Xll PREFACE. 



they are modifications of one and the same 

 essence, which is the active principle in light, 

 and in all the phenomena of nature. 



Since the time of Sir Isaac Newton, it has 

 been generally supposed that all the molecular 

 changes of matter may be referred to attractive 

 and repulsive forces. Yet the cause of attrac- 

 tion and repulsion has not been identified with 

 any known principle capable of demonstra- 

 tion. The orbits of planets, and the times of 

 their revolutions have been measured with 

 mathematical precision ; but the cause of their 

 eternal motion has never been clearly distin- 

 guished from projectile and gravitating forces. 

 The relative magnitudes of the particles or 

 atoms of ponderable matter have been, to a 

 certain extent, ascertained by the refined 

 analyses of modern chemistry : yet the cause 

 of chemical affinity is still confounded with 

 inherent properties, occult qualities, and unde- 

 fined powers. The composition of plants and 

 animals has been discovered, and their intimate 

 structure explored : yet the organizing prin- 

 ciple remains a profound mystery. What was 

 said by Descartes two hundred years ago, is 

 equally true at the present day. " In Philo- 

 sophia nihil adhuc reperiri, de quo non in 

 utramque partem disputatur." 



