258 IMMOBILITY, MOBILITY, ETC. 



the state of activity into which it had been 

 excited, into the passive and quiescent state 

 which is natural to it. This is the end 

 which invariably takes place, when one mass of 

 solid matter is made to act upon another; the 

 first loses as much of its own motion as it im- 

 parts to the second, insomuch that the quan- 

 tity of change which is excited in the one, 

 entirely depends on the quantity of power com- 

 municated from the other. This capacity to 

 be acted upon, this indifference to motion or 

 rest, is called mobility ; and the change which 

 the body undergoes during the transition from 

 one place to another, is called motion.* 



The effect or change which is produced by a 

 moving power on a solid substance, altogether 

 depends on the nature of its construction. If 

 it cracks or breaks without yielding, it is said 

 to be brittle, such as glass or flint. When a 

 body yields without cracking, it is considered 

 as flexible ; this is the case with lead, with iron, 

 and with a variety of other bodies. This capa- 

 city to be bent by the agency of external pow- 

 ers, which particular bodies contain, without 



* It is surprising Mr. Locke should have misapplied, as he 

 has done, the term mobility, and confounded capacity and 

 power together. Mobility he calls a power to be moved, in- 

 stead of a capacity or aptitude to be moved. In like manner 

 Sir Isaac Newton calls it a vis inertia. 



