EXPANSIBILITY. 263 



pansive power is immediately developed from 

 its confinement and displayed by its activity ; 

 spreading and dilating to the utmost limits 

 which the imagination can conceive; it commu- 

 nicates motion to the mobility of different bodies, 

 and causes pressure on all. 



It is this original and essential power which 

 abiding in gaseous and aeriform bodies, gives 

 them the generic character of expansibility, 

 strictly so called; which identifies their nature 

 and distinguishes them from those belonging to 

 every other class. The difference between 

 them appears to me as great as they are impor- 

 tant ; and will be rendered apparent, if com- 

 pared together. The motion or force which is 

 manifested by steel and other bodies considered 

 as elastic, is altogether produced by the unna- 

 tural direction given to the particles of matter 

 of which they are composed ; the expansibility 

 of gaseous bodies proceeds from the spontane- 

 ous and natural tendency of expanding, inherent 

 in them. In the one, the direction of the mo- 

 tion produced, always corresponds to the par- 

 ticular direction of the particles of matter of 

 which the elastic substance is constituted. In 

 the other, the dilatation which takes place, ex- 

 tends equally in every direction. In the for- 

 mer, the degree of motion produced is confined 

 within the narrowest limits ; in the latter, the 

 expansion is indefinite in its extent; while the 



