390 NATURE AND MAN. 



limited number of groups or classes of atoms, distinguished by 

 their several attributes ; each group, however, consisting of an 

 almost infinite number of individuals precisely resembling one 

 another in their properties. " Now, when we see a great number 

 " of things precisely alike, we do not believe this similarity to 

 " have originated except from a common principle independent 

 " of them ; and this conclusion, which would be strong even were 

 "there only two individuals precisely alike in all respects and/^?;* 

 '■'■ ever^ acquires irresistible force when their number is multiplied 

 " beyond the power of imagination to conceive. If we mistake 

 "not, the discoveries alluded to effectually destroy the idea of an 

 " eternal self-existent matter, by giving to each of its atoms the 

 " essential characters at once of a manufactured article and a 

 '■^subordinate agent." * 



Thus, then, whenever we witness any change in the material 

 world for which we desire to account, we are led by scientific 

 reasoning to seek for the force which produced it ; and only when 

 we have succeeded in finding this, do we consider that we have 

 rationally explained the phenomenon. But whence the force ? 

 Science now teaches us to look for the source of it in the trans- 

 formation of some other kind of energy ; as when the production 

 of heat by the burning of coal is turned, in the steam engine, to 

 the maintenance of mechanical motion, which, communicated to 

 a dynamo-machine, generates an electric current, which, in its 

 turn, may be made to produce heat, light, mechanical motion, or 

 chemical action. But, as Sir John Herschel pointed out, " In our 

 " own performance of a voluntary movement, we have a conscious- 

 " ness of immediate and persofial causation which cannot be dis- 

 " puted or ignored ; and when we see the same kind of act 

 " performed by another, we never hesitate in assuming for him 

 " that consciousness which we recognize in ourselves." 



The Physiologist, above all others, is forced, as it seems to 

 me, by the experience of every day, of every hour, and even of 

 every minute, to recognize the mutual convertibility of physical 

 and moral agency ; — the pricking of our skin with a pin producing 

 a change in our state of feeling ; and a mental determination 

 calling a muscle (or set of muscles) into a contraction which 

 * Discourse on the Study of Natural Philosophy," p. 38. 



