1046 Dynamic Theory. v 



cells cannot be separated crosswise by a straight line. So that owing 

 entirely to the shape of the cells and the position which they occupy 

 toward each other, the cohesion of the wood is very much greater 

 lengthwise than crosswise. When iron is rolled it becomes stringy and 

 tough in the direction of the rolling and brittle across the "grain," its 

 molecules or particles are forced into a shape and position analogous to 

 that of the wood cells, and like the wood its cohesion is much stronger 

 in one direction than in the other. Those qualities of bodies which de- 

 pend upon the relative position of their particles toward each other, are 

 the effects of some sort of energy to which they have been exposed. 

 Thus steam, water and ice are three forms of the same substance under 

 various forms of condensation depending upon temperature. 



The difference in the effects upon light in its passage through crystals 

 was pointed out (on pp. 397-398) as due to the peculiarities in the 

 molecular structure of the crystals. We have refraction, reflection, ab- 

 sorption, selective absorption, plane and rotary polarization, and other 

 phenomena obviously due to the way in which the molecules of the 

 body are put together, and consequently the form of the intermolecular 

 spaces. The undulations of the ether when they reach the crystal from 

 a luminous body obviously communicate motion to the ether in the 

 body. If this motion is propagated through the body, and communi- 

 cated to the ether beyond, the body is transparent. When a part of 

 the motions of a beam are absorbed and a part transmitted, or reflected, 

 the body appears of the color of the transmitted or reflected rays. 



The molecular constitution of bodies also governs the manner in 

 which they are affected by heat (see pp. 400, 401) as well as the qual- 

 ity, that is, the wave lengths of heat they emit when hot. Instead of 

 rays in a continuous progressive series of wave lengths the body may 

 emit a selected assortment or discontinuous series, missing some here 

 and there. 



Form also governs in regard to sound, as pointed out pp. 375 to 378, 

 and in chap. 86. The different shapes of the various instruments of 

 music cause the different sorts of sound when acted upon by appropri- 

 ate forms of energy. Different lengths of strings, or reeds, or pipes, 

 give sounds of different pitch, etc. Not only the mass-form of the in- 

 struments, but also the molecular arrangements of their materials are 

 concerned in the production of variety in sounds, causing harmonics, 

 &c. A silver bell has a very different sound from that of an iron or 

 copper one. 



It happens in many cases that the application of energy to a body 

 results in changes in the body itself, as well as in giving new forms or 

 directions to the motion when reflected from it. Considerable hills or 

 dunes of sand on some sea coasts are moved by the action of the wind 



