AEPINUS ATOMTZED. 



861 



§ 44. Return to § 42 ; and consider the diagraiu as representing a 

 crystal in its natural uustressed condition, consisting of a vast train of 

 assemblages of atoms with centres in the plane of the paper, and in pa- 

 rallel planes on each side of it. We now see that the forces experienced 

 by the electrions of one qnartet from ail the snrronnding atoms in the 

 plane of the paper woulcl, if uncompensated, displace the geometrical 

 centre of gravity of the quartet to one side or other of the plane of the 

 paper, and we infer that the forces experienced from ail the atoms on 

 the two sides of tins plane give this compensation to keep the centre of 

 gravity of the quartet in the plane. Stretch now the assemblage to any 

 degree eqnally in ail directions. The quartets remain eqnilateral with 

 their centres of gravity in the plane of the paper and parallel planes. 

 Lastly stretch it farther eqnally in ail directions parallel to the plane 

 of the paper, with no component motion perpendicular to this plane. 

 This last stretching diminishes the influence of ail the atoms whose 

 centres are in the plane of the paper tending to displace the centres of 

 gravity of their electrions in one direction from this plane ; and therefore 

 leaves ail the atoms out of this plane to predo minât e, and to cause a 

 definite calculable displacement of the centres of gravity of ail the quar- 

 tets in the contrary direction to the former. 



§ 45. To realize the opérations of § 44, eut a thin hexagonal plate 

 from the middle between two opposite corners of a cubic crystal, or 

 parallel faces of an octahedron. Fix clamps to the six edges of this plate, 

 and apply forces pulling their pairs equally in contrary directions. The 

 whole material of the plate becomes electro- polar with electric moment 

 per unit bulk equal to 4 New, of which the measurable resuit is uniform 

 electrostatical potentials ] ) in vacuous ether close to the two sides of 

 the plate, differing by 4 t. 4 Nexl; where t dénotes the thickness of 

 the plate, x the calculated displacenent of the centre of gravity of each 

 quartet from the centres of the atoms parallel to the two faces of the 

 plate, e the electric mass of an electrion, and N the number of atoms 

 per cubic centimètre of the substance. This crystal of the cubic class is, 

 in Voigt's mathematical theory, the analogue to the electric effect dis- 

 covered in quartz by the brothers Cl'rte, and measured by aicl of thin 

 métal foils attached to the two faces of the plate and metallically con- 

 nected to the two principal électrodes of au electroineter. 



l ) See my Electrostatics and Magnetism. § 512 cor. o. 



