LECTURE I. 31 



The opinion is still further refuted by ob- 

 serving, that these vivacious contractions 

 will equally take place, upon the same ex- 

 citement, in the exhausted receiver of an 

 air-pump and in the open air. They may 

 also be excited under water, under oil, in 

 a great variety of gases ; in short, under 

 circumstances which exclude the presence 

 of any chemical agent from without, to 

 which such changes could reasonably be 

 imputed. 



Secondly, the contraction of irritability 

 has been supposed to be a property of the 

 muscular fibres. Properties are generally 

 considered as permanent qualities. Thus, 

 the property of gravitation is continually 

 operating, equally when bodies remain at 

 rest and when it produces motion in them, 

 equally whilst I support this book in my 

 hand, and when I suffer it to fall on the 

 table. If, however, so curious an occa- 

 sional property could belong to matter, we 

 should naturally expect that it would be- 

 long to some peculiar quality, or arrange- 

 ment of matter. But irritability is con- 



