OPACITY is a condition of bodies by which they are incapable of transmitting light through them. It seems to depend upon the nature or disposition of the particles of bodies, but its precise cause is, at present, far from being understood. According to Newton, opacity may arise from the unequal densities of the particles of certain sub stances, in consequence of which the rays of light on entering those substances suffer such refractions and reflections as compel them there to remain, and cause them to be finally absorbed ; while, in bodies of a homogeneous nature, as glass, diamond, &e., the light experiences so much leas of these irregular actions that, except when the thickness of the medium is very great, it is enabled to peas quite through them.
The entire absorption of all the light which enters a substance, merely by the multiplied refractions or reflections which it undergoes within the mass, is difficult to conceive ; and the advocates of the undulatory theory ascribe opacity to the unfitness of the pores, or intervals between the molecules of s body, for permitting the vibrations of the particles of ether) without disturbing the molecules, which thus appropriate to themselves the via rim which would otherwise have belonged to the transmitted ethereal vibrations. The same
persons consider tranitpareneT to consist in such a disposition of the molecules of a body that the incident waves of ether can be propagated with a certain degree of freedom through the mats : some impediment to the propagation of the waves may exist in the most transparent substances; and hence when such substances have more than a certain thickness, the waves cease to be transmitted through them. [Taaes rAneaCY ADSORPTION or LIMIT.]