418 RICE ART. K 



we are facing the very difficult physical problem of explaining 

 by what mechanism such actions are exerted. We may imagine 

 that an elastic medium is free from everything in the nature of 

 external force, even gravity; we can hardly say, in view of the 

 customary notions of molecules and intermolecular forces, that 

 across the surface which separates two parts of the medium no 

 forces are exerted. Therefore in using the word "stress" as a 

 general term for the actions and reactions across dividing surfaces 

 which accompany strain and vanish when the strain vanishes, 

 we must regard stress as referring to change in the integral of 

 the intermolecular forces exerted across some finite portion of 

 such a surface, if we adopt a molecular theory of the constitution 

 of matter. However, in thermodynamical reasoning we avoid 

 the use of such conceptions, and we take it as a fundamental 

 assumption, well backed by experience, that there is for any 

 solid or fluid medium a condition of equilibrium to which the 

 system can be brought which can be termed conventionally the 

 unstrained state, and from which the medium can be strained 

 by the application of external forces, this process giving rise to 

 reciprocal internal forces across any conceptual surface dividing 

 the medium into two parts. Of such external forces the most 

 obvious example is gravity. This is sometimes referred to as a 

 "body force," being proportional to the mass of each element of 

 volume considered as pulled by the earth, moon, sun, etc. 

 Other types of external forces are the thrusts on the surface of a 

 body exerted by some liquid or gaseous medium surrounding it, 

 or on certain parts of the surface by a solid body in contact 

 with it. The pulls exerted by chains, ropes, etc., may be con- 

 sidered as body forces exerted throughout small parts of the 

 body; e.g., if a pull is exerted by means of a string fastened to a 

 nail embedded in the body, we can regard the medium as 

 actually existing throughout the small hole made by the nail, 

 and a body force existing in that small volume. Or alterna- 

 tively they might be regarded as surface pulls exerted across a 

 definite small portion of the bounding surface of the body. If a 

 body is electrified or magnetized the forces exerted by external 

 magnets and conductors, charged or conveying current, are 

 also external forces. Such external forces must be clearly dis- 



