UNITS AND STANDARDS OF MEASUREMENT. 371 



Unit of Density. 



Before we can measure and define the phenomena of 

 nature, we require a third independent unit, which shall 

 enable us to define the quantity of matter which occupies 

 any given space. All the motions and changes of nature, 

 as we shall see, are probably so many manifestations of 

 energy ; but energy requires some substratum or material 

 machinery of molecules, in and by which it may be 

 exerted. Very little observation shows that, as regards 

 force, there may be two modes of variation of matter. 

 The force required to set a body in motion, varies in 

 simple proportion to the bulk or cubic dimensions of the 

 matter, but also according to its quality. Two cubic 

 inches of iron of uniform quality, will require twice as 

 much force to produce a certain velocity in a given time 

 as one cubic inch ; but one cubic inch of gold will require 

 more force than one cubic inch of iron. There is then 

 some new measurable quality in matter apart from its 

 bulk, which we may call density, and which is, strictly 

 speaking, indicated by its capacity to resist and absorb 

 the action of force. For the unit of density we may 

 assume that of any substance which is uniform in quality, 

 and can readily be referred to from time to time. Pure 

 water at any definite temperature, for instance that of 

 snow melting under an inappreciable pressure, furnishes 

 a natural and invariable standard of density, and by 

 testing equal bulks of various substances compared with a 

 like bulk of ice-cold water, as regards the velocity pro- 

 duced in a unit of time by the same force, we should 

 ascertain the densities of those substances as expressed in 

 that of water. 



Practically the force of gravity is used to measure 

 density ; for a simple and beautiful experiment with the 



B b 2 



