589 
of Edinburgh, Session 1865 - 66 . 
The great defect in the theories of cr3^stalline refraction hitherto 
proposed, viz., the theories of Fresnel, Cauchy, Neumann, Macul- 
lagh, and G-reen, is the neglect of the action of the material mole- 
cules. In these theories the ether within crystals is supposed to 
possess special properties different from those which it possesses in 
vacuo, such as ])ossessing different degrees of elasticity in different 
directions ; the ether in every body being supposed to possess an 
elasticity peculiar to itself. In none of these theories are any con- 
siderations advanced to show how the ether might he supposed to 
have acquired the special properties which it is found necessary to 
assume that it possesses, in order that these theories may account 
for phenomena. So that, even if they were satisfactory in other 
respects, an important desideratum would still be left. A few 
remarks are then made on the question whether the ether is a 
continuous or discontinuous medium. In the present paper the 
general equations of motion are obtained on both suppositions. In 
vacuo the equations of motion are known to be of the same form 
whichever supposition is adopted. 
With respect to the molecular action between matter and the 
ethereal medium, it is supposed to be sensible at only very small 
distances. That this is true, in general, for molecular forces, is 
shown by such facts as the following : — When a solid, as a piece of 
marble, is reduced to powder, no amount of pressure will make the 
powder again cohere into a solid mass. Two clean surfaces of lead 
may be made to cohere, but not if there is the slightest film of 
oxide. There are a number of other facts of the same kind. The 
height CO which the fluid rises, or is depressed, in a capillary tube is 
independent of the thickness of the tube. Also, to take the case of 
water — if the thinnest film of grease be present in the tube, the 
water is depressed instead of elevated, showing that the sphere of 
action of the molecular forces of the glass on the water is less than 
the thickness of the thinnest film of grease. The strength of a 
wire, also, is dependent only on its section. Also, if we take a 
crystal of Iceland spar, and reduce it by cleavage, or otherwise, to 
as small dimensions as possible, it is found that the crystals succes- 
sively obtained are in every respect similar in their optical pro- 
perties to the original crystal. The portions of the crystals, 
therefore, removed by cleavage, have no effect on the optical pro- 
