THE EXCITING CAUSE 217 



no living body could exist. In this it succeeds so long as the state of 

 the parts of the hving body does not prevent it. 



We find, moreover, that this expansive fluid, especially when its 

 abundance gives it some intensity of action, is the principal factor 

 in the enormous multiplication of hving bodies of which I have just 

 spoken. Hence it is universal that in the hot chmates of the earth the 

 animal and plant kingdoms exhibit an extremely remarkable wealth 

 and abundance, whereas in the frozen regions of the earth they only 

 exist in a state of the greatest poverty. 



A further fact in favour of the principle just established is that there 

 is a great difference between the summer and winter of our own 

 chmates as regards the number of animals and plants. 



Although caloric is really the first cause of life in the bodies which 

 possess it, yet it could not alone account for its existence nor keep up 

 the movements which constitute it ; there is needed in addition, 

 especially for animals, the influence of a fluid to excite their acts of 

 irritabihty. Now we have seen that electricity possesses all the 

 properties necessary for constituting this exciting fluid, and that it 

 is distributed sufficiently widely notwithstanding its variations, to 

 ensure that hving bodies shall always be provided with it. 



It may be that some other invisible fluid combines with electricity 

 in making up the cause which is able to excite vital movements and all 

 the organic activities ; this is quite possible, but I see no need for sup- 

 posing it. 



It seems to me that caloric and the electric substance together are 

 quite sufficient to constitute the essential cause of hfe, the one by 

 setting the parts and internal fluids in a proper condition for the 

 existence of hfe, and the other by arousing in the course of its move- 

 ments through the body the various stimuh which give rise to the 

 organic activities and the active part of hfe. 



If we were to try to explain how these fluids work, and to determine 

 definitely the number of those that enter into the composition of the 

 exciting cause of all organic movements, we should be abusing the 

 power of our imagination and arbitrarily creating explanations which 

 we have no means of verifying. 



It is enough to have shown that the exciting cause of the movements 

 which constitute hfe does not reside in any of the visible fluids in the 

 interior of hving bodies, but that it takes its origin as follows : 



1. From caloric, which is an invisible penetrating, expansive, ever- 

 active fluid that percolates slowly through the supple parts, dis- 

 tending them and making them irritable ; and that is constantly 

 being dissipated and renewed and is never entirely absent from any 

 body that possesses hfe ; 



