gravity.

[Illustration]

4. The pieces of it which are found have the figure of an oblique
parallelepiped; each of the six faces being a parallelogram; and it
admits of being split in three directions parallel to two of these
opposed faces. Even in such wise, if you will, that all the six faces
are equal and similar rhombuses. The figure here added represents a
piece of this Crystal. The obtuse angles of all the parallelograms, as
C, D, here, are angles of 101 degrees 52 minutes, and consequently
the acute angles, such as A and B, are of 78 degrees 8 minutes.

5. Of the solid angles there are two opposite to one another, such as
C and E, which are each composed of three equal obtuse plane angles.
The other six are composed of two acute angles and one obtuse. All
that I have just said has been likewise remarked by Mr. Bartholinus in
the aforesaid treatise; if we differ it is only slightly about the
values of the angles. He recounts moreover some other properties of
this Crystal; to wit, that when rubbed against cloth it attracts
straws and other light things as do amber, diamond, glass, and Spanish
wax. Let a piece be covered with water for a day or more, the surface
loses its natural polish. When aquafortis is poured on it it produces
ebullition, especially, as I have found, if the Crystal has been
pulverized. I have also found by experiment that it may be heated to
redness in the fire without being in anywise altered or rendered less
transparent; but a very violent fire calcines it nevertheless. Its
transparency is scarcely less than that of water or of Rock Crystal,
and devoid of colour. But rays of light pass through it in another
fashion and produce those marvellous refractions the causes of which I