become bright before the fire, as if it had no sediment belonging to it.

How pure and simple soever a body may be in its nature, its transparency will be lost if the construction of its parts is altered. Water in froth, and glass in powder, have no transparency, but instead of it a superficial whiteness. Froth is composed of many minute bubbles, which reflect the light in so many various ways, that by the irregularity and confusion of these reflections the transparency of the matter is lost. The same thing happens when water is converted into snow, which consists of small concreted icicles branching out into various figures: The hoar frost also, consisting of the several particles of dew frozen into ice, have the same white appearance. I suspect that this intense whiteness may arise from a blending of all the colours of light, arising from so many irregular reflections and refractions of its rays in froth and snow. The powder of glass is made up of little angular bodies with polished surfaces, which reflect the light variously, and produce the same effect as in the bubbles of water. When water is consolidated into ice, its pellucidity is nearly preserved, because the structure of its parts

is nearly the same as before. But if ice is irregularly formed, and abounds, as it generally does, with internal bubbles of an etherial fluid separated and detained in it, much of its transparency is lost, and a confused whiteness succeeds.

The quality of transparency is given by a wise ordination of Providence to that fluid substance of water, which is so necessary to the life of all animals, and is the most convenient vehicle of meats and medicines. By seeing through it so clearly, we are immediately informed whether any heterogeneous matters are accidentally intermixt with it, which might be contrary to the intention, or hurtful to the animal frame. We are enabled to judge of the purity of many other bodies by their transparency; and their inward constitution could never be so easily known on any other principle. For this property alone, glass is far more valuable than gold: for the value of gold is arbitrary, but the worth of glass is intrinsic; its cleanliness and transparency recommend it to our daily use for the common purposes of life; it renders visible the most curious and subtile processes in philosophy and chemistry ; and in optics it assists the aged, and gives

to man an insight into the wonders of the. creation, approaching toward that of superior beings. Whatever may become of all the strange relations about a second sight, by this we have a second sight which is not fabulous.

Matter is said to be continuous, when the parts of which it is composed touch or rest upon one another. When the parts are continuous in solid bodies, they must all move at once. A stick of wood, or rod of metal, cannot be moved at one end without moving the other at the same time. A musical string extended vibrates through its whole length when any part of it is struck. Sound may be communicated with a facility which can scarcely be imagined through solid bodies, when their parts are continuous. If the gentlest scratching be made with a pin at one end of the largest and longest piece of timber, it will be heard distinctly when the ear is applied close to the other end. if the parts are discontinued, as by a crack in a bell, or by sawing the timber through, the vibrations are interrupted, and the sound is dissipated. Continuity in fluids does not render it necessary that all the parts should be moved at once, because they can slide backwards

But

backwards and forwards beside each other, so that the vibrations, being extended to a certain distance, become weaker till they vanish; but the vanishing distance will be the fluid is more

greater in proportion as subtile and moveable.

In the element of light it may be hard to say where these vibra tions will stop, when they are propagated through a pure medium. Continuity is as necessary to the senses of feeling and seeing objects, as of hearing sounds the blind man feels what is out of his reach by means of a stick, which he uses as a continuous medium betwixt himself and the object; and all vision is but a more refined sort of feeling distant objects, by means of that line of light which is extended from the object to the eye.

The particles of all vapours are discontinuous, and, being out of contact, can make no impression on one another, but must derive all their expansive force from some other medium, which extends them with violence, whence they are not found to exert any force, but with the concurrence of heat or fire, and being condensed by cold, they will easily return into a continuous mass. How far different bodies are continuous by their G 4

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