I have very often in a Morning, when there has been a great _hoar-frost_,
with an indifferently magnifying _Microscope_, observ’d the small _Stiriæ_,
or Crystalline beard, which then usually covers the face of most bodies
that lie open to the cold air, and found them to be generally _Hexangular
prismatical_ bodies, much like the long Crystals of _Salt-peter_, save
onely that the ends of them were differing: for whereas those of _Nitre_
are for the most part _pyramidal_, being terminated either in a point or
edge; these of Frost were hollow, and the cavity in some seem’d pretty
deep, and this cavity was the more plainly to be seen, because usually one
or other of the six _parallelogram_ sides was wanting, or at least much
shorter then the rest.
But this was onely the Figure of the _Bearded hoar-frost_; and as for the
particles of other kinds of _hoar-frosts_, they seem’d for the most part
irregular, or of no certain Figure. Nay, the parts of those curious
branchings, or _vortices_, that usually in cold weather tarnish the surface
of Glass, appear through the _Microscope_ very rude and unshapen, as do
most other kinds of frozen _Figures_, which to the naked eye seem exceeding
neat and curious, such as the Figures of _Snow_, frozen _Urine_, _Hail_,
several _Figures_ frozen in common Water, &c. Some Observations of each of
which I shall hereunto annex, because if well consider’d and examin’d, they
may, perhaps, prove very instructive for the finding out of what I have
endeavoured in the preceding Observation to shew, to be (next the _Globular
Figure_ which is caus’d by _congruity_, as I hope I have made probable in
the sixth _Observation_) the most simple and plain operation of Nature, of
which, notwithstanding we are yet ignorant.