PREFACE. Xi 



ster ; or whether it consist in the mere vibra- 

 tions of some unknown aether, as supposed by 

 Huyghens, Hooke, Euler, Young, Sir John 

 Herschel, Arago, and others. Nor is it yet 

 finally settled whether white light is composed 

 of seven primitive rays, as supposed by New- 

 ton, or of only three fundamental colours, as 

 maintained, with many cogent reasons de- 

 duced from experiment, by Sir David Brew- 

 ster. On the same subject, the advocates of 

 the undulatory or wave theory are equally at 

 variance among themselves : for while Huyg- 

 hens maintained that solar light is composed 

 of only two elementary colours, yellow and 

 blue, Hooke reduced all its modifications to 

 red and violet ; whereas Young reduced the 

 whole to red, green, and violet. 



What is still more remarkable, neither of the 

 two great rival parties has ever yet attempted to 

 ascertain the relations of light to caloric and 

 electricity ; nor in what way they perform so 

 many wonderful effects in the moving drama 

 of the universe. But if they be connected in 

 action, they must be united in theory. And it 

 will be a leading object of the following work 

 to prove, by a careful generalization of facts, 

 that caloric and electricity are mutually con- 

 vertible into each other ; consequently, that 



