16 WELLS'S NATURAL PHILOSOPnT. 



matter in a given space as the other, it is said to be twice 

 as dense. 



There is a direct connection between the density of a body and its porosity. 

 A body will be more or less dense, according as its particles are arranged 

 closely together, or are separated from each other ; and hence it is clear, that 

 the greater the density the less the porosity, and the greater the porosity the 

 less the density. 



17. If the particles of a body do not touch each other, then, if it is subjected 

 to pressure, they may be forced nearer, and made to occupy less space. 



This wo find to be the fact. All matter may be compressed. The most 

 solid stone, when loaded with a considerable weight, is found to be com- 

 pressed. The foundations of buildings, and the columns which sustain great 

 weights in architecture, are proofs of this. Metals, by pressure and hammer- 

 ing, are made more compact and dense. Air, and all gases, are susceptible of 

 great compression. "Water, and all liquids, are much less easily compressed 

 than either soUd or gaseous bodies. 



18. By Compressibility, therefore, we mean 

 pressibim™? that property of matter in virtue of which a 



body allows its volume or size to be diminished, 

 without diminishing the number of the atoms or particles 

 of which it is composed. 



19. Again, if the particles of matter of which 

 pan^biiityT ^ body is composcd do not touch each other, it is 



clear that they may be forced further apart. 

 This we find to be the case with all matter. Expansibility 

 is, therefore, that property of matter in virtue of which a 

 body allows its volume or size to be increased, without in- 

 creasing the number of the atoms or particles of which it 

 is composed. 



All bodies, when submitted to the action of heat, expand, and 

 Illastrations occupy a larger space than before. To this increase in dimen- 

 bility ? sions there is no limit. Water, when sufBcieutly heated, passes 



into steam, and the hotter the steam the greater the space it 

 will occupy. All bodies, if subjected to a sufficient degree of heat, will pasr 

 from the state of solids or liquids, into the state of vapor, or gases. 



20. Inertia signifies the total absence in a 

 ^*rJa? ^"' ^otly of all power to change its state. If a 



body is at rest, it can not of itself commence 

 moving ; and if a body be in motion, it can not of it- 

 self stop, or come to rest. The motion, or cessation of 



