356 SCIENTIFIC THOUGHT. 



receive energy, where does it come from ? if we lose 

 energy, where does it go to ? " It was recognised that 

 the great store of energy on which we at present depend 

 is the heat of the sun, which is partly used or wasted by 

 daily radiation, partly stored in the separated energies 

 of chemical substances, such as were produced by the 

 agency of solar heat in bygone ages ; the deposits of 

 coal in the bowels of the earth being a prominent 

 and important example. Where does the heat of the 

 sun come from, and how is it maintained ? These were 

 some of the questions which began to be asked. The 

 genesis of the cosmos, as suggested by Laplace and fanci- 

 fully elaborated by popular writers, had taken note only 

 of the matter in the sun and in the planetary system, 

 and had disregarded the heat l or energy which the sun 

 supplied, and on which the historical changes on the sur- 

 face of our globe have almost entirely depended. " But 

 physical laws are for our mental vision," as Helmholtz 

 says, " like telescopes which penetrate into the farthest 

 night of the past and the future." 2 Shortly before the 

 pioneers of the mechanical theory of heat published their 



1 " When Playfair (in his ' Illus- 

 trations of the Hut Ionian Theory ') 

 spoke of the planetary bodies as 

 being perpetual in their motion, 

 did it not occur to him to ask, 

 What about the sun's heat ? Is 

 the sun a miraculous body ordered 

 to give out heat and to shine for 

 ever ?" (Lord Kelvin in 1868, " On 

 Geological Time," ' Popular Lec- 

 tures and Addresses,' vol. ii. p. 45.) 

 " The old nebular hypothesis sup- 

 poses the solar system and other 



densation of fiery nebulous matter. 

 This hypothesis was invented be- 

 fore the discovery of thermodyna- 

 mics, or the nebulae would not have 

 been supposed to be fiery ; and the 

 idea seems never to have occurred 

 to any of its inventors or early 

 supporters that the matter, the 

 condensation of which they sup- 

 posed to constitute the sun and 

 stars, could have been other than 

 fiery in the beginning" (id., 1871, 

 ibid., vol. L p. 184). 



similar systems through the uni- j - See ' Vortriige und Reden,' 

 verse which we see at a distance as i Aufl., vol. i. p. 57. 

 stars to have originated in the con- . 



