y proportionably growing one less than another toward the
middle or center of the Shell, where there was a very small round white
spot. I could not certainly discover whether the Shell were hollow or not,
but it seem'd fill'd with somewhat, and 'tis probable that it might be
_petrify'd_ as other larger Shels often are, such as are mention'd in the
seventeenth _Observation_.

* * * * *

Observ. XII. _Of _Gravel_ in Urine._

I Have often observ'd the Sand or Gravel of Urine, which seems to be a
_tartareous_ substance, generated out of a _saline_ and a _terrestrial_
substance _crystalliz'd_ together, in the form of _Tartar_, sometimes
sticking to the sides of the _Urinal_, but for the most part sinking to the
bottom, and there lying in the form of coorse common Sand; these, through
the _Microscope_, appear to be a company of small bodies, partly
transparent and partly _opacous_, some White, some Yellow, some Red, others
of more brown and duskie colours.

The Figure of them is for the most part flat, in the manner of Slats or
such like plated Stones, that is, each of them seem to be made up of
several other thinner Plates, much like _Muscovie Glass_, or _Englsh Sparr_
to the last of which, the white plated Gravel seems most likely; for they
seem not onely plated like that, but their sides shap'd also into _Rhombs_,
_Rhomboeids_, and sometimes into _Rectangles_ and _Squares_. Their bigness
and Figure may be seen in the second _Figure_ of the seventh _Plate_, which
represents about a dozen of them lying upon a plate ABCD, some of which, as
a, b, c, d seem'd more regular than the rest, and e, which was a small one,
sticking on the top of another, was a perfet _Rhomboeid_ on the top, and
had four _Rectangular_ sides.

The line E which was the the measure of the _Microscope_, is 1/32 part of
an _English_ Inch, so that the greatest bredth of any of them, exceeded not
1/128 part of an Inch.

Putting these into several liquors, I found _oyl of Vitriol_, _Spirit of
Urine_, and several other _Saline menstruums_ to dissolve them; and the
first of these in less than a minute without _Ebullition_, Water, and
several other liquors, had no sudden operation upon them. This I mention,
because those liquors that dissolve them, first make them very white, not
_vitiating_, but rather rectifying their Figure, and thereby make them
afford a very pretty object for the _Microscope_.

How great an advantage i