86 ELEMENTS OF STATIC ELECTRICITY. 



Now the molecules of a metal rod may be com- 

 pared to an infinite number of these lines of balls ; and 

 it may be assumed that a heat impulse, or an electric 

 impulse, given at one end, moves along these lines 

 in some way analogous to that in which the impulse 

 moves along the lines of balls. 



We are not obliged to confine ourselves to any spe- 

 cific method of movement; but may suppose a wave 

 movement, similar to that which takes place in the 

 slack rope, or on the water, if it shall seem to accord 

 best with known facts and phenomena. 



In the polarization of light, it is supposed that the 

 waves assume a certain phase, in conformity with the 

 special arrangement of the molecules of the crystal. 

 Hence if the crystals are cut from the same block, and 

 placed in the same position, the phase will be the same 

 for each, and the light will pass through. But if the 

 second is turned at right angles to the first, the phase 

 produced by passing through the first will not be in con- 

 formity with the arrangement of the molecules in the 

 second, and hence the light cannot pass through. 



Suppose the arrangement -of the molecules to be in 

 layers, or strata, like the sheets composing a ream of 

 note-paper, placed in a vertical position ; the waves 

 of ether would assume a vertical phase, and, meeting 

 the second crystal, placed in the same position, would 

 pass through. But if the second were turned, so as to 

 bring its strata to a horizontal position, the vertical 

 waves would be broken, and could not pass through. 



Instead of the ether we may suppose the molecules 

 themselves thrown into waves, whose phase would con- 

 form to the structure of the crystal, and the same result 

 would evidently follow. 



