An EXPLANATION of the FOURTH PLATE
FIG. I.
Some curious Forms of small Diamonds, or shining Sparks in Flints
B Reaking a Flint-Stone by Accident,
a Cavity was found therein, all crusted over with
a pretty candy'd Substance, A A, &c. some Parts of which, such as B B B B, on
turning them to the Light, reflected its Rays in a very glittering and lively Manner : And
bringing it to the Microscope, the whole Surface of the Cavity appeared beset with a Mul-
titude of little chrystalline or Diamond-like Bodies, curiously shaped and polished, as the
Drawing represents them.
The vivid Repercussions of Light were, on Examination, observed to be reflected partly
from the plain external Surfaces of these regularly figured Bodies, and partly from within
the pellucid Bodies themselves, that is, from some Surfaces thereof opposite to those Sur-
faces which were next the Eye.
But these Sparks being so small, that no certain Experiments could easily be made with
them, Dr. HOOKE procured several of the shining Stones or Chrystals found in great
Quantities in Cornwall, and therefore called most commonly Cornisb Diamonds ; which
growing in the hollow Cavities of Rocks, much after the same manner as these did in the
Flint, and having regularly-shaped Surfaces nearly of the same Form with theirs, he ima-
gined might afford some convenient Help towards ascertaining the Properties of such Kinds
of Bodies.
By these he found, that the brightest Reflections of Light proceeded from within the
pellucid Body ; that is, the Rays admitted through the pellucid Substance, in their getting
out on the opposite Side, were very vividly reflected by the contiguous and strong re-
flecting Surface of the Air ; so that more Rays were reflected to the Eye by this Surface,
(though the Ray in entering and getting out of the Chrystal had suffered a double Re-
fraction) than there were from the outward Surface of the Glass, where it had suffered no
Refraction at all.
It is proper here to take notice,
that our Author mentions his Examination of several
Sorts of Sands with his Microscope ; amongst which he found divers most curiously
shaped, as these in the Flint were ; and which, he therefore supposes, not made by the
Comminution of larger chrystalline Bodies, but formed by the Concretion or Coagulation
of Water, or some other Fluid.
Sand, however, generally seems nothing else but exceeding small Pebbles, or some small
Pieces of bigger Stones, angled for the most part irregularly, without any certain Shape,
and having its little Grains frequently flawed and broken.
There are many Sorts of Sands, (as many perhaps as there are of Stones) which differ
from one another both in Colour, Figure, and Size. And as amongst Stones some are
called precious for their Excellency, so also amongst Sands, there are some that deserve the
same Epithet for their Beauty. The Grains of Sea-Sands are very large, and afford great
Variety of all Shapes and Colours, both opake and transparent. River-Sands are smaller-
grained, of different Colours and Forms ; and the Inland or Pit-Sands, vary also exceed-
ingly, being some white, some brown, some yellow, &c.
The white, or Writing-Sand, appears through the Microscope like transparent Pieces of
Allum, Sal-Gem, or Chrystal, but most commonly irregular. The coarser Sands are usu-
ally more opake, but even amongst them many Grains may be found both clear and beau-
tiful. Some Kinds of black Sand are brought from the East-Indies, and likewise from Vir-
ginia and other Parts of America, with polished shining Surfaces, many of which will be
attracted by the Load-stone ; and there are certain reddish Sands (brought from abroad
also) which present a delightful Sight to the Eye, not unlike a Jeweller's Box of Trea-
sure, wherein you see Rubies of a Rose-Colour, others of a deeper Red, Sappbires, Eme-
ralds, Hyacinths, Topazes, and in short, all Sorts of transparent Stones
| Spectacle de la Nat? Dial. XIX. |
