OPACITY, in philosophy, a quality of bodies which renders them impervious to the rays of light. It has been suppos ed that opacity consists in this, that the pores of the body are not all straight. 'Phis doctrine, however, is deficient: for though to have a body transparent, its pores must be straight, or rather open every way; yet it is inconceivable how it should happen, that not only glass and diamonds, but even water, whose parts are so very moveable, should have all their pores open and pervious every way; while the finest paper, or the thinnest gold leaf, should exclude the light, for want of such pores.
So that another cause of opacity must be sought for. Now all bodies have vast ly more pores or vacuities than are ne cessary for an infinite number of rays to pass freely through them in right lines, without striking on any of the parts them selves. For since water is nineteen times lighter or rarer than gold; and yet gold itself is so very rare that magnetic effluvia pass freely through it, without any opposition, and quicksilver is readily received within its pores, and even water itself by compression, it must have more pores than solid parts ; consequently wa ter must have at least forty times as much vacuity as solidity. The cause, therefore, why some bodies are opaque, does not consist in the want of rectilinear pores, pervious every way, but either in the un equal density of the parts, or in the mag nitude of the pores, and their being either empty, or filled with a different matter; by means of which the rays of light, in their passage, are arrested by innumera ble refractions and reflections, till at length falling on some solid part, they become quite extinct, and are utterly absorbed. Hence cork, paper, wood,
&c. are opaque ; while glass, diamonds, &c. are pellucid. For in the confines or joining of parts alike in density, such as those of glass, water, diamonds, &c. among themselves, no refraction or re flection takes place, because of the equal attraction every way ; so that such of the rays of light as enter the first surface, pass straight through the body, except ing such as are lost and absorbed, by striking on solid parts; but in the border ing of parts of unequal density, such as those of wood and paper, both with re gard to themselves, and with regard to the air, or empty space in their larger pores, the attraction being unequal, the reflections and refractions will be very great ; and thus the rays will not be able to pass through such bodies, being continually driven about, till they be come extinct.
That this interruption or discontinuity of parts is the chief cause of opacity, Sir Isaac Newton argues, appears from hence, that all opaque bodies immediately be gin to be transparent, when their pores become filled with a substance of nearly equal density with their parts. Thus, paper, dipped in water or oil, some stones steeped in water, linen cloth dipped in oil or vinegar, &c. become more transparent than before.