6 OF MATTER AKD ITS PROPERTIES. 



measure prevented by some interfering cause. 

 Fluids have been defined to be bodies whose parts 

 yield to the smallest force impressed, and by 

 yielding, are easily moved among each other. 

 It should be observed, however, that all fluids 

 offer some resistance to bodies moving through 

 them. The essential difference between the state 

 of solidity and that of fluidity is not accurately 

 known ; it has been supposed by some, that the 

 particles of fluids are spherical, and hence admitted 

 of free motion over each other, whilst those of 

 solids are angular, which occasioned an entangling : 

 but solids and fluids are often convertible into each 

 other; as water into ice, metals into a state of 

 fusion, mercury into a solid state by freezing, &c. ; 

 and it cannot be supposed that in these cases the 

 figures of the particles are changed. 



Others have supposed that the particles of fluids 

 are never in actual contact, being kept apart by 

 caloric, or the matter of heat: this doctrine is ren- 

 dered plausible, by observing that most solids are 

 rendered fluid by the addition of a sufficient quan- 

 tity of heat ; and, on the contrary, most fluids are 

 reduced to the solid state by its abstraction ; but 

 while we are ignorant of the actual form and mode 

 of existence of the particles of matter, this subject 

 must remain extremely obscure. 



Fluids are distinguished into non-elastic and 

 elastic. Non-elastic Fluids are those whose dimen- 

 sions are not, at least as to ordinary sense, affected 

 by compression; hence they are also called incom- 

 pressible fluids ; such as water, oil, mercury, &c. 



Of non-elastic fluids, some are distinguished by 

 the property of liquidity or humidity; which im- 

 plies wetting or adhering to other substances: thus 

 water, oil, milk, &c. are liquids. Melted metals 



