The well-known specimens of “petrified wood,” common on our coasts, and occurring in some beautiful varieties of beech and acacia in the bays of South Devon, are a further example of infiltration; but the process here must have been somewhat different.
The presence of metallic particles, what lapidaries term the “moss,” in many of our agates, argues an impregnating fluid, thoroughly charged with mineral matter. In most cases this fluid was a ferruginous stream, such as may often be seen issuing from some hidden reservoir, and trickling down the face of a cliff of gault or greensand. This tinges everything that lies in its path with an indelible red stain; it then oozes on through the shingle, and reaches the verge of the sand. The nearest pool becomes saturated between tides, and the suspended crystals of salt, which are of a penetrating character (as the state of your beaching-boots will soon inform you), enter into chemical combination with the metallic rust, and help to conduct its particles into the heart of many a limestone pebble.
Dark inland pebbles, on the other hand, will discharge much of the oxide which they have imbibed, and may be observed to brighten and improve their complexions after a few months’ sponging and tossing in the purer sea-water.
Some of our handsomer pebbles, when cut in two, reveal blotches of metal, which are glossy after polishing on the wheel. This metal is occasionally native iron. The darker varieties of “moss” contain a good deal of manganese, and some silver.
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.