118 MOSSOTTi's THEORY. [SECT. xiv. 



Mossotti furnishes additional presumption in favour of a 

 universal ether, already all but proved by the motion of 

 comets and the theory of light. 



In aeriform fluids the particles of matter are more remote 

 from each other -than in liquids and solids ; but 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 the limit, it has hitherto 

 always retained its gaseous properties, and resumes its 

 primitive volume the instant the pressure is removed. 



If the particles approach sufficiently near to produce 

 equilibrium between the attractive and repulsive forces, but 

 not near enough to admit of any influence from their form, 

 perfect mobility will exist among them resulting from the 

 similarity of their attractions, and they will offer great 

 resistance when compressed ; properties which characterise 

 liquids in which the repulsive principle is greater than in 

 the gases. When the distance between the particles is still 

 less, solids are formed. But the nature of their structure 

 will vary, because at such small distances the power of the 

 mutual attraction of the particles will depend upon their 

 form, and will be modified by the sides they present to one 

 another during their aggregation. Besides these three con- 

 ditions of matter, there are an infinite variety of others 

 corresponding to the various limits at which the two con- 

 tending forces are balanced, which 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. 



The effort required to break a substance is a measure of 

 the intensity of the cohesive force exerted by its particles, 

 which is as variable as the intensity of the repulsive prin- 

 ciple. In stone, iron, steel, and all brittle and hard bodies, 

 the cohesion of the particles is powerful, but of small extent. 



