40 LECTURE I. 



means destroy the mechanical properties 

 which had before obtained. Thus we see 

 the toughest wood slowly decay, or sud- 

 denly consumed by fire. 



So numerous are the phsenomena in na- 

 ture, that suggest and enforce the belief 

 that subtile substances may and do pervade 

 others more gross and inert, and produce 

 effects in and upon them ; that no surprise 

 can be excited upon finding that contem- 

 plative men have in all ages adopted and 

 inculcated this opinion. Yet it was not 

 till of late years, by observing the phaeno- 

 raena of electricity and magnetism, that de- 

 cisive evidences of this proposition were ob- 

 tained. In considering these subjects, we 

 observe the utmost boundaries of human 

 knowledge, for all our information must 

 be derived from our senses, which can ne- 

 ver give us any cognizance of the atoms 

 that compose surrounding bodies. Yet this 

 horizon of our views has been always dis- 

 tinctly seen by long-sighted observers ; and 

 advanced as we are in knowledge, we can 

 see no further. Persons of different cha- 



