OPACITY

, a quality of bodies which renders them opake, or the contrary of transparency.

The Cartesians make opacity to consist in this; that the pores of the body are not all straight, or directly before each other; or rather not pervious every way.

This 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 fought for.

Now all bodies have vastly more pores or vacuities than are necessary for an infinite number of rays to pass freely through them in right lines, without striking on any of the parts themselves. For since water is 19 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 much more pores than solid parts: consequently water must have at least 40 times as much vacuity as solidity.

The cause therefore, why some bodies are opake, does not consist in the want of rectilinear pores, pervious every way; but either in the unequal density of the parts, or in the magnitude of the pores; and to their being either empty, or filled with a different matter; by means of which, the rays of light, in their pas-| sage, are arrested by innumerable 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 opake; while glass, diamonds, &c, are pellucid. For in the consines or joining of parts alike in density, such as those of glass, water, diamonds, &c, among themselves, no refraction or reflection 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, excepting such as are lost and absorbed, by striking on solid parts: but in the bordering of parts of unequal density, such as those of wood and paper, both with regard 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 become extinct.

That this interruption or discontinuity of parts is the chief cause of Opacity, Sir Isaac Newton argues, appears from hence; that all opake bodies immediately begin 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.