ON THE GENETIC VIEW OP NATURE. 359 



a valid explanation of the maintenance of solar heat. In 

 fact, " as to the sun, we can now go both backwards and 

 forwards in his history upon the principles of Newton 

 and Joule." l 



But further means for testing the correctness of these so. 



Spectrum 



theories were afforded by the invention, in 1859, of Anal y sii - 

 Spectrum Analysis. It was found that the composition 

 of the light of luminous bodies, as revealed by prismatic 

 scattering in the spectrum, enabled us to tell a good deal 

 about the nature of the body itself from which the light 

 emanated. We can tell whether the body is shining 

 with its own or with reflected light, what are the con- 

 stituents of the incandescent body, whether it is an 

 incandescent solid or an incandescent gas ; also whether 

 the body is in motion or not. The nebular hypothesis 

 supposed that the planetary system owed its origin to 

 incandescent, perhaps gaseous, matter, which, through the 

 force of attraction, was collected in different centres : the 

 discoveries of thermodynamics and of spectroscopy have 

 enabled us to expand and correct some of the assump- 

 tions of this theory, and to add new features to its 

 minuter elaboration. It is not necessary that the matter 

 which was originally scattered through space and was 

 gathered into attracting centres should be itself incan- 

 descent or gaseous ; it may have been cold and solid like 

 dust ; rising in temperature and becoming incandescent 

 only through the conversion of arrested motion into heat, 

 which again was maintained for some time through acces- 

 sion of new matter or progressive shrinkage, but which 

 must in a calculable time be radiated away, leaving a 



1 Lord Kelvin, foe. cit., vol. ii. p. 131. 



