From what has been said about impregnation, we may readily conclude that the _colours_ found in our agate-pebbles have been chemically inwrought, and that the symmetrical _patterns_, over which these colours are disposed, are not due to a succession of “layers,” as in the ribbon-jaspers and Scotch pebbles, but must be referred to the organization of some extinct zoophyte, whose skeleton is preserved in the existing fossil. The cause of translucency, or even _transparency_, in some pebbles is, no doubt, to be sought in their finer and more even texture, but especially in the latter quality. Sir Isaac Newton was of opinion that the opacity of certain substances is simply a result of their cross-grained composition. He held that in a transparent body the particles must be regularly and evenly disposed at equal intervals, so that a ray of light entering such a substance would pass steadily on, according to the known laws of gravity and motion, meeting with no obstruction beyond that of the homogeneous density of the medium which it had to traverse. Whereas, in opaque bodies, he supposes these constituent particles to be unevenly disposed, and that the ray which enters at the surface is, as it were, pushed about and thrust aside, and at length lost to sight. The purity of spring-water, when undisturbed, illustrates this beautiful theory of the great philosopher; and, among solid substances, good glass, especially plate-glass which, after fusing in the furnace, has been carefully run and settled in the frames. It may also be simply shown in the act of wetting or steeping certain dry substances. Thus, stout writing-paper, owing to its cross texture, is opaque when dry; but if we immerse a sheet of it in oil, and then hold it up to the light, we find it has become transparent. For the smooth medium thus imbibed has entered its pores, and _equalized_ the general texture. We find also a striking argument for the probable truth of Newton’s eagle-eyed conclusion in certain minerals which possess double refraction. ICELAND SPAR, one of these, is perhaps the most perfectly transparent solid with which we are acquainted. Now, Iceland Spar _transmits_ both the rays, as we see in the twofold image which is presented to us. But TOURMALINE is opaque, even the red specimens looking almost black; and we find that Tourmaline _absorbs_ one of the two rays of polarized light. This singular stone polarizes a beam of light if it enter in any direction but that of the longer axis. No solid with which we are acquainted is absolutely transparent. And there is no perfect _reflector_ known. The best-polished surface _absorbs nearly one-half_ of that quantity of light which strikes upon it.