On Material Molecules and the Etherial Medium. 263 
parameters or angular elements. For a crystal of the pris- 
matic system A' B' 0' are all unequal ; for a crystal of the 
pyramidal system two of them will become equal. In a 
crystal of the cubic sj^stem A' B' Q' will be all equal. 
Hence, substituting from (16), (19), and (20), in equations 
(6), the general equations of motion of the ether become 
d'^u . f d^u a^u d^u ) ^ ^ , 
d^u' d^u d^u' 
till/ ttU UjiO 
d^v , ( d'^v d^u d^v 1 -r, ^ , 
. , d?v' ^, d'^ v' ^, d^ v 
d^w , f d'^w d^w d^w 1 -r. , 
d^w d?"w' ^, d^ w' 
^ + ^ rf/^ + ^ rfZ^- • • (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 etherial 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 material 
