[From the Encyclopaedia Britannica.] 



LXXIV. Attraction. 



THAT the different parts of a material system influence each other's mo- 

 tions is a matter of daily observation. In some cases we cannot discover any 

 material connection extending from the one body to the other. We call these 

 cases of action at a distance, to distinguish them from those in which we can 

 trace a continuous material bond of union between the bodies. The mutual 

 action between two bodies is called stress. When the mutual action tends to 

 bring the bodies nearer, or to prevent them from separating, it is called tension 

 or attraction. When it tends to separate the bodies, or to prevent them from 

 approaching, it is called pressure or repulsion. The names tension and pressure 

 are used when the action is seen to take place through a medium. Attraction 

 and repulsion are reserved for cases of action at a distance. The configuration 

 of a material system can always be defined in terms of the mutual distances 

 of the parts of the system. Any change of configuration must alter one or 

 more of these distances. Hence the force which produces or resists such a 

 change may be resolved into attractions or repulsions between those parts of 

 the system whose distance is altered. 



There has been a great deal of speculation as to the cause of such forces, 

 one of them, namely, the pressure between bodies in contact, being supposed 

 to be more easily conceived than any other kind of stress. Many attempts 

 have therefore been made to resolve cases of apparent attraction and repulsion 

 at a distance into cases of pressure. At one time the possibility of attraction 

 at a distance was supposed to be refuted by asserting that a body cannot act 

 where it is not, and that therefore all action between different portions of 

 matter must be by direct contact. To this it was replied that we have no 

 evidence that real contact ever takes place between two bodies, and that, in 

 fact, when bodies are pressed against each other and in apparent contact, we 



