the great blast furnaces of Penn- 

 sylvania, which rapidly fuse thousands of tons of refractory 

 iron ore, is but that of the wax candle when placed in con- 

 trast with the heat of the sun. Its light is so intense that 

 if we arrange one of our electric arc lights in a way so that 

 it may be projected upon its disk at noonday, it forms a 

 dark spot, destitute of luminosity. The spots seen on the 

 solar surface, and upon which a vast amount of research has 



