PREFACE. vii 



the same subtile fluid is obtained from the 

 atmosphere by respiration, he laid the foun- 

 dation of its true theory. But as he did not 

 extend his researches to its agency in the 

 mechanical, chemical, and vital phenomena of 

 nature, his labours have not been followed by 

 results corresponding with the magnitude of 

 his discovery, if we except the improvement 

 of the steam engine by Watt. 



Nor has any one, either among the ancients 

 or moderns, ever attempted to give a regular 

 and systematic history of the mode in which 

 caloric operates in all the molecular and 

 aggregate forces of matter. Mr. Whewell 

 observes, in his late History of the Inductive 

 Sciences, that " we have no hypothesis regard- 

 ing thermotics, which, being assumed in order 

 to explain one class of phenomena, has been 

 found to account exactly for another." Yet 

 he adds, " it is one of the cardinal points on 

 which the doors of physical knowledge must 

 turn, which have hitherto remained closed." 



Within the last hundred years, electricity 

 has been regarded by many persons as a key 

 by which to unlock all the secret cabinets of 

 nature. We have had electrical theories of 

 chemistry, of life, and of the universe. Yet 



