On Material Molecules and the Etkerial Medium. 263 



parameters or angular elements. For a crystal of the pris- 

 matic system A' B' C are all unequal ; for a crystal of the 

 pyramidal system two of them will become equal. In a 

 crystal of the cubic system A' B' C will be all equal. 



Hence, substituting from (16), (19), and (20), in equations 

 (6), the general equations of motion of the ether become 



d 2 u . ( d 2 u a 2 u d 2 u ) ^ _ , 



a , d 2 u' T)/ d 2 u ~, d 2 u' 

 + ~dx" z + W 1 + ° 



. , d?v d 2 v ~, d 2 v 



+ HaS' z + + 0 W* 



d 2 w . [ ' d 2 w d 2 w d 2 w 1 T1 . 



+ A w* + B W* + ' • • (21) 



The coefficients of v w', in the second and third equa- 

 tions, have in reality different values from the coefficient 

 of u' in the first equation. But, as will afterwards be seen, 

 in the integration of the equations, these terms give rise to 

 others involving the square of the wave-length, and are 

 therefore only important in the explanation of the dispersion 

 of light. Hence we may, without sensible error, suppose 

 the coefficients of v and w to have the same value as the 

 coefficient of u'. 



We must now find the equations of motion of the material 

 molecules. Now, whatever be the nature of the molecular 

 forces of cohesion, &c., all such forces are here supposed, in 

 accordance with the tendency of modern science, to have 

 their origin either in the action which matter exerts upon 

 matter, or in the action which the ethcrial medium exerts 

 upon matter. We shall therefore suppose the material 

 molecules to be retained in their positions of equilibrium 

 by the forces exerted upon them by the other materia] 



