264 EXPANSIBILITY. 



addition of an external force is absolutely ne- 

 cessary to make a flexible body to become an 

 elastic one ; it is by the substraction of an ex- 

 ternal force that gaseous bodies are enabled to 

 expand. Elasticity, therefore, is merely m 

 effecto, a power which is derived, but which 

 does not exist essentially ; it is an excited, not 

 an inherent power, whose energy immediately 

 ceases as soon as the compressing cause is re- 

 moved, and the particles of which the elastic 

 body is composed have returned to their natural 

 and original situation. 



In matter, however, which is essentially 

 expansible, it is far otherwise ; instead of m 

 effecto, it is causa motus ; not derived from 

 without, but which subsists inherently within ; 

 not produced by external means; it is through 

 the resistance alone, opposed, by external 

 means, that this expansive power can be sus- 

 pended or suppressed. The difference may be 

 proved by simply placing a flexible, an elastic, 

 and an expansible body together, under the 

 same relative situations. If a small portion of 

 air enclosed in a large bladder is placed under 

 the receiver of an air-pump, with a piece of lead 

 or steel, the change which the air undergoes is 

 totally different from that of the other two. 

 In proportion as the air within the receiver 

 external to these bodies is abstracted by ex- 

 haustion ; it is found that neither the steel nor 



