SECT. XIV. ULTIMATE PARTICLES OF MATTER. 103 



under the hands of the lapidary ; but as it is inconceivable that 

 the particles of matter should act upon one another without some 

 means of communication, it is presumed that the interstices of 

 material substances contain a portion of the ethereal medium with 

 which the regions of space are filled. 



The various hypotheses that have been formed as to the nature 

 and action of the forces which unite the particles of matter, have 

 been successively given up as science advanced, and now nothing 

 decisive has been attained, although Professor Mossotti, of Pisa, 

 by a very able analysis, has endeavoured to prove the identity of 

 the cohesive force with gravitation. As the particles of material 

 bodies are not in actual contact, he supposes that each is sur- 

 rounded by an atmosphere of the ethereal medium, which he 

 conceives to be electricity ; moreover he assumes that the atoms 

 of the medium repel one another, that the particles of matter also 

 repel one another, but with less intensity, and that there is a 

 mutual attraction between the particles of matter and the atoms 

 of the medium, forces which are assumed to vary inversely as the 

 square of the distance. 



Hence, when the material molecules of a body are inappreciably 

 near to one another, they mutually repel each other with a force 

 which diminishes rapidly as the infinitely small distance between 

 the material molecules augments, and at last vanishes. When 

 the molecules are still farther apart, the force becomes attractive. 

 At that particular point where the change takes place the forces 

 of repulsion and attraction balance each other, so that the mole- 

 cules of a body are neither disposed to approach nor recede, but 

 remain in equilibrio. If we try to press them nearer, the repul- 

 sive force resists the attempt ; and if we endeavour to break the 

 tody so as to tear the particles asunder, the attractive force pre- 

 dominates and keeps them together. This is what constitutes the 

 cohesive force, or force of aggregation, by which the molecules of 

 all substances are united. The limits of the distance at which 

 the negative action becomes positive vary according to the tem- 

 perature and nature of the molecules, and determine whether the 

 body which they form be solid, liquid, or aeriform. 



Beyond this neutral point the attractive force increases as the 

 distance between the molecules augments till it attains a maxi- 

 mum ; when the particles are more apart, it diminishes ; and, as 

 soon as they are separated by finite or sensible distances, it varies 



