CHAPTER V. 



SUN-HEAT. 



Its Source and Limits. 



SUN-HEAT is another product of the same 

 retro-action between the sun and earth ; con- 

 sequently it has the same range and the 

 same boundaries as when it is viewed as 

 light. 



Tendencies to unsettle in Science. 



The scientists of to-day may well look after 

 the soundness of their favorite theories of 

 the great physical forces ; for the uncertain 

 tenure of old theories, by reason of recent 

 discoveries, is becoming but too manifest. 

 New phenomena are now observed which 

 require solutions not met by present hypoth- 

 eses. The nebular hypothesis which has so 

 long possessed the scientific mind has, by 

 the discovery of the moons of Mars, become 

 a thing of the past. According to M. 

 MAICHE, water is found to be no longer the 

 old-fashioned conventional oxygen and hy- 



