An EXPLANATION of the SEVENTH PLATE
A Piece of Charcoal
A Piece of Stick charred or burnt till it becomes black
, if broke short between the
Fingers, appears with a shining smooth Surface, resembling the Surface of black
Sealing-Wax ; which examined by a small Magnifier, exhibits Abundance of such Pores
as are visible to the naked Eye in many Kinds of Wood, ranged round the Pith as well
circularly as radiating from a Center. These appear every where in the Substance of the
Coal, drilling it from End to End, so that you may easily blow through it.
Manner of Charring Coal, vid. EVELYN'S Sylva, p. 100, 101,
But besides these many great and irregular Spots or Pores, if a Glass that magnifies
much be made use of, an infinite Number of exceedingly small and very regular Pores
will be discovered, so thick, so orderly set, and so close to one another, that very little
Room is left between them to be occupied by a solid Body ; for the intermediate Par-
titions of these Pores appear so thin in some Places, that a Honey-comb is not less solid,
tho' in others they are much thicker, in proportion to the Holes.
The exceeding Smallness and Closeness of these Pores may be conceived in some degree
by their Numbers ; for no less than one hundred and fifty of them were counted in a Line
not more than the eighteenth Part of an Inch long ; consequently, a Line of an Inch in
Length must contain two thousand seven hundred of them : and about five Millions seven
hundred twenty five thousand three hundred and fifty of the like Pores must be in a circular
Area of an Inch Diameter. Nay, Cocus, black and green Ebony, Lignum Vitæ, Guaja-
cum, &c. have their Pores still smaller, and more numerous ; so exquisite are the Pipes
or Sluices whereby the Juices of Vegetables are conveyed !
PLATE VII. FIG. 2.
A Piece of petrified Wood
THE Pores in this Object were not so much bigger than those in the foregoing Fi-
gure, as the Draught before us shews them ; for this was viewed by a Microscope
that magnified six times more than what was used for the Piece of Charcoal, and the
Drawing made in the same Proportion. Each Pore, however, was nearly half as large
again as those in the burnt Wood, and the Disposition of the whole exactly in the same
Figure and Order as the small Pores of Charcoal, but there were none of the larger Pipes
or Cavities before deseribed in that.
The Subject under Examination seemed to have been a Part of some large Tree, that
had been broken off by Rottenness, before it became petrified. And Dr. HOOKE declares,
that all he had seen of this Kind seemed to have been rotten before the Petrifaction began :
and that he was confirmed in this Opinion, by examining a vast large Oak, which with mere
Age was rotten as it stood, whose Wood in Colour, Grain, and Shape, appeared exactly
like this petrified Substance. He likewise observes, that all those microscopical Pores, which
in sappy and found Wood are filled with the natural Juices of the Tree, were found in
this (when viewed with magnifying Glasses) empty, like those of Charcoal, but much
larger than any he had seen in Charcoal.
Pieces of petrified Wood are however very different in Shape, Colour, Grain, Tex-
ture, and Hardness ; some being brown and reddish ; others grey like an Hone ; some
black, flint-like, hard and brittle ; others soft like a Slate or Whetstone.
In this Petrifaction the Parts seemed not at all altered from their Position whilst Wood,
having the Pores of Wood still remaining, with a manifest Difference between the Grain
and Bark ; but it differed from Wood in Weight, Hardness, Closeness, Incombustible-
ness, and Brittleness.
Its Weight was to common Water as Three and a Quarter to One ; whereas few
English Woods, when very dry, are quite equal in Weight to Water.