SECT. XIV. GRAVITATION OF PARTICLES. 105 



producing the motions varying simply as the density of the 

 ether. 



In aeriform fluids, although the particles are more remote from 

 each other than in liquids and solids, yet the pressure may be so 

 great as to reduce an aeriform fluid to a liquid, and a liquid to a 

 solid. Dr. Faraday has reduced some of the gases to a liquid 

 state by very great compression ; but although atmospheric air 

 is capable of a diminution of volume to which we do not know a 

 limit, it has hitherto always retained its gaseous qualities, and 

 resumes its primitive volume the instant the pressure is removed. 

 Substances are said to be more or less elastic, according to the 

 facility with which they regain their bulk or volume when the 

 pressure is removed ; thus liquids resist compression on account 

 of their elasticity, and in solids the resistance is much greater but 

 variable, and the effort required to break a substance is a measure 

 of the cohesive force exerted by its particles. In stone, iron, steel, 

 and all brittle and hard substances, the cohesion of the particles 

 is powerful but of small extent ; in elastic bodies, on the con- 

 trary, its action is weak, but more extensive. An infinite variety 

 of conditions may be observed in the fusion of metals and other 

 substances passing from hardness to toughness, viscidity, and 

 through all the other stages to perfect fluidity and even to vapour. 

 Since all bodies expand by heat, the cohesive force is weakened 

 by increase of temperature. The cohesion of matter or the 

 strength of substances forms an important branch of study in 

 engineering. 



Every particle of matter, whether it forms a constituent part 

 of a solid, liquid, or aeriform fluid, is subject to the law of gra- 

 vitation. The weight of the atmosphere, of gases and vapour, 

 shows that they consist of gravitating particles. In liquids the 

 cohesive force is not sufficiently powerful to resist the action of 

 gravitation. Therefore, although their component particles still 

 maintain their connexion, the liquid is scattered by their weight, 

 unless when it is confined in a vessel or has already descended to 

 the lowest point possible, and assumed a level surface from the 

 mobility of its particles and the influence of the gravitating 

 forces, as in the ocean, or a lake. Solids would also fall to pieces 

 by the weight of their particles, if the force of cohesion were not 

 powerful enough to resist the efforts of gravitation. 



The phenomena arising from the force of cohesion are innu- 



F 3 



