2 On the Constitution of Matter, 



function of the distance as to satisfy the most obvious propertiea 

 of matter. Leaving out of consideration for the moment, all 

 theories except that of gravitation, let us consider what are the 

 facts. Let us consider the mutual action between two atoms or 

 molecules, or particles, or whatever they may be, of a substance 

 such as mercury, which is capable under ordinary circumstances, 

 of existing iu the solid, the liquid or the gaseous state. As 

 mercury has weight, we can hardly doubt that at a sufficient 

 distance, two particles of that substance attract one another. At 

 some less distance they repel one another, whatever be the cause, 

 for the vapour of mercury, whether in vacuo, or when mixed 

 with the air, tends to diffuse itself. At a still less distance, 

 in the liquid state, the particles cohere slightly, or attract 

 one another, appearing to be in a relative position of unstable 

 equilibrium. At a slightly diminished distance, the mercury 

 becomes solid, and the attractive force considerable. The solid 

 mercury resists further compression, so that the action again 

 becomes repulsive. 



During the last century, Boscovich propounded a theory of 

 alternate attractions and repulsions; but I am not aware of the 

 exact nature of his investigations or speculations, never having 

 had an opportunity of consulting his works. They do not appear 

 to have borne fruit, or to have been received with much favour. It 

 is hardly correct however, to apply the word theory to these 

 attractions and repulsions ; we should rather say that they are 

 obvious facts, requiring some theory for their explanation. One 

 theory may be stated thus : the action between two particles is 

 some function of their distance x, which for considerable dis- 

 tances is sensibly equal to —> but for very small distances, 



changes sign several times, becoming finally large and negative, 

 or repulsive. Let us consider whether there is any other 

 tenable hypothesis. 



The Dynamical theory of Grases, due chiefly to the labors of 

 Claudius and Maxwell, helps us somewhat. This theory may be 

 considered as established, and as forming the most important 

 addition which has been made to our real knowledge of the laws 

 of inorganic matter in this generation. Maxwell, for reasons 

 assigned, assumes that the gaseous molecules repel one another 

 according to a certain law which makes the force insensible, 

 except at very small distances. The theory of elastic molecules 

 involves a similar assumption ; for the elasticity of the molecules 

 must be caused by a repulsive action between their atoms ; 

 unless we are to accept an elastic molecule as a finality, beyond 

 which our enquiries into the nature of matter cannot extend. 

 The condensible gases and vapours so closely resemble the permanent 



