502 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



The future of physical science seems to lie in the path upon 

 which three of our ablest British physicists have so boldly entered, 

 and in which they have already made such large advances. I may 

 therefore be permitted briefly to touch upon the successive steps in 

 this lofty generalization, and to indicate the goal to which they 

 tend. 



It has been long known that many of the forces of nature are 

 related. Thus, heat is produced by mechanical action, when that is 

 applied in bringing the atoms of bodies nearer by compression, or 

 when it is expended in friction. Heat is developed by electricity r 

 when the free passage of the latter is impeded ; it is produced 

 whenever light is absorbed; and it is generated by chemical 

 action. A like interchangeability probably exists among all the 

 other forces of nature, although in many the relations have not 

 been so long perceived. Thus, the development of electricity from 

 chemical action dates from the observations of Gralvani ; and the 

 production of magnetism by electricity from the discovery of 

 Oersted. 



The next great step was to perceive that the relation of the 

 physical forces was mutual ; and that of any two, compared toge- 

 gether, either may stand to the other in the relation of cause. 



With respect to heat and mechanical force, this has been long 

 known. When a body is compressed by mechanical force, it gives 

 out heat ; and, on the other hand, when it is healed, it dilates, and 

 evolves power. The knowledge of the action of electricity, in dis- 

 solving the bonds of chemical union, followed closely upon that of 

 the inverse phenomenon : and the discovery of electro-magnetism 

 by Oersted was soon followed by that of magneto-electricity by 

 Faraday. With reason, therefore, it occurred to many minds that 

 the relations of any two of the forces of nature were mutual; 

 that that which is the cause, in one mode of interaction, may be- 

 come the effect, when the order of the phenomena is changed ; 

 and that therefore, in the words of Mr. Grove, one of the able 

 expounders of these views, while they are " correlative," or recipro- 

 cally dependent, " neither, taken abstractedly, can be said to be the 

 essential cause of the others." 



But a further step remained to be taken. If these forces were 

 not only related, but mutually related, was it not probable that the 

 relation was also a definite one ? Thus, when heat is developed by 

 mechanical action, ought we not to expect a certain definite proper- 



