RELATIONS TO OTHER SCIENCES 73 



The idea that force must be a pressure between contiguous portions 

 of substance is derived directly from the notion of the impenetra- 

 bility of matter. This is why the incompressible medium of Lord 

 Kelvin's theory seems so simple a conception; it is the naked em- 

 bodiment of the idea of impenetrability associated with inertia. 



It is entirely natural that such ideas as impenetrability and inertia, 

 borne in upon us as they are by our experience of matter in bulk, 

 should affect our theorizing, but it should never be forgotten that 

 as fundamental postulates they have no more authority than any 

 others that might be assumed that will coordinate the same facts 

 of observation. 



But passing from this more speculative region we find a pretty 

 general agreement on the rough outlines of the structure of matter. 

 With one notable exception most physicists and chemists agree in 

 the idea that matter is atomic or molecular in structure, and that 

 these molecules are in a state of more or less energetic translator^ 

 motion, bounding and rebounding from each other. This seems to 

 be the mechanical hypothesis which coordinates the largest number 

 of facts. 



A portion of matter is conceived as in a condition of equilibrium 

 under three pressures: the cohesive pressure due to mutual attrac- 

 tion between all molecules which are not farther apart than 50 to 

 300 millionths of a millimeter; the external pressure, which also acts 

 to cause contraction; and the internal pressure, which balances the 

 two former, and is due to a repulsive force called the force of impact, 

 which is usually supposed to be exerted only between contiguous 

 molecules. 



In the solid and liquid states the cohesive pressure is usually very 

 great compared with the external pressure. In case of gases it nearly 

 vanishes. The force between molecules is thus conceived as an attrac- 

 tion which increases rapidly as they approach, until at a certain dis- 

 tance it is balanced by a repulsive force which, increasing still more 

 rapidly, is the controlling force at all less distances. 



Lord Kelvin has recently followed out a study of equilibrium con- 

 ditions in a group of atoms which are assumed to have no mutual 

 influence until within a certain distance, then to attract each other 

 with a force that increases as they approach still nearer, rising to 

 a maximum and then diminishing, and finally becoming a repulsion 

 when the atoms are very near. He remarks, "It is wonderful how 

 much toward explaining the crystallography and elasticity of solids, 

 and the thermo-elastic properties of solids, liquids, and gases, we find 

 without assuming in the Boscovitchian law of force more than one 

 transition from attraction to repulsion." 



The fundamental soundness of the conception of matter as having 

 a grained structure of some sort seems to be established by the re- 



