412 ACTUAL WORKING IN ORIGINAL RESEARCH. 



guardedly cast forth in an army, has turned the stream of 

 victory from one side to the other, and thereby disposed 

 of empires and whole nations.' l When heat enables che- 

 mical action to occur, it acts as an exciting cause, but 

 when chemical union produces heat, it operates as a true 

 cause. The same agent or force may in one case operate 

 as a true cause, and in another only as a determining one ; 

 thus heat operates as a true cause in disassociating the 

 constituents of a chemical compound or in producing an 

 electric current in a thermo-electric pile ; but its function 

 is merely that of an excitant when a change of tempe- 

 rature enables substances to unite together chemically. 

 In the two first cases an amount of heat disappears 

 proportional and equivalent to the degree of effect pro- 

 duced, but in the latter it does not. What I have 

 termed exciting causes are not true causes of the chief 

 effect under consideration, but only of a coincident phe- 

 nomenon, which is in itself a necessary static condi- 

 tion of the potential cause operating ; thus a certain 

 temperature is a necessary condition of the explosion of 

 gunpowder, and any cause which will produce that condi- 

 tion will excite or determine the explosion. Temperature 

 is usually a- true cause of chemical decomposition or dis- 

 association, but only a static condition of chemical union. 

 As realities often differ greatly from appearances, so 

 also apparent causes are often not the real ones, and in 

 such cases the real causes are the most latent or hidden. 

 A wound-up spring contains hidden mechanical power ; 

 easily condensible vapours contain stored-up heat ; com- 

 bustible or explosive substances or mixtures, and also sup- 

 porters of combustion, possess latent chemical force ; and 

 when these latent powers are liberated, the real causes are 

 often less manifest than the exciting ones. 



1 South. 



